ISSN: 0090-3973
Published Online: 25 January 2006
Page Count: 9

Effects of Different Load Schemes on the Fatigue Crack Growth Rate
Ljustell, P
Ph.D. Student and Professor, Royal Institute of Technology (KTH),

Nilsson, F
Ph.D. Student and Professor, Royal Institute of Technology (KTH),

(Received 15 March 2005; accepted 13 October 2005)

Abstract

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 K^I,max-method provides an upper bound of the fatigue crack growth rate in the stage II region. The results indicate that the constant K^I,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.

Keywords:
fatigue crack growth, crack closure, load scheme, constant KI,max-method

Paper ID: JTE14097
DOI: 10.1520/JTE14097
ASTM International is a member of CrossRef.