Manufacture of cellular solids using natural fibre reinforced composites

Abstract

This thesis explains the manufacturing of recyclable, eco-friendly composites and their fabrication into hollow cores. The composites have been manufactured using compression moulding and extrusion techniques; each representing batch manufacturing and continuous manufacturing respectively. A statistical design of experiments based on Taguchi method has been used to study the multivariable system involved in the process of continuous extrusion. Factorial design of experiments (DoE) has been used to determine the best material formulation to obtain maximum mechanical properties. The composite sheets produced after the DoE were pelletised in a hammer mill and reprocessed by passing them through another cycle of extrusion. The effect of recycling on the mechanical properties, which were determined by performing static tests as per ASTM standards, has been investigated. The extruded composite sheets have been thermoformed into half-hexagonal and sinusoidal profiles using matched-die and roll forming processes. As the process involves bending and stretching the sheet to conform to the geometry of the mould, it is usually accompanied by large strains. These strains have been analysed using grid strain analysis, and the strain path taken during the forming operation has been determined using strain space diagrams. Due to the stretching and bending of the composite sheet during thermoforming process, a stress field is induced in the material, which upon extraction in that state, would result in either spring-forward or spring-back of the material causing dimensional instability, but by holding the part in that deformed state for a period of time will allow the stresses in the materials to relax. This time-stress information (stress relaxation behaviour) has been experimentally investigated and modelled using springs and dashpots arranged in series and parallel. The spring-back and spring-forward phenomena, occurring in the formed part upon de-moulding, have been investigated using single curvature vee-bending experiments. The profiled sheets obtained after forming have been assembled and bonded into honeycomb cores using adhesives and ultrasonic methods. These cores have been sandwiched between two wood veneer facings to form eco-friendly sandwich panels. The compressive and shear properties of these sandwich panels have been modelled and experimentally investigated. The compressive behaviour of the sisal-PP honeycomb cores has been modelled considering the honeycomb cell wall as a linear elastic specially orthotropic plate/lamina under plane stress and as a quasi-isotropic material. A finite element model of the sandwich panel has been developed in ANSYS classic finite element environment, to study the behaviour of the panel and the core, under flexural loading. Some non-structural properties such as, sound absorption, structural damping and energy absorption have been experimentally determined. The sound absorption ability of the honeycomb has been experimentally evaluated using a standing plane wave impedance tube. Three configurations; one with hollow cores, and the other two filled with polyurethane foam and wood fibres, respectively have been tried. The natural frequencies and structural damping have been experimentally determined by subjecting the sandwich beam to harmonic vibrations. The energy absorption characteristic has been experimentally determined by subjecting the honeycomb cores to quasi-static compressive loading.