With the continuous progress of the test, under the action of cyclic load, cracks appear on the sample and continue to grow, and its residual life is also decreasing. When it is lower than the bearing capacity of the sample, the fatigue fracture occurs. The test data of cycle times and crack length under different stress ratios are continuously recorded by the extensometer clamped on the specimen and plotted in origin software. The relationship between the two groups of data is shown in the figure.
At the beginning of cyclic loading, the crack length under different stress ratios increases slowly, and the crack length with stress ratio r = 0.1 increases rapidly with the increase of cycles, while the number of cycles is less under the same crack length. The results show that the fatigue life of the specimen is 48913 cycles, and the crack length is 24.05 mm; The crack length with stress ratio r = 0.6 increases slowly with the number of cycles. To reach the same crack length with stress ratio r = 0.1, the number of cycles is more. When the number of cycles reaches 80000, the crack almost does not propagate. After 80000, the crack presents a positive proportion function growth pattern. When the number of cycles reaches 200000, with the increase of the number of cycles, The fatigue life of the specimen is 203587 cycles, and the crack length is 35.94 mm. Because of the different stress ratio, the growth rate of the crack length of the two curves with the number of cycles is different, but the development trend of the two curves is the same.
The results show that the stress ratio r = 0.1 is easier to cause crack initiation and propagation, and the stress intensity factor at the crack tip is larger when the crack length is the same; From crack initiation to specimen fracture, the crack propagation process can be divided into three stages: low speed propagation stage, stable propagation stage and high speed propagation stage. The stable propagation stage is the key part to study fatigue crack propagation. The Paris formula shows that the crack propagation rate under stress is faster than that under r = 0.1, When the stress ratio r = 0.1, the length of crack causing fatigue failure is 24.05 mm, and the fatigue life is 48913 cycles.
The Paris formula can be used to calculate the fatigue crack growth rate and predict the fatigue life of the structure. It lays a foundation for the study of fatigue crack propagation of low-speed and heavy-duty gears.