Dynamic Mixed-Mode Crack Initiation and Growth in PMMA and Polycarbonate

Abstract

Mixed-mode dynamic crack initiation and growth in polymethymethacrylate (PMMA) and polycarbonate (PC) are studied experimentally. A simple specimen geometry and loading configuration is used to generate various mode-mixities during dynamic crack initiation and fracture. A Hopkinson pressure bar is used to rapidly load free-standing edge cracked samples in reverse impact configuration. Using eccentric loading relative to the crack line, different mode-mixities at crack initiation are achieved by increasing the initial crack length while keeping all other experimental parameters unchanged. A relatively new full-field optical technique, Digital Gradient Sensing (DGS), along with ultrahigh-speed photography is used to perform full-field measurements. DGS can measure instantaneous angular deflections of light rays due to elasto-optic effects and provides two orthogonal stress gradients under plane stress conditions. The mode-I and -II stress intensity factor histories of PMMA are evaluated via overdeterministic analysis of optically measured data. By quantifying critical stress intensity factors evaluated at crack initiation, dynamic fracture envelopes can be developed.