Abstract
A novel impact bend test procedure is described for determining the dynamic fracture-initiation toughness, KId, at a loading rate KI of the order of 106MPa√m/s. The split Hopkinson pressure bar technique was used to measure dynamic loads applied to a fatigue-precracked bend specimen. The dynamic stress intensity factor history for the bend specimen was evaluated by means of a dynamic finite element code. The onset of crack initiation was detected using a strain gage near a crack tip. The KId value was determined from the critical dynamic stress intensity factor at crack initiation. A series of dynamic fracture tests were performed for a Ti-6246 alloy and a 7075-T6 aluminum alloy. The KId values obtained for both the alloys were compared with the corresponding values obtained under quasi-static loading conditions. The fracture surface morphology was examined with the help of a scanning electron microscope to rationalize the apparent differences in the measured fracture-initiation toughness in response to the change in loading rate. Furthermore, the validity of the KId determination was confirmed by measurements of the crack tip plastic zone size in the interior of the fracture specimens based on the microhardness tests.
