Implicit integration scheme for porous viscoplastic potential-based constitutive equations

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 19s 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

Identification and comparison of different constitutive laws for high speed solicitation

This paper deals with a methodology for the identification of constitutive laws for metallic materials under dynamic loading at high strain rates. Different constitutive elastoplastic behaviors are compared to represent several materials such as 42CD4 steel or aluminum alloy A2017T3. The classical radial return algorithm is used to integrate the elastoplasticity law with a mixed isotropic/kinematic hardening. The theory has been used to implement the constitutive law in Abaqus/Explicit Vumat subroutine. In the second part we present the different types of elastoplastic hardening laws such as Johnson-Cook, modified Johnson-Cook, Zerilli-Armstrong or Simo. We discuss the different forms of the laws and their taking into account of strain rates, speed and temperature. The iterative procedure for the identification uses an experimental set of parameters (from a Taylor impact test) which are compared with an other set of numerical values. The method to converge to the optimal solution uses a combined Monte-Carlo and Levenberg-Marquardt algorithm. The study ends with a comparative discussion about the laws and the problems encountered during their identification for two materials: a 42CD4 steel, and an aluminum 2017T3

Probabilistic approaches and reliability design of power modules

The weak point for the standard power 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). So far, many researches are aimed at defining accurate finite element simulation with constitutive equations of materials behaviour and fatigue failure relation connecting the inelastic strain and the number of cycles before failure. Even if these relations can be clearly identified, we can see that the validation of the finite element model is difficult due to the scatter of input data. In fact, the fatigue life of solder joints strongly depends on geometric shape, solders behaviour (due to the process) and applied load. The aim of this paper is to estimate the probability of failure of power module with the structural reliability methods. Thus the geometric, materials and loading variables are considered as random variables and the failure mode is modelled with the called limit state function. The two methods, response surface method and neural network method, are used here to evaluate the reliability of the lead-free solder. The sensitivities of the mean and the standard deviation for each random variable have been evaluated

Non destructive investigation of defects in composite structures by full-field measurement methods

International audienceno abstrac

Non destructive investigation of defects in composite structures by full-field measurement methods

International audienceno abstrac

Reliability of the connections used in IGBT modules, in aeronautical environment

In this paper, two IGBT modules assembling technologies with double side cooling capabilities and high level of integration were proposed for aeronautic applications after a state of the art and failures analysis. These technologies are compared using design of experiment based on non-linear finite element analysis with various materials, with respect to their potential failures under thermal and power loading profiles. The configurations optimizing the lifetime and reliability level were pointed out by loading profile and failure mode. Recommendations were then done in order to choose the optimal configuration of assembly for each application. Finally, these design rules were followed by the study of parts dimensions effects on the design outputs in order to help dimensioning the IGBT modules