臨界応力拡大係数 K_<ISCC> による高力ボルト遅れ破壊感受性の評価

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The fracture mechanics approach was used to analyze the delayed fracture behavior of heat-treated SAE1527 steel with tensile strength of 156 kg/mm^2. The threshold stress intensity factor K_<ISCC> was determined using wedge-loaded DCB specimens in various test environments on the basis of a 5000 hr exposure criterion. Although the incubation time τ_1 varied considerably by changes of the test environment, K_<ISCC> was found to be virtually independent of the test environment, ranging from 65 to 85 kg/mm^3/^2. The K_<ISCC> 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 22 mm in diameter under standard tightening condition, the crack tip stress intensity factor K_I for a crack initiating from a thread root was calculated based on a few assumptions. A critical crack length I_C which can cause onset of delayed fracture crack propagation at K_<ISCC> was about 15 μ which is the same order of length as a prior-austenite grain size of conventionally heat-treated low-alloy steels. Thus it is confirmed that highly stressed and threaded fasteners of low-alloy steels with tensile strength around 150 kg/mm^2 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.
社団法人日本鉄鋼協会の論文
1976-01-01

臨界応力拡大係数 K_<ISCC> による高力ボルト遅れ破壊感受性の評価

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