Some Observations on The Fracture Characteristics and Fatigue Crack Growth Rates in a High Strength Aluminum Alloy.(Dept.M)

Because of the importance of assessing the structural integrity and life expectancy of engineering components, there has been considerable effort in the last thirty years in the characterization of fatigue cracking and fracture resistance for different structural materials. The present study is directed to the determination of the type of dependence of the fracture resistance and the fatigue crack initiation and propagation behavior on previous heat treatment performed on the material. High strength aluminum alloy, type 7075, has been selected as the test material. A successful T6 treatment, consisting of solution treatment followed by quenching and an aging period, results generally in good strength and toughness properties. Aging temperature was kept constant while aging time was varied from two to ten hours. Single-edge notched specimens have been used for fatigue crack propagation tests while double-edge notched specimens have been used for fracture toughness tests. Test results indicate that variation in the aging time for the alloy results in an appreciable variation in its fracture toughness and the rate of crack growth. Longer aging periods lead to higher crack growth rates and lower fracture toughness. The test results may be explained as being due to variations in the local driving force acting at the crack tip. This force is known to be influenced by several factors such as crack closure, cyclic plasticity, and crack deviation from its regular growth direction.