Development of a method for extracting fracture toughness from instrumented Charpy impact tests in the ductile and transition regimes

Abstract

The Charpy impact test is often used to qualify the structural materials used for the fabrication of industrial components, in particular steel-based components. This relatively simple test used for more than a century was originally designed to contribute to ensure steel quality. Later, it was used to determine the so-called ductile-to-brittle transition temperature (DBTT) to characterize the fracture resistance of body-centered cubic (bcc) structural materials. In the nuclear field, it was also used in surveillance programs monitoring the property degradation of the reactor pressure vessels avoiding prohibitive fracture toughness tests that would require large specimens. With the advances in fracture mechanics achieved over the last decades, it is possible nowadays to reliably measure fracture toughness from significantly smaller specimens. In particular, the master curve offers a simple method to derive the fracture toughness – temperature dependence curve that can be further used for integrity assessment. A number of empirical correlations were established by relating the DBTT measured with Charpy impact tests to the master curve reference temperature, T0. However, all those correlations are essentially empirical and rely heavily on experimental databases on which they were derived.The main objective of this work is a more direct derivation of quasi-static fracture toughness from the Charpy impact test that can be directly compared to measured quasi-static fracture toughness data. This way, the consistency between the multiple properties delivered by the various tests guarantees the overall quality of the experimental data used to characterize the mechanical properties of structural materials. The proposed procedure relies on the J-integral derived from the instrumented Charpy impact test and on the normalization of the crack resistance curve by accounting for dynamic effects, notch/crack configuration effects, loading rate effects, side grooving effects and eventually specimen size effects. The results obtained on several materials in the unirradiated and irradiated conditions are very promising and provide an interesting property-to-property correlation tool, not only as a quality control but they also offer a cheap and fast estimate of fracture toughness for material design purposes.