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The increasing use of J-resistance curves in design and ductile instability has inspired the need for a detailed and, indeed, reliable determination scheme for such curves. The basis for this paper emerged as part of a study of size effects on J-resistance curves. The three-part presentation includes descriptive sections for both the elastic unloading compliance and the key-curve methods, with the third part forming a comparative study of both procedures. Three-point bend specimens were used for testing the unloading compliance method using the crack mouth opening displacements for crack extension predictions. Using this specimen type, an empirically-based crack extension curvature correction procedure is introduced to correct crack length predictions for curved fronts. The load-line displacements were used for the crack tip energy evaluations with appropriate on-line corrections for indentation. The implementation of the key-curve method is reported and using compact tension specimens throughout, the results are compared to that of the elastic unloading compliance and the multiple specimen method. Finally, the virtues and drawbacks of both methods are compared and recommendations made. While the continuous nature of the key-curve method might be desirable for certain critical instability analyses, the justification of the experimental and numerical effort required is questioned.
normalized load, key curves, unloading compliance, fracture, three-point bend, compact tension, compliance, calibration, computer interaction, J, -resistance, elastic-plastic fracture, test methods
Post doctoral research assistant, Imperial College of Science, Technology and Medicine, London,