Delayed Fracture of High-strength Bolts

Abstract

Investigation of the delayed fracture characteristics was made with high strength bolts tightened on the fixture plate and exposed to rural and coastal industrial zones, fresh water, and seawater respectively for about nine years. At the same time, bolt materials subjected to the same heat treatment as the exposed bolts were submitted to four types of accelerated testing in laboratory. Both bolts and bolt materials are composed of seventeen grades of low alloy steels having tensile strengths ranging from 80 to 140kgf/mm2.The results of observations are:(1) Neither failure nor cracking was found in bolts with tensile strength less than 120kgf/mm2.(2) Since the critical strength above which delayed fracture initiated in bolts exposed agrees with that obtained from accelerated laboratory tests, it is considered that the critical strength for crack initiation can be determined by accelerated laboratory tests and the critical strength for crack propagation can be examined by exposure test of tightened bolts.(3) High atmospheric temperature, large amount of rainfall and excessive nut turning promote a delayed fracture in bolts.(4) Cracking along prior austenite grain boundaries and the results of analysis of corrosion products on the bolt surfaces suggest that the occurrence of delayed fracture is attributable to hydrogen.(5) There is a difference in the effects of Zn plating, Cd plating and Zn base phosphate coating on delayed fracture between exposure tests and accelerated laboratory tests.(6) The applied axial load on bolts tightened exceeding their yield strength and then exposed for about nine years is decreased approximately by 10%, in addition to 3 to 8% initial decrease occurred immediately after tightening. Delayed failure of washers had no effect on residualaxial load in bolts.