Research on fatigue crack of low speed and heavy load gear

Taking low-speed and heavy-duty gear as the research object, the fatigue crack propagation and fatigue fracture of gear teeth are studied. Firstly, the compact tensile specimen made of 42CrMo steel was used to carry out the fatigue crack growth test, and the numerical simulation was carried out to explore the feasibility of the test. The large module gear similar to the low speed and heavy load gear was designed, and the bending fatigue test was carried out on the bending fatigue testing machine. The microstructure of fatigue fracture was observed and analyzed by scanning electron microscope. The main conclusions are as follows

1) In the fatigue crack propagation test of 42CrMo steel compact tensile specimen under mode I constant amplitude loading, the stress ratio r = 0.1 is easier to cause crack initiation and propagation; Under the same crack length, the stress intensity factor at the crack tip is larger; The fatigue crack growth rate is faster; When fatigue failure occurs, the fatigue crack length is shorter, which is 24.05 mm, and the fatigue life is shorter, which is 48913 cycles.

2) Under the condition of numerical simulation, the Mises equivalent stress cloud near the crack tip is approximately symmetrical and has obvious gradient. Under the condition of the same crack length, the equivalent stress cloud area of the crack tip is larger with the stress ratio r = 0.1, and the area increases with the increase of the crack length. Compared with the experimental value, the value of stress intensity factor at the crack tip obtained by numerical simulation is always smaller, and the calculated fatigue life is longer, but the error is smaller, which can prove the feasibility of the experiment.

3) The bending fatigue test of low-speed and heavy-duty gear is carried out by using the bending fatigue testing machine. The test data show that each group of test data obey the lognormal distribution. The fatigue life increases with the decrease of stress level. When the reliability is 50%, the median fatigue life is longer, but the reliability is lower; When the reliability is 99.9%, the safety life is short. By using mathematical statistics knowledge, the fatigue life can be obtained when the reliability is 84.1%. At this time, the fatigue limit of the test gear is 232.68 MPa.

4) The results show that the stable growth zone is characterized by cleavage fracture, and the rapid growth zone is characterized by a plastic brittle mixed fracture with dimples and tearing shuttles: the secondary cracks initiate at the grain boundary and then propagate along the grain boundary. With the increase of the stress intensity factor k at the crack tip and the initiation and aggregation of new fatigue cracks, the secondary cracks in the fracture surface are formed, The crack propagation path will change from intergranular propagation to transgranular propagation, and part of the crack will stop under the obstruction of the grain; The granular slag inclusion in the metal affects the uniformity, continuity and isotropy of the material, which provides a place for the initiation and propagation of fatigue cracks.

The research has laid a good foundation for the design, production, manufacture, later use and fatigue life prediction of low speed and heavy load gears. In the future, we can study the fatigue crack growth characteristics of samples and gears under different thickness conditions from the perspective of size, and summarize the influence of thickness on fatigue crack growth.

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