Feasibility Study of Design and Manufacturing of Composite Coil Springs

This thesis investigates the manufacture of a number of novel Carbon-Fibre Reinforced Plastics (CFRP) composite coil spring prototypes, and it is aimed to study the feasibility of their use as helical spring within the automotive suspension system. As the optimum mechanical properties and maximum strength of carbon fibres are along their axial direction, the best configuration to lay up the reinforcement material in a helical geometry was found to be in the ±45 degree with respect to the helix axis. In addition, a comparison study was performed between the fabricated composite coil springs and their alternatives: steel coil springs and a composite leaf spring – manufactured in this work – counterparts. Furthermore, the numerical design of a coil spring was carried out using Creo Pro-Engineering CAD/CAM software. Out of the seven fabricated composite coil springs, two were made from prepreg woven carbon fibre fabrics, one from a braided dry carbon fibre sleeve, and the other four, from a prepreg unidirectional (UD) carbon fibre fabric. The manufacturing methods used for the production of these specimens included: autoclaving under pressure (20kN) and at an elevated temperature (140oC); impregnating the dry carbon fibre sleeve with resin epoxy and hardener agent and, curing the compound at room temperature for 24 hours; and using a wet-lay-up process and eventually curing the composite at 140oC for about one hour. Observation of the surface morphology of the fabricated composite springs revealed that the samples made out of UD-prepreg reinforcement materials were far more uniform in terms of the shape and surface finish, and resembled the most the shape of a helix spring; in addition to having advantages such as, allowing for a better control over the curing temperature, resin type, and being less time and labour consuming. Moreover, interestingly, the UD CF springs showed to be physically the most robust type compared to the woven and braided ones; yet, also exhibiting superior mechanical properties (compression and fatigue) close to those of the conventional steel springs, which were also test in this work. Furthermore, it was found that the mechanical strength and strain energy obtained from the UD CF springs were much higher than those of the braided springs. Furthermore, it was shown that for low-scale manufacturing purposes, the use of the hand lay-up manufacturing technique provides certain advantages such as freedom of changing the geometry or size (width and thickness) of the spring, control over the fibre orientation and type, and leading to a far less expensive manufacturing process compared to the automated fabrication methods, for example, such as using a CNC machine to wrap the reinforcement material around a helical mandrel. Nevertheless, some drawbacks were also associated with the use of this technique, including, inconsistency in the geometry and overall mechanical properties of the final springs, time consuming, and an inevitable introduction of physical defects such as bridges and wrinkles on the fibres during the hand lay-up process associated with the human error.