O.A. Khondker

Effects of Fabrication and Processing Techniques of Aramid/Nylon Weft-Knitted Thermoplastic Composites on Tensile Behaviour

Preliminary investigations on the fabrication, processing and tensile behaviour of Aramid/Nylon weft-knitted thermoplastic composites were reported in this paper. An intermediate material called “micro-braided yarn” (MBY) was used to fabricate thermoplastic composites with textile reinforcements. Using a tubular braiding machine Aramid/Nylon66 (AF/PA66) micro-braided (MB) yams were produced. These MB yarns were subsequently used to produce weft-knitted fabrics having 1×1 rib architecture. Three types of knitted composites were fabricated by combining micro-braiding and compression molding techniques and mechanical tests were conducted to study their tensile behaviour. Tensile properties were also compared with those of Aramid/Epoxy and Aramid/Nylon film-stacked knitted composites. Cross-sectional observations on the selected AF/PA66 MB knitted specimens, by optical microscopy, have confirmed that molding condition at 290°C under the pressure of 2 MPa for 20 minutes could achieve better matrix fusion and improved state of resin impregnation. The overall changes in the mechanical properties were found to be broadly related to several factors such as, continuity in the reinforcing fibres within the preform structure, molding techniques, process-parameters, state of pre-tensioning prior to consolidation and to the fibre/matrix interfacial adhesion.

Initial Fracture Behaviour of the Weft-Knitted Textile Composites Having Welt-Knit Architectures

Initial fracture behaviour of the weft-knitted glass/epoxy composites was investigated to study their dependence upon the welt-knit architectures with varying structural parameters, such as the length of the non-loop welt-stitches and the lineal density of held-stitches within the welt-length. Strain gages were bonded onto the test specimens at different locations of the structural unit-cell. Knee points of the stress-strain curves were closely studied in order to characterize the first onset of local failure. Post-failure examination was carried out on the test specimens using stereo-optical and scanning electron microscopy to analyze their microscopic fracture behaviour. No significant changes were recorded in the stiffuess and strength values with increasing welt-lengths and number of held stitches, in either test directions. In general, crack propagated almost normal to the direction of the two principal loading axes. According to the SEM micrographs, fracture plane for the wale-tested specimens coincided along the length of the welt-stitches and continued through the fibre crossover regions of the same course, whereas for the course-tested specimens, fracture plane ran along the fibre crossover regions transverse and adjacent to the length of the welt-stitches. Initial fracture modes in either test directions combined transverse and debonding cracks, inside and between the fibre bundles, respectively. These observations agreed well with the knee point responses obtained from the local stress-strain curves.