| In the past, the disagreement of near-threshold FCG rate data generated from constant Kmax tests, high load ratio constant R tests, and ΔKeff based data was a mysterious issue. Because of the disagreement, a variety of test or analysis methods were created to correlate FCG rate data. It was suspected that the ASTM threshold test method using load reduction was inducing remote crack closure due to plastically deformed material, which caused elevated thresholds and slower rates than steady-state behavior. The first goal of this study was the development of a test method to eliminate remote closure during threshold testing. In order to minimize remote closure effect, compression-precracking method was used to initiate a crack from a starter notch on compact specimens. Two materials with different fatigue crack surface profiles were tested and the results generated from the conventional ASTM precracking method and the compression-precracking test method were compared.;In order to understand the disagreement of near-threshold data, crack-opening load measurements were performed from locally installed strain gages instead of the remote gage. Some careful specimen preparations were performed to avoid out-of-plane bending, to maintain straight crack fronts, and to ensure testing system linearity. It was known that remote gages, such as crack-mouth-opening-displacement-gages were insensitive to measuring load-strain records near threshold. By using local gages, the crack closure effects were clearly observed even in high load ratio tests, like or higher than 0.7, and constant Kmax tests, which were believed to be crack closure free. By measuring load-reduced-strain records from local gages, crack-opening loads were able to correlate FCG rate data and showed that ΔKeff-rate data was unique for a wide variety of materials. By comparing (ΔKeff)th values, it may provide reasonable guidance for the material resistance against FCG. Because of “high-R crack closure”, some theories considered in the past may need to be reconsidered. First, constant Kmax tests are not entirely crack-closure free. Second, there are no Kmax effects that appear in ΔKth-Kmax relations. Research has shown that the three dominate crack-closure mechanisms (plasticity-, roughness- and debris-induced crack closure) FCG rate behavior in the threshold regime at any load ratios.;Key words: fatigue crack growth, threshold, load ratio, fatigue crack closure, load history, remote closure, effective stress intensity factor range, compression precracking, load reduction test, constant Kmax test, metallic materials. |
