Influence of stress intensity and crack speed on fracture surface topography: mirror to mist transition

The transition from very smooth “mirror” crack growth to the early stages of roughening associated with “mist” has been investigated using a range of surface topography techniques. The fracture mechanics properties of the brittle, glassy and isotropic epoxy resin used in this work were characterized using compact tension (CT) and double torsion (DT) tests (KIc=0.65 MN m−3/2). In the DT test, the mist to mirror transition occurred over a large section of the test sample and this facilitated examination by optical microscopy, scanning electron microscopy, atomic force microscopy and non-contact laser profilometry. Measurements on Wallner lines and river lines were used to map the crack velocities and directions over the fracture surface. The transition from mist to mirror, for a decelerating crack, occurred at a crack velocity, vc=0.1 vt, where vt is the shear wave velocity. There was a sharp change in roughness at the transition but no discontinuity in the crack deceleration behaviour. Two main topographical features were observed at the transition: firstly, undulations in the mirror region which decreased in amplitude away from the transition for a decelerating crack and, by implication, vice versa; secondly, a progressive decrease in river line density (for a decelerating crack). Detailed atomic force microscope profilometry was used to determine the surface topography associated with these features. The results provide an insight into the development of crack instabilities under dynamic conditions and a basis for interpreting the progressive development of roughness up to macroscopic bifurcation.