Developmental Research on Hydrogen Induced Fracture in High Tension Bolts

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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.