Spherical particles of size 7 microns to 200 microns are used to reinforce epoxy matrix at a constant volume fraction 10% and two different filler-matrix strengths, weak and strong. Optical interferometry in conjunction with high-speed photography has provided information regarding instantaneous crack tip positions and deformations when samples are subjected to impact loading. The crack velocity and stress intensity factor histories are extracted from the interferograms. Elastic characteristics remain unaffected by either the particle size or filler-matrix adhesion. Both weakly and strongly bonded particles in the matrix show higher values of fracture toughness relative to unfilled matrix material. Filler particle size affects fracture toughness significantly when the particles are used in the uncoated (or, weakly bonded) state. Additionally, a particle size of 35 microns is seen to enhance the fracture toughness most when compared to both smaller and larger size uncoated particles. Dynamic fracture toughness is linked to surface roughness parameters.
A linear relationship between fracture toughness and surface roughness is seen when particle size effects and filler-matrix adhesion effects are factored out. The overall surface roughness Ra does not correlate with macro-measurements and only a component of Ra, defined as "fracture induced roughness," Raf does. A model for calculating Raf based on volume fraction, particle size, inter-particle spacing and overall surface roughness is introduced. A linear relationship between steady state fracture toughness and the quantity Raf D-1/2 (D being the average particle diameter) has been found to exist.
Fractured surface micrographs from scanning electron microscopy indicate that crack tilting and twisting dominate when the filler is strongly bonded to the matrix, while crack front twisting and blunting occur with weakly bonded filler. Weaker filler-matrix interfaces also act as distributed attractors of a propagating crack resulting in greater surface roughness. Toughening mechanisms are investigated using numerical simulations. Interaction of a crack with perfectly bonded rigid isolated inclusions and clusters of inclusions in a brittle matrix is studied to investigate the role inclusions play in crack paths, stress intensity factors (SIFs), and energy release rates.
Effects of particle size and eccentricity relative to initial crack orientation are examined first as a precursor to the study of particle clusters. Simulations are accomplished using a new crack-growth prediction tool based on the symmetric-Galerkin boundary element method, a modified quarter-point crack-tip element, the displacement correlation technique for evaluating SIFs, and the maximum tangential stress criterion for crack-growth direction prediction. The numerical simulations demonstrate a complex interplay of crack-tip shielding and amplification mechanisms leading to significant toughening of the material. Interaction of a crack with weakly bonded rigid isolated inclusions and clusters of inclusions is currently under investigation.