Taking the transmission gear of heavy equipment as the research object, the fatigue crack propagation and fatigue fracture of gear teeth are studied. The compact tensile specimen made of 42CrMo steel was tested for fatigue crack growth and numerically simulated to verify the feasibility of the test; Then, the micro morphology near the fracture of the sample was observed and analyzed by scanning electron microscope; Finally, a large module gear similar to the transmission gear of heavy equipment is designed, and the bending fatigue test and corresponding numerical simulation are carried out on the bending fatigue testing machine. Finally, the numerical simulation of fatigue crack propagation of a pair of meshing gears is verified. The main conclusions are as follows:
1) In the fatigue crack growth test of 42CrMo steel compact tensile specimen under mode I constant amplitude loading, low stress ratio is more likely to cause crack initiation and propagation. Under the same crack length, the stress intensity factor at the crack tip is greater. The Paris formula of stable growth stage also shows that the crack growth rate is faster under the condition of small stress ratio, In this case, the fatigue life of the specimen is shorter.
2) The macroscopic morphology of fatigue fracture is divided into three obvious regions: crack source region, stable propagation region and instantaneous fracture region. In the microscopic state, the crack initiates at the grain boundary and then propagates along the grain boundary. In the same region, there are multiple cracks propagating in the same or different directions at the same time. In the process of crack propagation, with the increasing of fire factor at the crack tip and the initiation and aggregation of fatigue cracks. The propagation path will change from intergranular propagation to transgranular propagation. The increase of crack length leads to the increase of stress. At this time, the specimen bears tearing stress. The micro morphology shows the plastic brittle mixed fracture morphology of dimple and tearing edge, that is, the tearing of internal fibers; The granular slag inclusion defect in metal affects the uniformity, stability and isotropy of materials, and provides a place for the initiation and propagation of fatigue cracks.
3) Under the condition of numerical simulation, the Mises equivalent stress nephogram at the crack tip is consistent with the theory. The stress distribution on both sides of the crack tip is approximately symmetrical and has an obvious gradient. Under the condition of the same crack length, the smaller stress has a larger area than the equivalent stress nephogram at the crack tip. If the crack length increases, Its area also shows an increasing trend. Compared with the experimental values, the stress intensity factor at the crack tip under the same crack length is always smaller under the same stress ratio obtained by numerical simulation, and the calculated fatigue life is longer, but the error is small, which can prove the feasibility of the test and lay a solid foundation for the bending fatigue test of gear.
4) When the load is constant, the increase of crack radius is bound to increase the maximum value of K I at the crack front; If the long axis radius of the semi elliptical crack is increased, the K I at the depth of the crack will increase, and the closer to the gear tooth surface, the less obvious the change of the size of K I. this situation leads to the decreasing trend of the K I at the front edge of the whole crack. With the increase of torque, the stress intensity at the crack tip also increases, and still presents a symmetrical distribution, which is low in the middle and high at both ends.
5) According to the bending fatigue test data of 42CrMo gear under different compressive loads, it can be concluded that the higher the survival rate of the gear, the lower the fatigue life of the gear. The trend between the two is just the opposite. The fatigue life of the gear sample with higher survival rate under the same load is shorter, and vice versa. Through the Mises equivalent stress nephogram of gear teeth under different load levels obtained by numerical simulation, the causes of tooth root fatigue cracks are analyzed and obtained; In the post-processing of simulation, the cloud diagram of fatigue life can be obtained, that is, the bending fatigue life of gear can be obtained. The effectiveness of numerical simulation is verified by comparing the simulation results with experiments.
6) The root crack propagation is studied based on linear elastic fracture mechanics and simulated. The most obvious influence on the crack propagation trajectory is the initial crack position. With the initial crack position gradually shifting to the tooth top, the crack propagation trajectory gradually moves to the tooth top; When the initial crack angle has little effect on the crack propagation trajectory, the gear crack propagation trajectory is similar under different crack inclination angles; When the initial crack dip angle is 60 °, the crack tip displacement is the largest and the crack propagation is the easiest.
This study lays a good foundation for the design, production, manufacture, later service and fatigue life prediction of transmission gears in heavy equipment.