| Effects of intermittent rest period at 300℃, overload in tension and overload followed by rest period on fatigue crack growth in 4340 steel were investigated. The intermittent lest period stopped the growth of fatigue cracks completely near the threshold level of △K. The alleviation effect of rest period on the rate of crack growth was more distinct at the lower level of △K. The higher overload ratio was (i.e., OLR = 3), the more the alleviation effect. It is suggested that compressive residual stress at the ahead of crack tip was produced during overloading period, and it is enhanced roughness of the crack surface due to crack deflection after the overloading, which, in turn, ceased or retarded crack propagation during the subsequent cycling. The most retardation of fatigue crack growth was observed when overload and subsequent rest period were combined. The mechanisms are discussed. |
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