Experimental results have shown that polymer composites that have high fracture toughness tend to have high fatigue wear resistance. The work of fracture found in nacre (mother of pearl) is several orders of magnitude larger than the ceramic (aragonite) it is made of. The organic protein layers in the composite play a significant role in the mechanical response of nacre to stress. In this study, we hope to understand if an energy absorbing interphase similar to that found in nacre could have potential for toughening traditional, glass-particle-reinforced polymer composites. A multi-scale finite element model (FEM) has been developed to study the interaction between the crack and the reinforced particles. In this model, crack nucleation and propagation and the effect of particle/matrix/interphase material properties can all be characterized by the cohesive element and its traction-separation behavior. Loss of load carrying capacity begins when local deformation reaches a certain value, leading to the degradation of the material. Completely degraded elements form a traction-free crack surface. The most important advantage of this methodology for modeling fracture behavior is that macroscopic fracture criteria are not needed. 3-point bending macro-scale FEM serves to calibrate the deformation gradient of the study zone in front of the crack tip. A microscopic unit cell model was used to simulate the crack propagation. Three types of interphase were compared: (1) matrix and particle bonded without interphase, (2) matrix and particle bonded with silane interphase, and (3) matrix and particle bonded with beta-peptide (highly stretchable) interphase. Results show that the stress distribution around the filler and the bulk mechanical properties of the composite can be affected by changes in interfacial properties. Particle-reinforced polymer composites with a more compliant and stretchable interphase (e.g. beta-peptide) will help absorb local strain energy while remaining intact, allowing less damage within the matrix. This type of interphase decreases crack propagation speed and results in an increase of fracture toughness.

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