用抛光的恒载荷试样对40CrNiMoA和AISI4330M两种高强度钢在水介质中应力腐蚀裂纹的产生和扩展进行了金相跟踪观察,研究了裂纹亚临界扩展的形貌。结果表明:表面裂纹前端产生阴影区(塑性变形区),它的形貌随钢种不同而不同。对于40CrNiMoA,阴影区是扩展、闭合,再扩展、再闭合。而对于4330M,阴影区不闭合,近乎平行扩展。只有在刚开始加载、阴影区呈耳朵状时,它才反映了裂纹尖端的塑性区。在以后绝大部分的扩展过程中,阴影区并不代表裂纹尖端的塑性,而只反映产生剪切唇所引起的变形,且与裂纹的“突进”(Pop-in)密切有关,必须和小范围屈服断裂力学中裂纹尖端的塑性区相区别。
Using polished cantilever specimens made of two high strength steels 40CrNiMoA and AISI4330M, the nucleation and propagation on stress corrosion cracking in distilled water have been traced by metallographie shadowing technique. The morphology of sub-critical crack growth has also been observed. It was indicated that the shadow zone, i.e. a plastic deformation area in the front of crack tip, reveals. its different morphologies in various steels. In 40CrNiMoA, the shadow zone grows and joins one another with the crack propagation. In 4330M, it does not join together, but develop along two parallel paths. The catlike shadow zone immediately appeared at the crack tip after loading can be considered as the plastic zone at the crack tip. However, the shadow zone hereafter observed during whole process of the crack propagation seems not to present the plastic zone at the crack, but to represent the deformation induced by. the formation of shear lip and can be related closely to the pop-in. This should be distinguished from the plastic zone at the crack tip in the linear elastic fracture mechanics.
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