The low-frequency tension compression fatigue test of gray cast iron adopts MTS810 axial tension compression fatigue test machine. It uses integrated power cylinder in combination with sensor, which has high stiffness and accuracy. At the same time, it is equipped with teststariis controller, which can realize the automatic control of the test height. The test material is low alloy HT250. Before the test, the axial force in loading shall be calculated according to the predetermined loading load and the minimum working area of the sample. Calculation method of axial force: loading axial force (f) = loading load (MPA) × Minimum working area of sample (mm2). The stress ratio r = – 1, and the loading force waveform is sine wave. A total of 8 specimens were tested by low frequency tension compression fatigue test.
According to the basic requirements of fatigue test, generally, there is no fracture when the number of cycles is more than 10000000 under a certain load, that is, it is considered that there will be no fracture under this load. However, in the low-frequency fatigue test, because the loading frequency is 20Hz, the test time required for one sample to complete 10000000 cycles is about 10 days. In order to shorten the test cycle, the number of cycles is reduced to 5 × One sample can be completed in about 5 days after 1000000 times, which can be used as the benchmark for whether the sample is qualified under a certain load. The basis for shortening the number of test cycles is shown in the symmetrical cycle fatigue curve in Figure 2-3. From the fatigue life curve of gray cast iron in the figure, it can be seen that the fatigue curve of gray cast iron tends to be flat with the increase of cycle times, and gradually transitions from 100000 to horizontal. When the fatigue curve is at water level, it indicates that the sample can go through infinite cycles without fracture. The number of test cycles is 5 × 1000000 and 10000000 are in this horizontal section, so the number of low-frequency tension compression fatigue test cycles is reduced to 5 × 1000000 times. Load all samples successively until the sample completes the cycle or fracture, and record the test data.