Previous studies of the growth of small fatigue cracks in aluminum alloy 7075-T6 are reviewed to compare observed behavior and explanations for that behavior. The results of small crack growth tests conducted in this work are then reported. Crack growth was monitored periodically between initial crack sizes of approximately 10 μm to final sizes of approximately 0.5 mm in rotating bending and plate specimens subjected to fully-reversed cycling. Both longitudinal and transverse stress components were present in the central region of the plate specimens, while rotating bending specimens experienced nearly uniaxial cyclic stress. Specimens were taken from rolled 7075-T651 with a pancake microstructure. Tests were conducted at a number of different strain amplitudes. Small cracks grew faster than large cracks for the same stress intensity range ΔK, as expected from previous research by others. When compared on the basis of ΔK, growth rates in the plate specimens varied from being little different than those in rotating bending specimens to approximately four times higher, depending on strain amplitude. The growth rates of cracks at different locations around the circumference of the rotating bending specimens varied little, in spite of the different microstructural orientations of the different locations. In both plate and rotating bending tests, some cracks exhibited marked decelerations and accelerations in growth rate, while others did not. For those cracks that did not exhibit such oscillations in growth rate plus those cracks that had grown large enough for such oscillations to have subsided, growth rates were found to be proportional to crack size and to amplitude of maximum shear strain to a power of approximately 4.5. Growth rate data from both rotating bending and plate specimens were well correlated by that strain parameter.
Small cracks; Biaxial stress; Fatigue crack growth