Bruno Bennani

Identification of the damage parameters for anisotropic materials by inverse technique: application to an aluminium

Abstract A damage model is integrated into the explicit finite element framework to predict the damage evolution which occurs under dynamic loading in the crash or stamping process. This damage model is based on the description of the nucleation, growth and coalescence of the microvoids. This damage process leads to the progressive loss of the stress-carrying capacity leading to rupture. The model is adapted to take the material behaviour anisotropy and damage anisotropy into account. An inverse method is developed to identify the material parameters by correlating an experimental and numerical measurement with a tensile test on a notched specimen. This identification methodology is applied to an aluminium part cut from an extruded tube. The material parameters are identified and the ductile rupture is predicted.

Ductile damage and fracture finite element modelling of elasto-viscoplastic voided materials

Abstract Micro-structure void volume fraction is taken into account with finite element models developed for high strain rate elasto-viscoplastic problems. Void nucleation rate depends on matrix effective strain rate, void growth on material strain rate and elasto-viscoplastic potential proposed for porous material, void coalescence on matrix effective plastic strain rate and ductile fracture on critical void volume fraction. A radial return algorithm is proposed for the implementation into explicit finite element frameworks. Reference finite element modelling of ductile damage increases and fracture occurrence is performed for high-speed upsetting of steel multi-tubular.

DAMAGE OCCURRENCE UNDER DYNAMIC LOADING FOR STRAIN RATE SENSITIVE MATERIALS

To predict damage evolution occurring under dynamic loading, a damage model is implemented inside the explicit finite element framework. The damage model is based on the description of the growth, the nucleation and the coalescence of the microvoids. The microvoid growth is related to the plastic incompressibility relation. The microvoid nucleation is either controlled by the plastic strain or by the stress. The microvoid coalescence is described by a specific function. This damage process leads to the progressive loss of the stress carrying capacity of the structure. The ductile fracture occurs when the stress carrying capacity of the structure vanishes. The sensitivity of damage volution under dynamic loading in the case of porous strain rate sensitive material is analysed using single tensile tests. The dynamic bending test of a cantilever beam with a U-cross-section is performed. The influence of the strain rate on the deformed shape and on the loss of the structures stress carrying capacity is shown.

Elasto-viscoplasticity Behaviour of a Structural Adhesive Under Compression Loadings at Low, Moderate and High Strain Rates

Crashworthiness performance of automotives by introducing new materials is of high interest based on the global mass’ reduction of cars. Bonded techniques are tested at present with respect to the reduction of the mass/energy’s ratio combined with recycling processes. In this study, a compression test program is performed on a bulk structural adhesive under a very large range of strain rates, thus [0.1; 5000]/s. Uniaxial compression tests at 0.1 and 50/s are achieved using a high-speed hydraulic testing machine. A split Hopkinson bar’ device made of PA66 is experimented so as to characterize soft materials in the upper domain of strain rates, thus [500; 5000]/s. A special mold is used for the preparation of the adhesive plates. Two sheets are prepared (thickness 4xa0mm with 1xa0MPa pressure and 6xa0mm with 4xa0MPa pressure) in order to consider the effect of the pressure during the curing process. Water jet technique is used to extract cylindrical samples from the plates with accurate dimensions. The identification of the compression behaviour of the structural adhesive is lead with objective to find material parameters of a modified G’sell model for finite element modeling of crashworthiness.

Damage framework for the prediction of material defects: identification of the damage material parameters by inverse technique

Publisher Summary This chapter discusses a coupled damage-mechanical model for strain rate dependant voided material. The microvoid nucleation, growth, and coalescence are modeled. The ductile fracture is predicted at the end of the damage process. The improvement of the numerical simulation for crashes or forming processes under dynamic loading takes the prediction of the ductile rupture into account. The numerous material damage parameters must be defined to make the damage model available. An inverse method has been developed by coupling an optimizer and the finite element code. An experimental tensile test of the notched specimen is used to determine the variation of the inner radius of the specimen in function of the elongation. This measurement is compared with that obtained by numerical simulation and the material damage parameters are identified by minimizing the gap between these two measurements.