Energy Absorption Properties Of Warp-knitted Spacer Fabrics Reinforced Polyurethane Composites

Warp-knitted spacer fabrics possess superior compression resistance, impact resistance and high productivity. Therefore, the composites reinforced by these fabrics exhibit promising mechanical performances. Due to the better interlaminar shear strength, the composite delamination can be fully avoided when subjected to impact loading. As for superior anti-impact properties, integrity and low-cost, the composites reinforced with warp-knitted spacer fabrics are expected to become a new kind of cushioning material. However, the researches on warp-knitted spacer fabric composites are limited compared to other textile-structure reinforced composites. Therefore, this thesis is important to investigate cushioning properties of polyurethane composites reinforced with warp-knitted spacer fabrics and would offer theoretical guidance and experimental supports for designing cushioning properties of the composites.To better understand the effect of structure and parameter on the cushioning behaviors of warp-knitted spacer fabric polyurethane composites, eight warp-knitted spacer fabrics with different structures and parameters were designed and produced, including three lapping movements of spacer-guide-bar, four surface structures of spacer fabrics, two thicknesses of fabrics and two diameters of spacer yarns. The composite samples from eight warp-knitted spacer fabrics and three polyurethane materials were fabricated by hand lay-up method.Due to the low molecular groups existed in polyurethane matrix and the defects of manufacturing technology, the sample density may be uneven after curing of composites, which can reduce the mechanical properties of composites, including the anti-compression, shear and impact resistance behaviors. Thus, the composite uniformity plays an important role in the mechanical properties of composites. A compact X-RAY density profile analyzer (IMAL DPX300-LTE, Italy) was used to evaluate the density of composites. It can be seen that the density values in surface layers of composites are higher than that in spacer area. This phenomenon can be explained with the fact that the density values in the outer layers of spacer fabrics are higher as compared to that in spacer area, resulting in higher density values in the surface layers of composites. However, it is obviously observed that the density values in the spacer area of composites are nearly constant along thickness direction, indicating that the density distribution of composites was uniform in the spacer area of composites. Additionally, the density values in top and bottom layers of composites are also very close to each other. All the results significantly demonstrated that the density is fairly well-distributed in the finished composites, which can meet the end-use requirements.The forming composites have been characterized for compression properties by using the Ke-xin WDW3020universal material test instrument to investigate the compression resistance and energy absorption properties of composites. Furthermore, the influence of structure and parameter on the anti-compression and energy absorption behaviors of composites has been thoroughly investigated as well. The results show that the structures and parameters have strong influence on the compression performances of composites, the composites with larger underlapping of spacer yarns, hexagonal mesh in surface layers, finer spacer yarn, lower opening-agent content in polyurethane foam are suitable for absorbing energy at a lower stress level with higher efficiency. In contrast, the composites with smaller overlapping of spacer yarns, rhombus mesh in surface layers, coarser spacer yarn, higher opening-agent contained in polyurethane foam can be used to absorb energy at a higher stress level. The effect of sample thickness on the energy absorption properties varies with the changes of stress level. All the findings obtained indicate that the energy absorption properties of composites can be adjusted to meet the end-use applications by varying the structures and parameters of composites.Based on the Winkle elastic foundation theory, a compression analysis model of composites was established according to the arrangement of spacer yarns. The compression stress-strain curves obtained from the theoretical model were compared to the experimental results. The results show that the theoretical model is in good agreement with the experimental data, indicating that the predicted model can successfully simulate the compression behaviors of composites and offer the references for designing and producing the warp-knitted spacer fabric polyurethane composites for cushioning applications.As cushioning material, the impact resistance and energy absorption behaviors of composites are important. Taking GB/T8171-2008as the reference, standard plane striker, was designed and installed in the drop striker impact tester (Instron Dynatup9250HV) in order to simulate the impacts between striker and cushioning materials. The influences of structures and parameters on impact properties of composites at different impact energies were investigated. Moreover, damage performance and residual compressive strength of composites were also studied to describe the damage after impact. The results show that different composites exhibit vary peak impact-force and energy absorption properties. The composites made with larger underlapping of spacer yarns, rhombus mesh in surface layers, larger fabric thickness, coarser spacer yarn, higher opening-agent content in polyurethane foam display lower peak impact-force, thus can absorb more impact energy during the impact process. However, it is stressed that the energy absorption properties will not rise with increasing of underlapping number of spacer yarns all the time. The composites with over-larger underlapping of spacer yarns are easy to enter the dense area when suffered impact load, resulting in the decrease of energy absorption. The energy absorption capacities increase as the size of mesh in outer layers increases. However, the larger size of mesh in surface layers, the more spacer yarns exposure, which makes the spacer yarns damage more easily, leading to a decrease of impact resistance capacities of composites.The compression-after-impact curves of composites exhibit the same configuration as the compression curve without impact, except the peak force value. The sequence of residual compression strength of composites is the same as that of composites before impact, indicating that the composites with higher compression strength also keep higher residual compression strength.Based on the compression and impact tests, the finite element method and ANSYS software were involved to analyze the compression and impact performances of warp-knitted spacer fabric composites. It is assumed that the composites are homogeneous and continuous on macrostructure, a representative volume element having a spacer yarn cycle and polyurethane was taken to set up analysis model. The spacer yarn model was established firstly according to their actual arrangements, and then the polyurethane was modeled. And the final analysis model can be obtained by gluing these two parts together. The spacer yarn model consisted of several pieces of three dimensional geometrical curves. To avoid that the nodes inside the spacer yarns was not coinciding, the two neighbor spacer yarns could not be intersected. Besides the modeling, the definition of material characteristics is also very important. In this study, the non-liner polyurethane was defined as the isotropic materials according to the principle of elastic-plastic mechanics, while the reinforced warp-knitted spacer fabric was divided into two parts: surface of fabric and spacer yarn were defined as orthogonal anisotropic material and isotropic one respectively. The Von Mises contour nephogram and stress-strain (force-displacement) curves were obtained from the finite element analysis. According to the Von Mises contour nephogram, it can be found that the warp-knitted spacer fabrics in the composites bore the main compressive loads. When the composites suffered impact loads, the peak loads were found in the contact area between the striker and composites, and the loads spread to the edge of composites. The predicted compression stress-strain curves and impact force-displacement curves by the finite element analysis model were in reasonable agreement with experimental results.