Flow Localization in Thermal Viscoplastic Materials

本研究的主要目标是利用分析和计算方法进一步发展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，以及提高用于模拟剪切带形成的计算算法的准确性和效率。所谓的“热-机械机制”主要被认为是在高变形率下导致流动局部化的机制。针对应变率相关本构关系的全张量形式的积分，提出了有效的数值算法。这些数值算法特别考虑了由热软化或由孔洞生长或微裂纹形成引起的“进行中的断裂”引起的应变软化现象。考虑了变形的两个主要阶段；即启动前制度(I)和启动后制度（II）。状态1的研究涉及剪切带发生的必要和充分条件，以及几个因素对可能发生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。状态II的研究涉及带变窄现象和局部绝热的损失，导致带内的准稳态条件。建立了特征带厚度和特征带间距的模型。此外，材料性能的变化在波段内被解释，并分析了任何局部相变对体的宏观行为的影响。

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.