Ultrasonic testing of natural fibre polymer composites: effect of fibre content, humidity, stress on sound speed and comparison to glass fibre polymer composites

This work investigates the application of ultrasonic testing UT in evaluating natural fibre thermoplastic composites NFTC. The characterisation of sound propagation speed in the composite is intended to be a tool for evaluating the NFTC namely fibre content, fibre distribution and external factors' effects like relative humidity and stress. The quality of fibre distribution homogeneity can be assessed by mapping the returning signals of the emitted longitudinal ultrasonic wave. This study presents the measured sound speeds for a composite system of flax and polypropylene (PP). Flax fibre loadings rang from 0 to 60 wt%. Humidity factor is studied at 10, 50 and 95 % relative humidity RH levels. Different stress levels of 0, 10 and 20 MPa are applied on composite samples in an orthogonal direction to the longitudinal wave propagation direction. Using non-immersion: contact UT; an increasing linear trend of calculated longitudinal sound speed by 1 % is indicated per every 11 wt% increase of the applied flax fibres. This rate increased with more RH levels. More stress decreases the sound speed but in a trend parallel to the non-stressed samples. Finally, the distribution quality of the fibre is assessed using the ultrasonic technique. Results are compared with the reference corresponding PP-glass fibre composite. The high E-modulus and the density of glass fibres are opposite factors. Thus, the sound speeds for the both cases of 30 wt% flax and glass fibres are close and not more than 0.6 % different.