As can be seen from the figure, when the stress ratio r = 0.1 and the fatigue crack length α When increasing from 15mm to 20mm, the value of stress intensity factor at the crack tip increases from 22 to 33.3. When fatigue crack length α When increasing from 20mm to 25mm, the stress intensity factor at the crack tip increases from 33.3 to 51.2. When the stress ratio r = 0.6 and the fatigue crack length a increases from 15mm to 20mm, the value of the stress intensity factor at the crack tip increases from 7.5 to 12.2 α When it increases from 20mm to 25mm, the value of stress intensity factor at crack tip increases from 12.2 to 20.4. Through the above analysis, it can be concluded that the crack length is positively correlated with the crack tip intensity factor, and the growth rate increases gradually with the increase of crack length. This is because with the increase of crack length, the fracture area of compact tensile specimen is larger and larger, the contact area is smaller and smaller, and the load is always uniform, resulting in the tensile stress of the specimen is larger and larger. The experimental data can well show the growth trend of accelerating the change of stress intensity factor at the crack tip.
The data obtained in the experiment when the stress ratio is 0.1 and 0.6 are plotted with Origin software, and the abscissa and ordinate are the logarithm of the stress intensity factor at the crack tip and the fatigue crack growth rate, as shown in the figure. It can be seen that the relationship curves of the two are “s” type under different stress ratios. The lower threshold of fatigue crack growth and the upper threshold of fatigue crack growth divide the curve into three stages: low-speed propagation, stable propagation and high-speed propagation.
The stress ratio is the main reason for the change of fatigue crack growth rate. It can be seen from the figure that after data logarithmicization, there is delamination between the fatigue crack growth rate and the crack tip stress intensity factor data group, and the fundamental reason is the difference of stress ratio. The stress ratio affects the relationship between them, and has a greater influence on the relationship between them in the range of low stress intensity factor. The closure of crack related to stress ratio is the expression of the effect of the first two on stress, and the closure effect mainly comes from the plastic deformation at the crack tip. It can be seen from the figure that when the value of stress ratio R keeps rising, the curve moves towards Δ K moves in a lower direction, and R has a greater impact on stage I and III and less impact on stage II.