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Observations of tensile to shear transition behavior of fatigue crack fronts have been recorded numerous times during tension-tension cycling of sheet materials. Based on recent crack growth data, correlation of shear lip development and associated crack tip stress intensities has been obtained for a variety of aluminum, steel, and titanium alloys. Assumption of a crack growth transition zone region allows for direct application to deviations in crack growth rate versus stress-intensity relationships at low crack tip intensities. A stress-controlled specimen of 7075-T6 aluminum sheet was fatigue cycled from tensile to shear and back to tensile within the transition zone; crack front behavior was microscopically observed. Macroscopic and microscopic correlations were established with discontinuities in stress intensity versus crack growth semilog relationships. The beginning of shear lip development in 2024-T3 aluminum appears to be independent of thickness and dependent only on the stress-intensity factor at transition.
fatigue (materials), crack popagation, aluminum alloys, steels, titanium alloys, crack growth rate, fracture, transitional behavior, microstructure, stress intensity
Wilhem, D. P.
Senior engineer, Northrop Corp., Hawthorne, Calif.