Properties of natural fibre composites for structural engineering applications
Introduction
Advanced composites, owing to their high specific strength –to-weight and specific-stiffness-to-weight ratios, are very popular for different types of engineering applications. High strength fibres commonly used in aircraft and aerospace engineering industries, are carbon, glass and Kevlar. These fibres provide strong tensile strength to support tensile and bending stiffness of composite structures. Polymer-based matrix act as a cushioning material to protect these high strength and brittle fibres against an impact. Besides, matrix is used to absorb vibration energy to keep rigid structures safe under seismic attacks. Despite the fact that advanced composites bring advantages to many engineering applications, there are three major issues commonly criticized by the public:
- i)
Advanced composites are hardly to be recycled, which may cause serious environmental problems after disposal;
- ii)
the structures made of advanced composites may be over-strength in particular using carbon fibre reinforced polymer (CFRP) composites and
- iii)
there is relatively high materials cost of advanced composites for domestic products.
Therefore, fibres extracted from the nature have emerged in the past decades aiming at replacing traditional high strength synthetic fibres to form a new class of natural fibre reinforced polymer (NFRP) composite. The growing awareness of environmental concerns is also another element to force the engineering sectors to develop new materials from natural resources that are either reusable or renewable. Regarding this, natural fibres and biodegradable polymers have become an attractive topic recently.
In the last few decades, many research groups in the world have developed promising materials, which are made of biodegradable fibres and polymers to form new types of bio-composites (some articles have stated as “green composites”), to replace conventional materials. In many countries, newly established policies have also encouraged business units to pay much attention to save energy and materials to sustain the life of our planets [1]. Incentive in tax reduction is also another way to motivate different commercial units to design and make products with “Green” components. The cost of natural fibres is relatively low as they are abundant and from renewable resources compared with other synthetic fibres. Therefore, such remarkable advantages of NFRP composites enhance their commercial and research potentials. Accordingly, natural fibres and biodegradable polymers have become emerging materials in the composite community.
Section snippets
Types of natural fibre
Natural fibres can be classified into two types: animal-based fibres and plant-based fibres. Cocoon silk, chicken feather, wool and spider silks are commonly used as animal-based fibres, which mainly target to biomedical applications such as implants. These bio-products are required to be either bio-resorbable (some articles name it as “bio-degradable”), which means the ability to break down and assimilate back into the body or biocompatible, to avoid being harmful to human body. Lau et al. [2]
Structure of natural fibres
Natural fibres have complicated structures in microscopic view. As showed in Fig. 1, by grinding a kenaf bark fibre, a core of lumen is wrapped by different layers of cell wall with different microfibril orientations, which give the strength to the fibre subject to different loads [3]. Similar to other layer systems, the microfibrillar angle governs the tensile strength of nature fibres. Ramesh [4] has provided a detail table to compare the properties of different plant-based natural fibres
Interfacial bonding properties of NFRP composites
The mechanical properties of NFRP composites are directly governed by their interfacial bonding properties between the natural fibre and its surrounding matrix. Many different types of micro mechanical tests such as single fibre fragmentation test, single fibre pull out test, the micro-bond test and the single fibre compression tests are commonly used to examine the interfacial bonding properties of fibre reinforced composites.
Beckermann and Pickering [6] have studied the bonding property of
Models for NFRP composites
Many attempts have been used to interpret the mechanical properties of NFRP composites in past few years. One of the commonly-used theories for laminate composite is the “Rule of Mixture”, in which the mechanical properties of a composite can be determined by purely summating the properties of fibre and matrix with considering their volume fractions.
Munde and Ingle [20] have summarized and compared different theoretical models, including Hirsch's model, Halpin-Tsai equation,
Flammability
Although using natural fibres could subsequently reduce the overall product cost due to the reduction in raw material and synthesis process costs, their flammability still restricts its applications for many structural components, particularly for those structures that are required to bear lives of human. Even for the traditional carbon fibre reinforced polymer composites, with the presence of carbon fibre, which is a high thermal conductive material would accelerate the ignition rate of a
Applications of NFRP composites
Using natural fibres in engineering applications is challenging due to extensive issues addressed in previous sections. High degree of moisture absorption (5–10%), flammability, consistency of raw materials and their properties, and bonding characteristics between the natural fibres and polymeric matrix are the major issues that resist the popularity of using NFRP composites in real life.
Locality, weather condition during the growing period, the part of plant that are harvested, and the
Conclusion
Natural fibres can be obtained in the nature, making them to be new materials for reinforced polymer composites for different engineering applications [43], [44], [45], [46], [47], [48]. These fibres are renewable, lightweight and low cost with moderate strength, which make them become promising materials beyond glass fibre for secondary structural components. However, as they are not synthetized through precise manufacturing process, the mechanical and material properties are the issues. The
Acknowledgement
This project is supported by a Research Grants from the Swinburne University of Technology and Asia University.
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