The mutual interactions between intrinsic (damage) and extrinsic (shielding) mechanisms are essential for understanding fatigue crack growth in ductile materials. In the latter case, plasticity-induced crack closure is the dominating retardation mechanism in the Paris regime. The transition between plane strain and plane stress states leads to a locally different fracture surface contact, which is difficult to access during experiments. In this work, plasticity-induced crack closure under constant amplitude loading of an AlCu4Mg (AA2024) aluminium sheet material is studied from a three-dimensional perspective. An elastic–plastic 3D finite element model of an M(T)160 specimen with a bilinear isotropic hardening model is used to study the evolution of plasticity during fatigue crack growth. Cyclic crack propagation is simulated with the releasing-constraint method.

In the results, plasticity-induced crack closure is present up to a load ratio R = 0.3. Detailed investigations of the contact pressure distributions indicate a strong dependence on the stress intensity factors and the sheet thicknesses. The contact pressure distributions on the fracture surface can be divided into three characteristic regions. Near the plastic zone, plastic energy is still induced during the unloading process after the crack is already closed. However, the crack surface contact does not affect the shape of the plastic zone. Additionally, further plastic energy accumulates in the plastic wake region during crack closure within subsequent load cycles, described here as cyclic plastic wake. In summary, the finite element model can reproduce the major features of fatigue crack closure like K_op or K_cl and the three-dimensional and load-dependent evolution of plasticity-induced crack closure.

内在（损坏）和外在（屏蔽）机制之间的相互作用对于理解延性材料中的疲劳裂纹扩展至关重要。在后一种情况下，塑性引起的裂纹闭合是巴黎机制中的主要延迟机制。平面应变和平面应力状态之间的过渡导致局部不同的断裂表面接触，这在实验过程中难以接近。在这项工作中，从三维角度研究了 AlCu4Mg (AA2024) 铝板材料在恒定振幅载荷下的塑性诱发裂纹闭合。使用具有双线性各向同性硬化模型的 M(T)160 试样的弹塑性 3D 有限元模型来研究疲劳裂纹扩展过程中塑性的演变。

在结果中，塑性诱发的裂纹闭合在载荷比R  = 0.3 时出现。对接触压力分布的详细研究表明强烈依赖于应力强度因子和板材厚度。断面上的接触压力分布可分为三个特征区域。在塑性区附近，在裂纹已经闭合后的卸载过程中仍会产生塑性能。但是，裂纹面接触并不影响塑性区的形状。此外，在随后的载荷循环中，在裂纹闭合期间，塑性尾流区域会进一步积累塑性能量，此处称为循环塑性尾流。综上所述，有限元模型可以再现疲劳裂纹闭合的主要特征，如K _op或K _cl以及塑性诱发裂纹闭合的三维和载荷相关演化。