Due to the high load-bearing capacity and light weight, composite leaf spring with variable width and variable thickness has been increasingly used in the automobile industry to replace the conventional steel leaf spring with a heavy weight. The optimum structural design of composite leaf spring is particularly favorable for the weight reduction. In this study, an effective algorithm is developed for structural optimization of composite leaf spring. The mechanical performance of composite leaf spring with designed dimensions is characterized using a combined experimental and computational approach. Specifically, the composite leaf spring with variable width and variable thickness was prepared using the filament winding process, and the three-dimensional finite element (FE) model of the designed composite leaf spring is developed. The experimental sample and FE model of composite leaf spring are tested under the three-point bending method. From experimental and simulation results, it is shown that the bending stiffness of the designed leaf spring meets the design requirement in the automotive industry, while the results of stress calculation along all directions meet the requirements of material strength requirement. The developed algorithm contributes to the design method for optimizing the stiffness and strength performance of the composite leaf spring.

In order to improve the transverse mechanical performance of ordinary foam core sandwich, Z-pins of carbon fiber were inserted into the foam core forming a novel X-truss/foam sandwich construction. Mechanical behaviors of this new composite are mainly determined by its structure parameters, which offer many degrees of freedom to designers. Here, flatwise compression, shear, and bending tests were carried out to investigate the influence of structure parameters on mechanical behavior. These structure parameters include Z-pin inserted angle, volume fraction, and orientation of X-truss, etc. Based on the rule of mixture, equations have been derived to estimate the compression and shear properties, which conform to experimental results. With the increase of Z-pin inserted angle, compression performance is improved, while shear and bending performance decrease. The 45° angle offers the best properties of shear and bending behavior, whereas the 90° angle presents the best compression performance. In addition, using the theory of the Winkler foundation beam, a prediction equation of compression strength has been derived which indicates that larger diameter Z-pins can offer more efficient enhancement than smaller diameter ones in terms of compression behavior.

The load dependences of dynamic viscoelasticity of casting novolac epoxy 648 and its composites, reinforced by glass fiber (GFRP) or carbon fiber (CFRP), were investigated by dynamic mechanical thermal analysis under high loads. The fiber property and load pattern were considered to analyze the complicated variations in storage modulus and loss tangent with the change of loads. The load effects are comprehensively addressed by a phenomenological theory about suppression effect of static load on the motion of microstructure units, combined with the influence of shear stress and interface properties between fiber and resin.

Novel X-truss/foam sandwich structures were prepared by Z-pin inserted and bias stitching method, respectively, in order to improve the transverse mechanical behavior. With such reinforcements, the collapse resistance capability of the core and the bonding strength between the core and the skins are obviously increased. Thus the principal disadvantages of conventional sandwich structures are solved. The Z-pins and stitches embedding in rigid polyurethane foam configured a truss, which was made of carbon fiber/epoxy resin. The architectures of specimens made from Z-pin inserted and bias stitching methods are very similar but not identical, which make their mechanical performances different from each other. In order to evaluate these two methods, out-of-plane mechanical tests, such as compression, shear, and tension were practiced and failure modes were compared. In some conditions, the specimens made from the Z-pin inserted process have a higher compression modulus, but a lower tension and shear strength compared to that made from the bias stitching process. Moreover, there are big differences in their failure modes.