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首页> 外文期刊>Acta Mechanica >Localized fracture phenomena in thermo-viscoplastic flow processes under cyclic dynamic loadings
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Localized fracture phenomena in thermo-viscoplastic flow processes under cyclic dynamic loadings

机译：循环动力载荷下热粘塑性流动过程中的局部断裂现象

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The main objective of the paper is the investigation of localized fatigue fracture phenomena in thermo-viscoplastic flow processes under cyclic dynamic loadings. Recent experimental observations for cycle fatigue damage mechanics at high temperature and dynamic loadings of metals suggest that the intrinsic microdamage process does very much depend on the strain rate and the wave shape effects and is mostly developed in the regions where the plastic deformation is localized. The microdamage kinetics interacts with thermal and load changes to make failure of solids a highly rate, temperature and history dependent, nonlinear process. A general constitutive model of elasto-viscoplastic damaged polycrystalline solids developed within the thermodynamic framework of the rate type covariance structure with a finite set of the internal state variables is used (cf. Dornowski and Perzyna [16]. [17]. [18]). A set of the internal state variables is assumed and interpreted such that the theory developed takes account of the effects as follows: (i) plastic nonnormality; (ii) plastic strain induced anisotropy (kinematic hardening); (iii) softening generated by micro-damage mechanisms (nucleation, growth and coalescence of microcracks); (iv) thermomechanical coupling (thermal plastic softening and thermal expansion); (v) rate sensitivity; (vi) plastic spin. To describe suitably the time and temperature dependent effects observed experimentally and the accumulation of the plastic deformation and damage during a dynamic cyclic loading process the kinetics of microdamage and the kinematic hardening law have been modified. The relaxation time is used as a regularization parameter. By assuming that the relaxation time tends to zero, the rate independent elastic-plastic response can be obtained. The viscoplastic regularization procedure assures the stable integration algorithm by using the finite difference method. Particular attention is focussed on the well-posedness of the evolution problem (the initial-boundary value problem) as well as on its numerical solutions. The Lax-Richtmyer equivalence theorem is formulated, and conditions under which this theory is valid are examined. Utilizing the finite difference method for a regularized elasto-viscoplastic model, the numerical investigation of the three-dimensional dynamic adiabatic deformation in a particular body under cyclic loading condition is presented. Particular examples have been considered, namely a dynamic adiabatic cyclic loading process for a thin plate with sharp notch. To the upper edge of the plate is applied a cyclic constraint realized by rigid rotation of the edge of the plate while the lower edge is supported rigidly. A small localized region, distributed asymmetrically near the tip of the notch, which undergoes significant deformation and temperature rise, has been determined. Its evolution until occurrence of fatigue fracture has been simulated. The propagation of the macroscopic fatigue damage crack within the material of the plate is investigated. It has been found that the length of the macroscopic fatigue damage crack distinctly depends on the wave shape of the assumed loading cycle. [References: 63]
机译：本文的主要目的是研究循环动态载荷下热粘塑性流动过程中的局部疲劳断裂现象。对高温和金属动态载荷下的循环疲劳损伤力学的最新实验观察表明，固有的微损伤过程在很大程度上取决于应变率和波形效应，并且主要发生在塑性变形局部的区域。微损伤动力学与热和载荷的变化相互作用，使固体的破坏具有很高的速率，温度和历史相关性，是非线性过程。使用在速率型协方差结构的热力学框架内开发的，具有有限内部状态变量的弹性-粘塑性损伤多晶固体的一般本构模型（参见Dornowski和Perzyna [16]。[17]。[18] ）。假定并解释了一组内部状态变量，以使开发的理论考虑到以下影响：（i）塑性非正态性; （ii）塑性应变引起的各向异性（运动硬化）; （iii）由微损伤机制（微裂纹的形核，生长和聚结）产生的软化; （iv）热力耦合（热塑性软化和热膨胀）; （v）利率敏感性; （vi）塑料旋转。为了适当地描述实验观察到的与时间和温度有关的影响以及动态循环加载过程中塑性变形和破坏的累积，微损伤的动力学和运动硬化定律已被修改。弛豫时间用作正则化参数。通过假定松弛时间趋于零，可以获得速率无关的弹塑性响应。粘塑性正则化程序通过使用有限差分法确保稳定的积分算法。特别关注的是演化问题（初始边界值问题）的适定性及其数值解。制定了Lax-Richtmyer等价定理，并检验了该理论有效的条件。利用有限差分法对正则弹黏塑性模型进行了研究，给出了循环载荷条件下特定物体三维动态绝热变形的数值研究。已经考虑了特定的例子，即具有尖锐缺口的薄板的动态绝热循环加载过程。对板的上边缘施加周期性的约束，该约束通过板的边缘的刚性旋转而刚性地支撑下边缘而实现。已经确定了一个小的局部区域，该区域不对称地分布在缺口的尖端附近，该区域经历了明显的变形和温度升高。模拟了它的演变，直到出现疲劳断裂为止。研究了宏观疲劳损伤裂纹在板材料内的传播。已经发现，宏观疲劳损伤裂纹的长度明显地取决于假定的载荷循环的波形。 [参考：63]

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