Threshold Characteristics of Stress Corrosion Cracking under Dynamic Combined Multiaxial Loading Conditions

对4135高强钢在3.5% NaCl溶液中进行了光滑试样的循环应力腐蚀开裂试验。加载类型为弯曲（R = -1）、扭转（R = -1）、同相张拉（R = 0）和扭转联合（R = 0）。<tau>_<max>与<sigma>_<max>的比值范围从零（张力）到无穷大（扭转）。在<tau>_<max> <大于或等于> 780 MPa的扭转下，试样发生垂直于纵向的穿晶断裂。在500mpa <等于或小于> <tau>_<max> < 780MPa时，腐蚀坑底部产生纵向剪切裂纹，认为是溶蚀引起的，然后在腐蚀坑顶端形成以最大主应力为主的晶间循环SCC裂纹。在扭转<tau>_<max> < 500 MPa、弯曲和拉扭联合加载时，腐蚀坑底部以最大主应力或最大拉应力范围为主导的循环SCC裂纹萌生。在假定腐蚀坑为尖锐裂纹的条件下，应力强度因子范围<DELTA>_<IFSCC>确定了腐蚀坑底部起裂的力学条件。同样，当坑的K_<IIIFSCC>值超过K_<IIIFSCC>值时，溶蚀引起的纵向裂纹也随之产生。

Cyclic stress corrosion cracking (SCC) tests of smooth specimens were conducted on 4135 high-strength steels in 3.5% NaCl solution. Loading types were bendin (R = -1), torsion (R = -1), and combined in-phase tensio( (R = 0) and torsion (R = 0). The ratio of <tau>_<max> to <sigma>_<max> ranged from zero (tension) to infinity (torsion). In torsion at <tau>_<max> <greater than or equal> 780 MPa, specimens were fractured by transgranular cracking normal to the longitudinal direction. At 500 MPa <equal or less than> <tau>_<max> < 780MPa, a longitudinal creack by shear stresses, which is considered to be caused by dissolution, was initiated at the bottom of corrosion pit, and then intergranular cyclic SCC crack dominated by maximum proincipal stress was started at the tip of it. In torsion at <tau>_<max> < 500 MPa and in bending and combined tension and torsion loading, however, cyclic SCC crack dominated by maximum principal stress or maximum tensile stress range was initiated at the bottom of corrosion pits. Mechanical condition dominating crack initiation at the bottom of corrosion pit was determined by stress intensity factor range <DELTA>_<IFSCC>, which was obtained by assuming the corrosion pit as a sharp crack. Similarly, a longitudinal crack by dissolution was initiated when a K_<III> value of a pit exceeded the K_<IIIFSCC>.