Mechanical Behavior of Highly-Flexible Elastomeric Composites with Knitted-Fabric Reinforcement

This paper examines critical issues associated with the fabrication and forming of highly-flexible polymeric composites, reinforced with knitted-fabric structures. Knitted-fabric reinforcements have not generally been preferred over more traditional woven reinforcements in high-performance composites, mainly because of their lower stiffness/strength performance when embedded in a rigid, thermosetting matrix material. However, with their unique formability, knitted fabrics promise great potential in applications where large deformation of the structure is desirable; such as energy/impact absorption and forming applications. One very attractive feature of knitted composite materials, is the large displacements that the underlying knitted fabric can potentially undergo before exhibiting a significant increase in stiffness. The unusual extensional behavior of knit fabric is attributed to the fact that the fibers are more-or-less free to slide over each other before the yarns become highly oriented, eventually “locking” in a packed formation. When the loops become highly elongated, the knit fabric achieves its maximum resistance to in-plane deformation, and exhibits a stiffness closely related to the elastic stiffness of the straightened fiber/yarn bundles. The unique formability of knitted fabrics is mainly due to this yarn movement. The highly “stretchable” behavior of knitted textile reinforcement materials can be used to great advantage in thermoforming composite structures. In order to fully utilize the exceptional stretch properties of the knitted-fabric, the matrix material should be able to deform at least as much as the fabric, and the knitted yarn movements need to be restricted by the matrix as little as possible. In this study, a multi-level finite element procedure was developed to analyze and control the deformation characteristics of plain weft knit reinforced composites. A database of mechanical properties for various knit geometries was obtained. Using these results, it is shown that carefully “tailored” knit fabric reinforcement can be used to improve mechanical performance and facilitate polymer forming processes, such as thermoforming. In this study, elastomeric materials such as polyurea and thermoplastic elastomer (TPE) were used to fabricate composites with knitted-fabric. Two different types of arrangements were experimentally studied: knitted fabric embedded in the elastomer and a sandwich of knitted fabric between elastomeric skins. It is shown that by fully utilizing the high stretchability of the knitted fabric reinforcements, attractive material properties can be obtained especially for energy/impact absorption and forming applications. The improvement of thermoforming process stability with the use of carefully tailored knitted fabric reinforcements is also presented.