Fracture Behavior of Particulate Polymer Composites (PPCs) and Interpenetrating Polymer Networks (IPNs): Study of Filler Size, Filler Stiffness and Loading Rate Effects

Abstract

Particulate Polymer Composites (PPCs) generally consist of nano- or micro-fillers of various sizes and shapes randomly dispersed in a polymer matrix. Introduction of second phase fillers of different stiffness and alteration of filler-matrix adhesion characteristics offer a cost effective way of tailoring strength, stiffness and fracture toughness of such composites. However, filler size, filler stiffness and loading rate (static or dynamic) could vary the mechanical response in general and fracture behavior in particular. Furthermore, failure of PPCs is intrinsically linked to the interactions of a matrix crack with foreign particles. Accordingly, an in-depth understanding of the fracture behavior of PPCs is essential for design and safety of the mechanical members made of these materials. The first part of this research investigates the role of nano- and micro-filler size, and loading rate effects on silica-filled epoxy PPCs. The digital image correlation method in conjunction with high-speed photography (250,000 – 300,000 frames/sec) is used to quantify crack-tip deformation histories during impact loading. The measured displacement fields are analyzed to extract stress intensity factor histories for dynamically propagating cracks. The quasi-static fracture tests show improved fracture toughness of ~170% for nanocomposites and ~55% for micro-particle filled ones at 10% volume fraction, relative to the neat epoxy. The dynamic crack initiation toughness, on the other hand, is consistently lower for nanocomposites than the micro-filler counterparts. The post-mortem analyses of fracture surfaces reveal higher surface ruggedness for nanocomposites under quasi-static loading. However, the opposite is evident for inertial loading cases. Next, the synergistic effects of amino-functionalized multi-walled carbon nanotubes and polyol diluent on fracture of two- and three-phase (hybrid) epoxy nanocomposites are investigated at quasi-static and impact loading conditions. Measurements show improved crack initiation toughness in modified-epoxies relative to the neat resin with the highest enhancement of ~90% in case of hybrid nanocomposites. Fractography reveals a combination of toughening mechanisms including shear yielding, crack deflection, CNT bridges and pull-outs. In the second part, to gain fundamental understanding of the counterintuitive loading rate effects on fracture responses of PPCs, experimental simulations of dynamic crack growth past inclusions of two different elastic moduli, stiff (glass) and compliant (polyurethane) relative to the matrix (epoxy), are carried out. The crack growth behavior as a function of inclusion-matrix interfacial adhesion strength and the inclusion eccentricity relative to the initial crack path is studied. The measurements show that the crack front is arrested by a symmetrically located compliant inclusion for half of the duration needed for complete specimen fracture. The dynamically growing crack is attracted and trapped by the weak inclusion-matrix interface in case of both stiff and compliant inclusions when located symmetrically, whereas it is (i) deflected by the stiff inclusion and (ii) attracted by the compliant inclusion, when located eccentrically and strongly bonded to the matrix. The compliant inclusion cases show higher fracture toughness than the stiff inclusion counterparts. Measured crack-tip mode-mixities correlate well with the observed crack attraction and repulsion mechanisms. Fracture surfaces reveal much higher surface roughness and ruggedness after crack-inclusion interaction for the compliant inclusion case than the stiff counterpart. Lastly, the fracture behavior of transparent Interpenetrating Polymer Networks (IPNs) with poly(methyl methacrylate) (PMMA) as the stiff phase and polyurethane (PU) as the ductile phase and varying PMMA:PU ratios in the range of 90:10 to 70:30 are studied. Quasi-static and dynamic fracture tests show that an optimum range of PMMA:PU ratio in the IPNs can produce enhanced fracture toughness (~60%) when compared to PMMA. All IPNs show higher impact energy absorption capability (a 3 to 4 fold increase) relative to PMMA. The impact damage features reveal shear-crazing and through-the-thickness cracking as the dominant failure modes responsible for greater energy absorption in IPNs.