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采用Ａ５３７钢单边缺口试样，研究了以不同方式叠加在主载荷△Ｐ上的小幅载荷在疲劳裂纹扩展过程中的作用，并采用激光散斑技术测量了循环加载过程中的裂纹闭合效应。结果表明：３０％△Ｐ以下的小幅载荷叠加在主载荷底部时，对由主载荷引起的裂纹扩展速率ｄａ／ｄＮ几乎没有影响，３０％△Ｐ以上的小幅载荷可使裂纹扩展速率增加。叠加在主载荷顶部的各小幅载荷均可使裂纹扩展速率加大。通过对加载条件下裂纹闭合效应的测量，并结合裂尖的应力应变状态的变化进行分析认为：小幅载荷对裂纹扩展速率的影响受裂纹闭合程度的制约，其损伤作用只体现在裂尖材料产生不可逆塑性形变的外加载荷范围内。
A537 steel unilateral notched specimens were used to study the effect of small loads superposed on main load △ P in fatigue crack propagation in different ways. Laser crack speckle technique was used to measure the crack closure effect during cyclic loading. The results show that when the small load below 30% △ P is superimposed on the bottom of main load, it has almost no effect on the crack growth rate da / dN caused by the main load. The small crack growth rate increases more than 30% △ P. The small loads superimposed on the top of the main load increase the crack propagation rate. By measuring the crack closure effect under the loading condition and combining with the change of the stress and strain state of the crack tip, it is considered that the influence of small load on the crack growth rate is restricted by the degree of crack closure, and the damage effect is only reflected in the crack tip material Irreversible plastic deformation within the applied load range.