Carbon fiber composite multistrand helical springs with adjustable spring constant: design and mechanism studies

The helical spring is one of the fundamental mechanical elements in a wide range of industrial applications. In this study, a novel helical spring made of multistrand carbon fiber/epoxy composite were designed and fabricated by an integrated formation process to achieve flexible adjustment of the spring constant without changing geometric size. Five types of composite helical springs were fabricated by controlling different reinforcement twist and fiber volume content (Vf), forming three twistless composite multistrand helical springs (CMHSs) with Vf of 45%, 50%, and 55% and two multistrand twisted CMHSs with twist coefficient of 4 and 6r/10 cm and Vf of 55%. Quasi-static compression tests showed that the spring constant could be adjusted effectively by changing the Vf and reinforcement twist. By applying finite element analysis, the general correlation of the effects of Vf and reinforcement twist on the spring constant of the CMHSs were further analyzed and discussed. It was proved that the increase of reinforcement twist and Vf can both improve the spring constant, among which the effect of reinforcement twist dominated due to the stress homogenization of the twisting structure. Furthermore, a prediction of spring constant as a function of twist ranging from 0 to 40r/10 cm were conducted based on experimental and simulation results, which was of engineering significance for the application of composite helical springs.