The stress-level effect on fatigue-crack growth under constant-amplitude loading

The fatigue-crack-closure concept has been successfully used with stress-intensity factors to predict the growth of cracks under a wide variety of load histories and in complex crack configurations. Both test and crack-closure analyses have shown that the stress-intensity-factor-range-against-rate curves are affected by the stress ratio (R), the applied stress or load level (Smax or Pmax), and the crack-front constraint (plane-stress or plane-strain behavior). However, most life-prediction codes use only linear-elastic fracture mechanics (LEFM) concepts, which neglect stress-level effects, to make life predictions. Thus, under some loading conditions, such as negative R ratios or high-applied stress levels, non-conservative life predictions are made using only LEFM procedures. Fatigue-crack-growth tests have been conducted on middle-crack tension M(T) specimens made of 2024-T3 thin-sheet (B = 2.3 mm) aluminum alloy over a wide range in applied stress levels (0.1–0.5 times the flow stress of the material) and for two stress ratios (R = 0.05 and −1). The FASTRAN life-prediction code, using either the crack-closure model or LEFM procedures, and the AFGROW code, which uses only LEFM procedures, were used to make crack-growth predictions from an initial crack size to failure in the M(T) specimens. The results from AFGROW and FASTRAN, using LEFM procedures, agreed very well with each other. The crack-closure model predicted all results with ±20%, whereas, the codes using LEFM procedures (neglecting stress-level effects) resulted in non-conservative life predictions as large as a factor-of-3 from the test results.