Investigation of the effects of shed-rate, initial K-max, and geometric constraint on Delta K-th in Ti-6Al-4V at room temperature

A correct definition of threshold stress intensity is critical to understan ding the growth of small cracks in advanced gas turbine engine applications . A study of the effect of specimen geometry, constraint, and load shedding rate on the measured threshold stress intensity of Ti-6Al-4V (AMS 4928) wa s conducted to compare conventional through cracks (compact tension specime n) to surface flaw defects (K-b specimen). At room temperature, stress rati os of 0.1 and 0.8 were examined. The effect of shed rate was assessed and i t was found that shed rates as low as -20 in(-1) (-0.8 mm(-1)) give accurat e threshold measurements in both geometries at both stress ratios. These re sults indicate that the more gradual shed rate of -2 in(-1) (-0.08 mm(-1)) recommended in ASTM E647 (Standard test method for measurement of fatigue c rack growth rates, Annual Book of ASTM Standards, E647-95a, 1997:557-93) ma y be conservative for high strength titanium alloys. A study of the effect of starting stress intensity factor on the resultant threshold value was al so completed, It was found that starting at high initial stress intensity f actors gave false values for threshold. (C) 1999 Elsevier Science Ltd. All rights reserved.