An investigation of the influence of anisotropy and frequency on damping in short glass fibre reinforced polypropylene

This paper describes the measurement of the logarithmic decrement and the dynamic moduli of injection moulded short glass fibre reinforced polypropylene. Beam specimens were cut at three angles (0°, 45°, 90°) from injection moulded plaques containing 0%, 8% and 19% by volume of glass fibres. The logarithmic decrement was measured from the decaying free transverse vibrations of cantilever beam specimens in the frequency range 60 Hz to 250 Hz. The tests were conducted at room temperature in a near vacuum to reduce the effect of air damping. The dynamic moduli of the fibre reinforced specimens have been compared with upper and lower bounds derived by Gibson and Yau. The loss modulus for the 0° specimens exceeds the upper bound for both fibre volume fractions. Increasing the fibre volume fraction reduced the logarithmic decrement but increased the storage and loss modulus. For unfilled polypropylene both the logarithmic decrement and loss modulus showed a greater dependence on the specimen angle than the storage modulus. The degree of anisotropy of the logarithmic decrement and the dynamic moduli increased with increasing fibre volume fraction. The glass reinforced specimens show a maximum logarithmic decrement at a specimen angle of 45°. The storage modulus was in reasonable agreement with the static modulus.