Flow Localization in Thermal Viscoplastic Materials

这项研究的主要目标是利用分析和计算方法进一步加深对金属和聚合物塑性变形过程中剪切应变局部化机制的理解，并提高用于模拟剪切带形成的计算算法的精度和效率。所谓的“热-机械机制”主要被认为是在高变形速率下导致流动局部化的机制。发展了有效的数值算法来积分全张量形式的应变率相关的本构关系。这些数值算法特别考虑了应变软化现象，或者是热软化现象，或者是由于孔洞生长或微裂纹形成而产生的“进行中的裂缝”。考虑了变形的两个主要阶段，即起爆前阶段(I)和起爆后阶段(II)。状态I的研究涉及剪切带开始出现的充要条件，以及几个因素对开始出现剪切带的临界应变水平的影响。这些因素包括热传导、热软化、应变率敏感性、弹性、缺陷和边界条件。区域II的研究关注的是能带变窄和局域绝热损失的现象，这导致了能带内的准稳态条件。建立了计算特征带厚度和特征带间距的模型。此外，材料性质的变化被考虑在带内，并分析了任何局部相变对物体宏观行为的影响。

The primary goal of this study is to further develop using analytical and computational procedures, the understanding of the mechanisms responsible for shear strain localization during the plastic deformation of both metals and polymers, as well as to improve the accuracy and efficiency of the computational algorithms used to simulate shear band formation. The so-called "thermo-mechanical mechanism" is primarily considered the mechanism responsible for flow localization at high rates of deformation. Efficient numerical algorithms are developed for the integration of the full tensorial form of strain-rate dependent constitutive relations. These numerical algorithms specifically account for strain softening phenomena by either thermal softening or by "fractures in progress" due to void growth or microcrack formation. Two main stages of the deformation are considered; namely, the pre-initiation regime (I) and the post- initiation regime (II). The study of regime I is concerned with the necessary and sufficient conditions for the onset of shear bands and the influence of several factors on the critical strain level at which bands may initiate. Such factors include heat conduction, thermal softening, strain rate sensitivity, elasticity, imperfections, and boundary conditions. The study of regime II is concerned with the phenomenon of band narrowing and the loss of local adiabaticity that leads to a quasi-steady-state condition within the bands. A model is constructed to derive the characteristic band thickness and the characteristic band spacing. Further, the change in material properties is accounted for within the bands, and the influence of any local phase transformation on the macroscopic behavior of the body is analyzed.