Observation of mixed mode fracture with center notched disk specimens

Abstract

Center notched disk (CND) specimens were loaded diametrically under Brazilian test configuration. By changing the notch inclination angle with respect to the loading direction the mode of fracture was varied from mode I (tensile) to mixed mode (tension-shear and compression-shear). The tests were conducted at two rates of loading. First, a quasi-static rate of loading was used under closed loop testing conditions. High rate of loading was applied using an instrumented Charpy machine. The selected impact rate of loading did not modify the failure mode. Failure was still the result of propagation of a single crack. At quasi-static rate, both laser holographic interferometry and reflection photoelasticity with conventional TV imaging were used to observe mixed mode fracture in the center notched disk specimens. Reflection photoelasticity and high speed photography were used to monitor the crack growth at impact rate. Laser holographic interferometry was also used to calibrate reflection photoelasticity recordings on similar specimens at comparable rates of loading. For all notch inclination angles, the peak load increased by a constant factor relative to the peak loads at quasi-static rate of loading. However, the increase of apparent stress intensity factor at crack initiation for tensile cracking is almost double that corresponding to shear cracking.