Fatigue Crack Initiation Behavior in Notched Member of Commercially Pure Titanium

Abstract

Fatigue tests were carried out on a notched specimen of an annealed commercially pure titanium under push-pull loading, in order to clarify fatigue notch characteristics, especially the reason why the metal is extraordinarily insensitive to notch in fatigue. The initiation and initial growth of a crack at the notch root area were observed successively with optical-and scanning-electron-microscopes using a replication technique. Observations have shown that the fatal crack starts propagating as a single crack only after one-grain-size cracks initiate at several neighboring grains and then join with each other. Crack opening displacements (CODs) were also measured for similar cracks. Measurements have shown that CODs are small and unchange almost before joining, but at joining they increase abruptly and then monotonously with a number of cycles. It is suggested that pure titanium has a very thick surface layer relating to crack initiation, leading to extraordinarily low sensitivity to notch. The behavior is considered to be mainly attributed to the strong constraint for microscopic plastic deformation between adjacent crystals, because pure titanium has a hcp crystal structure and has only a few number of slip systems.