OBSERVATION OF MIXED-MODE FRACTURE WITH CENTER NOTCHED DISK SPECIMENS

Center notched disk (CND) specimens were loaded diametrically under Br azilian test configuration. By changing the notch inclination angle wi th respect to the loading direction the mode of fracture was varied fr om mode I (tensile) to mixed mode (tension-shear and compression-shear ). The tests were conducted at two rates of loading. First, a quasi-st atic rate of loading was used under closed loop testing conditions. Hi gh rate of loading was applied using an instrumented Charpy machine. T he selected impact rate of loading did not modify the failure mode. Fa ilure was still the result of propagation of a single crack. At quasi- static rate, both laser holographic interferometry and reflection phot oelasticity with conventional TV imaging were used to observe mixed mo de fracture in the center notched disk specimens. Reflection photoelas ticity and high speed photography were used to monitor the crack growt h at impact rate. Laser holographic interferometry was also used to ca librate reflection photoelasticity recordings on similar specimens at comparable rates of loading. For all notch inclination angles, the pea k load increased by a constant factor relative to the peak loads at qu asi-static rate of loading. However, the increase of apparent stress i ntensity factor at crack initiation for tensile cracking is almost dou ble that corresponding to shear cracking.