Abstract
Abstract— The crack tip cyclic heat generation rate, fatigue crack growth rate, and the crack closure stress were measured in a β‐annealed Ti‐6Al‐4V for R ratios of ‐0.1, 0.1 and 0.2. The R= ‐0.1 and 0.1 cases exhibited macroscopic crack closure and the R = 0.2 case exhibited no macroscopic crack closure as measured by strain gage techniques. There was a transition in the slope of the da/dN vs ΔK curve for the closure cases and no transition for the no‐closure case. The crack tip heat generation rate above the transition for the closure cases was an order of magnitude greater than for the no‐closure case. Also the crack tip heat generation rate below the transition was greater for the closure cases than for the no‐closure case. The transition in the heat generation rate and the crack growth rate was attributed to a transition from static frictional locking of shear branch cracks to sliding of the branch crack surfaces. Below the transition the crack faces are locked open creating a large roughness‐induced closure component. When the branch cracks are allowed to undergo reverse shear, the amount of roughness‐induced closure markedly decreases and the fracture surface becomes less tortuous. The relative sliding generates the increased heat.
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