Crack-propagation rate in 7075-T6 tension plates was determined for central cracks initially oriented in 45, 60, and 90 deg, to the width direction of the tension plates which were loaded cyclically. Opening and sliding mode of stress-intensity factors, K1 and K2, were determined by the method of direct stiffness for curved cracks generated from these initially slanted cracks. Crack-propagation rates, Δa/ΔN, were then plotted against the maximum opening mode of stress-intensity factor, K1, in the presence of sliding mode of stress-intensity factor, K2. Comparison between the corresponding crack-propagation rate in control specimens without K2 showed that the propagation rate is definitely increased in the presence of K2.

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