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The effects of stress ratio (R) and maximum applied stress-intensity factor (Kmax) on fatigue crack growth rate (da/dN) have been investigated. Fatigue crack growth behavior was analyzed by plotting applied cyclic stress-intensity factor (ΔK) versus applied Kmax for an α+β 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 (≈ 10-10 m/cycle) and for Kmax < 4.6 MPa√m the threshold cyclic stress-intensity factor (ΔKth) is greatly influenced by crack closure. For closure free fatigue crack growth (Kmax > 4.6 MPa√m), a subtle but distinct decrease in ΔKth is observed with increasing Kmax. Plots of ΔK versus Kmax 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 ΔKth.
fatigue crack growth, fatigue crack growth threshold, crack closure, K, max, effects, stress ratio effects
Senior EngineerResident Research Associate, Lockheed Martin Engineering & SciencesNational Research Council, HamptonHampton, VAVA
Senior Scientist, NASA-Langley Research Center, Hampton, VA