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An investigation was performed to assess the effects of water vapor on fatigue crack growth in 7475-T651 aluminum alloy plate at frequencies of 1 and 10 Hz. Twenty-five mm thick compact specimens were subjected to constant amplitude fatigue testing at a load ratio of 0.2. Fatigue crack growth rates were calculated from effective crack lengths determined by using a compliance method. Tests were conducted in hard vacuum and at water vapor partial pressures ranging from 94 Pa to 3.8 kPa.
Fatigue crack growth rates were frequency-insensitive under all environmental conditions tested. For constant stress intensity factor ranges, crack growth rate transitions occurred at low and high water vapor pressures. Crack growth rates at intermediate pressures were relatively constant and showed reasonable agreement with published data for two Al-Cu-Mg alloys. The existence of two crack growth rate transitions suggests either a change in rate-controlling kinetics or a change in corrosion fatigue mechanism as a function of water vapor pressure. Reduced residual deformation and transverse cracking on specimens tested in water vapor versus vacuum may be evidence of embrittlement within the plastic zone due to environmental interaction.
aluminum alloy, compliance, corrosion fatigue, fatigue crack growth rate, fractography, frequencies, vacuum, water vapor
Materials Research Scientist, Langley Research Center, Hampton, Va.