An Experimental Study of Compression Behavior of Warp-knitted Spacer Fabric

This paper presents an experimental study of the compression behavior of a typical warp-knitted spacer fabric which is specially developed as a cushioning material for human body protection. The fabric has a highly heterogeneous and discontinuous structure and exhibits a complicated compression behavior. Different test boundary conditions and sample sizes were first adopted to conduct compression tests of the fabric. Then, the compression behavior of the fabric was analyzed based on its compression load-displacement curve obtained under a selected test condition. The potential compression mechanism of the fabric was identified with support of the image analysis of the fabric structure at different compression stages. The study provides some useful information for designing warp-knitted spacer fabrics for impact protection. INTRODUCTION Warp-knitted spacer fabric is a novel kind of threedimensional textile structure consisting of two separate outer fabric layers which are joined together but kept apart by spacer yarns [1]. Recently, this kind of fabric has been proposed as cushioning material for human body protection due to the good compressibility, high moisture conductivity and excellent thermoregulation capability [2, 3]. The compression property is one of the most important of conventional fabric’s basic mechanical properties, and is closely related to fabric handle [4]. Different from conventional fabrics, the compression behavior is vital to the cushioning performance of a warpknitted spacer fabric. The previous studies showed that the overall compression load-displacement relationship of a warp-knitted spacer fabric could be engineered to have three main stages, i.e., linear elasticity, plateau and densification [2, 5-7]. This is a typical behavior required by a cushioning material to dissipate the kinetic energy of the impacting mass while keeping the maximum load below some limit [8]. However, the effects of test conditions on the results were not considered in those studies, and a suitable method for simultaneously observing the deformation of spacer yarns within the fabric during the overall compression process is still lacking [2, 5, 6]. As a consequence, no well accepted interpretation is provided for a reasonable understanding of the deformation mechanism and structure-property relationships of warp-knitted spacer fabrics in compression. As a warp-knitted spacer fabric has a highly heterogeneous and discontinuous structure, its compression behavior can be affected by different test boundary conditions and sample sizes. There is no recognized test standard specially developed for testing the compression behavior of warp-knitted spacer fabrics. A literature review shows that different test methods were adopted by different researchers in their investigations. For instance, Ye et al. conducted the compression tests of spacer fabrics for car seats by using a contact area of 10 cm at a speed of 10 mm/min without indication of sample sizes [6]. Mecit and Roye tested the compression properties of spacer fabrics for concrete applications by using three different contact areas (10, 20 and 25 cm), and three different sample sizes in both circular and square shapes (100, 300 and 450 cm) with and without stabilization of samples at two different compression speeds (10 and 100 mm/min) [9]. Their study demonstrated that the compression speed, sample size and contact area did not have significant influences on the test results. However, this conclusion is only valid for the spacer fabrics for concrete applications. In such a kind of fabric, the outer layers are grid nets, and the contacts between spacer yarns and outer layers as well as the contacts among spacer yarns are not evident due to the low areal density of outer layers. In addition, this kind of spacer fabric also has a symmetric structure in both the through-thickness and in-plane directions. Therefore, evident effects of test boundary condition and sample size cannot be observed. However, the warp-knitted spacer fabrics designed for cushioning