Effect of Notch Radius on Fatigue Life and Early Fatigue Crack Behavior of Pure Titanium Implanted with Nitrogen Ions.

Abstract

The aim of this study was to investigate the effects of surface modification by ion implantation and notch radius on the fatigue life, crack initiation and early crack growth behavior for pure titanium. Three-point bending fatigue tests were carried out using an electro hydraulic fatigue testing machine under a frequency of 20 Hz and a stress ratio of R=0 in laboratory air at room temperature. The fatigue specimens used were smooth and notched specimens with the notch radii of 0.1, 0.25, 1 and 2 mm. The fatigue crack initiation and the early crack growth were successively observed using an optical microscope adopting a plastic replica method. The main results obtained are summarized as follows. (1) The fatigue life of pure titanium was decreased by the ion implantation. (2) The fatigue crack initiated from the notch root irrespective of ρ, Δσ and ion implantation. (3) The early crack growth patterns were divided into two types : the first type, in which the main crack grew continuously until the final fracture, and the second type, in which the main crack initiated from the notch root and then the secondary crack appeared in front of the main crack. The second type of crack growth tended to be observed with increasing Δσ and decreasing ρ. (4) The fatigue life of the second type of crack growth was greater than that of the first type of crack growth. (5) The first and the second types of crack growth were actually controlled by the same condition of mechanics which is the σ/Δσ value of about 1.5.