A DirectJ-RCurve Analysis of Fracture Toughness Tests

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Abstract
A method is proposed to evaluate fracture toughness in a J-R curve format directly from load displacement records without the need for automatic crack length measurement devices. This method uses the calibration curve in which the flow properties of a material for a stationary crack provide the relationships between load, displacement, and crack length. Crack growth is then evaluated by comparing load and displacement pairs for the growing crack case with this stationary crack curve. This is similar to the approach in which a common “key curve” was used to determine crack length; however, in this case, individual calibration curves are developed for each specimen. The calibration curve is described by separate elastic and plastic components of displacement. The relationship between load and elastic displacement is described by a compliance function and between load and plastic displacement by a power term with constant coefficient and exponent. Three methods are suggested for evaluating the power term exponent and coefficient. The first assumes that the exponent coming from a fit of the true-stress true-strain tensile test results can be used, the second determines all information directly from the test record, and a third does the same but uses a plastic zone adjustment.The method is applied to a set of results from an A508 steel in which the J-R curve had previously been determined using the elastic compliance method of crack length measurement. These tests were conducted on compact specimens with a range of sizes varying by a factor of 20 (10T to 1/2T). The method is also applied to a 4340 steel and stainless steel weld metal. The results show that second approach works best. Although further evaluation of the method is needed, it showed promise as a replacement for automatic crack length measurement techniques in the evaluation of J-R curves.

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