FATIGUE-CRACK PROPAGATION THRESHOLD - WHAT IS IT AND HOW IS IT MEASURED

The present understanding of a ''fatigue crack propagation threshold i mplies a nonpropagation condition of a fatigue crack and in addition s ome property usable for design (similar to the ''fatigue strength'' in Wohler-type fatigue). The first task is the definition of a nonpropag ation condition, to be denoted as Delta K-th. In the next step, a ''pr operty usable for FCP design'' is defined which is a nonpropagation co ndition characterized as the largest amplitude Delta K which does not produce fatigue crack propagation rates larger than 10(-7) mm/cycle un der any cyclic loading condition. The parameter so defined is termed t he threshold and denoted as Delta K-T. Every Delta K-T is a nonpropaga tion condition, but not every nonpropagation is a threshold value Delt a K-T. The existence of such a parameter Delta K-T has to be proven by special, extensive tests, which are presented with the respective res ults. Once the proof related to Delta K-T has been furnished, measurem ent of Delta K-T is done by the simplest technique available. Nonpropa gation conditions Delta K-th and threshold Delta K-T data for Al- and Ti-alloys are presented. Furthermore, test procedures and specimen pre parations are outlined.