Compliance ratio method of estimating crack length in dynamic fracture toughness tests

Crack extension during fracture toughness tests of ferritic structural steels cannot be determined from measurements of unloading compliance or electric potential change when the specimen is dynamically loaded. Measurements of crack extension in fracture toughness tests are also very difficult when the test temperature is high or the test environment is aggressive. To circumvent this limitation, researchers for years have been developing key curve and normalization function methods to estimate crack extension in standard elastic-plastic fracture toughness test geometries. In the key curve method [1,2] a load-displacement curve is measured for a so-called "source" specimen that is subsize or has a blunt notch so that the crack will not initiate during elastic-plastic loading. The load and displacement are then converted to normalized stress-strain units to obtain a key curve that can be used to predict crack extension in geometrically similar "target" specimens of same material loaded at similar loading rates and tested under similar environmental conditions. More recently, Landes and coworkers [3,4] proposed the normalization method. During the past three years the ASTM Task Group E 08.08.08 has prepared a draft Appendix H for the ASTM E 1820 specification that presents the normalization method as an alternative to the standard E 1820 unloading compliance procedure. Although the normalization method works well in many cases, it has serious drawbacks: the load, displacement, and crack length at the end of the test must be measured; the prescribed functional form that is fitted to the initial and final data may not be accurate for all materials and the iterative method of inferring crack length from the combination of the data and the normalization function is complex. The compliance ratio (CR) method developed in Ref 5 is used in this paper to determine key curves for predicting crack extension in dynamically loaded specimens, It consists of first testing a statically loaded source specimen using the unloading compliance procedure as specified in ASTM 1820. Then the so-called CR load-displacement curve is calculated for the source specimen, which is the load-displacement record that would have been obtained if the crack had not extended. Nondimensionalizing the CR load by the maximum load and the displacement by the elastic displacement at the maximum load, P*/P-max and from the source specimen yields the rate adjusted key curve. Assuming that the key curves of the statically and dynamically loaded specimens are one and the same, the compliance is calculated with a reverse application of the compliance ratio method and the crack length is obtained using the equations in ASTM E 1820. The CR method is found to be much simpler than the normalization method described in the draft Appendix H of ASTM 1820.