考虑应变梯度的缺口低周疲劳寿命预测模型与实验验证

The fatigue life of the engineering structure often depends on the notched components. It is necessary to study the mechanical behavior and fatigue failure criteria considering the multiaxial complex stress state and the strong strain gradient at the notch-root, which is closely related to the failure of the notched component. The project is focused on the strain gradient to discuss the method for the fatigue life prediction of notched specimen under the multiaxial stress state. (1) Combining with microscopic observation of the deformation, investigate the gradient effects of deformation at the notch-root under the cyclic loading, and establish the reasonable cyclic plastic constitutive model containing the strain gradient, and then get more reasonable stress and strain field of the notch area. (2) According to the fatigue life test and the analysis of stress and strain, research the effects of strain gradient on failure resistance, and take the role of strain gradient into account in the fatigue life equation, and then select the fatigue failure criterion which is consistent with the test. Establish the model for fatigue life prediction with strain gradient under the multiaxial stress state. The project will develop the cyclic plastic constitutive model, which is closer to the nature of the material deformation, and improve the cyclic plasticity theory. It will suggest a numerical method which can reasonably reflect the influence of strain gradient on fatigue life of metal materials. On basis of the method, the low cycle fatigue life of metal structure can be evaluated better.

实际工程结构中，控制结构寿命的往往是缺口构件，其破坏与缺口处多轴复杂应力状态及强烈的应变梯度密切相关，有必要考虑这些因素对缺口根部区域力学行为和疲劳破坏条件开展研究。本项目以应变梯度为研究重点，探讨处于多轴应力状态下的缺口试样疲劳寿命预测方法：（1）结合试样变形观测，研究缺口根部区域变形场在循环加载下的梯度效应，建立合理考虑应变梯度作用的循环塑性本构模型，从而能得到更合理的缺口区域应力、应变场。（2）结合疲劳寿命试验和反映梯度的应力应变分析，研究应变梯度的存在对材料抗破坏能力的影响规律，在疲劳寿命方程中考虑应变梯度的作用，筛选与试验相一致的疲劳破坏判据，建立多轴应力状态下考虑应变梯度的疲劳寿命预测模型。本项目将发展更贴近材料变形本质的循环塑性本构模型，完善和发展循环塑性理论；建议一种能够合理反映应变梯度对金属材料疲劳寿命影响的数值计算方法，在此基础上可以更合理地评估金属结构的低周疲劳寿命

实际工程结构中含有很多的缺口构件，构件缺口处存在明显的应力集中现象，其疲劳寿命常控制着整个结构的疲劳寿命。为解决这类构件的安全服役问题, 学者通常用各种缺口试样来研究含缺口构件的力学行为，虽然总结得到了一些关于疲劳寿命的经验法则和公式，但仍未得到较合理的疲劳机制分析模型。本项目计划发展更贴近材料变形本质的循环塑性本构模型，完善和发展循环塑性理论；建议一种能够合理反映应变梯度对金属材料疲劳寿命影响的数值计算方法，在此基础上可以更合理地评估金属结构的低周疲劳寿命。. 本项目为了针对金属材料在低循环荷载下的粘塑性、疲劳寿命相关机理，开展了考虑应变梯度作用的宏观循环粘塑性模型和金属低循环疲劳机制的研究。推导了Chaboche率相关循环本构模型，并且首次加入了等效应变梯度对材料硬化性质的影响。. 介绍了等效应变梯度在有限元中的计算过程及方法，并结合ABAQUS用户接口采用Intel Fortran成功的将含有应变梯度项的材料模型编译成用户材料子程序。通过缺口试样的模拟计算并与不考虑应变梯度的材料模型的计算结果进行对比，结果表明改进的本构模型计算结果更贴近实验现象，更能合理的解释缺口试件相对光滑试件疲劳寿命长的原因，同时也建议了考虑应变梯度的缺口试样疲劳寿命方程。 .