Fatigue Crack Growth from Narrow-Band Gaussian Spectrum Loading in 6063 Aluminum Alloy

Abstract

Constant amplitude and narrow-band Gaussian loadings are applied to extruded 6063 aluminum crack-growth specimens in an effort to characterize the effective stress intensity levels during random loading. Crack-growth rates are determined for constant amplitude loadings at stress ratios (R) of 0.09, 0.3 and 0.5, and for a variable amplitude loading simulated to match a narrow-band Gaussian spectrum. Crack-opening stress levels measured by the compliance method during the constant amplitude loading are found to differ substantially for -T5 and -T6 heat treatments due to a change from intergranular to transgranular crack growth. Crack-opening load ratios correlate well with the maximum applied stress intensity factor, Kmax, for the -T5 material. The Kmax dependence leads to an effective halving of the crack-growth exponent. Calculated variable amplitude lives are much shorter when this correlation is taken into account (an acceleration effect) and show a greater difference between loading blocks condensed by racetrack filtering at threshold levels of two and four standard deviations, similar to what was observed in the tests. Crack-opening-load measurements in one specimen with the narrow-band Gaussian (variable amplitude) loading failed to detect any closure. A substantial difference in the closure behavior of nominally identical R = 0.3 tests indicates that closure may occur irregularly in the extruded aluminum. Calculated crack-growth lives, assuming no closure in the variable amplitude tests, are much shorter than the test results. Including closure in the variable amplitude loadings greatly improves the predictions.

On the Fatigue Analysis of Wind Turbines

“…Starting from a rather small crack of 0.025 mm (0.001 in), the crack will grow to critical length (essentially infinite growth rate) in less than 6 months, see Fig. 29 Veers and Van Den Avyle [121] have investigated the application of the constant amplitude data to spectral loading conditions. Using the constant amplitude data obtained by Van Den Avyle and Sutherland [116], the behavior of 6065-T5 aluminum is characterized using Eq.…”