Crack growth and closure behavior of short fatigue cracks

Crack growth and closure behavior of short cracks are investigated for various stress ratios, using in-plane bending specimens of 2024-T351 aluminum alloy. Artificially prepared two-dimensional, short through-thickness cracks are used. Crack length and closure of short cracks are measured continuously during the test by employing an unloading elastic compliance technique and a personal computer system. For most of all the stress ratios tested, short cracks grow faster than long cracks in the low stress intensity factor range region, and the growth rates of short cracks merge with the long crack growth curve with increases in the value of stress intensity factor range. The variation in growth rates between short and long cracks is reduced as the value of stress ratio is increased, and finally disappears at a stress ratio of R = 0.5. The growth rates of short cracks are well described by the effective stress intensity factor range. The growth rates of short cracks in terms of the effective stress intensity factor range coincide well with the long crack data. However, the closure behavior of short cracks is significantly different from that of long cracks. The crack opening ratio of a short crack decreases from an initial high value corresponding to a fully open crack linearly with the value of effective stress intensity factor range, and merges with the long crack results. Based on the test results obtained, a relatively simple procedure is proposed to predict closure and growth behavior of short cracks. The validity of the proposed procedure is examined using the test data of other workers. In addition, some methods proposed by other workers are also discussed.

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