Experimental investigation of the alternate recurrence of quasi-static and dynamic crack propagation in PMMA.

The alternate (stick-slip) cracking phenomenon in Poly(methyl methacrylate) (PMMA) was investigated using high-speed imaging and digital image correlation (DIC). PMMA is known to show a great variety of fracture behaviors by even small changes in loading conditions. With TDCB-shaped samples and under a range of constant extension rates, the phenomenon of alternate cracking is observed. Here, loops of successive quasi-static and dynamic crack propagations are found within a single fracture experiment suggesting a `forbidden' velocity regime. For the first time, such material/structural cyclic fracture behavior is examined through the lens of linear elastic fracture mechanics (LEFM) by using in-situ High-Speed (HS) DIC. Energy release rates and crack velocities during fracture experiments are derived from full-field measurements using Williams' series expansion. Fracture surfaces of post-mortem samples have been systematically analyzed using optical microscopy. The investigation of the actual limits of the `forbidden' velocity regime in terms of critical velocity and energy release rate in relation to post-mortem crack length features is achieved by holistic experimental data on alternate cracking. This work provides key experimental data regarding the improved understanding of a unified theoretical framework of crack instabilities.