损伤引起的反向应变率效应及其对本构关系和热粘塑性失稳的影响

对于材料在高应变率下的应变率无关和应变率负敏感现象,本文根据铸镁合金的宏观与微观相结合的试验结果,提出了一个基于损伤弱化的反向应变率效应机制,并建议了一个计及这一反向效应的热粘塑性本构方程。由此可以解释材料的表观应变率强化、表观应变率无关和表观应变率弱化三类基本情况。相应地,随损伤引起的反向应变率效应的增强,热粘塑性失稳也有三种基本情况:或在更高应变率或更大应变下发生绝热剪切,或由临界准则转化为临界应变准则,以及或在高应变率下不会发生绝热剪切。

THE INVERSE STRAIN-RATE EFFECT INDUCED BY DAMAGE AND ITS INFLUENCE ON CONSTITUTIVE RELATION AND THERMO-VISCOPLASTIC INSTABILITY

Regarding the rate-independent and negative rate sensitivity of materials at high strain-rates, a mechanism of inverse strain-rate effect induced by damage-weakening is suggested , based on the macro-microscopically experimental results for a cast magnesium alloy. Taking such an inverse effect into consideration, a damage modified thermoviscoplastic constitutive equation is suggested, and three basic cases are analysed, i. e. apparently rate-strengthening, apparently rate-independent and apparently rate-weakening. Correspondingly, with increasing the inverse rate effect, three basic cases for thermo-viscoplastic instability are discussed, i. e. adiabatic shearing at higher strain-rate and/or larger strain, adiabatic shearing at a critical strain and no a-diabatic shearing at high strain-rate or impact condition.