Determination of crack initiation and crack growth stress-life curves by fracture mechanics experiments and statistical analysis

摘要

It is well known that the fatigue lifetime of a notched component consists of the crack initiation lifetime (crack size equal to the characteristic microstructure size) and the crack growth lifetime. The crack growth lifetime is easily calculated if the initial crack size is known. A much more difficult task is to define a suitable initial crack size and to determine the initiation lifetime of such a crack. In the present contribution, a method is proposed to determine the crack initiation as well as the crack growth lifetime within a conventional fracture mechanics setup. To this purpose, single edge notched bending (SENB) specimens were manufactured with two different notch geometries. For measuring the crack length, the direct current potential drop (DCPD) technique was used. The DCPD results are not only used for crack growth assessment, but also for determining the point - or range, respectively - of crack initiation. By a statistical assessment of DCPD measurements at different load stress levels, it is possible to determine crack initiation and crack growth stress-life (S/N) curves, and in particular to assess the influence of the notch geometry on the crack initiation lifetime. The lower statistical tolerance limits of these curves may be used as criteria for mechanical design and determination of inspection intervals.