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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]

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]