Calibrating a material model for AD995 alumina from plate impact VISAR profiles

Nous présentons une validation/calibration d'un modèle de matériau pour l'alumina AD995. Nous utilisons les résultats de cinq expériences d'impact de plaques symétrique menées par Grady [1], désignées CE 56 à 60. Notre modèle de matériau est semblable à celeu employé par Johnson et Holmquist [5]. Il a une courbe de rupture fragile pour le matériau sans endommagement, et une courbe d'écoulement pour le matériau ruiné (granuleux). La réponse iriscoplastique de matériau endommagé aux contraintes de cisaillement, au de la courbe d'écoulement quasistatique, est maxwellienne. Nous calibrons la réponse viscoplastique pour correspondre au test CE 58, puis vérifions la validité du modèle de matériau, en prédisant les résultats pour les quatre autres tests. Un assez bon accord est obtenu.We present a validation/calibration of a material model for AD995 alumina. We use five of Grady's symmetric impact test data designated CE56 to CE60. Our material model is similar to that employed by Johnson and Holmquist [5]. It has a fracture surface for the intact material, and a flow surface for the fractured (granular-like) material. The viscoplastic response of the fractured material to shear stresses beyond the quasistatic yield surface is Maxwellian. We calibrate the viscoplastic response to match test CE58, and then check the validity of the material model by predicting the results for the four other tests. Agreement is quite good

Modeling Bauschingher's effect in planar impact

Bauschinger's effect (BE) was first noticed as a decrease in yield stress upon unloading from a plastic state in standard tension-compression tests. It was later established that BE is quite general in terms of materials and modes of loading. Generalizing, BE may be regarded as anisotropic plasticity (in stress space) caused by the plastic flow itself. BE was extensively researched as part of an ongoing effort to understand and model cyclic plasticity. Cyclic plasticity models are based on the concept of kinematic hardening, are usually very complex, and contain many material parameters to be calibrated from tests. Here we're concerned with modeling BE in dynamic situations, specifically planar impact tests. In these tests BE is manifested by the so called Quasi-Elastic (QE) response upon unloading from the shock plateau level. Our approach is based on ideas put forward since the 1930s. First we show that our model, which we call Effective Grains Model (EGM), can reproduce the main modes of response in the plastic range, including BE. Then we apply it to planar impact tests and show that it can reproduce the QE response

Surface-burn model for shock initiation

An investigation of a surface-burn of the shock-induced decomposition initiation and detonation of heterogeneous explosives is described. The model assumes a microscale process with hot spots ignited by viscoplastic heating at the boundaries of collapsing pores. A relatively thin reaction zone, or burn surface, is driven by the conduction of the heat of reaction, and has a surface-burn velocity with an Arrhenius dependence on the temperature of the unreacted solid component. Global reaction rates are derived from the microscale model with an empirical burning topology function and a macroscopic reactant-product mixture defined by pressure equilibrium, ideal mixing of specific volume and internal energy,and isentropic response of the unreacted constituents. With simplifying assumptions, the model is extended to treat multi-component explosives. The model is implemented into a method of characteristics hydrocode and shown to be effective in simulating several examples of initiation experiments on TATB explosives. 10 refs., 9 figs