SYMPOSIA PAPER Published: 24 August 2020

Experimental Method for Direct Evaluation of the -Contour Integral


A method for direct experimental evaluation of the J-contour integral has been developed and used to measure J as a function of strain in tensile panels of high-strength steel (σy = 900 MPa) under elastic-plastic loading conditions. The principle of the present method is to measure the integrand terms of J at suitable intervals along an appropriate contour and then to evaluate the integral. Because the resulting J values are based directly on the definition of J, no assumptions about the crack size or the stress-strain fields in the vicinity of the crack tip are necessary. This advantage is crucial in cases of very short cracks (less than 10 percent of the panel width), which are of special interest for structural applications. The integrand terms are measured with strain gages and linear variable differential transducers (LVDTs) placed on an appropriate contour surrounding the crack. Load, LVDT-displacement, strain-gage, and crack-mouth-opening-displacement measurements are acquired, stored, and printed at each deformation increment, and the J-integral is evaluated numerically by a minicomputer. J-integral values are plotted against the average specimen strain in real time. Directly measured J-integral values and values calculated using linear elastic fracture mechanics agreed within 12 percent for average strain values between 10 and 90 percent of yield. The experimental uncertainties become significantly less for strains of several times yield than for strains below yield. This technique has been used to measure J as a function of strain in edge-cracked and center-cracked tensile panels with short cracks at strains beyond yield, cases for which closed form solutions for J are not available.

Author Information

Read, D., T.
Fracture and Deformation Division, National Bureau of Standards, Boulder, Colo., US
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Developed by Committee: E08
Pages: 199–213
DOI: 10.1520/STP791B1983013
ISBN-EB: 978-0-8031-7720-8
ISBN-13: 978-0-8031-0729-8