Crack behavior investigation and crack opening stresses for plane stress, plane strain and 3D cases

Two and three dimensional finite element programs of fatigue crack growth and closure were applied to multi axial elastic perfectly plastic aluminum 2024 alloy under constant amplitude crack extensions. Crack opening and closing stresses of nodes on the crack surface plane were investigated for different stress ratios and thicknesses for the desired unloading and reloading portions of load histories. The finite element programs incorporate linear strain isoperimetric rectangular and cubic elements. The plasticity part of the analysis uses initial stress approach under small strain assumptions. The crack was extended one element size as the applied load reached the maximum stress of each load cycle. The variation of local stresses at the crack tip plus the behavior of v-displacements of nodes on the crack surface plane are also investigated. The results show that the closure behavior of multiaxial loading is lower than that of uniaxial case. The magnitude of maximum shear stress at the crack tip of multiaxial case which is lower than that of uniaxial loading. The crack growth in the axial mode will be slower than in the multiaxial mode.