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The application of J-integral methods to fatigue crack growth testing was first presented by Dowling and Begley . They demonstrated that under certain conditions the J-integral range (ΔJ) applied to a cracked body correlates well with the measured crack growth per cycle in tests performed in the elastic-plastic regime. This paper presents an automated test system for computer-controlled low-cycle fatigue crack growth evaluation of ductile materials using cyclic-J, and details the procedure for this technique. The system consists of a hydraulic test machine, a minicomputer, and a microcomputer. The test machine is controlled in either clipgage or stroke mode with the control signal of the minicomputer channeled through a digital-to-analog converter. The microcomputer is dedicated to compliance calculation. Analysis of crack length, closure load, and applied ΔJ is performed each cycle and the control signal is adjusted to achieve the desired J-range. Crack growth results are periodically stored on disk along with load-crack opening displacement data for the cycle.
This automated system has been utilized for rising J-range, J-range shedding, and constant J-range tests using compact tension specimens of two high-strength structural steels. Results from these tests show agreement with the linear extrapolation of crack growth data obtained under linear-elastic conditions, and demonstrate the promise of such an approach for low-cycle fatigue crack growth characterization.
low-cycle fatigue crack growth, cyclic , J, J-integral range, elastic-plastic fracture, automated testing, crack propagation
Associate Professor, U.S. Naval Academy, Annapolis, MD
Metallurgist, David W. Taylor Naval Ship Research and Development Center, Bethesda, MD