Integration-point reformulation of an adiabatic shear band initiation criterion
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摘要: 绝热剪切带(ASB)临界剪切应变判据中的剪切应变由试件表面网格变形测得,为解决该应变变量难以直接用于有限元积分点计算的问题,提出一种ASB起始判据的积分点转写方法。根据应变莫尔圆和J2关联流动关系构造最大塑性剪切应变,以其增量为权重,仅在计算临界剪切应变时对积分点应力状态历程加权。当最大塑性剪切应变与加权临界剪切应变之比不小于1时,积分点满足ASB起始条件。上述计算过程通过LS-DYNA二次开发实现,并采用不同槽角剪切压缩试件进行检验。结果表明,随着槽角增大,ASB起始位置总体上由切槽中部向槽端转移,与试验观察的变化趋势一致。增量加权方案减弱了早期应力状态波动对临界剪切应变的影响,所得比值高值区较瞬时和算术平均方案更集中,且不同网格下的潜在起始位置和高值区方向基本一致。积分点转写使依赖试验测量的临界剪切应变变量能在有限元积分点内直接计算,为复杂载荷下ASB起始位置的数值识别提供了方法。
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Abstract: An integration-point reformulation method is proposed for the adiabatic shear band (ASB) initiation criterion. The original criterion uses shear strain measured from a grid marked on the specimen surface. This strain cannot be used directly in finite-element integration-point calculations. The maximum plastic shear strain is calculated using Mohr’s circle for strain and the J2 associated flow rule. Its increment is used as the weight. The weighting is applied only to the stress-state history used to calculate the critical shear strain. An integration point meets the ASB initiation condition when the ratio of the maximum plastic shear strain to the weighted critical shear strain is not less than unity. The calculation procedure is implemented in LS-DYNA through a user-defined material subroutine. Shear-compression specimens with different notch angles are used for evaluation. The results show that the ASB initiation site generally shifts from the center of the shear zone toward a notch tip as the notch angle increases. This trend agrees with experimental observations. The increment-weighted scheme reduces the effect of early stress-state fluctuations on the critical shear strain. It also produces a more concentrated high-ratio region than the instantaneous and arithmetic-mean schemes. The potential initiation sites and orientations of the high-ratio regions remain generally consistent across different mesh sizes. The integration-point reformulation allows the critical shear strain variable derived from experimental measurements to be calculated directly at finite-element integration points. The resulting criterion provides a numerical method for identifying ASB initiation sites under complex loading. -
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