An Enhanced Normalization Method for Dynamic Fracture Toughness Testing

Abstract

Fracture toughness testing of ductile materials can be difficult in situations where it is not possible to measure crack extension during the test, such as under high rate loading or in aggressive environments. In these situations, an alternative method of inferring crack extension must be used to generate the tearing resistance curve, and thereby determine ductile crack initiation. ASTM test method E1820-01 uses the Normalization method to generate the plasticity function for the specimen, which can then be used to calculate crack extension. This method relies heavily on accurate measurement of the load, displacement and crack length at the end of test. These measurements are used to generate a point on the normalized load-displacement plot, known as the “anchor point”. If a test ends with unstable crack extension, the anchor point cannot be determined and the method cannot be applied. If there is a large amount of crack extension in a test, the uncertainty in the derived plasticity function increases, which can lead to non-conservative J-R curves. An alternative approach that can be used to reduce the dependency on the anchor point is the Compliance Ratio (CR) method. This method uses a quasi-static test with unloading compliance to generate a load-displacement record with no crack extension, or “key curve”. Once this key curve is obtained, it can be scaled to account for the effect of loading rate. The specimen compliance at any point in a dynamic test record is determined by comparing the key curve with the actual load at the same displacement, and from this the crack extension can be determined. There are problems with this approach when the quasi-static key curve does not match the dynamic test record early in the test where blunting transitions to ductile tearing.