Mode I crack fatigue growth test and fracture morphology analysis of compact tensile specimen of heavy load transmission gear

By referring to the national standard GB / t6398-2000 test method for fatigue crack growth rate of metallic materials, the sample is cut into standard parts. The normal stress was applied and the crack propagation experiment was carried out. The growth of mode I crack, the change of stress intensity factor at the crack tip and the crack growth rate of 42CrMo steel were studied. The research can provide some reference for the safe life of 42CrMo steel members with crack defects.

The fatigue crack growth test of compact tensile specimens under mode I constant amplitude loading is carried out. The results show that the stress ratio r = 0.1 (small stress ratio state) is easier to lead to the initiation of fatigue cracks; In the process of propagation, when the crack length is the same, the stress intensity factor at the crack tip is larger than that at the crack tip; From the generation of crack to the fracture of sample, there are three stages: low-speed propagation, stable propagation and high-speed propagation, which run through the whole crack propagation process. Among them, the stable growth stage has a greater influence on fatigue crack growth. The Paris formula shows that the crack growth rate at low stress ratio is faster than that at high stress ratio, and the crack length leading to fatigue failure of the sample is shorter, which is α= 24.05mm, and the fatigue life is 48913 cycles. In the high-speed propagation stage, the sample breaks instantaneously. The fatigue crack growth rate is deduced through the Paris formula to predict the fatigue life of the structure.

The fatigue fracture of gear teeth is analyzed by scanning electron microscope. The crack source area presents a relatively bright state due to the mutual extrusion and grinding of both sides of the fracture; A secondary crack appears at the edge of the cleavage fracture surface in the stable propagation region, indicating that the twin boundary provides a place for crack initiation. Secondary cracks often originate at grain boundaries and propagate along grains, and grains hinder the propagation of secondary cracks. The rapid growth zone shows a plastic brittle mixed fracture morphology including dimples and tearing edges, which is the result of equiaxed dimples growing, gathering and finally connecting with each other. The secondary cracks are relatively wide. The growth process also goes through intergranular propagation to transgranular propagation and finally stops at the obstruction of grains. The inclusion of metallurgical defects affects the isotropy of the material, During the loading process, stress concentration leads to the emergence of secondary cracks, accelerates the propagation of fatigue cracks and provides a place for the initiation and propagation of fatigue cracks.