Experience With a Load Change Technique for Crack Growth Rate Measurements

Crack propagation behaviour is a very important property for the assessment of aero-engine disc materials. At low temperatures the crack growth rate in nickel-based super- alloys is essentially time-independent and the crack growth rate can be determined at high frequencies. At the intermediate and high temperatures occuring in service, the time per cycle plays an important role. Therefore dwell times must be considered in crack growth measurements. This leads to very long test times. The paper describes experience with a method which aims at reducing the test time. The essential idea is to measure crack growth rates very accurately only over a short crack growth increment and then to change for other loading conditions. The appropriate use of this method leads to a number of da/dN vs △K curves for different loading conditions obtained from a single specimen test.