Improved test method for very low fatigue-crack-growth-rate data

ABSTRACT Currently, in North America, the threshold crack‐growth regime is experimentally defined by using ASTM Standard E647, which has been shown in many cases to exhibit anomalies due to the load‐reduction (LR) test method. The test method has been shown to induce remote closure, which prematurely slows down crack growth and produces an abnormally high threshold. In this paper, the fatigue‐crack growth rate properties in the threshold and near‐threshold regimes for a titanium alloy, Ti‐6Al‐4V (STOA), are determined by using the LR test method and an improved test method. The improved method uses ‘compression–compression’ precracking, as developed by Pippan, Topper and others, to provide fatigue‐crack‐growth rate data under constant‐amplitude loading in the near‐threshold regime, without load‐history effects. Tests were conducted over a wide range in stress ratios (R = 0.1–0.7) on compact C(T) specimens for three different widths (25, 51 and 76 mm). The slitting method was used on 51 mm C(T) specimens to confirm that the material did not contain significant levels of residual stresses from forming and/or machining. A crack‐mouth‐opening‐displacement gage was used to monitor crack growth. Data from the ASTM LR method gave near‐threshold values that were found to be dependent upon the specimen width. However, data from the compression precracking constant amplitude (CPCA) loading method gave near‐threshold data independent of specimen width. A crack‐closure analysis was performed for both the LR and CPCA data, to correlate data at the various stress ratios. The CPCA data correlated well with the effective stress‐intensity factor range against rate relation, whereas the LR data exhibited significant threshold fanning with both stress ratio and specimen width.