THE EFFECT OF MICROSTRUCTURE AND FRACTURE SURFACE ROUGHNESS ON FATIGUE CRACK PROPAGATION IN A Ti‐6A1‐4V ALLOY

Abstract

AbstractCharacteristics of fatigue crack propagation (FCP) have been studied on materials with three different microstructures of a Ti‐6A1‐4V alloy, prepared with different heat treatments. The effect of microstructure on the FCP behaviour was attributed to the development of crack tip shielding, primarily resulting from the role of crack path morphology in inducing crack closure and crack deflection. Roughness‐induced crack closure played an important role on the near‐threshold FCP behaviour at a stress ratio of 0.05, but the FCP data plotted in terms of the effective stress intensity factor range, δKeff (allowing for crack closure), still exhibited the effect of microstructure. Fractographic examinations were performed, using a scanning electron microscope (SEM) with the aid of image processing, which enabled a three‐dimensional reconstruction of the fracture surface using a stereo pair of SEM micrographs. Fracture surface roughness was evaluated quantitatively by the ratio of the real area of the reconstructed fracture surface to its projected area. As fracture surface roughness was taken into account in evaluating the FCP data in addition to crack closure, the effect of microstructure disappeared, indicating that the intrinsic FCP resistance was the same in all the materials. Thus, it was concluded that fracture surface roughness was a dominating parameter in controlling the FCP of the Ti‐6A1‐4V alloy.