An analysis of cyclic stress-strain response correlated with static mechanical properties and development of new method for assessing fatigue life

循环应力-应变曲线是根据弹塑性断裂力学计算裂纹扩展速率和预测疲劳寿命所必需的。鉴于此，本研究的目的是开发一种新的循环应力应变曲线数值模拟方法，并在此基础上进行疲劳寿命评估。引入动态位错力学，对铜的循环应力应变响应进行了数值模拟。然后，将其应用于周向缺口构件的循环应力应变分析和寿命估算。主要结果如下：(1)利用静态力学特性成功地计算了循环应力-应变响应。(2)采用弹塑性有限元方法计算了周向缺口构件的应力-应变响应，其中将(1)中描述的本构方程引入有限元计算。(3)从弹塑性断裂力学的角度分析了缺口根部表面裂纹的形状变化。该方法较好地预测了在给定试验条件下，裂纹扩展过程中缺口根部发生的半椭圆形裂纹向环向裂纹的转变。(4)基于有限元应力分析，对周向缺口构件的裂纹萌生寿命进行了评估。结果表明，缺口根部产生的等效塑性应变主导了缺口构件的裂纹萌生寿命。(5)通过表面裂纹和周向裂纹的扩展分析，较好地评估了圆柱形和尖端缺口构件的低周疲劳寿命。

The cyclic stress-strain curve is necessitated to calculate a crack growth rate based on elastic-plastic fracture mechanics and to predict the fatigue life. In view of this, the purpose of the present study is to develop a new method for the numerical simulation of the cyclic stress-strain curve and for the fatigue life assessment based on it. The cyclic stress-strain response of copper was numerically simulated by introducing the dynamic dislocation mechanics. Then, this was applied to the cyclic stress-strain analysis of the circumferentially notched components and to the life estimation. The main results obtained are summarized as follows.(1),The cyclic stress-strain response was successfully calculated by using the static mechanical properties through the proposed method.(2) The stress-strain responses of the circumferentially notched components were calculated by means of elastic-plastic finite element method , where the constitutive equation stated in the item (1) was introduced into the F.E.M. calculations.(3) The change in the shape of the surface crack originated at trie notch root was successfully analyzed in terms of the elastic-plastic fracture mechanics. The transition from the semielliptical to circumferential crack, which occurred at the notch root in the crack extension process under a given test condition, was satisfactorily predicted in the proposed analysis.(4) The crack initiation life of the circumferentially notched components was evaluated on the basis of the finite element stress analysis. Consequently, it was revealed that the equivalent plastic strain generated at the notch root dominates the crack initiation life of the notched components where the multiaxial stress develops.(5) The low-cycle fatigue lives of the dully and acutely notched cylinder components were satisfactorily evaluated through the analysis of the growth of the surface and circumferential cracks.