Abstract
The fracture mechanics approach was used to analyze the delayed fracture behavior of heat-treated SAE1527 steel with tensile strength of 156kg/mm2. The threshold stress intensity factor KISCC was determined using wedge-loaded DCB specimens in various test environments on the basis of a 5000 hr exposure criterion. Although the incubation time τ1 varied considerably by changes of the test environment, KISCC was found to be virtually independent of the test environment, ranging from 65 to 85kg/mm3/2.
The KISCC values obtained in the present investigation were applied to the design of tightening high strength bolts as an example of structural components subjected to the similar environments. For high strength bolts 22mm in diameter under standard tightening condition, the crack tip stress intensity factor KI for a crack initiating from a thread root was calculated based on a few assumptions. A critical crack length IC which can cause onset of delayed fracture crack propagation at KISCC was about 15 μ which is the same order of length as a prior-austenite grain size of conventionally heat-reated low-lloy steels. Thus it is confirmed that highly stressed and threaded fasteners of low-lloy steels with tensile strength. around 150kg/mm2 and higher are remarkably sensitive to delayed fracture even in an outdoor environment. These results bring about an understanding of in-service failures of high strength fasteners and arise an increasing demand for a strict inspection practice for them.
