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对在 10 - 5～ 10 - 7mm cycle疲劳裂纹扩展曲线da dN—ΔK和疲劳裂纹扩展门槛值ΔKth测量中常用的降载法进行研究。每一级载荷疲劳裂纹扩展二次 (Δa1i,ΔN1i)和 (Δa2i,ΔN2i)。用多点多项式分别拟合 (Δa1i,ΔN1i,ΔK1i)和(Δa2i,ΔN2i,ΔK2i) ,i=1,2 ,3 ,… ,n。每一种材料得到两条da dN—ΔK曲线和两个ΔKth。结果表明 ,降载法中的前一级载荷仍明显有过载作用 ,干扰材料疲劳裂纹扩展 ,影响测量数据的真实性。本文提出解决方法———“二次扩展法” ,即在每一级载荷 ,舍去受前一级载荷影响大的第一次循环数据 ,而用受前一级载荷影响较小的第二次循环数据作为疲劳裂纹扩展曲线da dN—ΔK和计算疲劳裂纹扩展门槛值ΔKth的数据
The loading method commonly used in the measurement of da dN-ΔK and fatigue crack growth threshold ΔKth in the fatigue crack growth curve of 10 - 5 ~ 10 - 7mm cycle was studied. Each load fatigue crack growth quadratic (Δa1i, ΔN1i) and (Δa2i, ΔN2i). (Δa1i, ΔN1i, ΔK1i) and (Δa2i, ΔN2i, ΔK2i), i = 1, 2, 3, ..., n are fitted by the polynomial polynomial, respectively. Two da dN-ΔK curves and two ΔKths were obtained for each material. The results show that the load of the previous stage in the load-shedding method is still obviously over-loaded, which interferes with the fatigue crack growth of the material and affects the authenticity of the measured data. This paper proposes a solution - the “second expansion method”, that is, in each level of load, discard the first cycle of data affected by the load of the previous level is large, but with a load less affected by the second Subcyclic data is used as the fatigue crack growth curve da d -ΔK and the fatigue crack growth threshold value ΔKth