Influence of inlay yarn type and stacking sequence on mechanical performance of knitted uni-directional thermoplastic composite prepregs

The aim of this study is to develop and investigate mechanical properties of knitted unidirectional thermoplastic composite prepregs. Knitted prepregs were fabricated by using thermoplastic yarns (high density polyethylene and polypropylene) and high performance yarns (kevlar, basalt and carbon) in double jersey inlay structure. This is a new approach to combine the reinforcing fiber with resin forming thermoplastic fiber during the knitting operation. The structures were stacked further in three stacking sequences at different angles (0/0/0/0, 0/90/0/90, 0/90/90/0), and hot compression was used to convert them into composite prepregs by melting the thermoplastic component. Mechanical properties e.g. tensile strength and modulus, flexural strength, flexural modulus, impact energy absorbed etc. were investigated in detail. Full factorial experimental design was used in order to study the effect of main yarn, inlay yarn and direction of stacking/plying on mechanical properties of composites. Analysis of variance (ANOVA) was conducted by Minitab 17 software to estimate the significance of testing direction (T), type of inlay yarn (I), type of main yarn (M) and stacking sequence (S) on mechanical properties. Overall highest tensile and flexural strengths were observed for Carbon fiber based samples followed by Kevlar and Basalt respectively. Theoretical estimation of elastic modulus shows similar trend as the experimental results. The inter-laminar shear strength is maximum when the fiber orientation changes in each layer. Type of main yarn and inlay yarn have significant contribution on impact related properties. Depending upon type of material, these composites can be used in aerospace, automotive, civil and sports goods.