Evaluation of Fatigue Crack Growth and Crack Closure Behavior in AISI 4340

To obtain fatigue crack closure data, fatigue crack growth tests for AISI 4340 are performed under constant amplitude loading at various stress ratios and the behavior of crack closure is investigated. For determination of crack opening load under constant amplitude loading, ASTM compliance offset method is applied because its simplicity among the various methods.It is observed that crack opening ratios of AISI 4340 for constant amplitude loading are nearly constant at stress ratio R. Based on the characteristic crack closure behavior, Fatigue crack growth rates(da/dN) at various stress ratios are well correlated with effective stress intensity factor range(ΔKeff). It was confirmed that the crack closure behavior is the primary factor controlling fatigue crack growth.