An indirect technique for determining closure-free fatigue crack growth behavior

The effects of stress ratio (R) and maximum applied stress-intensity factor (K-max) on fatigue crack growth rate (da/dN) have been investigated. Fatigue crack growth behavior was analyzed by plotting applied cyclic stress-intensity factor (Delta K) versus applied K-max for an alpha+beta titanium alloy (Ti 6-2-2-2-2) at several values of da/dN. This analysis revealed two distinct regions of fatigue crack growth behavior. At threshold da/dN (approximate to 10(-10) m/cycle) and for K-max < 4.6 MPa root m the threshold cyclic stress-intensity factor (Delta K-th) is greatly influenced by crack closure. For closure free fatigue crack growth (K-max > 4.6 MPa root m), a subtle but distinct decrease in Delta K-th is observed with increasing K-max. Plots of Delta K versus K-max were also generated for AA 2024 test data produced in two different studies; These data also revealed two distinct regions of fatigue crack growth behavior consistent with the Ti 6-2-2-2-2 results. This methodology can be used to. determine closure free fatigue crack growth and to monitor subtle changes in Delta K-th.