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Abstract: This paper deals with a viscoplastic potential-based model allowing thermomechanical damage behavior modeling of porous materials. The model describes rate dependent effects, hardening, creep as well as defects coalescence and propagation. Kinematic and isotropic hardening effects are taken into account by a set of internal state variables. The integration and implementation of the model into the FE code using a fully implicit integration scheme is exposed. Finally, it''s used to predict mechanical behaviour degradation of solder layers used in power electronic packaging. Stress-strain behaviour and the evolution of volumic fraction of voids for the material under cyclic loading are presented. [Copyright &y& Elsevier]

Temperature-induced solder joint fatigue is a main reliability concern for aerospace and military industries whose electronic equipment used in the field is required to remain functional under harsh loadings. Due to the RoHS directive which eventually will prevent lead from being utilized in electronic systems, there is a need for a better understanding of lead-free thermomechanical behavior when subjected to temperature variations. As solder joints mechanical properties are dependent of their microstructural characteristics, developing accurate solder joint fatigue models means to correctly capture the microstructural changes that undergo the solder alloy during thermal cycling. This study reports the Sn3.0Ag0.5Cu (SAC305) solder joints microstructural evolution during damaging temperature cycles. Electron BackScatter Diffraction (EBSD) analysis was conducted to assess the SAC305 microstructure corresponding to a specific damage level. Investigated microstructural features included the β-Sn grain size and crystallographic orientation, as well as the grain boundary misorientation and Ag 3 Sn intermetallic compound (IMC) size. As-reflowed and damaged components were also mechanically characterized using nanoindentation technique. The microstructural analysis of SAC305 solder joints prior to thermal cycling showed a highly textured microstructure characteristic of hexa-cyclic twinning with two β-Sn morphologies consisting of preferentially orientated macrograins known as Kara's beach ball, along with smaller interlaced grains. The main observation is that recrystallization systematically occurred in SAC305 solder joints during thermal cycling, creating a high population of misoriented grain boundaries leading to intergranular crack initiation and propagation in the high strain regions. The recrystallization process is accompanied with a progressive loss of crystallographic texture and twinning structure. Ag 3 Sn IMCs coalescence is another strong indicator of SAC305 solder damage since the bigger and more spaced the IMCs are the less dislocation pinning can prevent recrystallization from occurring. [ABSTRACT FROM AUTHOR]

An experimental investigation of two potential candidate materials for the diamond die attachment is presented in this framework. These efforts are motivated by the need of developing a power electronic packaging for the diamond chip. The performance of the designed packaging relies particularly on the specific choice of the solder alloys for the die/substrate junction. To implement a high temperature junction, AuGe and AlSi eutectic alloys were chosen as die attachment and characterized experimentally. The choice of the AlSi alloy is motivated by its high melting temperature T m (577 °C), its practical elaboration process and the restrictions of hazardous substances (RoHS) inter alia. The AuGe eutectic solder alloy has a melting temperature (356 °C) and it is investigated here for comparison purposes with AlSi. The paper presents experimental results such as SEM observations of failure facies which are obtained from mechanical shear as well as cyclic nano-indentation results for the mechanical hardening/softening evaluation under cyclic loading paths. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, errors related to digital image correlation (DIC) technique applied to measurements of displacements are estimated. This work is based on the generation of synthetic images representative of real speckle patterns. With these images, various parameters are treated in order to determine their impact on the measurement error. These parameters are related to the type of deformation imposed on the speckle, the speckle itself (encoding of the image and image saturation) or the software (subset size). [Copyright &y& Elsevier]

Abstract: The weak point for Insulated-Gate Bipolar Transistor (IGBT) modules in terms of reliability is thermal fatigue in solder joints due to the thermal stress induced by constitutive materials with different coefficients of thermal expansion (CTE). Now, many researches aimed to define accurate finite element simulation with constitutive equations of material behavior and fatigue failure relation connecting the inelastic strain and the number of cycles before failure. Even when these relations are clearly identified, the validation of the finite element model is difficult due to the scatter of input data. In fact, fatigue life of solder joints strongly depends on geometric shape, solders behavior (due to the process) and applied load. The aim of this article is to estimate the probability of failure of power module with structural reliability methods by considering geometric, material and loading variables as random variables. Since in a non-linear context, the finite element calls are expensive in terms of computer run time, an FE strategy is proposed here to replace conventional 3D mesh of layer by 3D-shell. To reduce computation time, response surface method, which approximates the output strain with respect to input random variable (RV) with the design of experiment (DOE) procedure, is used to perform reliability analysis. This reponse surface allows at the end to perform Monte Carlo random simulation process for fitting Weibull and fatigue life distribution on the output inelastic strain. [Copyright &y& Elsevier]

In this paper a two-dimensional and a three-dimensional finite element models of unsteady-state metal cutting are presented. These models take into account dynamic effects, thermo-mechanical coupling, constitutive damage law and contact with friction. The simulations concern the study of the unsteady-state process of chip formation. The yield stress is taken as a function of the strain, the strain rate and the temperature in order to reflect realistic behavior in metal cutting.Unsteady-state process simulation needs a material separation criterion (chip criterion) and thus, many models in the literature use an arbitrary criterion based on the effective plastic strain, the strain energy density or the distance between nodes of parts and tool edge. The damage constitutive law adopted in models presented here allows defining advanced simulations of tool''s penetration in workpiece and chip formation. The originality introduced here is that this damage law has been defined from tensile and torsion tests, and we applied it for machining process. Stresses and temperature distributions, chip formation and tool forces are shown at different stages of the cutting process.Finally, we present a three-dimensional oblique model to simulate the unsteady-state process of chip formation. This model, using the damage law defined before, allows an advanced simulation close to the real cutting process. The final part shows a milling application.An arbitrary Lagrangian Eulerian formulation (ALE) is used for these simulations; this formalism combines both the advantages of Eulerian and Lagrangian representations in a single description, it is exploited to reduce finite element mesh distortions. [Copyright &y& Elsevier]

Abstract A non-linear discrete hybrid approach based on the association of hypoelastic continuous elements (non-linear shear behaviour) with specific connectors (non-linear tension stiffness) is developed. It allows the simulation of a two-dimensional (2D) woven reinforcement forming via an accurate explicit finite element analysis. This approach allows the simulation of 2D unbalanced fabrics uncoupling tensile and shear behaviour. It only needs a few parameters to be identified, and shows a good agreement with the experiments. The identification of the model parameters is investigated, and their relevance is analysed in reference tests. To determine the continuous element behaviour, a VUMAT hypoelastic model is implemented in Abaqus/Explicit. This model allows the prediction of fibre stresses and the accurate determination of shear angle in large deformations. Identification and validation of the model are performed using standard characterisation fabric tests. The experimental characterisation provided the numerical data to produce a representational prediction of the deformed fabric geometry and shear angle distribution. Further, the behaviour of the carbon woven reinforcement is identified. A bias extension test is used to both calibrate and validate the model. The capability of the model is illustrated to simulate deep drawing, and to compare with the experimental results of hemispherical forming. [ABSTRACT FROM AUTHOR]

The Structural Health Monitoring (SHM) in civil engineering faces several challenges. The main issue lies in defining a reliable and precise methodology of damage detection and localization in order to allow preventive maintenance or to enable the definition of repair actions. In this paper, a new methodology of SHM is proposed. Using Vibration-Based Damage Detection Methods (VBDDM), a damage detection and localization algorithm is elaborated and tested on a Finite Element Model (FEM) of an existing building. In a first case, the damage is introduced artificially by a local reduction of stiffness, while in the second case, the damage is calculated according to a real seismic signal from the italian L’Aquila earthquake. The advantages and disadvantages of each dynamic monitoring technique are discussed and the usefulness of the algorithm is highlighted. [ABSTRACT FROM AUTHOR]

Mechanical suitability of ohmic contacts among the select metallization systems, deposited on a p-type heavily boron-doped homoepitaxial diamond layer, was evaluated via mechanical tests on the nanoscale. Two candidate metallization systems were considered: Si/Al and Ti/Pt/Au. Metallizations were performed using two different techniques: plasma-enhanced chemical vapour deposition and “lift-off”. Effectiveness of the techniques was assessed via mechanical tests on the microscale and the nanoscale. Nano-indentation experiments were performed to determine the mechanical properties of the layers. Nano-scratching experiments were used to evaluate the mechanical adhesion on the diamond substrate. Scanning electron microscopy was applied for observation of the morphology of the surface and the indent and for detecting defects. [ABSTRACT FROM AUTHOR]

Abstract: This work examines the thermal fatigue effects on different configurations of power modules used in harsh aeronautical environment. They are used in various applications where the temperature cycling due to the working environment is the most limiting fact. In this case, it is highlighted that the topology assembly choice is a critical point to reach the lifetime required for the final application. In addition, it is proposed to correlate the probabilistic finite elements calculus to the experimental accelerated ageing tests on test vehicle, in order to determine the best configuration of assembling stack consisting of baseplate/RoHS solder/metallized substrate. [Copyright &y& Elsevier]

Abstract: In order to predict the flight parameters and to improve the life time of long duration super-pressure balloon, a research program on modelization and experimental simulation of the balloon envelope mechanical behavior is carried out. The balloon is a 10m diameter type made with multilayer polymeric films and tapes. A facility was developed to measure, via a stereo-correlation system, the 3D displacement and the in plane solicitation strain of a 1.5m diameter balloon envelope part in realistic flight conditions, i.e. pressure, temperature and loading at the sample boundaries. A time dependant non-linear Maxwell model of the polymeric material behavior was identified from uniaxial creep and relaxation tests and implemented in a Finite Elements code, simulating the sample tested in the facility. The Poisson ratio of the transparent and supple balloon film has been measured with an image correlation system. Experimental results are obtained both at room and in cold conditions (−60°C) for various values of differential pressure. Vertical displacement and in plane 2D strain of apex part deduced from the numerical modeling are compared to experimental results. [Copyright &y& Elsevier]

Long duration super-pressure balloons constitute a great challenge in scientific ballooning. For any type of balloons (spherical, pumpkin, …), it is necessary to have a good knowledge of the mechanical behavior of envelopes regarding the level and the lifetime of the flight. For this reason CNES, ONERA and ENIT are carrying out a research program of modelization and experimentation in order to predict the envelope shape of a balloon in different conditions of temperature and differential pressure. This study was conducted in two parts. During the first one, we defined, with parameters obtained from unidirectional tests, the mechanical laws (elasticity, plasticity and viscosity properties of polymers) of materials involved in the envelope. These laws are introduced in a finite element code, which predicts the stress and strain status of a complex envelope structure. During the second one, we developed an experimental set-up to measure the 3D strain on a balloon subsystem, which includes envelope, assemblies and apex parts, in real flight conditions. This facility, called NIRVANA, is a 1 m3 vacuum chamber with cooled screens equipped with a stereoscopic CCD measurement system. A 1.5 m diameter sample can be tested under differential pressure, regulated temperature (from +20 to -120 °C) and a load (up to 6 tonnes) applied on tendons. This paper presents the first results obtained from the modelizations and measurements done on an envelope sample submitted to axisymmetrical stress due to the differential pressure. This sample consists of a 50 μm multilayer polymer film with an assembly, used in 10 m diameter STRATEOLE super-pressure balloons. The modelization gives results in good accordance with the experiments and will enable us to follow this work with cold conditions, time dependence (creeping) and more complex structures. [Copyright &y& Elsevier]