Micromechanics of toughening in ductile/brittle polymeric microlaminates: Effect of volume fraction

Ductile/brittle microlaminates, comprised of alternating layers of ductile polymer [e.g., polycarbonate (PC)] that inelastically deform by shear yielding, and brittle polymer [e.g., styrene-acrylonitrile (SAN), polymethylmethacrylate (PMMA)] that undergo crazing in tension, offer a strategy for tensile toughening by capitalizing on the synergistic interactions between crazing and shear yielding. This work presents a micromechanical model for PC/PMMA ductile/brittle laminates that captures the constituent volume fraction dependence of the macroscopic behavior, as well as the underlying micro-mechanisms of deformation and tensile failure, in particular the synergy between crazing and shear yielding. The finite element implementation of the model considers both two-dimensional and three-dimensional representative volume elements (RVEs), and incorporates continuum-based physics-inspired descriptions of shear yielding and crazing, along with failure criteria for the ductile (PC) and brittle (PMMA) layers. The interface between the ductile and brittle layers is assumed to be perfectly bonded. The 3D RVE models successfully capture the macroscopic tensile stress–strain behavior of the ductile/brittle laminates at varying constituent volume fractions and the corresponding underlying micromechanisms of deformation and failure. The transition from brittle to ductile laminate behavior is in agreement with experimental data and is found to occur as the fraction of the ductile layer is increased to 0.60. The simulations reveal the brittle to ductile cross-over to be governed by the ability of the ductile layers to constrain the dilation and growth of crazes in the brittle layer. In particular, after the critical volume fraction of the ductile constituent is attained, surface edge crazes are inhibited from tunneling across the width of the specimen which then leads to reduced crazing/cracking and increased shear yielding. 2D RVE models are unable to adequately account for the deformation and toughness of PC/PMMA microlaminates as a function of the constituent volume fractions, as they neglect any distribution of inelastic events in the laminate-width (out-of-plane) direction.