Franck Lauro

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.

Improvement of damage prediction by anisotropy of microvoids

Abstract The finite element method is nowadays widely applied to crashworthiness or sheet metal forming. In order to accurately predict the damage and failure evolution occurring in such simulations, a realistic material description is required. For this purpose, this paper presents a damage model for elasto-plastic materials, taking into account both the material and the damage anisotropy. The damage is then expressed as the microvoid volume fraction due to growth, nucleation and coalescence of microvoids. The anisotropy of the material is introduced by means of Hill’s potential in the yield function and the anisotropy of the damage by the shape of the microvoids. The void is defined as an ellipsoid which could evolve in shape and direction according to the loading direction and the geometry of the structure. A radial return algorithm is proposed for the implementation of a new constitutive law for elasto-plastic porous materials for convected shell elements into an explicit finite element framework. To compare numerical and experimental results of bending tests on a non-axisymmetric aluminium extruded section, identification of the damage parameters is required. This is done by using an inverse method by correlating experimental and numerical macroscopic measurements which are strongly dependent on the parameters. Tensile tests on a thin notched specimen are used as a mechanical test to measure the macroscopic response. Due to the anisotropy aspect of the damage parameters, three macroscopic measurements in the three orthotropic directions L, L–T and T are considered. The behaviour law and damage parameters are identified. Bending tests are then performed and give good correlation with experimental data.

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.

Mechanistic insights on nanosilica self-networking inducing ultra-toughness of rubber-modified polylactide-based materials

Abstract Developing novel strategies to improve the impact strength of PLA-based materials is gaining a significant importance in order to enlarge the range of applications for this renewable polymer. Recently, the authors have designed ultra-tough polylactide (PLA)-based materials through co-addition of rubber-like poly(ϵ-caprolactone-co-d,l-lactide) (P[CL-co-LA]) impact modifier and silica nanoparticles (SiO2) using extrusion techniques. The addition of silica nanoparticles into these immiscible PLA/P[CL-co-LA] blends altered their final morphology, changing it from rubbery spherical inclusions to almost oblong structures. A synergistic toughening effect of the combination of P[CL-co-LA] copolymer and silica nanoparticles on the resulting PLA-based materials therefore occurred. To explain this particular behavior, the present work hence aims at establishing the mechanistic features about the nanoparticle-induced impact enhancement in these immiscible PLA/impact modifier blends. Incorporation of silica nanoparticles of different surface treatments and sizes was thereby investigated by means of rheological, mechanical and morphological methods in order to highlight the key parameters responsible for the final impact performances of the as-produced PLA-based materials. Relying on video-controlled tensile testing experiments, a toughening mechanism was finally proposed to account for the impact behavior of resulting nanocomposites.

Poly(lactic acid)-Based Materials for Automotive Applications

As a result of increasingly stringent environmental regulations being imposed on the automotive sector, ecofriendly alternative solutions are being sought through the use of next-generation bioplastics and biocomposites as novel vehicle components. Thanks to its renewability, low cost, high strength, and rigidity, poly(lactic acid), PLLA, is considered a key material for such applications. Nevertheless, to compete with traditional petroleum-sourced plastics some of the properties of PLLA must be improved to fulfill the requirements of the automotive industry, such as heat resistance, mechanical performance (especially in terms of ductility and impact toughness), and durability. This review focuses on the properties required for plastics used in the automotive industry and discusses recent breakthroughs regarding PLLA and PLLA-based materials in this field.

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.

Anisotropic damage applied to numerical ductile rupture

ABSTRACT Damage models models are applied for elasto-viscoplastic materials. They take microvoids volume fraction evolution into account by means of the growth, the nucleation and coalescence of microvoids. The anisotropy of the material is introduced with the Hills potential in the modified yield function. The anisotropy of the damage is taken into account in one model with the shape of the microvoids. The void is defined as an ellipsoid which could evolve in shape and direction according to the loading direction and the geometry of the structure. These models are applied on unit cell specimens and on a double non- axisymmetrical V-notched specimen. The influence of the microvoids shape and their orientations on the final ductile rupture is shown.

Viscoelastic–viscoplastic model for short-fiber-reinforced composites with complex fiber orientation

ABSTRACT In this article, strain-rate sensitivity of short-fiber reinforced composites (SFRC) with complex fiber orientation is modeled. The principal aim is to allow an easier implementation than that of classical techniques, in case of complex reinforcement characteristics and nonlinear matrix behavior. The interest for such an approach is particularly acute for high-strain-rate simulations using explicit formulation. The composite behavior is computed based on an additive decomposition of the state potential between linear elastic short fiber media and a viscoelastic–viscoplastic matrix medium. Finite element simulations demonstrate model’s ability to reproduce behavior trends of SFRC with distributed fiber orientations, for a wide strain-rate range.