Objectives  In order to research the uneven distribution problem of initial crack depth along the thickness direction of a plate,

  Methods  based on the 3D Virtual Crack Closure Technique (VCCT), this paper takes a semi-circular crack as an example for carrying out a numerical simulation on the tip stress field of a crack subjected to alternating tensile loads.

  Results  For the horizontal equivalent stress field of the crack in the plastic deformation area, it is found that the von Mises equivalent stress at the calculation point decreases as the distance from the crack tip increases, where it is basically subjected to linear or bilinear distribution. However, the stress of the crack in the non-plastic deformation area is the cubic function of the distance. The cross-section of the crack always extends in the normal direction along the crack's edge, where it is subjected to maximum stress. The internal stress features a highly nonlinear distribution with regard to the distance from the center of the crack, and the external stress decreases as the distance increases. As the distance reaches up to 1.67 times greater than the crack radius, the concentration of stress disappears. As the tensile loads are 2/3 and 1/3 times greater than the material yield stress, the maximum stress at the edge of the crack is always equal to 1.45 and 2 times the load respectively.

  Conclusions  The results of this paper can provide valuable references for the ultimate strength calculation of plate structures with initial cracks.