Fatigue cracks often occur when the component is under the condition of alternating load for a long time. The initial crack length A0 may be less than the critical length AC. however, due to the cyclic load, the crack will gradually expand. After a certain period of time, the crack will expand from the initial size A0 to the critical size AC, which eventually leads to the fracture of the component. Therefore, it is necessary to analyze and study the fatigue crack growth rate test, and with the help of the curve of fatigue crack growth rate, lay the foundation for solving the problem of fatigue crack growth law and fatigue life of components or materials.
At present, 42CrMo steel is often used in the design and manufacture of low-speed and heavy-duty gears. In order to explore the fatigue crack growth and fatigue life of low-speed and heavy-duty gears, it is necessary to obtain the crack growth rate performance of 42CrMo steel at room temperature. In this chapter, 42CrMo steel is taken as the research object, which is made into compact tensile specimens, and the crack growth performance of the material is obtained through fatigue crack growth test; The Paris formula of 42CrMo steel crack in stable growth stage is established by drawing and fitting the relevant data by Origin software. It lays a foundation for discussing the fracture failure mechanism of gear teeth and predicting the fatigue life of low speed heavy duty gear teeth from the material point of view.
The fatigue crack propagation test under I-type constant amplitude loading is carried out for compact tensile specimens. The results show that the stress is more likely to cause the crack initiation and propagation than r = 0.1. When the crack length is the same, the stress intensity factor at the crack tip is larger; From the initiation of crack to the fracture of the specimen, the crack growth process is divided into three stages, namely, the low-speed growth stage, the stable growth stage and the high-speed growth stage. The stable growth stage is the key part of the study of fatigue crack growth. Through the Paris formula, the crack growth rate is faster than that under r=0.1, The crack length of the specimen was 24.05 mm and the fatigue life was 48913 cycles.
The fatigue crack growth rate can be calculated by Paris formula, and the fatigue life of the structure can be predicted. The results provide a foundation for the study of fatigue crack propagation of low speed heavy duty gears.