Essential to the assessment of the fatigue integrity of critical components is the prediction of their crack propagation lives This requires the effective and efficient characterization of the fatigue crack growth behaviour of the material when using a small number of specimens A test facility, able to control the range of applied load (ΔP ) and range of stress intensity factor (ΔK ), has been used to determine the fatigue crack growth rates in corner notched specimens of Udimet 720 for stress ratios of 05, 01 and −10 Initially, the results were analysed in terms of the three-point secant data reduction method Subsequently, the benefits to be gained by the use of alternative methods involving regression analysis have been examined The effectiveness of the procedures has been assessed in terms of the extent to which they reduce the width of the scatter band representing the fatigue crack growth behaviour and the accuracy with which the experimental observed fatigue lives are predicted On this basis, the results from the ΔK controlled tests were superior to those obtained under ΔP control, when analysed by similar data reduction methods In addition, the results from ΔK tests were improved when the secant method was replaced by a linear regression method, which allowed the sample interval to be reduced and more levels of constant ΔK to be examined in a single specimen

The characterization of fatigue crack growth behaviour by constant ΔK control testing

Acoustic Emission Analysis on Fatigue Threshold Behavior

This paper investigates the effect of load shedding steps on constant-ΔK testing. ASTM fatigue crack growth specimens have been used in constant-ΔK tests for many fatigue investigations. In the tests, load levels need to be gradually stepped down to keep the stress intensity factor constant. This work points out that care should be taken in choosing the length of the step in order to control ΔK variation. Formulas for ΔK as well as crack growth rate variability are derived. The experimental application of these techniques is also presented.

F. J. Zwerneman

Papers

Load shedding effect on constant-ΔK testing