Developmental Research on Hydrogen Induced Fracture in High Tension Bolts

抗拉强度超过1300MPa的F13T高抗拉螺栓自应用于钢结构和桥梁领域以来，出现了延迟失效的现象。这些失效大多数被认为是由扩散氢引起的，因为这些失效通常发生在污染的环境中，如海滨或工业区。由于缺乏冶金学知识或对扩散氢的吸收机理的认识，空气中的氢、在金属中的扩散过程、在局部高应力场中的积累和裂纹的萌生。因此，为了求解应力作用下氢在金属中的扩散过程，不仅需要考虑氢浓度梯度，而且需要考虑应力分布。进行了包括应力场贡献在内的数学推导，并将结果应用于有限元法（FEM），采用数值方法计算了扩散氢在螺纹底部附近聚集所需的孕育时间，并进行了预充试样的延迟断裂试验。F13T螺栓的试验材料为经热处理的含硼高强度钢。在HCl或H_2SO_4电解液中阴极充氢数小时后，在设计应力下施加恒载荷，通过声发射测量观察裂纹的萌生和扩展直至断裂，并将有限元分析得到的螺纹底部氢的扩散过程与实验结果进行了比较。并且在直到断裂的孕育时间方面与该结果显示出良好的一致性。

The delayed failures of the F13T high tension bolts whose tensile strength were over 1300 MPa have been reported since they were introduced into the field of steel structures and bridges. Most of these failures have been thought to be induced by the diffusible hydrogen because these failures often occurred under polluted circumstances like seaside or industrial areas.Although many studies about hydrogen induced crack (HIC) have been reported in a hydrogen rich weldmetal, behavior of diffusible hydrogen in high tension bolt has not yet been solved satisfactorily due to the lack of metallurgical knowledge or mechanism about absorption of hydrogen from the air, diffusible process in metal, accumulation in the locally higher stressed field and initiation of crack. Therefore, there is the need to take into account not only the hydrogen concentration gradient but also the stress distribution in order to solve the diffusional process of hydrogen in metals under stress.Mathematical development including the contribution of the stress field was done, results were applied into the finite element method (FEM), and a numerical method was used to calculate the incubation time required for diffusible hydrogen to accumulate around the bottom of a thread.Delayed fracture tests with pre-charged specimens were also performed. The testing material used for F13T bolt was high strength steel contained Boron for heat treatment. After each specimens was cathodically charged with hydrogen for several hours in HCl or H_2SO_4 electrolyte, they were stressed with a constant load under its design stress, and observed initiation and propagation of crack by measuring acoustic emission till they were fractured.The diffusible process of hydrogen around the bottom of thread obtained by FEM analysis were compared with these experimental results, and showed a good agreement with that in view of incubation time till fracture.