Analysis of failure mechanisms in platelet-reinforced composites

The short-term mechanical strength of platelet-reinforced polymer composite s was modeled using classical two-dimensional stress-transfer analysis. The stress field in the platelet and at the platelet/matrix interface was desc ribed in the presence of a matrix crack perpendicular to the interface. Mod eling takes into account the tensile strength of the platelet, its adhesion to the matrix, and also considers the internal stress state resulting from processing. Platelet rupture and interface delamination were considered to be the two key failure mechanisms, depending on the ratio of platelet stre ngth to interface strength. The transition between the two failure events w as predicted to occur at a critical platelet length, the value of which dep ends on the elastic properties of the platelet and matrix, on the platelet geometry and strength, on the platelet/matrix adhesion, and on the internal stress state. The approach was applied to the case of low volume fraction silicon oxide platelets/poly(ethylene terephthalate) composites, where the size of the platelets was accurately controlled either below or above the p redicted critical length. Compression molded composites, with perfect align ment of the platelets, and injection molded composites, were prepared and t ested. The toughness of the compression molded composites was found to be a ccurately predicted by the strength model, with a 100% increase in the case of platelets smaller than the critical length compared to larger platelets . Injection molded composites with platelets larger than the critical lengt h were found to fail without yielding. By contrast, when the platelets were smaller than the critical length, the injection molded composites exhibite d excellent ductility. The general agreement obtained between the predicted and observed toughening transition shows the importance of filler size and stress state on the strength of platelet-reinforced composites. (C) 2001 K luwer Academic Publishers.