Different types of cracks have their own stress and displacement: type 1 crack is subjected to tensile stress perpendicular to the crack surface, which makes the upper and lower surfaces of the crack surface open relatively; The mode II crack is subjected to the shear stress parallel to the crack surface and perpendicular to the crack front, so that the crack slides relatively along the x-axis on the upper and lower surfaces; The mode III crack is subjected to the shear stress parallel to the crack surface and parallel to the crack front, so that the upper and lower surfaces of the crack are staggered along the z-axis. In practical engineering structures, fatigue cracks may not be a single crack type, but there may be two or three crack types at the same time. This fracture mode is collectively referred to as composite crack.
The main difference between compound mode crack propagation and classical giffiti mode crack propagation is whether the crack propagates along a new branch. Therefore, the fracture criterion of composite mode crack is carried out around the following problems: crack propagation direction and crack propagation conditions. Researchers have explained the composite fracture criterion from different angles, such as Erdogan’s maximum circumferential tensile stress theory, nuismer’s energy release rate criterion and SIH’s strain energy density factor criterion.
Chinese scholars have made further research on crack fracture on the basis of the three criteria. Zhao Yishu modified the initial crack propagation direction of the strain energy density factor criterion, and pointed out that the initial crack propagation direction is the direction of the minimum distance from the crack tip to the elastic-plastic boundary. Zhao Yanhua et al. Pointed out that in some cases, the strain energy density factor criterion is quite different from the test results, and took the second invariant of the deflection stress tensor as the judgment basis to study the crack cracking angle and load, which are verified by experiments. Jiang Yuchuan believes that the reason for the yield of metal materials is related to the specific energy of shape change, but not to the volume strain energy. It is pointed out that the real power of crack propagation comes from the specific energy of shape change, and the criterion of specific energy density factor of shape change is established.
From the above, it can be seen that mode I crack is the main cause of low stress fracture, and it is also the main object of test and research conducted by scholars for many years. Open and composite cracks are the focus of research.