(1) The fatigue life curve of ordinary HT250 is shown in Figure 1.

(2) The fatigue life curve of low alloy HT250 is shown in Figure 2.

(3) The fatigue life curve of HT300 is shown in Figure 3 -.

It can be seen from Fig. 1, Fig. 2 and Fig. 3 that the overall trend of fatigue life curves of three grades of gray cast iron samples is the same, but the corresponding load values are different. In the high stress section, the cycle times decrease rapidly with the increase of loading load. After the loading load reaches the horizontal section, the cycle times begin to increase steadily until it reaches 1000000 cycles. The sample does not fracture after 10000000 cycles. It is generally considered that the sample will not fatigue fracture under a certain load.