Numerical calibration of the direct current potential drop (DCPD) method in fracture mechanics fatigue tests

Abstract

Fatigue strength and fracture mechanics concepts are linked together by the so-called cyclic R-curve, i.e. the crack size dependence of the fatigue crack propagation threshold. The cyclic R-curve requires to identify experimentally the extension of a propagating crack in the mechanically short crack regime. One of the most used techniques adopted in experimental tests is the direct current potential drop (DCPD) method, according to which the electrical resistance of the tested specimen increases due to crack propagation and the resulting electric potential change is used to derive the crack length thanks to proper calibration curves. In this work, the DCPD technique was applied in fracture mechanics fatigue tests of carbon steel bars weakened by a single-edge, semi-elliptical pre-crack. First, it was assumed that the semi-elliptical pre-crack propagates under fatigue axial loading in such a way to keep an iso-stress intensity factor (SIF) KI crack front. Accordingly, the corresponding crack pattern was derived by means of 3D structural FE analyses using the Peak Stress Method (PSM). Afterwards, the DCPD calibration curves were derived through 3D electrical FE analyses. The effects of the locations of the current and of the potential probes were investigated. Finally, a three-probe dual channel DCPD technique was applied to compensate any temperature variation of the tested specimen.