THE EFFECTIVE FATIGUE THRESHOLD - SIGNIFICANCE OF THE LOADING CYCLE BELOW THE CRACK OPENING LOAD

Experiments were carried out to demonstrate the validity of a new concept for the evaluation of the effective fatigue threshold values for loading conditions with K(min) below the crack opening value. For this, load-CTOD curves were determined for different R-ratios keeping K(th,max) constant and reducing K(min) below K(th,min), which resulted in a resumption of the fatigue crack growth. This observation is explained on the basis of a new closure evaluation, which takes into account the total range of CTOD to affect the driving force for fatigue crack growth. Three methods to measure the effective threshold values are proposed and critically compared. Experimental data are given for Cu, high-strength Al alloys and a dual-phase steel.