Fatigue crack growth threshold concept and test results for Al- and Ti-alloys

The fact that fatigue crack propagation (FCP) rates approach zero asymptotically as the fracture mechanics loading parameter Delta K decreases toward certain limiting values is the reason for the scientific interest in these so-called 'threshold' values. In respect to the practical use of these 'limiting values', the author divides them into two groups, namely in 'non-propagation conditions' Delta K-th and in a material property 'FCP-threshold' Delta K-T. It is shown that Delta K-T must be an integral part in the functional correlation between FCP-rates and the cyclic loading conditions expressed in; Delta K-eff. The experimental determination of Delta K-th on. six Al-alloys and four Ti-compressor disk alloys show that Delta K-T exists for all the alloys investigated. The effect of temperature on Delta K-T was investigated on one Ti 6Al4V- and one Ti 8Al 1V 1Mo disk material at room temperature (RT) and 260 degrees C and at RT, 260 degrees C and 360 degrees C,respectively. No temperature effect on Delta K-T was found for the temperature range investigated.