Local crack-tip strain approach to fatigue crack propagation

✍ Scribed by Furuya Yasubumi; Shimada Heihachi

• Publisher: Elsevier Science
• Year: 1987
• Tongue: English
• Weight: 647 KB
• Volume: 26
• Category: Article
• ISSN: 0013-7944
• DOI: 10.1016/0013-7944(87)90017-8

✦ Synopsis

A progressive crack-tip strain approach to fatigue crack propagation is presented. Localstrain damage accumulation curve for crack initiation is used as a "cyclic damage accumulation" and "fracture criterion" in the heavily deformed region, R,?, just adjacent to the crack-tip. The changes of cyclic strain ahead of the crack-tip is experimentally measured by the real-time finegrid-method.

Using the average strain range in R, and linear cumulative damage law of local strain, the number of cycles N* necessary for the material failure over the distance, R,, is calculated from local-strain damage accumulation curve. The average crack propagation rate, da/dN, in R, is estimated as the ratio R,,/N*. As a result, very good agreements between the measured and predicted dajdN are obtained in cases of two kinds of steels, MANTEN and RQC-100. This fact suggests the possibility that we can estimate the fatigue crack propagation rate by combining the cyclic strain ahead of the crack-tip with local-strain damage accumulation curve, and that, we can unify the two fatigue stages, crack initiation and propagation, from the view point of the local crack-tip strain.

NOMENCLATURE

Cycles number for fatigue crack initiation Cycles number for fatigue crack propagation Cycles number of total fatigue life of specimen (N, = N, + N,,) Macroscopic total strain range of specimen Local strain range at the notch-root on crack initiation process Strain range at the propagating crack-tip Average local strain (AC,) damage accumulation value for crack initiation Size of heavily deformed region ahead of the crack-tip Average strain range in R, ahead of the crack-tip Cycles number for the material in R, to be failed by crack propagation Local-strain damage accumulation curve for fatigue crack initiation Fatigue crack propagation rate