Effect of filler geometry on fracture mechanisms in glass particle filled epoxy composites

• A process is developed to uniformly embed slender fillers into a polymer matrix.
• Slender filler composites exhibit better stiffness and fracture characteristics.
• Spherical reinforcements are better in improving the flexural strength of a composite.
• Filler geometry as well as the process zone influences toughening mechanisms.
• Filler orientation induced mode-mixity improves composite fracture characteristics.

Quasi-static experiments are conducted on epoxy systems, embedded with milled-fibers and spherical glass particles, to study the effect of filler shape and volume fraction on the flexural and fracture characteristics of polymer composites. While the flexural modulus and fracture toughness of composites monotonically increase with filler volume fraction, the values are consistently higher for the milled-fiber case. On the contrary better flexural strength is achieved by reinforcing spherical particles. Besides reinforcement dependent toughening mechanisms the effect of process zone evolution in the form of filler/matrix debonding is evident from the fracture surface micrographs. Due to instantaneous fracture in spherical particle composites the process zone is restricted to a narrow strip in the vicinity of the initial crack front whereas the slow stepwise crack growth in slender filler case let the process zone evolve in pockets over the entire fracture surface. In general crack deflection process is dominant in spherical particle composites whereas fiber bridging in slender filler case is followed by fiber pull-out, fiber fracture and matrix cracking. Computational study of crack–inclusion interaction indicates that the crack-leg bridging has extensive influence on fracture parameters when compared to the crack-tip shielding. The fracture parameters get exponentially magnified with the crack-tip approaching the inclusion if spaced at less than a characteristic distance. The skewed orientation of slender fillers induces mode-mixity at microscopic scale which in turn improves macroscopic fracture behavior of composites.