材料期刊网

本文用三点弯曲试样测定15MnVN钢在淬火和200℃低温回火状态下疲劳裂纹扩展速率。试验在循环载荷比R=0.5的恒幅条件下进行,并利用金属塑性变形在光滑表面上产生浮凸的现象,用相衬显微镜测定裂纹顶端单调塑性区尺寸。在平面应力状态下垂直于裂纹面方向的单调塑性区尺寸与相应的(K_(max)/σ_s)~2成正比,其比例系数为0.21。根据Rice循环塑性区尺寸与单调塑性区尺寸关系,得到垂直裂纹面方向的循环塑性区尺寸与相应裂纹扩展速率之间为一指数关系。 在裂纹扩展过程中,相对裂纹尺寸a/W约等于0.687时裂纹顶端从小范围屈服进入大范围屈服;a/W<0.687裂纹顶端的塑性变形约在θ=30—60°方向角内向两翼伸展;当a/W>0.687,不仅两翼向更大的θ角展开,而且在θ=0°附近塑性变形也有相当发展。 金相和扫描电镜观察结果,具有板条状马氏体组织的15MnVN钢疲劳裂纹扩展是以穿晶型的解理开裂方式进行的,是一种再生核扩展机制。

An evaluation was made of the fatigue crack propagation rate at constant amplitude of cyclic load ratio R=0.5 for the three-point bend specimens of 15MnVN steel quenched and tempered at 200℃. The monotonic plastic [zone size of the fatigue crack tip was measured under observation of phase contract microscope with the relief phenomenon out from the smooth [surface of metal during plastic deformation. The monotonic plastic zone size, perpendicular to the direction of crack surface, under plane stress is proportional to the corresponding (K_(max)/σ_s)~2. Its proportional coefficient is 0.21. According to the relationship between Rice's reversed cyclic and the monotonic plastic zone size, an exponential relation is obtained between the reversed cyclic plastic zone size perpendicular to the direction of crack surface and the related fatigue crack propagation rate. In the process of fatigue crack propagation, when the relative crack size α/W is about 0.687, the plastic zone of crack tip varies from a small [to broad scale. At α/W<0.687, the plastic deformation of crack tip spreads out in two sides of angle θ about 30—60°. While α/W>0.687, it spreads out not only in enlarged angle but also in considerably extent near θ=0°. The metaUographic and SEM observations show that the fatigue crack of 15MnVN steel with platelet martensite structure propagates along transgranular cleavage, i. e. a regenerated nuclear propagation mechanism.