Abstract
Literature suggests that, in addition to well-known dependencies on metallurgy and environmental conditions, the applied loading rate (which may be quantified by the change in stress intensity with time; dK/dt) can strongly influence hydrogen environment-assisted cracking (HEAC) behavior, especially for alloy/environment combinations that exhibit only modest HEAC susceptibility. Critically, this loading rate dependence complicates the rigorous implementation of current fracture mechanics-based standardized testing methods for assessing HEAC susceptibility, as ASTM E1681 (generally dK/dt < 0), ASTM F1624 (dK/dt = 0), and ISO 7539-9 (dK/dt > 0) all employ different loading rate profiles. Yet, despite this potentially significant effect on current testing practices, quantification of loading rate dependencies is limited to small number of material/environment combinations and mechanistic understanding of the causal factors governing this dependence remains incomplete.
To address this mechanistic knowledge gap, a slow-rising K testing approach was utilized to measure HEAC kinetics in a Ni-Cu superalloy (Monel K-500) immersed in 0.6 M NaCl and polarized to -950 mVSCE at dK/dt ranging from 0.2 to 20 MPa√m/hr. Results confirm (1) the onset of intergranular HEAC at all tested dK/dt and (2) a strong influence of dK/dt on the measured crack growth rate, with the fastest dK/dt exhibiting a 30-fold increase in crack growth rate relative to the slowest dK/dt. Two characteristic regimes of behavior based on the applied dK/dt were observed. Specifically, a ‘linear’ regime where the measured crack growth rates linearly increased with dK/dt was noted for dK/dt from 0.2 to 6 MPa√m/hr, while a ‘plateau’ region where growth rates were independent of dK/dt was observed for dK/dt > 6 MPa√m/hr. Assessment of the observed K vs. crack extension relationships and targeted interrupted testing (where dK/dt > 0 experiments were switched to dK/dt = 0) provided critical insights into the processes governing the observed loading rate dependence. Critically, hydrogen diffusion-limited crack growth is only observed within the ‘plateau’ region, while the crack growth rates in the ‘linear’ region are governed by the rate at which the driving force progresses along the strain-controlled R-curve.
The general validity of these observations is then assessed via a brief comparison to subsequent studies examining the effect of loading rate on the HEAC behavior across a wide range of alloys, including AA7075-T651, AA5456-H116, Beta-C Ti, Pyrowear 675 steel, and Custom 465 steel. The collective implications of these results in the context of current standardized approaches for evaluating HEAC are then discussed.