FATIGUE CRACK GROWTH ANISOTROPY IN ANNEALED AND PRE-STRAINED COMMERCIALLY PURE TITANIUMS

Abstract

Fatigue crack growth tests were carried out on commercially pure titanium sheets with a drilled hole and a center notch, in order to clarify anisotropy of fatigue crack growth and the influence of pre-straining on the anisotropy. The crack growth resistance in rolling direction was higher than that in a direction transverse to the rolling direction for surface cracks longer than 2.5mm initiated at a drilled hole, and for cracks initiated at a center notch at ΔK higher than 10MPam^1/2, in contrast to the growth behavior of various steels. Fractograpy observations showed that the anisotropy became remarkable after transition from cleavage-like facet dominant fracture surface to ductile appearance dominant one. The transition occurred when reversed plastic zone size at the crack tip was comparable to the grain size of the metal during crack growth. Pre-straining had only a little effect on the crack growth anisotropy.