Volume 34, Issue 4 (July 2006)
Effects of Different Load Schemes on the Fatigue Crack Growth Rate
An experimental study was conducted on a nickel-based super alloy, Inconel 718, at an elevated temperature of +400°C. The basic question was how to conduct the experiments in order to obtain fatigue crack growth data as free from load history effects as possible in the stage II region. This was investigated by use of four different load sequences. An important question was to investigate if the so called constant KI,max-method provides an upper bound of the fatigue crack growth rate in the stage II region. The results indicate that the constant KI,max-method gives an upper bound in fatigue crack growth rate in the current measuring range but also gives a statistically significant lower exponent of the crack growth equation. A more reliable method to establish the fatigue crack growth parameters is to propagate the crack under constant stress-intensity factor range. This should be done at two different stress-intensity factor ranges for a few millimetres at each range. Also, ΔK-reducing procedure should not be used, when performing fatigue crack growth testing in the stage II region, in order to minimize the risk of transient crack closure effects.