Mode i fracture toughness of CNT-reinforced PMMA

The influence of carbon nanotube (CNT) concentration on the fracture toughness of poly(methyl methacrylate) (PMMA) was examined on single-edge V-notched-beam (SEVNB) specimens. Six groups of SEVNB specimens containing 0.5, 1, 2, 4, 8.5 wt% of CNTs and neat PMMA as a reference were tested. First, a notch was introduced into the specimens by a specially made disk whose edge is V-shaped with a 30^°1/2 angle and a 30 μm tip width. As suggested by an American Society for Testing and Materials Standard for polymers, induction of a natural crack was attempted, without success. Therefore, fracture toughness values were determined with the 'sharp' machined notch by means of a calibration formula. These were compared to values obtained using a stress concentration factor and found to differ by less than 3%. The latter calculation takes into account the geometry of the notch. Results showed a decrease in the fracture toughness values with an increase in the CNT concentration. For specimens in which a natural crack was attempted, referred to as a razor-cut notch, a significant increase in the apparent fracture toughness was observed, as a result of the induced damage.