裂纹闭合和循环塑性预应变及循环载荷压缩部分对裂纹扩展速率的影响

本文针对三种国产材料 Ｌｙ１１ｃｚ、 Ｌｙ１２ｃｚ 铝合金和 １８ Ｍｎ Ｈ Ｐ钢，通过实验初步考察了循环塑性预应变和循环载荷压缩部分对疲劳裂纹扩展的影响；采用电测法，测定了两种铝合金材料疲劳裂纹扩展的张开应力和有效应力强度因子幅值比 Ｕ。结果表明：（１）材料循环塑性预应变和循环载荷压缩部分，都使疲劳裂纹扩展速率提高；（２）常幅载荷下，在疲劳裂纹稳定扩展阶段，有效应力强度因子幅值比 Ｕ 与应力比 Ｒ 有关，与裂纹长度ａ 无关，并依赖于材料的力学性能。

Examining on some Factors Which Influences the Crack Expending Rate d a /d N

The influence of pre cyclic alternative plastic strain and the compression part of the loading cycle on the fatigue crack growth in the three materials, 18MnHP steel, LY11 cz and LY12 cz aluminum alloys, are investigated. The crack opening stresses for the two aluminum alloys are determined by means of foil strain gauge method and the effective stress intensity factor range ratios U are hence obtained. It is found that, (1) the fatigue crack growth is accelerated by the pre cyclic alternative plastic strain and the compression part of the loading cycle; (2) the effective stress intensity factor range ratio U , under constant amplitude cyclic loading and in the stable stage of crack growth, depends on the stress ratio R but is independent of the crack length a. U is also influenced by the mechanical properties of material.