Volume 26, Issue 3 (May 1998)
Fatigue Crack Propagation Threshold: What Is It and How Is It Measured?
The present understanding of a “fatigue crack propagation threshold” implies a nonpropagation condition of a fatigue crack and in addition some property usable for design (similar to the “fatigue strength” in Wöhler-type fatigue). The first task is the definition of a nonpropagation condition, to be denoted as ΔKth. In the next step, a “property usable for FCP design” is defined which is a nonpropagation condition characterized as the largest amplitude ΔK which does not produce fatigue crack propagation rates larger than 10−7 mm/cycle under any cyclic loading condition. The parameter so defined is termed the threshold and denoted as ΔKT. Every ΔKT is a nonpropagation condition, but not every nonpropagation is a threshold value ΔKT. The existence of such a parameter ΔKT has to be proven by special, extensive tests, which are presented with the respective results. Once the proof related to ΔKT has been furnished, measurement of ΔKT is done by the simplest technique available. Nonpropagation conditions ΔKT and threshold ΔKT data for Al- and Ti-alloys are presented. Furthermore, test procedures and specimen preparations are outlined.