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

摘要

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-6A1-4V (STOA), are determined using an improved test method. To generate fatigue-crack-growth rate data under constant-amplitude loading in the near-threshold regime, without load-history effects, a "compression-compression" precracking method, developed by Pippan, Topper and others is used. Using this procedure, a pre-notched specimen is cycled under compression-compression loading to produce an initial fatigue precrack, which naturally stops growing. The specimen is then subjected to constant-amplitude fatigue loading to generate crack growth rate data in the near threshold regime at the desired stress ratio. Steady-state constant-amplitude (load-history free) crack-growth rate data is achieved after the crack has grown several compressive plastic-zone sizes. Tests were conducted over a wide range in stress ratios (R = 0.1 to 0.7) on compact C(T) specimens for three different widths (25, 51 and 76-mm). These results were compared with threshold data generated on the same material using the current ASTM LR test procedures. Applying the LR test procedures for threshold testing on the titanium alloy C(T) specimens resulted in threshold values that were found to be dependent upon specimen width, a clear violation of fracture mechanics concepts that fatigue crack growth rate behavior can be uniquely correlated with the alternating stress intensity range, AK, for a given stress ratio. However, the improved technique, compression precracking constant amplitude (CPCA) loading, produced near-threshold data that were independent of specimen width. A crack-closure analysis was preformed on both the LR and CPCA data. The CPCA results correlated into nearly a unique effective stress-intensity factor against rate relation, whereas the LR data was not and exhibited severe threshold fanning with the stress ratio and specimen width.