D-C Electric-Potential Method Applied to Thermal/Mechanical Fatigue Crack Growth

A d-c electric potential system for measuring crack length under thermal/mechanical fatigue-crack-growth (TMFCG) test conditions is described. A programmable d-c current supply and precision multimeter produce reliable electric-potential readings. H.H. Johnson's formula is used to calculate crack length from electric potential for the center-crack-tension M (T ) geometry. Calibration constants for the formula are determined from an initial optical crack-length measurement. The resolution of the system is 1.0 mu V which corresponds to a crack extension of approx 0.002 mm for the center-crack-tension geometry using a current of 10.00 A. Good crack-length accuracy and low data scatter are achieved by taking special precautions to minimize or eliminate errors in potential measurement due to thermal effects. Material resistivity changes are identified as the cause of short and long term changes in the measured electric potential for uncracked specimens. Crack-length accuracy is discussed in terms of short-term scatter and long-term drift. Results for Ni-base superalloy IN718 are presented. 13 ref.--AA