球墨铸铁疲劳裂纹萌生及扩展机制的探讨

用电子显微断口分析和裂纹扩展途径的金相观察，探讨了不同珠光体和铁素体相对量的球墨铸铁的疲劳断裂机制，特别是疲劳裂纹萌生及早期扩展机制。结果表明：疲劳裂纹容易在石墨球和基体之间的界面萌生和扩展，石墨球边及基体中的次生裂纹对da/dN的影响有两重性。在疲劳裂纹萌生及早期扩展阶段，铁素体基本以周期解理和细条纹为主，珠光体基体为渗碳体解理和铁素体片上的疲劳条纹，80%珠光体球铁为上两种机制的混合。疲劳裂纹早期扩展速度为△K (th)所控制，疲劳裂纹孕育期N 0为σ s和△K (th)所控制。光棒或钝缺口下的N 0主要取决于σ s，尖锐缺口试样的N 0主要取决于△K (th)。在疲劳裂纹稳定扩展阶段，铁素体为疲劳条纹，珠光体为条纹与解理，80%珠光体中牛眼铁素体为条纹和晶间断。疲劳条纹的宽度能反映da/dN的变化，但两者往往不一致。疲劳裂纹快速扩展为条纹与静断特征混合，快速扩展速率受K (1c)的大小所控制。

THE INVESTIGATON ON FATIGUE CRACK INITIATION AND PROPAGATION MECHANISM FOR NODULAR CAST IRON

An investigation on the fracture mechanism of three stage of fatigue crack propagation for nodular cast iron with different percentage of pearlite and ferrite has been carried out by means of electron fractographic analysis and metallugical microscope observation for fatigue crack propagating path. The following results are observed.The fatigue crack initiation and propagation occur preferentially on the boundaries betweengraphite nodule and matrix. The crack initiation and propagation are effected by both theshape and distribution of spheroidal graphite.In both the crack initiation and the early stage of fatigue crack propagation. The fracture offerriteris characterized by cyclic cleavage, and the fracture of pearlite is characterized by acombination of cementite cleavage and fatigue striation in ferrite. The rate of early stage ofcrack propagation are controlled by△K (th). The number of stress cyclic for initiation of fatigue crack No are controlled byσs and△K (th). No is governed by△K (th) in blunt nottched specimen,No is governed by△K (th) in case of sharp notches.In the stable stage of fatigue crack propagation, the ferrite is characterized by fatigue striation. The pearlite is characterized by fatigue striation and cleavage, the bull's eye ferrite of 80% pearlite nodular cast iron is characterized by fatigue striation and shear region. The fatigue striation spacing able to reflect variation of da/Dn.In fast stage of fatigue crack propagation, the fractographic is characterized by a combination of fatigue striation and static fracture. The rate of fast fatigue crack propagation is governed by fracture toughness.