THE RELATIONSHIP BETWEEN X-RAY RESIDUAL STRESS NEAR THE CRACK AND CRACK OPENING/CLOSING BEHAVIOR CONTROLLING FATIGUE CRACK PROPAGATION IN Ti-6Al-4V ALLOY

Abstract

Using the X-ray method of stress measurement for Ti-6Al-4V alloys, the residual stress near the crack was measured for annealed (AN) and solution treated and aged (STA) titanium alloys, under the condition that the measured X-ray stress was in satisfactory agreement with the applied stress under tension. The residual stress measured in the wake of the propagating fatigue crack, σr, was compressive, resulting in a smaller crack opening displacement, COD, than theorized. The measured σr and COD-values let us understand the fatigue crack propagation rate da/dN in terms of the effective stress intensity factor Keff. As a result, the da/dN under the same Keff-value was smaller in the AN specimen with zigzag crack growth than in the STA specimen with straight crack growth, although the da/dN-Keff relationship under various stress amplitudes was represented by a straight line in a log-log scale separately for the AN and STA specimens.