Initial fracture of the welt weft-knitted textile composites

Abstract

Abstract This paper investigates the initial fracture behaviour of the weft-knitted glass/epoxy composites for its 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 characteristic locations of the knit-structural unit cell. A statistical approach was applied to estimate knee points as the quantitative evaluation of the initial fracture. Knee points of the stress–strain curves were closely studied in order to characterize the first onset of the local failure. Post-failure examination was carried out on the test specimens using stereo-optical and scanning electron microscopy (SEM) to analyze the microscopic fracture behaviour. As expected, no significant changes were recorded in the stiffness and strength values with increasing welt-lengths and number of held-stitches, in either test directions. In general, crack propagated almost normal to the principal loading axes. According to 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 were the combination of transverse crack and debonding, inside and between the fibre bundles, respectively. These observations agreed well with the knee point responses obtained from the local stress–strain curves.