The normalization method has been used to develop J-R curves directly from load versus load-line displacement data without the need for on-line crack length measurement. This method, based on the original key curve-method, used the principal of load separation and an assumed functional form for the deformation function to determine a calibration equation for each specimen being evaluated. Originally the functional form was a power law with two unknown constants. A new functional form (the LMN function) containing three unknown constants is proposed for use in the method of normalization. This function requires three calibration points, one more than was needed by the power law function. The LMN function is described in this paper, and the method for determining the three needed calibration points is given in detail. The procedure for using this new normalization method is describedin a step-by-step manner. Some results are included which show that this approach to normalization gives a J-R curve which agrees well with those developed from the elastic compliance method used in the ASTM test standards.
A direct method for evaluating J-R curves from load displacement data is proposed. This method eliminates a need for automatic crack length monitoring equipment. The method uses the geometric normalization suggested by Ernst for deeply cracked bend specimens to develop calibration curves which relate load, displacement, and crack length. Given two of these parameters, the third can be determined from the calibration curves. These curves are developed by assuming a functional form with unknown constants and evaluating the unknowns at calibration points in the test. This method was previously proposed using a power law functional form. In this paper a more general functional form is proposed which combines a power law and straight line. The method is studied for a variety of materials and specimen sizes. From the results a recommendation is made on how to generally apply the method.
A method for experimentally determining the eta (η) factor based on separation constants has been recently proposed. This method has two important implications for elastic plastic fracture toughness testing. First, the method can be used to determine the η factors for any new test specimen geometry which might be added to existing test standards. Such specimens as disk compact, arc bend, and arc tension are used in the KIc test standard. They can be added to the J based standards if the specimen calibrations are known, one being the η factor calibration. In this paper a step by step procedure is given describing η factor calibration for an arbitrary specimen geometry based on a series of blunt notched specimens. The procedure proposed in this paper was then applied to existing blunt notch data for the traditional test specimen geometries, the compact, and single edge notched bend specimens. The results of the study show different values for η from these in the existing standards both in magnitude and trend with a/W. In addition they show a slight material sensitivity. The consequences of having incorrect η factors in the test standards are explored in a sensitivity study. These results are used to evaluate the importance of having correct η factors and recommendations are made.
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