Microscopic observation of failure in polymer-matrix composites under reflected and transmitted light

Abstract

Many studies on micromechanisms of failure in fibre-reinforced polymers have been performed to understand and thereafter to improve their mechanical performance. Several researchers [1, 2] used in situ scanning electron microscopy (SEM) to examine fracture morphology on the surface, which is not always the same as that in the interior [3]. Transmission electron microscopy (TEM) then seemed to be a promising tool for observation of the interior morphology [4]; in this case thinner specimens generally obtained by microtomy should be free from artefacts such as residual strains and damage, particularly at the fibre-matrix interface. A petrographic thin sectioning technique employing lapping and polishing of both sides of a sample was recently utilized in conjunction with polarizing transmission optical microscopy (PTOM) [5, 6]. Although the minimum size of an object visible (resolution) for PTOM is about 0.3/xm, the specimen preparation seems to be much easier than that for TEM. To make better use of the sectioning technique for investigation of failure mechanisms in composite materials, attempts should be made to combine PTOM with other microscopy methods. In this study we adopted a combination of polarizing reflection optical microscopy (PROM) and PTOM for fractographic analysis. Three kinds of injection-moulded, dumbbell-type, short glass fibre (SGF) reinforced thermoplastic composites were used for the study: a poly(ethylene terephthalate) (PET)-based, a poly(ether ether ketone) (PEEK)-based and a poly(cyanoaryl ether) (PCAE)-based composite. The embedded SGFs were about 10/xm in diameter and were treated with an aminosilane coupling agent and a thermoset sizing agent to provide good adhesion with the thermoplastic matrices. The fibre weight fractions for SGF-PET, S G F P E E K and S G F P C A E were 30, 10 and 10%, respectively. The average fibre lengths were approximately 160/xm. The fibres were considerably oriented in the mould-fill direction, i.e. the direction of the tensile test. The PET matrix was in an almost amorphous state. The respective crystallinity of PEEK and PCAE was about 28 and 30% and was obtained from differential scanning calorimetry (DSC) analysis. The size of spherulites was <1 /xm in diameter and they were slightly visible with PTOM. The gauge portion of the specimens was 60 mm × 12.5 mm with a thickness of 3 mm and was initially single-edge notched using a diamond