Nanoscale and effective mechanical behavior and fracture of silica nanocomposites

An experimental investigation of the composite and local mechanical and fracture behavior of an EPON based epoxy with 12 nm (primary) and 100 nm (secondary) fumed silica particles was carried out. The secondary particles promoted matrix stiffening at small weight fractions, which decreased at larger weight fractions and converged to that of the primary nanoparticles. The elastic modulus of the composites with 12-nm silica particles increased by as much as 30% for 15 wt.% silica loading. Atomic Force Microscopy coupled with Digital Image correlation (AFM/DIC) full-field strain measurements showed matrix strain localization in the vicinity of 100-nm fillers, which controlled the overall composite stiffness. In composites with 5 wt.% secondary particles, neighboring particles located at small proximities to each other behaved as single large particles or resulted in local matrix strain shielding. The tensile strength of all composites was independent of particle size and weight fraction, which was attributed to strong particle bonding and failure initiation in the matrix. The critical mode I stress intensity factor of 12-nm silica composites increased with silica weight fraction, by as much as 35% for 15 wt.% silica. SEM fractographs showed enhanced matrix yielding for 15 wt.% silica compared to significant roughening of the fracture plane and void formation at smaller weight fractions.