Effect of filler shape, volume fraction and loading rate on dynamic fracture behavior of glass-filled epoxy

The effect of filler shape and filler volume fraction on the dynamic fracture behavior of particulate polymer composites (PPC) has been studied. Mode-I dynamic fracture experiments were carried out on pre-notched glass-filled epoxy. An experimental setup comprising of a gas-gun and a long-bar was used to deliver one-point impact loading to unconstrained specimens. Pulse shapers were utilized to control the loading rate during impact loading. The dynamic crack initiation and propagation events were captured using high-speed photography (∼300,000 frames per second). Digital Image Correlation (DIC) method was utilized to measure in-plane displacement fields around the crack-tip and extract fracture parameters including stress intensity factor histories to examine the filler shape, volume fraction and loading rate effects. The results showed a pronounced improvement in crack initiation toughness for rod-shaped fillers producing ∼145% increase over unfilled epoxy at 15% Vf with flakes and spherical fillers showing ∼97% and ∼67% improvement, respectively. For all three different volume fractions - 5%, 10%, and 15% - considered, the rod-shaped fillers produced the highest crack initiation toughness as well as post-initiation stress intensity factors followed by flakes and spheres, respectively. A linear relationship between crack initiation toughness and log of filler aspect ratio was also recorded. In addition, for 10% Vf rod-shaped filler case, the effect of loading rate on dynamic fracture behavior has been examined. The loading rate study showed ∼113% and ∼50% increase in crack initiation toughness for the lowest and the highest loading rate cases, respectively, compared to that of neat epoxy.