On the influence of the elastic characteristics of composite materials on the vibrating properties

Economical composite materials such as PA66GF35 (glass-fibre-reinforced polyamide matrix) are being increasingly used in the automotive industry. Their good mechanical characteristics combined with low density, very high workability, low production cost and high availability are attractive prerogatives that induce engineers to adopt it in complex technological challenges. Injection moulding is the most common production technology used to realize composite components. While in industrial design this type of material is considered as isotropic, it is well known that injection moulding process gives orientation to the reinforcing fibres, leading to anisotropic mechanical behaviour. Starting from these considerations, attention has been turned to the vibrating properties of such materials and to the comparison between vibration modes and mechanical properties. In fact, composite materials are also used to produce components in the automotive field significantly affected by noise problems. Since the noise derives from a fluid-structure interaction, the own frequencies and the vibrating modes cover an important role on the Noise, Vibration and Harshness performance of the components. A comparison of the vibration modes of a plate in PA66GF35, numerically modeled both as isotropic and anisotropic material according to Folgar and Tucker theory, was carried out and compared with experimental measurements. The anisotropy of the composite material is demonstrated by the variation of the mechanical characteristics obtained from the static tensile tests. Results show that injection moulding confers different mechanical properties to real components due to the intrinsic fluid-dynamic phenomena of the production process.