Machinability of natural fibre reinforced polymeric composites

Machinability of natural fibre reinforced polymeric composites

From an industrial perspective, there are some issues with the machinability of synthetic fibre/polymer composites (glass fibres), including hole accuracy, delamination, appearance and energy consumption. These are mainly due to the abrasive nature of the synthetic fibres. Introducing natural fibres as reinforcement to the polymeric composite may overcome some of these issues, as natural fibres have less abrasive effects compared to synthetic fibres. Moreover, natural fibres are becoming an attractive candidate to replace synthetic fibres for several applications.

In the current study, epoxy composites based on date palm fibres were developed. The interfacial adhesion of the date palm fibres with the matrix is first evaluated for optimum fibre diameter and NaOH concentrations through a single fibre pull out test. With regards to machinability of the composite, drilling experiments were conducted on neat epoxy, glass/epoxy and date/epoxy composites. A new dynamometer was developed and fabricated locally. The influence of the cutting speed, feed rate and drill diameter on the machinability of the composites was evaluated in terms of hole accuracy, inner and outer delamination, specific cutting pressure and machining power. Scanning electron microscopy and optical microscopy were used to examine the damaged features in the experiments of interfacial adhesion and machinability. The ANOVA approach was used to identify the most effective parameters on the machinability of the composites.

The results revealed that NaOH concentration in the treatment solution affects the strength and the interfacial adhesion of the fibre with the matrix. Six per cent NaOH concentration is the optimum concentration in which there was less damage to the strength of the fibre and enhancement to the interfacial adhesion of the fibre with the matrix. Tensile properties of the epoxy were found to be improved with the addition of either natural or synthetic fibres. However, glass fibres improved the tensile strength of the epoxy significantly compared to the date palm fibre, even though glass fibres suffered from detachment and a pull out mechanism during loading conditions.

In the drilling process for all materials, there were three regions observed: inner, intermediate and outer. The thrust and torque behaviour with the drilling time was divided into these three regions. The peak values were observed in the intermediate regions in which the shears occurred at a higher level with the thrust force. The presence of glass fibres in the epoxy composites assisted in reducing crack propagation at the inlet regions; however, it highly deteriorated the outlet region, as detachments and a decomposition mechanism were observed. For neat epoxy, cracks and fractures were the main damage features noticed. Meanwhile, the presence of date palm fibre in the epoxy improved the machinability of the composites, as it required similar values of torque and thrust to that of neat epoxy, while glass fibres needed more power. Date palm fibres assisted in reducing cracks in the inner and outer regions of the hole of the composites, even though pull out and detachments were observed.

Hole accuracy is highly controlled by the operating parameters, as an increase in the drilling speed and feed rate resulted in high error percentage of hole accuracy for all materials. The addition of date palm fibres had a lower error percentage to the hole accuracy compared to the glass fibres, and this was mainly due to the abrasive nature of the glass fibres. Micrographs show fragmentations and pull out of fibres for neat epoxy and glass/epoxy composites, respectively. Meanwhile, date/epoxy exhibited fragmentation at the edge of the holes, which was considered less damage compared to the glass/epoxy. The specific cutting pressure reduced significantly with an increase in the feed rate. However, there was no clear effect from the drilling speeds on the specific cutting pressure. The most influential parameter was the drill diameter, as there was a large increase in the specific cutting pressure with an increase of the drill diameter. Conversely, the drilling process for all materials resulted in high roughness values for the inner surface of the holes. The highest roughness for the inner surface was recorded when the glass/epoxy was drilled. Nevertheless, the addition of the date palm fibre also contributed to an increase of surface roughness of the composites. However, there was less of an increase in the roughness of the inner surface with the addition of date palm fibres to the composite compared to glass fibres. Machining power was influenced by the higher operating parameter values; however, for the intermediate and low range operating parameter values, there was not much difference in the machining power of all materials. Interestingly, the machining power for the date/epoxy composite was competitive when compared to the neat epoxy, which is a promising result for the use of date palm fibre from a machinability perspective.