Tailoring the fracture response of two-phase network reinforced composites through irregularity

The mechanical behavior of composite materials is significantly influenced by their structure and constituent materials. One emerging class of composite materials is irregular network reinforced composites (NRC’s), whose reinforcing phase is generated by a stochastic algorithm. Although design of the reinforcing phase network offers tailorable control over both the global mechanical properties, like stiffness and strength, and the local properties, like fracture nucleation and propagation, the fracture properties of irregular NRC’s have not yet been fully characterized. This is because both the irregular reinforcing structure and choice of matrix phase material significantly affect the fracture response, often resulting in diffuse damage, associated with multiple crack nucleation locations. Here, we propose irregular polymer NRC’s whose matrix phase has a similar stiffness but half the strength of the reinforcing phase, which allows the structure of the reinforcing phase to control the fracture response, while still forming and maintaining a primary crack. Across a range of network coordination numbers, we obtain J-integral and R-curve measurements, and we determine that low coordination polymer NRC’s primarily dissipate fracture energy through plastic zone formation, while high coordination polymer NRC’s primarily dissipate energy through crack extension. Finally, we determine that there are two critical length scales to characterize and tailor the fracture response of the composites across the coordination numbers: (i) the size of the plastic zone, and (ii) the size and geometry of the structural features, defined as the areas enclosed by the reinforcing network.