The objective of this work has been to characterize the fatigue crack growth rate of Inconel 718 in the elastic and elastic-plastic regimes. The major new contribution here is to develop fatigue crack growth rate data on this alloy using shallow crack specimens subjected to cyclic loadings that involve material plasticity exceeding what is allowed by the standard Linear Elastic Fracture Mechanics (LEFM) procedures of ASTM E 647.
Compact (C(T)) and three point bend (SE(B)) specimen geometries were used in this investigation. The SE(B) specimens were used to obtain shallow crack data using crack to depth ratios (a/W) as small as 0.08 in standard bend specimens with W = 50.8 mm. Compliance methods were used to estimate the crack length during the cyclic testing. The C(T) specimens were used to investigate the effect of fully reversed loading, i.e., R = -1.0. These specimens were tested only in deep crack configurations with a/W > 0.3. Both C(T) and SE(B) specimens were used to obtain high cycle fatigue crack growth data as well as some component of the low cycle fatigue crack growth rate data. The cyclic elastic stress intensity range was used to characterize the crack growth driving “force” in the high cycle regime as defined in ASTM E 647. A cyclic J integral range, as originally utilized by Dowling and Begley , was used in the elastic-plastic regime.
High cycle and low cycle fatigue crack growth data were successfully obtained from tests on shallow crack SE(B) specimens. High cycle fatigue crack growth was not affected by specimen geometry or by crack length ratio, even for a/W ratios as low as 0.08. The low cycle fatigue crack growth rate was similar, but not identical, to what one would get by extrapolating the high cycle fatigue crack growth rate, as proposed by Dowling and Begley . Under conditions of increasing J range, the crack growth rate under elastic-plastic conditions was accelerated, while under decreasing J range conditions the crack growth rate was decelerated in comparison to the extrapolated high cycle fatigue crack growth rate measurements. The R ratio did not affect the crack growth rate for the two cases tested here, namely R = 0.1 and R = -1.0. More rapidly increasing J range conditions resulted in greater crack growth rate acceleration. In the elastic-plastic fatigue regime, shallow crack specimens demonstrated slower crack growth rates than deep crack specimens