Investigation of the effects of shed-rate, initial Kmax, and geometric constraint on ΔKth in Ti–6Al–4V at room temperature

A correct definition of threshold stress intensity is critical to understanding the growth of small cracks in advanccd 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) was conducted to compare conventional through cracks (compact tension specimen) to surface flaw defects (K b specimen). At room temperature, stress ratios of 0.1 and 0.8 were examined. The effect of shed rate was assessed and it was found that shed rates as low as -20 in (-0.8 mm -1 ) give accurate threshold measurements in both geometries at both stress ratios, These results indicate that the more gradual shed rate of -2 in -1 (-0.08 mm ') recommended in ASTM E647 (Standard test method for measurement of fatigue crack growth rates, Annual Book of ASTM Standards. E647-95a. 1997 :557--93) may he conservative for high strength titanium alloys. A study of the effect of starting stress intensity factor on the resultant threshold value was also completed. It was found that starting at high initial stress intensity factors gave false values for threshold.