Characterisation of flax polypropylene composites using ultrasonic longitudinal sound wave technique

The aim of this article is to study the feasibility of using the ultrasonic longitudinal sound wave in the definition of the fibre content and distribution in natural fibre composites. A flax polypropylene system is selected for this study due to its attractive properties and established market. Flax chopped fibres are mixed with polypropylene at different fibre loadings ranging from 0% to 50%. As a reference a 30 wt% glass fibre polypropylene composites is also manufactured. In order to avoid possible void obstacles to wave propagation and to simulate the common practice of natural fibre composites, fibres are alkalinised and a coupling agent is added. Rectangular plates of 80 x 220 x 4 mm size are injection moulded. Water immersion type of pulser/receiver ultrasonic equipment is used. The effect of fibre content is correlated with the sound speed of the longitudinal wave as well as the wave attenuation. The speed is calculated using the simple time of flight TOF method. The attenuation coefficient of the material is characterised by the aid of the A-scan. The longitudinal wave speed is found to increase by 1% every 7% increase in fibre weight content. The speed is relatively higher than the measured speeds for the same samples using the contact ultrasonic method. The attenuation coefficient is found to increase in a parabolic trend with the fibre content. The results are referenced with sound speed measurement for the same material of other contact ultrasonic method. Both C-scan pictures and attenuation maps of the investigated samples are used to evaluate the fibre distribution. Glass fibre is found to have more attenuation in comparison with similar natural fibre content in the composite. The effect of natural fibre content on the sound speed is modelled using the role of mixture concept. (c) 2012 Elsevier Ltd. All rights reserved.