Review and Consideration of Apparent Negative Crack Growth in Fracture Toughness Tests

Apparent negative crack growth is often encountered when performing elastic-plastic fracture toughness tests with the Elastic Compliance single-specimen technique. It consists of a decrease of specimen compliance (reduction of crack size) during the early portion of the test, before the attainment of maximum force and the onset of ductile crack extension. This phenomenon was recognized and discussed inside the fracture mechanics community since the mid-1980s, but widespread consensus was never achieved on its root causes and on the best approach to account for it in the analysis of an elastic-plastic fracture toughness test. It has been shown that both experimental (test setup) and material behavior aspects can be invoked to explain the decrease of elastic compliance that translates into decreasing crack size in the early loading stage. The current edition of ASTM E1820, Standard Test Method for Measurement of Fracture Toughness, does not offer provisions to handle this phenomenon, and users are left free to treat the issue as they see fit. In this study, several articles published in the last 40 years were reviewed, and different proposed methods were applied on 15 selected tests performed on specimens of different geometries and thicknesses. Comparisons between original crack sizes, ductile crack extensions, and critical toughness values are presented, and recommendations are provided for revising ASTM E1820 with due consideration of the occurrence of apparent negative crack growth.