Static and Dynamic Characterisation of Flax Fibre-reinforced Polypropylene Composites

Abstract

This work investigates the effects of fibre content and fibre orientation on the mechanical performances of flax fibre-reinforced polypropylene composites (FFPCs) along with fracture characteristics, particularly for unidirectional (UD) beam-like specimens. This work also investigates the effects of fibre content, fibre orientation and frequency on the dynamic behaviour of UD beam test specimens of FFPCs. Laminates of various fibre contents and orientations were manufactured by a vacuum bagging process, their static and dynamic properties are then obtained using various mechanical (tensile, flexural and impact (Charpy impact and drop weight impact)) and dynamic (dynamic mechanical analysis (DMA) and impact hammer technique (IHT)) analyses, respectively. The mechanical properties of the composites are strongly affected by fibre orientation with respect to the loading direction; for example, the tensile modulus decreases from 20 GPa to 3.45 GPa at an off-axis angle of 30° for a fibre volume fraction of 0.40. The largest mechanical properties (tensile, flexural and Charpy impact strength) are found in the case of 0° fibre orientation. For composites with fibre volume fractions in the range 0.31-0.50, tensile moduli are in the range 16-21 GPa and tensile strengths are in the range 125-173 MPa, while flexural moduli and strengths are in the ranges 12-15 GPa and 96-121 MPa, respectively, making them suitable for structural applications. The maximum impact strength of 52 kJ/m2 is observed for a fibre orientation of 0° and volume fraction of 0.50 in the case of in-plane impact load (i.e., Charpy impact), whereas the composites with 30° and 45° fibre orientations have the maximum energy absorption of about 17 J on average in the case of out-of-plane impact load (i.e., drop weight impact). The measured tensile moduli are compared with the predicted tensile moduli and a reasonable agreement is seen up to a fibre volume fraction of 0.40. The obtained results also suggest that the flax fibre composites are comparable to glass fibre composites especially in terms of specific stiffness. The dynamic characteristics were found from vibration measurements of beam test specimens using DMA and IHT to frequencies of 100 Hz and 1000 Hz, respectively. The frequency response of a sample was measured and the response at resonance was used to estimate the natural frequency and loss factor. The single-degree-of-freedom circle-fit method and the Newton’s divided differences formula were used to estimate the natural frequencies as well as the loss factors. The damping estimates were also investigated using a “carpet” plot. Experiments were subsequently conducted on a range of samples with different fibre volume fractions and orientations. The results show significant variations in natural frequencies and loss factors according to the variations in fibre orientation. Samples with 45°, 60° and 90° fibre orientations exhibit approximately the same natural frequencies. Composites with differing fibre orientations exhibit different loss factors for the various modes of vibration, and the maximum loss factor is obtained for the case of 45° fibre orientation, with the loss factor generally lying in the range of 2-7%. It was found that the loss factor increases with increasing frequency and decreases slightly with increasing fibre content. It was shown that the effect of twisting on damping is more significant (153% higher) than the bending. Numerical estimates of the response, and in particular the natural frequencies, were made using a Mechanical APDL (ANSYS parametric design language) finite element model, with the beam being discretised into a number of shell elements. If the fibre angle is 0° or 90° with respect to the beam axis, then, for an impact on the centreline, the motions are predominantly bending. For other fibre orientations, the impact induces both bending and torsion. The outcomes from this study indicate that flax fibre-reinforced composite could be a commercially viable material for applications in which noise and vibration are significant issues and where a significant amount of damping is required with a combination of high impact energy absorption and stiffness. The thesis concludes with suggestions for further research in the future, based on the findings of this study.