ON THE TRANSITION FROM NEAR‐THRESHOLD TO INTERMEDIATE GROWTH RATES IN FATIGUE

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Abstract
Abstract—Evidence is presented that the cyclic stress intensity threshold for fatigue crack growth in A1 2219‐T851 is associated with a critical maximum value of stress intensity,Kc. This relationship is discovered by measuring the local value of stress intensity at the crack tip which is less than the applied stress intensity because of fatigue induced compressive residual stresses in the plastic zone. Crack growth rates and values of the crack tip residual stress are measured as functions of load ratio. For local stress intensities greater thanKc, the growth rate follows a power‐law relationship, increasing monotonically with δK. For local stress intensities belowKc, growth rates are also sensitive to the cyclic stress range, δσ. If the stress range is small, a threshold to growth, typical of long cracks, is seen. When the cracks are short and δσ exceeds a critical value, growth rates are a complex function of both δσ and δK. This behavior is attributed to the effect of δσ on the propagation of the crack front past obstacles such as grain boundaries.