Recherche

Cugnoni Joël

Professeur HES associé

Compétences principales

Essais de structures et matériaux

Contrat principal

Professeur HES associé

Bureau: C61a

Haute école d'Ingénierie et de Gestion du Canton de Vaud
Route de Cheseaux 1, 1400 Yverdon-les-Bains, CH

Institut
Institut de conception mécanique et technologie des matériaux (COMATEC)

HES-SO Master
Av. de Provence 6, 1007 Lausanne, CH

Domaine
Technique et IT

Filière principale
Master of Science in Engineering

BSc HES-SO en Systèmes industriels - Haute école d'Ingénierie et de Gestion du Canton de Vaud

Conception
Modélisation et simulation (Modeling and numerical simulations)
Fabrication additive - Impression 3D
Matériaux composites

MSc HES-SO en Engineering - HES-SO Master

Fabrication additive - Impression 3D
Modélisation et simulation (Modeling and numerical simulations)

Terminés

Structures composites multifonctionnelles et connectées pour l'internet des objets

AGP

Rôle: Requérant(e) principal(e)

Financement: HES-SO Rectorat

Description du projet : Grâce à leur rapport rigidité-masse imbattable, les matériaux composites (p.ex carbone / époxy) sont rapidement devenus incontournables dans les domaines des transports (aéronautique, automobile, mobilité douce), les applications biomédicales (fixateurs, prothèses, imagerie) et dans les sports (vélos, skis). La construction de structures en matériaux composites s'apparente à un mode de fabrication additive qui implique la superposition de différentes couches de matériaux (plis ou noyaux sandwich) remplissant chacune un rôle spécifique en termes de fonctionnalités mécanique et thermique mais aussi potentiellement électrique, optique ou électromagnétique. Après dépose, la résine est réticulée sous pression sur un moule pour obtenir la forme et la résistance finale de la pièce. Cependant, les fonctionnalités mécaniques et électronique/communication sont généralement réalisées avec deux sous-systèmes distincts : une structure porteuse et un ensemble de câblage, connecteurs et composants électroniques (PCB) ce qui augmente le nombre de composants à assembler et le poids. L'idée principale de ce projet s'inspire de la forte similitude entre la fabrication de PCB et des structures composites, qui sont tous deux produits par assemblage de couches polymères renforcés de fibres. L'objectif de ce projet est de combiner les derniers développements de la fabrication composite (composites thin-ply et hybrides par dépose de plis robotisée) et des technologies de fabrication additive à base d'impression jet d'encres conductrices pour développer une technologie de fabrication permettant l'intégration de fonctions électriques/électroniques (capteur, microcontrôleurs, distribution de puissance) et de communication (antennes, bus) au sein même d'une pièce composite structurelle. Les défis à relever sont principalement le développement et la fiabilisation de méthodes de fabrication de circuits sur supports composites 3D par impression directe et/ou par impression et lamination de PCB imprimés sur films flexibles. L'objectif final est de prototyper une méthode de fabrication digitale et automatisée allant de la dépose de plis à l'impression du circuit électrique. Des techniques d'interconnexion dans l'épaisseur du laminé compatible avec la fabrication composite seront également développées, notamment en utilisant des résines époxy conductrices et/ou des inserts métalliques. Finalement, des méthodes de conception et dimensionnement doivent être mises au point pour s'assurer de l'intégrité du système. Les technologies nécessaires à la réalisation de ce concept sont aujourd'hui disponibles, mais l'intégration de ces techniques et surtout leur validation en termes de performance et fiabilité mécanique et électrique restent à valider en laboratoire et sur un démonstrateur (axe de machine / bras robot intégré) Cette technologie de fabrication de composite multifonctionnel a un fort potentiel grâce au gain de poids et donc d'économie d'énergie (aéronautique, spatial, transports), mais aussi en terme d'augmentation de valeur ajoutée par pièce (produit à haute valeur ajoutée et automatisable) et comme base pour l'intégration de l'internet des objets (monitoring, qualité, feedback). Plusieurs projets sont actuellement en phase de lancement en Europe sur ce sujet. Avec ce financement, notre équipe sera idéalement positionnée pour développer des projets de transfert technologiques avec l'industrie aérospatiale et des transports, l'industrie des machines mais aussi le biomédical au niveau Suisse et Européen.

, , , , , , , , , , , , , , , , , , , , , Equipe de recherche au sein de la HES-SO: Maturo Jonas Bircher Fritz Bürgy Olivier Huber Benjamin Renner Johannes Mauron Muriel Perritaz Bastien Compagnon Dimitri Schneuwly Vincent Bovay Justine Brügger Luca Schaad Nicolas Carrie Natalia Cugnoni Joël Nardin Raphaël Balestra Gioele Brodard Patricia Stefanucci Alfonso Lapaire Clovis Blum Remo Chandran Rajasundar Giuntoli Bruno

Partenaires académiques: COMATEC; FR - EIA - Institut IPRINT; Cugnoni Joël, COMATEC

Durée du projet: 01.09.2019 - 27.12.2021

Montant global du projet: 250'000 CHF

Statut: Terminé

2023

Implementation and parametric study of J-integral data reduction methods for the translaminar toughness of hierarchical thin-ply composites

Article scientifique ArODES

Guillaume Broggi, Joël Cugnoni, Véronique Michaud

Engineering Fracture Mechanics, 2023, vol. 282, article no. 109169

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Résumé:

Three different J-integral formulations to derive the experimental translaminar toughness of composites from compact tension tests with a large-scale fracture process zone are implemented and discussed. They improve the existing approaches by taking advantage of stereo-digital image correlation to acquire full-field displacement fields. A field fitting procedure based on robust and efficient piecewise cubic smooth splines addresses noise-related issues reported in previous studies. Additionally, the paper proposes a novel crack tip extraction procedure to report the energy release rate as a function of the crack increment, even if knowledge of the crack tip is not required for the proposed J-integral method. The three methods are discussed in light of a parametric study conducted on synthetic and experimental data, including artificially noisy data. The study reveals that the proposed J-integral methods are suitable for translaminar toughness evaluation of a wide range of materials without the need for restrictive assumptions. However, variations in propagation values were observed when applied to experimental data. Finally, guidelines are drawn to chose the most suitable parameters for the algorithms that are proposed as a Python package.

2018

Towards aerospace grade thin-ply composites :

Article scientifique ArODES

effect of ply thickness, fibre, matrix and interlayer toughening on strength and damage tolerance

Joël Cugnoni, R. Amacher, S. Kohler, J. Brunner, E. Kramer, C. Dransfeld, W. Smith, K. Scobbie, L. Sorensen, J. Botsis

Composites Science and Technology, 2018, vol. 168, pp. 467-477

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Résumé:

Thin-ply composites represent a promising approach to further improve the performance of carbon fibre composite structures thanks to their ability to delay the onset of matrix cracking and delamination up to the point of fibre dominated failure. However, this increased strength comes with a more brittle failure response which raises concerns on damage tolerance. Thus a careful material optimization is needed to address this trade-off. In this work, eight different formulations of thin-ply composites ranging from low modulus to high modulus carbon fibres are evaluated to understand the effects of the fibre and matrix constituents on the onset of damage and strength in unnotched tensile (UNT) tests of quasi isotropic laminates for ply thicknesses between 300 and 30 microns. The obtained experimental data are combined in master curve diagrams for simplified material selection process. It is observed that certain thin-ply composites with a ply thickness t < 134 μm can reach UNT strength corresponding to or approaching the ultimate strain of the fibres as well as UNT stress at onset of damage as high as 92% of the latter. Based on this knowledge, a novel aerospace grade toughened thin-ply composite system is developed which can reach a quasi-isotropic UNT strength above 1 GPa (>95% of the fibre strain). The newly developed composite is further optimized to improve damage tolerance by toughening the resin and selected interfaces. The effect of those modifications on damage tolerance are evaluated through compression strength after impact (CAI) tests and open hole tensile tests (OHT). It is found that an optimized interlayer toughened thin-ply composite based on 68 microns plies of intermediate modulus fibre can reach both outstanding strength properties with comparable or better CAI and OHT strength compared to current aerospace grade composites.

2017

An efficient method for fiber bridging traction identification based on the R-curve :

Article scientifique ArODES

formulation and experimental validation

Guillaume Frossard, Joël Cugnoni, Thomas Gmür, John Botsis

Composite Structures, 2017, vol. 175, pp. 135-144

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Résumé:

Fiber bridging is one of the main toughening mechanisms in mode I interlaminar and intralaminar fracture of laminated composites. Intact fibers exert closing forces on both faces of the crack, restraining crack propagation. An effective identification procedure is required to characterize bridging tractions and to develop accurate prediction models. The method proposed in this work is based on the resistance (R)-curve and assumes fiber bridging tractions decreasing non-linearly with respect to the crack opening displacement, following a parametric function. The distribution parameters are identified by a fixed-point iterative procedure where the energy release rate computed in a numerical model is matched with the values obtained experimentally in two points of the R-curve. The method is applied and validated through three cases, from low bridging intensity in thin-ply delamination to high bridging intensity in intralaminar fracture.

Deformation behavior of nano-porous polycrystalline silver. Part II :

Article scientifique ArODES

simulations

S. Zabihzadeh, Joël Cugnoni, L. I. Duarte, S. Van Petegem, H. Van Swygenhoven

Acta Materialia, 2017, vol. 131, pp. 564-573

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Résumé:

Three-dimensional finite element simulations of nano-porous silver structures are performed to understand the correlation between the porous morphology and the mechanical behavior. The nanostructures have been obtained from ptychographic X-ray computed tomography. The simulations allow distinguishing between the interplay and role of the ligament size, the pore morphology and the porosity, and therefore provide a better comprehension of the experimental observations. We show that the proposed model has a predictive character for mechanical behavior of nano-porous silver.

2016

Effects of large scale bridging in load controlled fatigue delamination of unidirectional carbon-epoxy specimens

Article scientifique ArODES

E. Farmand-Ashtiani, Joël Cugnoni, J. Botsis

Composites Science and Technology, 2016, vol. 137, pp. 52-59

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Résumé:

Fatigue delamination growth in composites is accompanied by large scale bridging (LSB) that yields important toughening effects. However, the extent of this mechanism depends on the laminate geometry rendering its modeling a challenging task. This work presents a combined experimental/numerical study on characterization of specimen thickness dependence of LSB in fatigue delamination. Double cantilever beam specimens of different thicknesses (h = 2, 4 and 8 mm), equipped with arrays of multiplexed fiber Bragg grating sensors, are subjected to mode I fatigue loads. Measured strain data with the sensors are employed to identify the bridging tractions and subsequently compute the energy release rate (ERR) due to the bridging as well as the ERR at the crack tip. The obtained results confirm that fatigue delamination growth strongly depends on the specimen geometry when LSB prevails. It is shown that both the extent of bridging and critical ERR at failure increase by increasing the specimen thickness while the maximum bridging traction at the crack tip is found independent of the specimen geometry. The identified traction-separation relations serve to establish a power correlation, between the crack growth rate and ERR at the crack tip which is independent of the specimen thickness.

Mode I interlaminar fracture of carbon epoxy laminates :

Article scientifique ArODES

effects of ply thickness

Guillaume Frossard, Joël Cugnoni, Thomas Gmür, John Botsis

Composites Part A: Applied Science and Manufacturing, 2016, vol. 91, part 1, pp. 1-8

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Résumé:

In this work, the influence of ply thickness on strain energy release rate (ERR) in delamination of carbon-epoxy laminates is addressed. Specimens with three ply thicknesses: 0.030, 0.075 and 0.150 mm are tested. While the ERR at onset of crack propagation is independent of ply thickness, the plateau level is much lower in thin-ply laminates than in thicker ply ones. This effect is attributed to changes in microstructure, caused by the tow spreading process involved in the fabrication of prepregs. Fiber and matrix rich regions observed only in thick ply laminates promote the development of large bundles of bridging fibers which exert large closing forces, leading to higher ERR in thick-ply laminates. Fiber bridging distribution is identified using an iterative procedure from R-curve results, and implemented in cohesive element models. This identification method provides reliable results, as the simulated load-displacement curves are in good agreement with experimental results.

Peri-implant bone adaptations to overloading in rat tibiae :

Article scientifique ArODES

experimental investigations and numerical predictions

Marco Piccinini, Joël Cugnoni, John Botsis, Patrick Ammann, Anselm Wiskott

Clinical Oral Implants Research, 2016, vol. 27, no. 11, pp. 1444-1453

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Résumé:

Objectives : (i) To assess the effects of mechanical overloading on implant integration in rat tibiae, and (ii) to numerically predict peri-implant bone adaptation. Materials and methods : Transcutaneous titanium implants were simultaneously placed into both tibiae of rats (n = 40). After 2 weeks of integration, the implants of the right tibiae were stimulated daily for 4 weeks with loads up to 5N (corresponding to peak equivalent strains of 3300 ± 500 με). The effects of stimulation were assessed by ex vivo mechanical tests and quantification of bone mineral density (BMD) in selected regions of interests (ROIs). Specimen-specific finite element models were generated and processed through an iterative algorithm to mimic bone adaptation. Results: Bilateral implantation provoked an unstable integration that worsened when mild (2–4N) external loads were applied. In contrast, a stimulation at 5N tended to “counterbalance” the harmful effects of daily activity and, if applied to well-integrated specimens, significantly augmented the implants' resistance to failure (force: +73% P < 0.01, displacement: +50% P < 0.01 and energy: +153% P < 0.01). Specimen-specific numerical predictions were in close agreement with the experimental findings. Both local and overall BMD variations, as well as the implants' lateral stability, were predicted with small errors (0.14 gHA/cm3 and 0.64%, respectively). Conclusions : The rats' daily activity detrimentally affects implant integration. Conversely, external stimulations of large magnitudes counterbalance this effect and definitively improve integration. These changes can be predicted using the proposed numerical approach.

Numerical prediction of peri-implant bone adaptation :

Article scientifique ArODES

comparison of mechanical stimuli and sensitivity to modeling parameters

Marco Piccinini, Joël Cugnoni, John Botsis, Patrick Ammann, Anselm Wiskoot

Medical Engineering Physics, 2016, vol. 38, no. 11, pp. 1348-1359

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Résumé:

Long term durability of osseointegrated implants depends on bone adaptation to stress and strain occurring in proximity of the prosthesis. Mechanical overloading, as well as disuse, may reduce the stability of implants by provoking bone resorption. However, an appropriate mechanical environment can improve integration. Several studies have focused on the definition of numerical methods to predict bone peri-implant adaptation to the mechanical environment. Existing adaptation models differ notably in the type of mechanical variable adopted as stimulus but also in the bounds and shape of the adaptation rate equation. However, a general comparison of the different approaches on a common benchmark case is still missing and general guidelines to determine physically sound parameters still need to be developed. This current work addresses these themes in two steps. Firstly, the histograms of effective stress, strain and strain energy density are compared for rat tibiae in physiological (homeostatic) conditions. According to the Mechanostat, the ideal stimulus should present a clearly defined, position and tissue invariant lazy zone in homeostatic conditions. Our results highlight that only the octahedral shear strain presents this characteristic and can thus be considered the optimal choice for implementation of a continuum level bone adaptation model. Secondly, critical modeling parameters such as lazy zone bounds, type of rate equation and bone overloading response are classified depending on their influence on the numerical predictions of bone adaptation. Guidelines are proposed to establish the dominant model parameters based on experimental and simulated data.

Multi-scale modeling of elasto-plastic response of SnAgCu lead-free solder alloys at different ageing conditions :

Article scientifique ArODES

effect of microstructure evolution, particle size effects and interfacial failure

Milad Maleki, Joël Cugnoni, John Botsis

Materials Science and Engineering:, 2016, vol. 661, pp. 132-144

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Résumé:

In microelectronics applications, SnAgCu lead-free solder joints play the important role of ensuring both the mechanical and electrical integrity of the components. In such applications, the SnAgCu joints are subjected to elevated homologous temperatures for an extended period of time causing significant microstructural changes and leading to reliability issues. In this study, the link between the change in microstructures and deformation behavior of SnAgCu solder during ageing is explained by developing a hybrid multi-scale microstructure-based modeling approach. Herein, the SnAgCu solder alloy is seen as a three phase metal matrix composite in which Ag3Sn and Cu6Sn5 hard intermetallics play the role of reinforcements and Sn the role of a ductile matrix. The hardening of the Sn matrix due to fine intermetallics in the eutectic mixture is modeled by incorporating the mean field effects of geometrically necessary dislocations. Subsequently, a two level homogenization procedure based on micromechanical finite element (FE) models is used to capture the interactions between the different phases. For this purpose, tomographic images of microstructures obtained by Focused Ion Beam (FIB) and synchrotron X-Ray in different ageing conditions are directly used to generate statistically representative volume elements (RVE) using 3D FE models. The constitutive behavior of the solder is determined by sequentially performing two scales of numerical homogenization at the eutectic level and then at the dendrite level. For simplification, the anisotropy of Sn as well as the potential recovery processes have been neglected in the modeling. The observed decrease in the yield strength of solder due to ageing is well captured by the adopted modeling strategy and allows explaining the different ageing mechanisms. Finally, the effects of potential debonding at the intermetallic particle-matrix interface as well as particle fracture on the overall strength of solder are discussed based on simplified unit cell FE models and experimental observations.

Mixed mode fracture behavior of welded wood joints investigated with the Arcan test

Article scientifique ArODES

Martin Rhême, John Botsis, Joël Cugnoni, Parviz Navi

Wood Science and Technology, 2016, vol. 50, no 1, pp. 53-69

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Résumé:

Friction welding of wood is an assembly method that is still under investigation and development. A possible application for welded wood joints is the fabrication of multi-layered panels (i.e., cross-laminated panels). In an effort to model the behavior of such products, work is needed to characterize the mechanical strength and fracture properties of welded joints produced with parallel and cross-grain orientations. The present work addresses combined experimental and numerical investigations into the strength and fracture characterization of welded wood joints. The Arcan test setup is used for the experimental mechanical characterization. Numerical and experimental strength analyses are carried out to investigate the effect of the wood’s fiber orientation and in-plane loading direction on the joint strength and fracture toughness. The results show that the orientation of the fibers does not affect the tensile and shear strength (2.3 and 7 MPa, respectively). In the case of fracture, the virtual crack closure technique is used in a finite element model to determine the critical values of energy release rate in pure and mixed modes. A mixed mode fracture criterion of the welded joint is determined.

2015

Delamination in cross-ply laminates :

Article scientifique ArODES

identification of traction–separation relations and cohesive zone modeling

E. Farmand-Ashtiani, D. Alanis, Joël Cugnoni, John Botsis

Composites Science and Technology, 2015, vol. 119, pp. 85-92

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Résumé:

Identification of traction–separation relations is at the forefront of damage and fracture mechanics of composite laminates. Such relations are of particular interest in fracture of laminates consisting of layers of different fiber orientations. In this study, an iterative method based on internal strain measurements and parametric numerical modeling is employed to identify a traction–separation relation in quasi-static mode I dominated delamination of a cross-ply carbon fiber epoxy laminate. The results demonstrate that crack propagation is accompanied by a large bridging zone which is smaller than the zone in a uniaxial composite specimen of the same materials and linear dimensions. While the initiation value of the energy release rate (ERR) is the same, the ERR at steady state propagation and the maximum stress in the bridging zone are respectively 1.7 and 3.1 times larger than the corresponding values of the unidirectional specimen. The obtained traction–separation relation is employed in a cohesive zone model to predict the loading response and crack growth. The adopted approach is a step towards a better understanding of delamination in cross-ply composites.

Monitoring of composite structures using a network of integrated PVDF film transducers

Article scientifique ArODES

Enrique Guzman, Joël Cugnoni, Thomas Gmür

Smart Materials and Structures, 2015, vol. 24, no. 5, 13 p.

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Résumé:

Aiming to reduce costs, polyvinylidene difluoride (PVDF) film patches are an emerging alternative to more classic piezoelectric technologies, like ceramic patches, as transducers to measure local deformation in many structural applications. This choice is supported by advantages such as the low weight and mechanical flexibility of PVDF, making this polymer suitable for embedding inside full scale polymer based composite structures. Piezoelectric transducer patches can be used as actuators to dynamically excite full-scale composite structures, and as sensors to measure the strain. The main objective of this paper is to verify that the PVDF transducers can provide exploitable signals in the context of structural health monitoring. In order to do so, two aspects of the design of transducer network are investigated: the optimization of the sensor network, for which the effective independence method is proposed, and the use of operational modal analysis (OMA), since it is a simple method to extract the natural frequencies of a structure from a time series. The results of the analysis are compared to a reference set issued from experimental modal analysis (EMA), a simple, well-known, classic method, which is carried out using accelerometers and an impact hammer. By statistical means, it is shown that there is no significant difference between the two methods, and an optimized PVDF transducer network combined with OMA can perform the dynamic analysis of a structure as well as a classic EMA setup would do. This leads the way to the use of low-cost PVDF embedded transducer networks for robust composite material characterization.

2014

Influence of gait loads on implant integration in rat tibiae :

Article scientifique ArODES

experimental and numerical analysis

Marco Piccinini, Joël Cugnoni, John Botsis, Patrick Ammann, Anselm Wiskott

Journal of Biomechanics, 2014, vol. 47, no. 13, pp. 3255-3263

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Résumé:

Implanted rat bones play a key role in studies involving fracture healing, bone diseases or drugs delivery among other themes. In most of these studies the implants integration also depends on the animal daily activity and musculoskeletal loads, which affect the implants mechanical environment. However, the tissue adaption to the physiological loads is often filtered through control groups or not inspected. This work aims to investigate experimentally and numerically the effects of the daily activity on the integration of implants inserted in the rat tibia, and to establish a physiological loading condition to analyse the peri-implant bone stresses during gait. Two titanium implants, single and double cortex crossing, are inserted in the rat tibia. The animals are caged under standard conditions and divided in three groups undergoing progressive integration periods. The results highlight a time-dependent increase of bone samples with significant cortical bone loss. The phenomenon is analysed through specimen-specific Finite Element models involving purpose-built musculoskeletal loads. Different boundary conditions replicating the post-surgery bone–implant interaction are adopted. The effects of the gait loads on the implants integration are quantified and agree with the results of the experiments. The observed cortical bone loss can be considered as a transient state of integration due to bone disuse atrophy, initially triggered by a loss of bone–implant adhesion and subsequently by a cyclic opening of the interface.

Thin ply composites :

Article scientifique ArODES

experimental characterization and modeling of size-effects

R. Amacher, Joël Cugnoni, John Botsis, L. Sorensen, W. Smith, C. Dransfeld

Composites Science and Technology, 201, vol. 101,pp. 121-132

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Résumé:

Thin-ply composites are rapidly gaining interest in the composite industry, not only because of the larger design possibilities that they offer, but also because of positive size effects that have been shown to improve performance in various loading conditions [1]. In this work, carbon fiber–epoxy composites of different ply thicknesses (30–300 g/m2 fiber areal weight) were produced from the same batch of Toray M40JB fiber and NorthTPT TP80ep matrix to study the influence of ply thickness on the ultimate strength and on the onset of damage in lamina, laminates and components. Uniaxial tension, open-hole compression and open-hole tensile fatigue tests on quasi isotropic [45°/90°/−45°/0°]ns laminates showed very significant improvements regarding the on-set of damage, and in some cases ultimate strength, when decreasing the ply thickness. These performance improvements are the result of major changes in the damage progression and failure modes of the laminates caused by a systematic delay or near suppression of transverse cracking and delamination growth in thin-ply composites. On the component level, thin-ply laminates enabled a marked improvement for bolted-joint bearing, especially in hot–wet conditions. Under impact, the 30 μm thin ply laminate exhibited a quasi-brittle failure with extensive translaminar cracking while a ply thickness of 100 μm was found as optimum to minimize the projected damage area. Ply thickness scaling of transverse and in-plane shear strength was identified based on classical laminate theory and unnotched tensile tests on quasi-isotropic specimens. The empirical scaling was found to follow a linear trend over a range of ply thicknesses from 30 to 250 μm. Due to the near suppression of delamination, the strength of thin-ply composites could then be modeled more effectively than thick ply composites using classical laminate theory or standard multilayer shell modeling.

A toolbox of oligopeptide-modified polymers for tailored elastomers

Article scientifique ArODES

Emmanuel Croisier, Su Liang, Thomas Schweizer, Sandor Balog, Marijana Mionic, Ruben Snellings, Joël Cugnoni, Véronique Michaud, Holger Frauenrath

Nature Communications, 2014, vol. 5

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Résumé:

Biomaterials are constructed from limited sets of building blocks but exhibit extraordinary and versatile properties, because hierarchical structure formation lets them employ identical supramolecular motifs for different purposes. Here we exert a similar degree of structural control in synthetic supramolecular elastomers and thus tailor them for a broad range of thermomechanical properties. We show that oligopeptide-terminated polymers selectively self-assemble into small aggregates or nanofibrils, depending on the length of the oligopeptides. This process is self-sorting if differently long oligopeptides are combined so that different nanostructures coexist in bulk mixtures. Blends of polymers with oligopeptides matching in length furnish reinforced elastomers that exhibit shear moduli one order of magnitude higher than the parent polymers. By contrast, novel interpenetrating supramolecular networks that display excellent vibration damping properties are obtained from blends comprising non-matching oligopeptides or unmodified polymers. Hence, blends of oligopeptide-modified polymers constitute a toolbox for tailored elastomers with versatile properties.

Multi-factorial models of a carbon fibre/epoxy composite subjected to accelerated environmental ageing

Article scientifique ArODES

Enrique Guzman, Joël Cugnoni, Thomas Gmür

Composite Structures, 2014, vol. 111, pp. 179-192

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Résumé:

Among materials being introduced in the aerospace industry, the carbon fibre reinforced plastics (CFRP) have a place of privilege because of their exceptional stiffness-to-mass ratio. However, the polymer-based matrix is vulnerable to damages by environmental conditions. This work exposes the experimental results of several accelerated environmental ageing protocols on CFRP panels. The main concern is to justify or reject by statistical means that a significant degradation of mechanical properties does occur over the time, and to establish a basic model to quantify the effects of different environmental factors of the composite ageing. The results considered here are the elastic properties evaluated over several weeks of accelerated artificial ageing. The stiffness degradation of the samples subjected to the aforementioned ageing protocols is statistically described by a non-linear multi-factorial model inspired by the Design of Experiments (DoE) theory. The evolution of constitutive properties (namely mass and elastic properties) over the time exhibits an asymptotic exponential increasing (or decreasing) pattern over the time. The usefulness of these mathematical models is their predictability, based only on theoretical considerations on moisture absorption. This path is further investigated in this paper, clearing up the way to a methodical prediction of ageing models.

Isothermal ageing of SnAgCu solder alloys :

Article scientifique ArODES

three-dimensional morphometry analysis of microstructural evolution and its effects on mechanical response

Milad Maleki, Joël Cugnoni, John Botsis

Journal of Electronic Materials, 2014, vol. 43, no. 4, pp. 1026–1042

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Résumé:

Due to the high homologous temperature and fast cooling rates, the microstructures of SnAgCu (SAC) solders are in a meta-stable state in most applications, which is the cause of significant microstructural evolution and continuous variation in the mechanical behavior of the joints during service. The link between microstructures evolution and deformation behavior of Sn-4.0Ag-0.5Cu solder during isothermal ageing is investigated. The evolution of the microstructures in SAC solders are visualized at different scales in 3D by using a combination of synchrotron x-ray and focused ion beam/scanning electron microscopy tomography techniques at different states of ageing. The results show that, although the grain structure, morphology of dendrites, and overall volume fraction of intermetallics remain almost constant during ageing, considerable coarsening occurs in the Ag3Sn and Cu6Sn5 phases to lower the interfacial energy. The change in the morphometrics of sub-micron intermetallics is quantified by 3D statistical analyses and the kinetic of coarsening is discussed. The mechanical behavior of SAC solders is experimentally measured and shows a continuous reduction in the yield resistance of solder during ageing. For comparison, the mechanical properties and grain structure of β-tin are evaluated at different annealing conditions. Finally, the strengthening effect due to the intermetallics at different ageing states is evaluated by comparing the deformation behaviors of SAC solder and β-tin with similar grain size and composition. The relationship between the morphology and the strengthening effect due to intermetallics particles is discussed and the causes for the strength degradation in SAC solder during ageing are identified.

Studies of hygrothermal degradation of a single fiber composite :

Article scientifique ArODES

an iterative approach with embedded optical sensors and numerical analysis

M. Lai, John Botsis, Joël Cugnoni

Composites Part B: Engineering, 2014, vol. 60, pp. 577-585

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Résumé:

The hydrothermal ageing of glass/epoxy interface is investigated using an experimental–numerical approach on cylindrical epoxy specimens with centrally located optical fibers. A 24 mm long Bragg grating sensor is inscribed on the optical fiber and used to monitor strains along the fiber, due to processing and subsequent ageing in water at 50 °C. The distributed strains are used to: (a) evaluate the residual strain field developed during processing, employing a parametric finite element identification scheme, (b) monitor the evolution of the moisture induced strains during ageing using linear and non-linear responses for the epoxy recorded experimentally, (c) track debond growth at the interface, generated during ageing, by adopting a concentration dependent cohesive finite element model. Good agreement is found between experimental data and simulations until 47 days of immersion (or 63% of saturation). Afterwards, the model is not quantitatively accurate but indicates well the trend of the experimental data.

2013

External mechanical microstimuli modulate the osseointegration of titanium implants in rat tibiae

Article scientifique ArODES

Giovanna Zacchetti, Anselm Wiskott, Joël Cugnoni, John Botsis, Patrick Ammann

BioMed Research International, 2013, vol. 2013

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Résumé:

Purpose. To assess the effect of external mechanical microstimuli of controlled magnitude on the microarchitecture of the peri-implant bone beds in rat tibiae. Materials and Methods. Tibiae of forty rats were fitted with two transcutaneous titanium cylinders. After healing, the implants were loaded to 1 to 3 N, five days/week for four weeks. These force levels translated into intraosseous strains of , , and . After sacrifice, the implants’ pullout strength was assessed. Second, the bone’s microarchitecture was analyzed by microcomputed tomography ( CT) in three discrete regions of interest (ROIs). Third, the effect of loading on bone material properties was determined by nanoindentation. Results. The trabecular BV/TV significantly increased in an ROI of 0.98 mm away from the test implant in the 1 N versus the 3 N group with an opposite trend for cortical thickness. Pull-out strength significantly increased in the 2 N relatively to the nonstimulated group. Higher values of E-modulus and hardness were observed in the trabecular bone of the 2 N group. Conclusion. The in vivo mechanical loading of implants induces load-dependent modifications in bone microarchitecture and bone material properties in rat tibiae. In pull-out strength measurements, implant osseointegration was maximized at 2 N .

Survivability of integrated PVDF film sensors to accelerated ageing conditions in aeronautical/aerospace structures

Article scientifique ArODES

E. Guzman, Joël Cugnoni, Thomas Gmür, Philippe Bonhôte, Alain Schorderet

Smart Materials and Structures, 2013, vol. 22, no. 6, 12 p.

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This work validates the use of integrated polyvinylidene fluoride (PVDF) film sensors for dynamic testing, even after being subjected to UV-thermo-hygro-mechanical accelerated ageing conditions. The verification of PVDF sensors' survivability in these environmental conditions, typically confronted by civil and military aircraft, is the main concern of the study. The evaluation of survivability is made by a comparison of dynamic testing results provided by the PVDF patch sensors subjected to an accelerated ageing protocol, and those provided by neutral non-aged sensors (accelerometers). The available measurements are the time-domain response signals issued from a modal analysis procedure, and the corresponding frequency response functions (FRF). These are in turn used to identify the constitutive properties of the samples by extraction of the modal parameters, in particular the natural frequencies. The composite specimens in this study undergo different accelerated ageing processes. After several weeks of experimentation, the samples exhibit a loss of stiffness, represented by a decrease in the elastic moduli down to 10%. Despite the ageing, the integrated PVDF sensors, subjected to the same ageing conditions, are still capable of providing reliable data to carry out a close followup of these changes. This survivability is a determinant asset in order to use integrated PVDF sensors to perform structural health monitoring (SHM) in the future of full-scale composite aeronautical structures.

Influence of the moisture content on the fracture characteristics of welded wood joint :

Article scientifique ArODES

Part 1: Mode I fracture

Martin Rhême, John Botsis, Joël Cugnoni, Parviz Navi

Holzforschung, 2013, vol. 67, no. 7, pp. 747-754

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Friction welding is a joining technique for wood materials. The positive aspects of this technique are the speed of processing and the absence of chemical or mechanical agents, but the welded joints are not water resistant. To understand better the effect of moisture on the fracture behavior of welded joints, their fracture characteristics have been investigated. The double cantilever beam specimens were tested, which permit to compute the mode I energy release rate of a welded joint. The results confirm the negative effect of moisture on the fracture properties of the joint. The data concerning the maximal tensile strength of the joining material were collected by uniaxial tests and implemented in a finite element model to establish a cohesive law, which describes the behavior of welded pieces in terms of moisture content.

Monitoring and characterization of the interfacial fracture in sandwich composites with embedded multiplexed optical sensors

Article scientifique ArODES

E. Farmand-Ashtiani, Joël Cugnoni, John Botsis

Composite Structures, 2013, vol. 96, pp. 476-483

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Interfacial debonding is one of the most critical damage modes that threatens the structural performance of sandwich composites. In this work, integrated multiplexed fiber Bragg grating (FBG) sensors of 1 mm gauge length are used for detection and characterization of crack propagation along the face–core interface. Single cantilever beam specimens incorporating arrays of FBG sensors are subjected to monotonic and fatigue debonding tests. The experimental configurations are simulated by finite element method in order to verify the experimental procedures and analyze the strain data corresponding to the location of the embedded FBGs. It is demonstrated that the embedded FBGs can successfully reveal the strain field imposed by the interface crack into the sandwich face. The good agreement between the experimental results and concurrent numerical simulations suggests that such data can be used for monitoring of interfacial fatigue and fracture in sandwich structures.

Microstructure-based modeling of the ageing effect on the deformation behavior of the eutectic micro-constituent in SnAgCu lead-free solder

Article scientifique ArODES

Milad Maleki, Joël Cugnoni, John Botsis

Acta Materialia, 2013, vol. 61, no. 1, pp. 103-114

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The eutectic micro-constituent in SnAgCu solder governs the deformation behavior of the joint as it shows better deformation resistance than the Sn dendrites and occupies a high volume percentage of the whole solder. The main scope of this study is to develop a three-dimensional (3-D) homogenization model taking into account the microstructural evolution in the eutectic micro-constituent of SnAgCu solder in order to simulate the change in mechanical behavior of the joint caused by isothermal ageing. For this purpose, 3-D configurations of Ag3Sn and Cu6Sn5 intermetallic compounds (IMCs) in near-eutectic SnAgCu solder are visualized in the as-soldered condition and after ageing by focused ion beam/scanning electron microscopy tomography. The tomographic images are used to generate feature-preserving finite element meshes of the actual microstructures. The representative volume element size and constitutive behavior of the eutectic mixture in the two conditions are determined by a numerical homogenization procedure. The results show a considerable reduction in the yield stress level of the eutectic micro-constituent after ageing of the solder joint. It is shown that the increase in the inter-particle spacing and decrease in the aspect ratio of IMCs due to ageing cause a significant change in the strain distribution in the tin matrix, which leads to a lower contribution of IMCs in load-sharing and yield strength of aged solder. The elastic–plastic properties of as-soldered and aged eutectic mixtures are determined by nanoindentation. The results of homogenization are validated through comparison with experimental results and prediction of the dislocation detachment theory.

2012

Implementation of the “loaded implant” model in the rat using a miniaturized setup - description of the method and first results

Article scientifique ArODES

H. W. Anselm Wiskott, Philippe Bonhote, Joël Cugnoni, Stéphane Durual, Giovanna Zacchetti, John Botsis, Susanne S. Scherrer, Patrick Ammann

Clinical Oral Implants Research, 2012, vol. 23, no. 12, pp. 1352-1359

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Objective : To miniaturize the “loaded implant” model to permit its application to small rodents. In this model, two titanium implants are placed 8 mm apart with their heads protruding from the skin and are forced together by a dedicated actuator. To assess the effect of (i) the post-implantation healing period and the duration of stimulation and (ii) the intratissular strain level on the microtomographical bone parameters BV/TV, Tb.N., Tb.Th. and BIC. Materials and methods : Implants, 1 × 8 mm, were machined, inserted into the tibiae of rats and activated. A total of 123 animals were used. In series 1, the implants were left to heal for 2/4 weeks and then loaded to generate intratissular strains of 1125 ± 5% με for 4/8 weeks. Series 2 had their implants loaded to 750, 1500 and 2250 ± 5% με, respectively. Results : Bone to implant contact increased upon loading. In series 1, no difference was observed regarding the duration of healing or the stimulation period. In series 2, at 750 με, the bone parameters did not differ from baseline. At 1500 με, all four parameters increased. At 2250 με, three of four parameters decreased relative to 1500 με. Conclusions : (i) The loaded implant model can be miniaturized to the millimeter range; (ii) in the present model, implant activation beyond 4 weeks did not affect the bone parameters; (iii) mechanical stimulation increased bone to implant contact by up to 20%; (iv) the results obtained are consistent with the concept of an anabolic effect from 750 to 1500 με and deleterious effects at strains in the 2250 με range; and (v) strains at 2250 με did not lead to implant dis-integration.

Numerical and statistical estimates of the representative volume element of elastoplastic random composites

Article scientifique ArODES

Matteo Galli, Joël Cugnoni, John Botsis

European Journal of Mechanics - A/Solids, 2012, vol. 33, pp. 31-38

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In many applications elastoplastic composites are used in limited amounts, therefore it is important to have estimates of the size of their representative volume element both for modeling and experimental purposes. In this work the tensile response of particle reinforced random composites is simulated by microstructural finite element models. Several microstructural realizations are considered for each composition and volume, and the scatter in the response is used as representativeness metric. The microstructural morphology is characterized using methods and statistical descriptors that can be employed with micrographs of real materials. Numerical results show that the representative volume element dimensions can be estimated by verifying either the consistency of the stress–strain curve for single microstructural realizations and increasing material volume sizes or the convergence of the response of several microstructural realizations at the same material volume size. The analysis of the stress–strain state at the microstructural level shows that the plastic strain and the hydrostatic pressure in the matrix material depend hyperbolically on the interparticle distance. Microstructural analyses show that the matrix coarseness is correlated to the scatter in the mechanical response and therefore can be used to have approximate estimates of the representative volume element size.

Low energy impact damage monitoring of composites using dynamic strain signals from FBG sensors :

Article scientifique ArODES

Part I: Impact detection and localization

Jeannot Frieden, Joël Cugnoni, John Botsis, Thomas Gmür

Composite Structures, 2012, vol. 94, no. 2, pp. 438-445

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Fiber reinforced composite materials risk to suffer from subsurface, barely visible, damage induced by transverse relatively low energy impacts. This two-paper series presents a method for the localization of an impact and identification of an eventual damage using dynamic strain signals from fiber Bragg grating (FBG) sensors. In this paper, the localization method allowing to predict the impact position based on interpolation of a reference data set is developed and validated. The data utilized in the method are the arrival times of the asymmetric zero order Lamb waves at the different sensors. A high rate interrogation method based on intensity modulation of the Bragg wavelength shift is used to acquire the FBG signals. The localization method allows to predict the impact position with a good accuracy and therefore the inspection of the laminate can be limited to this region.

Low energy impact damage monitoring of composites using dynamic strain signals from FBG sensors :

Article scientifique ArODES

Part II: Damage identification

Jeannot Frieden, Joël Cugnoni, John Botsis, Thomas Gmür

Composite Structures, 2012, vol. 94, no. 2, pp. 593-600

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In part I of this two-paper series, a method for the localization of an impact using dynamic strain signals from fiber Bragg grating (FBG) sensors is presented. In this paper, an inverse numerical–experimental method allowing to identify the damage based on experimentally measured eigenfrequency changes is developed and validated. The damage identification is limited to a region in the vicinity of the impact position predicted by the localization method. The eigenfrequency changes are determined experimentally from dynamic strain signals obtained with embedded FBG sensors and the parameters of a homogenized damage model are adjusted to fit the numerical results to the experimental data.

2011

On the mutual effect of viscoplasticity and interfacial damage progression in interfacial fracture of lead-free solder joints

Article scientifique ArODES

Milad Maleki, Joël Cugnoni, John Botsis

Journal of Electronic Materials, 2011, vol. 40, no. 10, pp. 2081–2092

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The main goal of this paper is to shed light on the effect of strain rate and viscoplastic deformation of bulk solder on the interfacial failure of lead-free solder joints. For this purpose, interfacial damage evolution and mode I fracture behavior of the joint were evaluated experimentally by performing stable fracture tests at different strain rates employing an optimized tapered double cantilever beam (TDCB) design. The viscoplastic behavior of the solder was characterized in shear, and the constitutive parameters related to the Anand model were determined. A rate-independent cohesive zone damage model was identified to best simulate the interfacial damage progression in the TDCB tests by developing a three-dimensional (3D) finite-element (FE) model and considering the viscoplastic response of the bulk solder. The influence of strain rate on the load capability and failure mode of the joint was clarified by analyzing the experimental and simulation results. It was shown how, at the lower strain rates, the normal stress generated at the interface is limited by the significant creep relaxation developed in the bulk solder and thus is not sufficiently high to initiate interfacial damage, whereas at higher rates, a large amount of the external energy is dissipated into interfacial damage development.

Vibration-based characterization of impact induced delamination in composite plates using embedded FBG sensors and numerical modelling

Article scientifique ArODES

Jeannot Frieden, Joël Cugnoni, John Botsis, Thomas Gmür

Composites Part B: Engineering, 2011, vol. 42, no. 4, pp. 607-613

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Dynamic strain signals from embedded fiber Bragg grating sensors are used for experimental modal analysis in carbon fiber reinforced polymer plates. Throughout a series of impact tests, the change of eigenfrequencies due to damage is evaluated for different impact energies. A detailed numerical model including the damage pattern obtained from X-ray computed tomography images demonstrates that most of the frequency changes can be explained by a delamination type of damage, whereas the total delamination surface has an affine relation to the absorbed impact energy. A homogenized damage model, including two damage factors, allows to predict the change of natural frequencies for a known damage size.

Poroviscoelastic characterization of particle-reinforced gelatin gels using indentation and homogenization

Article scientifique ArODES

Matteo Galli, Elvis Fornasiere, Joël Cugnoni, Michelle L. Oyen

Journal of the Mechanical Behavior of Biomedical Materials, 2011, vol. 4, no. 4, pp. 610-617

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Hydrogels are promising materials for bioengineering applications, and are good model materials for the study of hydrated biological tissues. As these materials often have a structural function, the measurement of their mechanical properties is of fundamental importance. In the present study gelatin gels reinforced with ceramic microspheres are produced and their poroviscoelastic response in spherical indentation is studied. The constitutive responses of unreinforced gels are determined using inverse finite element modeling in combination with analytical estimates of material parameters. The behavior of composite gels is assessed by both analytical and numerical homogenization. The results of the identification of the constitutive parameters of unreinforced gels show that it is possible to obtain representative poroviscoelastic parameters by spherical indentation without the need for additional mechanical tests. The agreement between experimental results on composite gelatin and the predictions from homogenization modeling show that the adopted modeling tools are capable of providing estimates of the poroviscoelastic response of particle-reinforced hydrogels.

Crack – fiber sensor interaction and characterization of the bridging tractions in mode I delamination

Article scientifique ArODES

S. Stutz, Joël Cugnoni, John Botsis

Engineering Fracture Mechanics, 2011, vol. 78, no. 6, pp. 890-900

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Fiber bridging is regularly encountered in mode I delamination tests of unidirectional fiber reinforced composites. However, characterization of the bridging tractions is rather difficult. One way to indirectly evaluate the bridging traction distribution is to embed a fiber Bragg grating (FBG) sensor close to the crack tip and to measure the distributed strain along this FBG. The strain measurements from the FBG sensor are used to characterize the fiber bridging tractions by an identification method. In this work, the sensor is embedded in a unidirectional carbon/epoxy composite. Firstly, it is treated as an inclusion near the crack plane and a numerical analysis is performed to study its effect on the measured strain field and energy release rate. The results demonstrate that the sensor, located at about two fiber diameters from the crack plane, has a negligible effect on the fracture process. Secondly, among the identified linear, bilinear, and exponential bridging traction distributions, the exponential one is found to be a suitable model. Characterization of the bridging tractions allows to calculate the energy release due to the bridging fibers which is similar to the difference between the initiation energy release and the propagation value. The results also agree with the bridging tractions evaluated from the conventional energy release rate – crack opening displacement method.

Studies of mode I delamination in monotonic and fatigue loading using FBG wavelength multiplexing and numerical analysis

Article scientifique ArODES

S. Stutz, Joël Cugnoni, John Botsis

Composites Science and Technology, 2011, vol. 71, no. 4, pp. 443-449

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Bridging by intact fibers in composite materials is one of the most important toughening mechanisms. However, a direct experimental assessment of its contribution is not easy to achieve. In this work a semi-experimental method is proposed to quantify its contribution to fracture of unidirectional carbon fiber/epoxy double cantilever beam (DCB) specimens in mode I delamination under monotonic and 1 Hz fatigue loads. In each specimen, an embedded optical fiber with an array of eight wavelength-multiplexed fiber Bragg gratings is used to measure local strains close to the crack plane. The measured strain distribution serves in an inverse identification procedure to determine the tractions in the bridging zone in monotonic and fatigue loads. These tractions are used to calculate the energy release rate (ERR) associated with bridging fibers. The results indicate that the ERR due to bridging is about 40% higher in fatigue. The bridging tractions are further included in a cohesive element model which allows to predict precisely the complete load displacement curve of monotonic DCB tests. Using the principle of superposition and the identified tractions, the total stress intensity factor (SIF) is calculated. The results show that the SIF, at initiation, is very close to the one calculated at crack propagation and bridging by intact fibers is responsible for the entire increase in toughness seen in the DCB specimens used herein.

Hydro-mechanical coupling in the periodontal ligament :

Article scientifique ArODES

A porohyperelastic finite element model

Marzio Bergomi, Joël Cugnoni, Matteo Galli, John Botsis, Urs C. Belser, H.W. Anselm Wiskott

Journal of Biomechanics, 2011, vol. 44, no. 1, pp. 34-38

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Harmonic tension–compression tests at 0.1, 0.5 and 1 Hz on hydrated bovine periodontal ligament (PDL) were numerically simulated. The process was modeled by finite elements (FE) within the framework of poromechanics, with the objective of isolating the contributions of the solid- and fluid phases. The solid matrix was modeled as a porous hyperelastic material (hyperfoam) through which the incompressible fluid filling the pores flowed in accordance with the Darcy’s law. The hydro-mechanical coupling between the porous solid matrix and the fluid phase circulating through it provided an apparent time-dependent response to the PDL, whose rate of deformation depended on the permeability of the porous solid with respect to the interstitial fluid. Since the PDL was subjected to significant deformations, finite strains were taken into account and an exponential dependence of PDL permeability on void ratio – and therefore on the deformation state – was assumed. PDL constitutive parameters were identified by fitting the simulated response to the experimental data for the tests at 1 Hz. The values thus obtained were then used to simulate the tests at 0.1 and 0.5 Hz. The results of the present simulation demonstrate that a porohyperelastic model with variable permeability is able to describe the two main aspects of the PDL’s response: (1) the dependency on strain-rate—the saturated material can develop volumetric strains by only exchanging fluid and (2) the asymmetry between tension and compression, which is due to the effect of both the permeability and the elastic properties on deformation.

Identification of the elastic and damping properties in sandwich structures with a low core-to-skin stiffness ratio

Article scientifique ArODES

Marco Matter, Thomas Gmür, Joël Cugnoni, Alain Schorderet

Composite Structures, 2011, vol. 93, no. 2, pp. 331-341

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A mixed numerical–experimental identification procedure for estimating the storage and loss properties in sandwich structures with a soft core is developed. The proposed method uses at the experimental level a precise measurement setup with an electro-dynamic shaker and a scanning laser interferometer, and at the computational level an original structurally damped shell finite element model derived from the higher-order shear deformation theory with piecewise linear functions for the through-the-thickness displacement. The parameter estimation is derived from adequate objective functions measuring the discrepancy between the experimental and numerical modal data. Through a sensitivity analysis it is shown that for sandwich structures with a soft core only one specimen is required for characterizing the dominant properties of both the core and the skins. The procedure is then applied to two test cases for which all the influent elastic properties and the major damping parameters could be estimated with a fairly good precision.

2010

Three-dimensional morphometry of strained bovine periodontal ligament using synchrotron radiation-based tomography

Article scientifique ArODES

Marzio Bergomi, Joël Cugnoni, H. W. Anselm Wiskott, Philipp Schneider, Marco Stampanoni, John Botsis, Urs C. Belser

Journal of Anatomy, 2010, vol. 217, no. 2, pp. 126-134

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The periodontal ligament (PDL) is a highly vascularized soft connective tissue. Previous studies suggest that the viscous component of the mechanical response may be explained by the deformation-induced collapse and expansion of internal voids (i.e. chiefly blood vessels) interacting with liquids (i.e. blood and interstitial fluids) flowing through the pores. In the present work we propose a methodology by means of which the morphology of the PDL vascular plexus can be monitored at different levels of compressive and tensile strains. To this end, 4-mm-diameter cylindrical specimens, comprising layers of bone, PDL and dentin covered by cementum, were strained at stretch ratios ranging from λ = 0.6 to λ = 1.4 and scanned using synchrotron radiation-based computer tomography. It was concluded that: (1) the PDL vascular network is layered in two distinct planes of blood vessels (BVs): an inner layer (close to the tooth), in which the BVs run in apico-coronal direction, and an outer layer (close to the alveolar bone), in which the BVs distribution is more diffuse; (2) during tension and compression, the porosity tissue is kept fairly constant; (3) mechanical straining induces important changes in BV diameters, possibly modifying the permeability of the PDL and thus contributing to the viscous component of the viscoelastic response observed under compressive forces.

A PSDT shell finite element formulation including structural damping

Article scientifique ArODES

Marco Matter, Thomas Gmür, Joël Cugnoni, Alain Schorderet

Computers Structures, 2010, vol. 88, no. 15–16, pp. 902-908

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This paper describes efficient C0-compatible finite elements for modelling structurally damped laminated composite plates and shells under harmonic vibrations. Based upon a shear deformation theory of any order p (PSDT), the elements are well adapted for evaluating, from given elastic and damping properties of the material, the global modal response (mode shapes, natural frequencies and modal loss factors) of multilayered structures. Numerical test cases are presented in order to validate the proposed formulation. Results show that this shell model is accurate and more efficient than the 3D solid formulation for analyzing thin to moderately thick structurally damped plates or shells.

High-speed internal strain measurements in composite structures under dynamic load using embedded FBG sensors

Article scientifique ArODES

Jeannot Frieden, Joël Cugnoni, John Botsis, Thomas Gmür, Dragan Coric

Composite Structures, 2010, vol. 92, no. 8, pp. 1905-1912

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Internal strain measurements in cross-ply carbon-epoxy composite plates under dynamic loads are carried out using embedded FBG sensors. The principle of the FBG interrogation is based on intensity demodulation achieved via a Fabry-Pérot filter. To account for the non-linearity of the filter, the system is calibrated and the amplitude of the strain data is validated. Strains are acquired at a rate of 100 kHz with a noise level as low as 2 με and used for modal analysis and strain monitoring in low energy impact. The experimental results under impact and modal analysis compare very well with pertinent numerical models and modal analysis obtained from laser vibrometer measurements.

The role of the fluid phase in the viscous response of bovine periodontal ligament

Article scientifique ArODES

Marzio Bergomi, Joël Cugnoni, John Botsis, Urs C. Belser, H. W. Anselm Wiskott

Journal of Biomechanics, 2010, vol. 43, no. 6, pp. 1146-1152

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The mechanical response of the periodontal ligament (PDL) is complex. This tissue responds as a hyperelastic solid when pulled in tension while demonstrating a viscous behavior under compression. This intricacy is reflected in the tissue's morphology, which comprises fibers, glycosaminoglycans, a jagged interface with the surrounding porous bone and an extensive vascular network. In the present study we offer an analysis of the viscous behavior and the interplay between the fibrous matrix and its fluid phase. Cylindrical specimens comprising layers of dentine, PDL and bone were extracted from bovine first molars and affixed to a tensile-compressive loading machine. The viscous properties of the tissue were analyzed (1) by subjecting the specimens to sinusoidal displacements at various frequencies and (2) by cycling the specimens in ‘fully saturated’ and in ‘partially dry’ conditions. Both modes assisted in determining the contribution of the fluid phase to the mechanical response. It was concluded that: (1) PDL showed pseudo-plastic viscous features for cyclic compressive loading, (2) these viscous features essentially resulted from interactions between the porous matrix and unbound fluid content of the tissue. Removing the liquid from the PDL largely eliminates its damping effect in compression.

Representative volume element size of elastoplastic and elastoviscoplastic particle-reinforced composites with random microstructure

Article scientifique ArODES

Joël Cugnoni, M. Galli

CMES Computer Modeling in Engineering Sciences, 2010, vol. 66, no. 2, pp. 165-185

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With the progress of miniaturization, in many modern applications the characteristic dimensions of the physical volume occupied by particle-reinforced composites are getting comparable with the reinforcement size and many of those composite materials undergo plastic deformations. In both experimental and modelling contexts, it is therefore very important to know whether, and up to which characteristic size, the description of the composites in terms of effective, homogenized properties is sufficiently accurate to represent their response in the actual geometry. Herein, the case of particle-reinforced composites with elastoviscoplastic matrix materials and polyhedral randomly arranged linear elastic reinforcement is considered since it is representative of many metal matrix composites of technical interests. A large parametric study based on 3D finite element microstructural models is carried out to study the dependence of the Representative Volume Element (RVE) size on the mechanical properties of the constituents, the reinforcement volume fraction and the average strain level. The results show that RVE size mainly depends on the reinforcement volume fraction and on the macroscopic strain level. The estimated RVE size for elastoplastic composites with 5% to 10% volume fraction of reinforcements is found in the range of 5-6 times the average size of reinforcement particles, while for higher volume fraction, e.g. 15% to 25%vol., the RVE size increases rapidly to 10 to 20 times the reinforcement size. Moreover insights on the influence of mesh refinement and boundary conditions on finite element homogenization analysis are obtained.

2009

A study of the shear response of a lead-free composite solder by experimental and homogenization techniques

Article scientifique ArODES

V. Sivasubramaniam, M. Galli, Joël Cugnoni, J. Janczak-Rusch, John Botsis

Journal of Electronic Materials, 2009, vol. 38, no. 10, pp. 2122–2131

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The current study proposes a combined experimental and modeling approach to characterize the mechanical response of composite lead-free solders. The influence of the reinforcement volume fraction on the shear response of the solder material in the joint is assessed. A novel optimized geometry for single lap shear specimens is proposed. This design minimizes the effect of plastic strain localization, leading to a significant improvement of the quality of experimental data. The constitutive model of the solder material is numerically identified from the load–displacement response of the joint by using inverse finite element identification. Experimental results for a composite solder with 0.13 reinforcement volume fraction indicate that the presence of the reinforcement leads to a 23% increase of the ultimate stress and a 50% decrease of the ultimate strain. To interpret experimental data and predict the elastoplastic response of the composite solder for varying particle volume fraction, a three-dimensional (3D) homogenization model is employed. The agreement between experiments and homogenization results leads to the conclusion that the increase in the ultimate strength and the decrease in ductility are to be attributed to load sharing between matrix material and particles with the development of a significant triaxial stress state which restricts plastic flow in the matrix.

Numerical-experimental identification of the elastic and damping properties in composite plates

Article scientifique ArODES

Marco Matter, Thomas Gmür, Joël Cugnoni, Alain Schorderet

Composite Structures, 2009, vol. 90, no. 2, pp. 180-187

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A novel method is presented for estimating the elastic and dissipative parameters of composite plates and shells through a mixed numerical-experimental identification procedure based on the natural frequencies, modal damping factors and mode shapes of the specimen under investigation. The modal quantities are measured with an optimized contact-free experimental setup using a loudspeaker for exciting the structure and a scanning laser interferometer for measuring the dynamic response. The corresponding numerical predictions are obtained with an accurate shell element model derived from the higher-order shear deformation theory and including a structural damping mechanism. Adequate objective functions relying on the discrepancy between the experimental and numerical modal data are developed for the parameter estimation. The proposed procedure is next applied to three test cases and the results obtained show that all the elastic properties and the major damping parameters can be estimated with a high accuracy.

Monitoring of hygrothermal ageing effects in an epoxy resin using FBG sensor :

Article scientifique ArODES

a methodological study

D. Karalekas, Joël Cugnoni, John Botsis

Composites Science and Technology, 2009, vol. 69, no. 3–4, March 2009, pp. 507-514

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This study demonstrates the feasibility of using fibre Bragg grating (FBG) sensors to monitor the effects of hygrothermal ageing on the axial strains in a cylindrical epoxy specimen with a centrally located optical glass fibre. A 24 mm long FBG, inscribed on the fibre, is used to monitor the moisture absorption in the epoxy, immersed in distilled water at 50 °C for 2330 h. Hygromechanical simulations support the experimental strain data when the first part of the absorption process is neglected or when a variable coefficient of moisture expansion is used. Also, the effects of using average moisture or when it is distributed according to Fick’s law are negligible. The FBG response also indicates the appearance of progressive debonding after sufficient exposure to moisture.

2008

Interfacial intermetallic growth and strength of composite lead-free solder alloy through isothermal aging

Article scientifique ArODES

V. Sivasubramaniam, N. S. Bosco, Joël Cugnoni, John Botsis

Journal of Electronic Materials, 2008, vol. 37, no. 10, pp. 1598–1604

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The effects of particle reinforcement of Sn-4.0wt.%Ag-0.5wt.%Cu (SAC405) lead-free solder on interfacial intermetallic layer growth and strength of the ensuing joints through short-term isothermal aging (150°C) were studied. Composite solders were prepared by either incorporating 2 wt.% Cu (3 μm to 20 μm) or Cu2O (∼150 nm) particles into SAC405 paste. Aggressive flux had the effect of reducing the Cu2O nanoparticles into metallic Cu which subsequently reacted with the solder alloy to form the Cu6Sn5 intermetallic. While all solders had similar interfacial intermetallic growth upon reflow, both of the composite solders’ growth rates slowed through aging to reach a common growth rate exponent of approximately 0.38, considerably lower than that of the nonreinforced solder (n = 0.58). The nanoscale reinforced solder additionally exhibited the highest tensile strength in both the initial and aged conditions, behavior also attributed to its quick conversion to a stable microstructure.

Bone reactions to controlled loading of endosseous implants :

Article scientifique ArODES

Anselm H. W. Wiskott, Joël Cugnoni, Susanne S. Scherrer, Patrick Ammann, John Botsis, Urs C. Belser

Clinical Oral Implants Research, 2008, vol. 19, no. 11 pp. 1093-1102

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Résumé:

Objectives: To validate an experimental setup designed to apply load onto bone tissue using osseointegrated implants in a rabbit model. Specifically, (1) to design an apparatus capable of generating controlled forces, (2) to assess implant placement, maintenance and loading and (3) to evaluate outcome variables using three radiological methods. Material and methods: New Zealand White rabbits were used. Two dental implants were inserted 15–18 mm apart in the animals' tibiae. After 3 months of healing, the implants were loaded normal to their long axes using a pneumatically activated device. A 15 min load regimen at 1 Hz was applied 5 days per week. Every week the applied load was increased by 5 N up to week 8 and by 10 N up to 100 N by week 14. Groups of animals (n=3) were sacrificed at load levels 25, 50 and 100 N. One unloaded controlateral implant in each group provided the baseline data. The rabbits were computer tomography (CT) scanned and radiographed using conventional frames every 4–5 weeks. After sacrifice, a volume of interest (VOI) located in the inter-implant zones and a VOI set as a ring surrounding the distal implant were analyzed using micro computer tomography (μCT). Results: A variety of osseous responses was observed, ranging from minor alterations to significant increases in porosity and lamelling of the cortical layer. μCT data of the inter-implant VOI demonstrated an initial increase in total volume (upto 50 N) followed by stabilization. Concomitantly, bone volumetric density first decreased and then augmented until the end of the experiment. This phenomenon was not observed in the peri-implant VOI, for which volumetric density augmented from the beginning to the end of the experiment. Conclusions: 1. In future trials the loading devices must be constructed so as to sustain heavy cyclic loads over prolonged periods. 2. When properly handled, rabbits are cooperative animals in this application. In a third of the sites, signs of inflammation were observed. 3. In the inter-implant VOI, the cortical bone tended to react in two phases: first, as an increase in porosity and lamelling and second, as an augmentation of bone volumetric density. The peri-implant VOI adapted only by augmenting volumetric density.

Monitoring of process induced strains in a single fibre composite using FBG sensor :

Article scientifique ArODES

a methodological study

D. Karalekas, Joël Cugnoni, John Botsis

Composites Part A: Applied Science and Manufacturing, 2008, vol. 39, no. 7, pp. 1118-1127

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A partially embedded long fibre-Bragg grating (FBG) into an epoxy cylindrical specimen is used to monitor the strain distribution evolution along its axis when subjected to subsequent cycles of thermal ageing at 70° and 110 °C. Assuming a total volume reduction of 2.5% at full cure, the epoxy conversion at the end of the thermal cycles was found ∼85%. Using strain data from the FBG, the coefficient of thermal expansion was evaluated as ∼11.5 × 10−5 C−1. Based on the strain data from the FBG, numerical simulations of an equivalent thermo-elastic matrix were performed to obtain the residual stresses in the specimen at the end of the thermal cycles. The results demonstrate the capability of FBG sensors in providing important information on the degree of cure and evolution of fabrication induced strains inside the epoxy.

Identification of the matrix elastoplastic properties in reinforced active brazing alloys

Article scientifique ArODES

Matteo Galli, Joël Cugnoni, John Botsis, J. Janczak-Rusch

Composites Part A: Applied Science and Manufacturing, 2008, vol. 39, no. 6, pp. 972-978

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The mechanical response of the reinforced active alloy IncusilTMABA®–SiC is studied. Specimens with up to 27 vol.% SiC particles are produced and tested in tension. The presence of the reinforcement leads to modifications in the alloy mechanical response and microstructure, with a decrease in the volume fraction of Ti-containing phases. To study the in situ response of the matrix material in the composites an inverse homogenization approach is proposed. The identified responses of the matrix materials are softer than that of unreinforced IncusilTMABA®. These results are experimentally confirmed by studying the effect of Ti-addition to IncusilTM15, a braze alloy whose composition is close to that of IncusilTMABA® without Ti.

Modal validation of sandwich shell finite elements based on ap-order shear deformation theory including zigzag terms

Article scientifique ArODES

Mattia Sulmoni, Thomas Gmür, Joël Cugnoni, Marco Matter

International Journal for Numerical Methods in Engineering, 2008, vol.75, no. 11 pp. 1301-1319

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This paper describes a set of improved C0-compatible composite shell finite elements for evaluating the global dynamic response (natural frequencies and mode shapes) of sandwich structures. Combining a through-the-thickness displacement approximation of variable high order with a first-order zigzag function, the proposed finite elements are suited for modelling sandwich plates and doubly curved shells with a non-uniform thickness and are more accurate than conventional models based on the first- and third-order shear deformation theories, especially in sandwich panels with highly heterogeneous properties. The new finite element model is then validated by a comparison with the standard shell and 3D solid models. From these investigations, it can be concluded that adding a zigzag function even to high-order polynomial approximations of the through-the-thickness displacement is a useful tool for refining the modelling of sandwich structures. In addition, the proposed formulation is sufficiently versatile to represent with the same level of accuracy the behaviour of thin-to-thick laminated shells as well as of strongly heterogeneous sandwich structures.

2007

Delamination detection and characterisation of bridging tractions using long FBG optical sensors

Article scientifique ArODES

Larissa Sorensen, John Botsis, Thomas Gmür, Joël Cugnoni

Composites Part A: Applied Science and Manufacturing, 2007, vol. 38, no. 10, pp. 2087-2096

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Measurements of non-uniform strains due to the propagation of mode-I delamination and identification of bridging tractions in double cantilever beam (DCB) composite specimens made of uniaxially reinforced AS4/PPS are presented. A fibre Bragg grating (FBG) of 22 or 35 mm is embedded parallel to the reinforcing fibres, during consolidation and above the delamination plane. Strain distributions along the sensor are obtained by optical low-coherence reflectometry and inverse scattering. The specimen’s residual strains in the sensor are used as an initial reference, even when it causes birefringence. Observations and strain measurements indicate fibre bridging across the delamination plane. The measured distribution of strains in the FBG sensor and numerical modeling are used in an inverse procedure to determine the actual distribution of bridging stresses during delamination.

Inverse method based on modal analysis for characterizing the constitutive properties of thick composite plates

Article scientifique ArODES

Joël Cugnoni, Thomas Gmür, Alain Schorderet

Computers Structures, 2007, vol. 85,no. 17–18, pp. 1310-1320

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This paper presents a powerful method for evaluating the constitutive properties of composite laminates through a mixed numerical–experimental identification procedure based on both the extracted mode shapes and the corresponding natural frequencies of the structure. The modal quantities are measured with a precise contact-free experimental technique and extracted numerically with an accurate shell element derived from the higher order shear deformation theory. The elastic properties are estimated with a nonlinear least squares algorithm applied to modal identification criteria. With this novel approach of combining frequency and mode based error norms, the proposed identification method allows an accurate identification of both in-plane and transverse elastic properties with a single non-destructive test. The effectiveness of the procedure is highlighted by test cases on moderately thick to thick plates.

Improved modal characterization of the constitutive parameters in multilayered plates

Article scientifique ArODES

Marco Matter, Thomas Gmür, Joël Cugnoni, Alain Schorderet

Composites Science and Technology, 2007, vol. 67, no. 6, pp. 1121-1131

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This paper presents an accelerated modal numerical–experimental identification method for estimating the elastic parameters in multilayered composite plates. The proposed algorithm is based upon a contact-free measurement setup and a finite element model including a shear deformation theory of variable order. By minimizing selectively, according to the nature of the parameter, the residuals between the numerical and experimental frequencies and mode shapes, the method takes advantage of the sensitivity of the constitutive properties on the modal data. This optimization in two steps instead of a traditional one-step approach improves the accuracy of the elastic parameters estimated and enhances the convergence rate of the characterization method, as shown in two test cases applied to a unidirectional multilayered carbon/epoxy plate.

Experimental and numerical studies on size and constraining effects in lead-free solder joints

Article scientifique ArODES

Joël Cugnoni, John Botsis, V. Sivasubramaniam, J. Janczak-Rusch

Fatigue Fracture of Engineering Materials and Structures, 2007, vol. 30, no. 5, pp. 387-399

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The durability and reliability of lead-free solder joints depends on a large number of factors, like geometry, processing parameters, microstructure and thermomechanical loads. In this work, the nature and influence of the plastic constraints in the solder due to joining partners have been studied by parametric finite element simulation of solder joints with different dimensions. The apparent hardening due to plastic constraints has been shown to strongly depend on the solder gap to thickness ratio with an inversely proportional evolution. Due to interaction of several parameters, the macroscopic stress–strain constitutive law of lead-free solder materials should be determined in the most realistic conditions. In order to identify the elasto-plastic constitutive law of Sn–Ag–Cu solders, a sub-micron resolution Digital Image Correlation technique has been developed to measure the evolution of strain in solder joints during a tensile test. Experimental results of the stress–strain response of Sn–Ag–Cu solder joints have been determined for several solder gaps. The measured load–displacement curves have been used in an inverse numerical identification procedure to determine the constitutive elasto-plastic behaviour of the solder material. The effects of geometrical constraints in a real solder joint with heterogeneous stress and strain fields are then studied by comparing the apparent (constrained) and constitutive (non-constrained) stress–strain relationships. Once the size dependant constraining effects have been removed from the stress–strain relationship, the scale effects can be studied separately by comparing the constitutive elasto-plastic parameters of joints with a variable thickness. Experimental stress–strain curves (constrained and unconstrained) for Sn–4.0Ag–0.5Cu solder in joints of 0.25–2.4 mm gap are presented and the constraining and the size effects are discussed.

2006

Size and constraining effects in lead-free solder joints

Article scientifique ArODES

Joël Cugnoni, John Botsis, Jolanta Janczak-Rutsch

Advanced Engineering Materials, 2006, vol. 8, no. 3, pp. 184-191

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A novel in-situ characterization technique combining Digital Image Correlation strain measurement and inverse numerical identification was developed to determine the plastic constitutive properties of Sn-Ag-Cu lead-free soldering material inside real joints. The effects of size and geometrical constraints on the constitutive and apparent stress-strain response of 0.1, 0.5 and 1mm joints were studied, showing clearly the important constraining and size effects in thin solder joints.

2004

Modal validation of a set of C0-compatible composite shell finite elements

Article scientifique ArODES

Joël Cugnoni, Thomas Gmür, Alain Schorderet

Composites Science and Technology, 2004, vol. 64,no. 13–14, pp. 2039-2050

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This paper presents efficient C0-compatible finite elements for modelling laminated composite shells under free vibrations. Derived from the first-order shear deformation theory (equivalent single-layer laminate model), the elements are well adapted for evaluating the global dynamic response (natural frequencies and mode shapes) of moderately thick multilayered shells. The components of their structural matrices are based on an exact integration per layer, which results in a higher solution accuracy than with standard explicit through-the-thickness schemes. The described finite element formulation, which can be easily implemented in commercial finite element codes, is next validated by means of several experimental modal test cases on thin to relatively thick plates or shells.

Identification by modal analysis of composite structures modelled with FSDT and HSDT laminated shell finite elements

Article scientifique ArODES

Joël Cugnoni, Thomas Gmür, Alain Schorderet

Composites Part A: Applied Science and Manufacturing, 2004, vol. 35, no. 7–8, pp. 977-987

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This paper presents an improvement and an extension to modal analysis of an existing multilayered composite shell finite element. Generalising the formulation to a set of elements, the proposed models are based upon the first- and higher-order shear deformation theories and are well suited for evaluating the global dynamic response of thin and thick laminated shells respectively. Characterized by a through-the-thickness displacement approximation of a freely chosen order, they display excellent convergence properties when the polynomial order is increased and present a higher computational effectiveness in comparison to the classical layerwise models. The models considered are compared to closed-form solutions based on the layerwise plate theory and the so-called zig–zag formulation. Experimental and numerical modal test cases on thin and thick plates are next investigated in order to validate the proposed shell models. Good agreement is found with the analytical, experimental and numerical references.

2015

Thin ply composites :

Conférence ArODES

experimental characterization and modeling

R. Amacher, Joël Cugnoni, John Botsis

Proceedings of the 19th International Conference on Composite Materials - ICCM19

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Résumé:

Thin ply composites are quickly gaining interests in the composite industry not only because of the larger design space that they offer but also because of positive size effects that have been shown to affect their performance in various loading conditions [1]. In this work, carbon-epoxy composites of different ply thicknesses (30g/m2, 100g/m2 and 300g/m2 fiber areal weight) were produced from the same batch of Toray M40JB fiber and NorthTPT TP80ep matrix to study the influence of ply thickness on the ultimate strength and onset of damage of lamina and quasi isotropic laminates. Characterization tests on unidirectional lamina showed only limited influence of the ply thickness on the elastic and ultimate strength properties except for longitudinal compression in which the thinner ply specimens showed some advantage because of a more uniform microstructure. Uniaxial tension, open-hole compression and open-hole tensile fatigue tests on quasi isotropic [45°/90°/-45°/0°]ns laminates showed however very significant improvements of on-set of damage, and in some cases ultimate strength, when decreasing the ply thickness..These performance improvements could be related to a major change in the damage progression and failure modes of the laminates caused by a systematic delay or near suppression of transverse cracking and delamination growth in thin-ply composites. Detailed meso-scale finite element models of quasi isotropic unnotched tensile tests were developed and demonstrated that the increased stability of transverse intralaminar cracks was the main cause of the improved onset of damage of thin ply composites.

Mode I fracture of thin-ply carbon-epoxy laminates: effects of ply thickness

Conférence ArODES

Guillaume Frossard, Joël Cugnoni, Thomas Gmür, John Botsis

Proceedings of the 20th International Conference on Composite Materials - ICCM20

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The resistance to delamination of unidirectional fibre reinforced laminates increases with crack growth due to large scale bridging. This toughening mechanism, which is reflected by the increasing resistance curves, is subject to size effects. In this work, the influence of the ply thickness on the energy release rate (ERR) in mode I delamination of unidirectional carbon-epoxy laminates is characterized. Three ply thicknesses are considered from thin-ply (0.030 g/m2) to thick-ply (0.150 g/m2). While the ERR at onset of crack propagation do not depend on the ply thickness, the plateau level varies considerably. This effect is attributed to the change of homogeneity of the microstructure, which affects the amount of bridging fibres. Due to the tow spreading procedure used during the production of the prepregs, resin rich regions are present in thick ply laminates, while the fibre distribution is more homogeneous in thin-ply laminates. The heterogeneity of the microstructure increases the probability to have bundles of fibres trapped between the crack faces and eventually leads to higher bridging energy and higher ERR plateau.

2014

A new structural health monitoring (SHM) system using integrated polyvinylidene difluoride (PVDF) transducer networks

Conférence ArODES

Enrique Guzman, Joël Cugnoni, Thomas Gmür

Proceedings of the 65th International Astronautical Congress

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2013

A comparison of remodeling approaches applied to an implanted rat tibia

Conférence ArODES

Marco Piccinini, Joël Cugnoni, John Botsis, Giovanna Zacchetti, Patrick Ammann, Anselm Wiskott

Proceedings of the 19th Congress of the European Society of Biomechanics

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2011

A semi-automatic procedure to develop specimen-specific finite element models from micro computer tomography images of implanted bones

Conférence ArODES

Marco Piccinini, Joël Cugnoni, John Botsis, Giovanna Zacchetti, Patrick Ammann, Anselm Wiskott

CBU 2011 proceedings

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2010

Analysis of hygrothermally induced fiber fracture in single fiber composite using fiber Bragg grating

Conférence ArODES

M. Lai, Joël Cugnoni, John Botsis

EPJ Web of Conferences ; ICEM 14 – 14th International Conference on Experimental Mechanics

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Hygro-thermal ageing of composite materials introduces a modification in the material properties of its constituents and the development of stresses that can lead to severe local damage like fracture of the reinforcing fibers and de-bonding of the latter from the matrix. The study of the complex stress field generated by this induced damage is helpful in designing more resistant materials. Modelling strain-stress filed in the vicinity of these discontinuities has led to the development of different analytical models like for instance shear lag models based on simplification assumptions. Recently the development of embedded optical sensors allowed to shed light on the assumptions made, since they can be used, at the same time, as reinforcement and sensors and thus being capable to give information on the strain distributions during the evolution of the damage. In this work a single fiber composite, whose reinforcement is an optical sensor, is used in order to investigate the complex strain field generated by the fiber fracture caused by the matrix swelling during water uptake.

2009

Internal strain measurements in CFRP plates subjected to impact load using FBG sensors

Conférence ArODES

Jeannot Frieden, Joël Cugnoni, John Botsis, Th. Gmür, D. Coric

Proceedings of the ICCM17

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The local strain response of a composite plate is monitored with embedded fibre Bragg gratings during low velocity impacts. The modal characteristics of the plate, i.e. its eigenfrequencies, coarse mode shapes and modal damping ratios are obtained from the transient response to an impact and are used for damage evaluation after impact.

On moisture induced propetry changes and stress analysis of a single fiber composite during ageing

Conférence ArODES

Marco Lai, John Botsis, D. Coric, Joël Cugnoni

Proceedings of the ICCM17

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Résumé:

The hygrothermal response of an epoxy is reported as function of average moisture uptake. The specimen used is a single fibre composite with axially located optical fibre that contains a Bragg grating sensor. Bragg grating strain data are compared with stress analysis accounting for moisture diffusion in the specimen.

2008

Reliability analysis of lead-free solders

Conférence ArODES

Amir M. Lajimi, Joël Cugnoni, John Botsis

Proceedings of the World Congress on Engineering and Computer Science 2008

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Résumé:

In this study, a deep analysis of the reliability of lead-free (SnAgCu) solders in comparison with tin-lead solders have been done for a particular aerospace application. Creep properties of this solder composition have been investigated, and results have been validated with a thermal shock test. An 0805 resistor has been considered to simulate creep deformation, an electronic board has been used for a thermal shock test to check continuity of the circuit for the specified mission, and finally a detailed metallographic study has been performed. Main constitutive relations have been implemented in a commercial finite-element analysis software (ABAQUS 6.5), to predict creep strain accumulation under thermal loads. Effects of implementing different constitutive relations in compare with life prediction models have been investigated. Overall, both solders are highly reliable with this number of thermal cycles; however, SnAgCu shows higher life time under this type of loading.

Durability analysis of SnAgCu solder joints for an aerospace application

Conférence ArODES

Amir M. Lajimi, Joël Cugnoni, John Botsis

Advances in Electrical and Electronics Engineering - IAENG Special Edition of the World Congress on Engineering and Computer Science 2008

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Résumé:

In this work, a deep analysis of the reliability of lead-free (SnAgCu) solders in comparison with conventional tin-lead (SnPb) solders was done for a particular aerospace application. Creep properties of this solder composition were investigated, and results were validated with a thermal shock test. A 0805 resistor was considered to simulate creep deformation, an electronic board was used for a thermal shock test to check continuity of the circuit for the specified mission, and finally a detailed metallographic study was done to examine microstructure of solder joints after thermal shock test. Main constitutive relations were implemented in commercial finite-element analysis software (ABAQUS 6.5), to predict creep strain accumulation under thermal loads. Moreover, effects of implementing different constitutive relations in compare with life prediction models were investigated. Overall, both solders are highly reliable with this number of thermal cycles; however, SnAgCu shows higher life time under this type of loading.

Characterisation of bridging tractions in mode-I delamination using distributed strain measurements and numerical modeling

Conférence ArODES

L. Sorensen, John Botsis, Th. Gmür, Joël Cugnoni, L. Humbert

Proceedings of the 13th European Conference on Composite Materials

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Results of bridging tractions in mode I delamination on uniaxially reinforced AS4/PPS specimens are presented. Bridging tractions determined using distributed strains and an inverse- numerical technique are compared to those obtained using a method based on the experimental measurements of energy release rate and crack opening displacement. Both experiments result in similar distributions of the bridging tractions. The two types of bridging laws are implemented in a numerical model that uses cohesive elements to calculate global force- displacement response. In both cases the cohesive model can describe the global load- displacement curve well. However, the J-integral method requires an additional assumption about the length of the bridging zone. Regardless of the law used to define the bridging tractions, the cohesive model requires certain assumptions regarding the initial stiffness of the cohesive elements and the maximum stress before softening. These parameters influence crack propagation and the resulting strains.

2006

Modal identification of the elastic properties in composite sandwich structures

Conférence ArODES

Marco Matter, Thomas Gmür, Joël Cugnoni, Alain Schorderet

Proceedings of the Composites Testing and Model Identification 2006 - CompTest2006

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Réalisations

Recherche

Cugnoni Joël

Professeur HES associé

Compétences principales

Simulations numériques

Fabrication additive

Composites

Conception mécanique

Essais de structures et matériaux

Professeur HES associé