Analysis of en échelon/hackle fringes and longitudinal splits in twist failed glass samples by means of fractography and electromagnetic radiation

This study introduces several innovations in the experimental study of fracture. (1) A new method of simulating fringe cracks; this is accomplished by the application of uniaxial compression on cylinders that contain out-of-plane pre-cuts along their walls. Previous investigators combined independent operations of mode III to mode I, whereas in the present experiment, a single remote compression was transformed into mixed modes I and III by local stress rotation along the pre-cuts. An enlargement of inter pre-cut angle causes an increase of sample strength. (2) Contrary to previous experiments and many field exposures in sediments that exhibit transitions from parent fractures to fringes, the present study simulates a reverse transition, from fringes to parent fractures (to tensile longitudinal (axial) splits). Thus, a change occurs from local mixed modes I and III to local single mode I. The new results may be applied to the interpretation of secondary fractures and fringes in naturally fractured granites. (3) Monitoring of the electromagnetic radiation (EMR) that was induced by the fractured samples enabled us to determine in real time, the sequence of events and the fracture velocities along the various failure stages. Strings of high-frequency EMR pulses (with a frequency of several MHz) were measured during the fringe formation, indicating small widths of en échelon cracks, while the subsequent longitudinal splitting initiated lengthy EMR pulses of lower frequency (some tens of kHz) indicating much wider cracks (which indeed were measured).