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The fact that fatigue crack propagation (FCP) rates approach zero asymptotically as the fracture mechanics loading parameter ΔK 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’ ΔKth and in a material property ‘FCP-threshold’ ΔKT. It is shown that ΔKT must be an integral part in the functional correlation between FCP-rates and the cyclic loading conditions expressed in ΔKeff. The experimental determination of ΔKth on six Al-alloys and four Ti-compressor disk alloys show that ΔKT exists for all the alloys investigated. The effect of temperature on ΔKT was investigated on one Ti 6Al 4V- and one Ti 8Al 1V 1Mo disk material at room temperature (RT) and 260°C and at RT, 260°C and 360°C, respectively. No temperature effect on ΔKT was found for the temperature range investigated.
fatigue crack propagation, threshold, non-propagation condition, material property
Senior Scientist, Institute for Materials Research, DLR — German Aerospace Research Center, Cologne,