Initial Stage of Fatigue Cracking in Pure Titanium under Bending and Torsion

Abstract

A fatigue test has been performed on pure titanium using round bar specimens and hollow cylindrical specimens under rotating bending and reversed torsion, respectively. Then the initial stage of fatigue cracking has been investigated by means of electron microfractography and of fracture mechanics.
The main results obtained are as follows:
(1) The crystallographic facets of Stage 1 cracking are found in the case of rotating bending but not found in the case of reversed torsion.
(2) The criterion of transition from Stage 1 to Stage 2 cracking can not be expressed only by the stress intensity factor in the case of rotating bending.
(3) The transition of macroscopic fracture appearance from shear mode to tensile mode occurs simultaneously together with the change of fractographic appearance from Stage 2a to Stage 2b in the case of reversed torsion. The criterion of transition can be expressed by the stress intensity factor.