Crack propagation in fine grained graphites under mode I and mixed-mode loading, as observed in situ by microtomography

Stable crack growth under mode I and mixed-mode loading of two fine grained advanced graphites, SNG742 and T220, has been studied within small (18 mm diameter) specimens of the diametral compression disc geometry to investigate the criterion for crack propagation. Cracks were propagated from a central notch, inclined at 0 degrees or 30 degrees to the loading axis, as the tests were observed in situ by X-ray computed microtomography. The three-dimensional (3D) displacement fields were measured by digital volume correlation. The crack shape, tip location and crack opening displacements were determined by 3D phase congruency edge detection of full field displacements. Linear elastic 3D finite element simulations calculated the J-integral and mode I, mode II and mode III stress intensity factors (SIFs) acting on the crack tip, using the full field displacements as boundary conditions. For SNG742 and T220 specimens with notch angle of 0 degrees, nearly pure mode I crack propagation was obtained. For specimens loaded with the 30 degrees notch angle, the SIFs changed from mixed mode I/II towards pure mode I as the crack propagated. The mode II SIF was less than 20% of the mode I SIF, and as the crack extended over a distance similar to 3 mm, the critical mode I SIF (fracture toughness) was insensitive to the mode II SIF, with SNG742 graphite having higher fracture toughness than T220 graphite. (C) 2022 Elsevier Ltd. All rights reserved.