Effect of inclusion size on failure mechanism and mechanical properties of polymeric composites containing micro and nano particles

The effect of particle size on the mechanical properties of polymeric composites was investigated experimentally and numerically. It was found from experiments that particle sizes at micro scale have little influence on the Young's modulus of the composite and that Young's modulus increases as the size of particles decreases at nano scale. It was also observed that tensile strength of the composite is significantly dependent on particle size. At 1 vol.% loading, the tensile strength increased as the particle size decreased. However, the trend for the composite with alumina nanoparticles of 3% volume fraction was found to be opposite. TEM and SEM micrographs showed higher likelihood of poor dispersions in the composite with 3 vol.% nanoparticles than that with 1 vol.%. Finite element analyses showed that total strain energy release rate for particle/matrix debonding growth decreases as particle size decreases and that sliding fracture mode becomes dominant as the debonding grows. It was found that interfacial fracture toughness does not depend on particle size in micron scale but increases substantially when the sliding fracture mode prevails. It was analyzed with molecular dynamics simulations that the Young's modulus enhancement by decrease of nanoparticle size may be attributed to stiff polymer layers around nanoparticles. It was also found that the stiff polymer layers around nanoparticle are more effective on improving the elastic modulus with smaller nanoparticles and stronger polymer-nanoparticle interactions.