Direct Current Potential Drop (DCPD)
Cormet’s reverse current direct current potential drop (DC PD) is used for monitoring the crack-growth rate of CT and SENB specimens. The operation principle of a DCPD instrument is simple: the growing crack decreases the unbroken material length and correspondingly increases the resistance measured over the specimen.
The specimens shall have prefatigued cracks created using servohydraulic fatigue instruments. Current feeding wires will be welded on the specimen top and voltage measurement wires on the specimen front face surface. Special attention must be paid to the wire feed-throughs as at least four wires will be taken out of the autoclave. ASTM E647 includes a good introduction for the CT and DCPD instrument operation.
The controlling software is integrated with Cormet loading device software, but the instrument can be controlled using an external loading device with a trigger signal. Cormet's software calculates the crack length using Johnson function. The software has also a procedure for the constant K-tests.
Check Cormet's blog related to the isolation resistance of DCPD measurement wires and the CT specimen.