The data of crack tip stress intensity factor (SIF) and crack length under different stress ratios collected through the test are plotted in origin software, as shown in the figure. It can be seen from the figure that there is a positive correlation between the stress intensity factor at the crack tip and the crack length, that is, the stress intensity factor at the crack tip increases with the increase of the fatigue crack length, and the growth rate is gradually accelerating. The reason is that with the increase of crack length, the fracture area and contact area of compact tensile specimen become larger and smaller, and the load is always uniform, which leads to the tensile stress of specimen becoming larger and larger. It is reflected in the test data that the increasing rate of stress intensity factor at the crack tip is increasing.
At the beginning of crack propagation, the measured value of crack length is 9 mm. At this time, the measured value of stress intensity factor on the sample with stress ratio r = 0.1 is greater than that on the sample with stress ratio r = 0.6. With the progress of the experiment, the crack length is increasing. Before the fatigue failure of the sample with r = 0.1, the slope of the two curves is higher, The slope of the curve of the specimen with small stress ratio is always greater than that of the specimen with large stress ratio, that is, the growth rate of the stress intensity factor at the crack tip of the specimen with small stress ratio is faster. When the crack length is about 23 mm, the specimen with stress ratio r = 0.1 has fatigue failure, and the stress intensity factor at the crack tip reaches the maximum. Compared with the specimen with stress ratio r = 0.6 at the same crack length, the stress intensity factor increases slowly, but the acceleration also increases. Finally, when the crack length reaches 36 mm, the fatigue failure occurs when the stress ratio r = 0.6. The results show that: when the stress range is small, the load is relatively stable, the initial value of the stress intensity factor at the crack tip is small, the growth is slow with the crack growth, and the crack growth length is longer when the fatigue failure occurs. When the stress range is large, the load fluctuation is large, and the initial value of stress intensity factor at the crack tip is large. With the growth of the crack, it grows faster, and the fatigue failure occurs when the crack growth length is short.