Elber proposed the concept of crack closure, and used the theory of crack closure to explain the fatigue crack growth behavior. Under constant amplitude alternating load, the fatigue crack growth behavior of some metal materials shows a very obvious stress ratio effect, that is, a higher stress ratio will lead to a faster fatigue crack growth rate. However, for some metal materials, the stress ratio effect of fatigue crack growth behavior is not obvious, which is mainly due to the rapid relaxation of average stress under asymmetric cyclic loading.
In the process of fatigue crack growth under normal stress ratio constant amplitude load, a tensile overload is applied, and the hysteresis or even complete stagnation of fatigue crack growth rate is observed. The overload ratio directly determines the hysteresis of fatigue crack growth rate caused by a single tensile overload peak, in which the overload ratio is the ratio of the overload peak to the maximum stress of constant amplitude load. Under a single tensile overload peak, the fatigue crack growth rate of some metal materials will show a short acceleration before attenuation. Bathias interpreted this phenomenon as additional damage caused by tensile overload peak at crack tip or overload plastic zone. For high and low amplitude load cases, when the stress ratio of high amplitude and low amplitude load cases is the same, a hysteresis phenomenon similar to the fatigue crack growth rate caused by a single tensile overload peak can be obtained; However, when the maximum loads of high amplitude and low amplitude load cases are the same, the high amplitude load case has little effect on the low amplitude fatigue crack growth rate. The hysteresis behavior of fatigue crack growth is very useful for the integrity of engineering structures. This behavior can be explained, such as crack tip passivation, strain hardening, crack closure and residual compressive stress.
The first mock exam of fatigue crack growth is a parameter in the unified model, and the corresponding values can be obtained through the basic fatigue test. Therefore, the first mock exam of fatigue crack growth can be considered as reliable and reliable. The appearance of fatigue crack changes the structural system of the sample. With the increase of crack length, the effective cross-sectional area at the necking decreases, resulting in the decrease of energy transfer efficiency and resonance frequency at both ends of the sample. Due to the alternating load, the bearing member will produce small cracks, and the cracks will gradually expand with the increase of the number of alternating load cycles. There are many kinds of crack growth shapes, and the deformation of cracks is also different. Generally, according to different loads, cracks are divided into three basic types: open type, slip type and tear type, as shown in the figure: