Recommendations for the measurement of R-curves using centre-cracked panels

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Abstract
Experimental techniques for obtaining the R-curve, a measure of fracture toughness used for selection and quality assurance of thin-section materials, are discussed in the light of experience at RAE, DFVLR, and NLR. Detailed recommendations are made for R-curve measurement using centre-cracked panels. To improve standardization, the procedure is more closely defined than in the proposed recommended practice for R-curve determination of the American Society for Testing Materials (ASTM), but machining costs are reduced by relaxing tolerances on specimen dimensions. The crack length is measured by either a potential-difference of a compliance method, and the specimen is loaded to give uniformly increasing load-point displacement of crack-opening displacement. Constant rate of increase of load is not recommended. Optical measurement of crack length is not recommended. The R-curve is calculated by a method which avoids giving undue weight to measurements made at zero or small crack extensions.

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