A practical method is proposed to calculate crack extensions during stable and subsequent unstable crack growth conditions in aluminum alloy 7075-T651 specimens. Also for aluminum alloy 2024-T351 specimens, the stable crack growth is calculated up to the limit stress. A crack initiated at a critical CTOD extends stepwise in successive increments of CTOD, so that the ratio of a CTOD increment to a crack-extension increment is a critical value, independent of an initial CTOD. The value attained instantaneously at each step of crack extension is different from measurable total CTOA. Some CTOD functions for MT and CT specimens of various sizes and crack lengths are calculated up to their limit loads using finite element analysis. The functions when plotted against applied stress vary little with crack length over a wide range except near the limit stress. The proposed method is based on the calculated functions for a mechanics solution. There seem to be two types in the final phase of fracture: (1) limit load for the 2024-T351 alloy and (2) fracture instability for the 7075-T651 alloy. When the total of CTOD increments during stable crack growth reaches a critical value, fracture instability occurs that characterizes crack extension without further increment of CTOD. Fracture constants are determined so that the calculations may be fitted to available test data with respect to effective and physical crack lengths. The determined fracture constants are transferred to other specimens to predict the failure loads, in good agreement with test loads. Discussions are made on specimen size effects and crack length measurement in the presence of crack tip tunneling.
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