The tension compression fatigue tests of three grades of gray cast iron are carried out on plg-100 high frequency fatigue testing machine. The axial tension compression loading mode is adopted, the stress ratio is r = – 1, and the loading waveform is sine wave. 16 effective data are measured for each grade of gray cast iron sample. The tension compression fatigue test starts from the stress level higher than the estimated fatigue strength, and then decreases step by step. When the reference value of material fatigue strength is unknown, the static tensile yield strength of the material, such as Rp0.2 or rel, can be selected as the nominal value of fatigue strength. The first specimen is at the preset stress level σ 0, if the sample breaks before reaching the specified life n = 10000000 cycles, and the fracture position is in the normal working section of the sample, the second sample needs to be at the lower stress level σ Test under 1. When the first sample reaches the specified life n = 10000000 cycles, the second sample needs to be at a higher stress level σ 2 conduct the test, and all test samples shall be loaded according to this rule. The difference of stress levels at all levels is called “stress increment”, and the stress increment shall remain unchanged throughout the test.

The test was carried out at level 4 stress level. After the test of the 6th or 7th sample is completed, the calculation can be started according to the above formula σ- 1, and successively calculate the fatigue strength values of the 8th, 9th, 10th,… Samples after test. When the change of these fatigue strength values becomes smaller and smaller and tends to be stable, the test can be stopped. The fatigue strength calculated after completing the test of the last sample is taken as the required fatigue strength. The accuracy of the lifting method fatigue test results depends on the determination of the stress increment. Generally, the stress increment can be 3% – 5% of the estimated fatigue strength of the test sample,

Or within 5% of the fatigue strength measured by conventional fatigue test method.

The tension compression fatigue test of gray cast iron mainly uses the lifting method to test the tension compression fatigue strength of three grades of gray cast iron. The stress increment of three grades of gray cast iron is 4, 4.25 and 5MPa in turn. The initial fatigue strength of three grades of gray cast iron is set as 75, 85 and 100MPa respectively.

The fatigue strength calculation formula is shown in the formula;

Where: K is the number of pairs; N is the number of effective samples; M is the stress level series; σ Is to solve the ith fatigue limit value (MPA) by pairing method; ν Is the number of effective specimens at the ith fatigue strength.

Draw the fatigue life curves (S-N curves) of three grades of gray cast iron. The fatigue life curve (S-N curve) consists of high stress section and horizontal section, in which the high stress section is measured by group method and the horizontal section is measured by lifting method.