Fatigue crack growth and closure of a high tensile strength steel containing short precracks.

Abstract

Using very small-sized specimens of a special type, the fatigue crack growth and closure of high tensile strength steel, HT80, containing short precracks, were studied with a refined, minicomputeraided unloading elastic compliance monitoring technique. The effect of the stress relief annealing in vacuum on the growth behavior of short fatigue cracks was also investigated. The closure behavior of short cracks in stress-relieved, precracked specimens was found to be strongly dependent on the precracking load condition. Particularly, when the final effective stress intensity range (ΔKeff) during precracking exceeded the initial ΔKeff of subsequent fatigue tests of the precracked specimens, the closure behavior of short cracks in precracked specimens was startlingly dissimilar to that of short cracks initiated in smooth specimens. This result implies that the short cracks in precracked specimens can not be always regarded as identical to normally initiated short cracks. However, if the stress intensity range (ΔK) during precracking does not exceed the initial ΔK of subsequent fatigue tests of precracked specimens, the closure behavior of short cracks in precracked specimens will be nearly identical to that of normally initiated short cracks in smooth specimens. The growth rates of short cracks in precracked specimens were well represented in terms of ΔKeff as were those in smooth specimens. The stress relief annealing in vacuum was found to slightly decrease the material resistance to growth of short cracks.