Fracture Toughness of High Density Polycarbonate Microcellular Foams

Abstract

Microcellular foams (MCFs) of polycarbonate (PC) with relative densities of 0.9 and 0.7 (MCF-0.9 and MCF-0.7) are produced by solid-state foaming. Microstructural characterization shows that they have bi-modal distribution of spherical cells, with median cell sizes of 3–4 mm and 6–9 mm for both cell populations. Tensile testing shows that ultimate tensile strength and Young’s modulus approximately ranked with relative density, although MCF-0.9 has a modulus similar to the unfoamed PC (uPC). Toughness measurements show that, when compared to uPC on a critical stress intensity factor basis, MCF-0.9 shows no reduction in toughness and MCF-0.7 shows a 35% reduction. When compared to uPC on strain energy basis, 12–15% increases in toughness are measured for both MCFs. Their fracture occurs by multiple initiation, growth, and coalescence of voids formed at cells acting as stress concentrators. A fine cell morphology results in prolonged growth and coalescence phases, and thus improves fracture toughness.