Effects of Pre-stretching on the Tensile Properties of Knitted Glass Fiber Fabric Reinforced Polypropylene Composite

Weft knitted glass fiber (GF) fabric reinforced polypropylene (PP) composite sheets were fabricated by hot pressing of stacked rib 1: 1 knitted fabric preform layers. The knitted preforms were made from GF/PP commingled yarn containing 50 vol% (≈73 wt%)GF. Before hot pressing, the preforms were stretched in the wale direction at various ratios. The tensile properties of the pre-stretched GF-PP composites were studied. It was found that there is a critical value of pre-stretch ratio as far as tensile stiffness and tensile strength are concerned. On increasing the pre-stretch ratio within the range of this critical value, both tensile stiffness and tensile strength increase. If the pre-stretch ratio goes beyond this critical value and continues to increase, both tensile stiffness and tensile strength display a tendency of slight decrease because of the break of more glass fibers. The failure strain of the composites decreases with the increase of the pre-stretch ratio because of the decrease of the stretchability of the knitted reinforcements.