negative thermal expansion coefficient, which may result in matrix microcracking during processing or environmental exposure due to the relative expansion difference between the fibres and the matrix . Figure 5: Typical stress-strain curves for different types of fibres  Aramid fibres represent the middle course between glass and carbon fibres in terms of strength, stiffness and costs. They are regularly used as stay cables or ballistic protection due to their extraordinary wear resistance. The main benefit of aramid fibres is their ability to undergo plastic deformation in compression, and exhibit ductile fracture in tension through the fibrillation of fibres in contrast with the sudden rupture of other types of fibres . Besides, aramid fibres are less notch sensitive, very lightweight, have high tensile strength and modulus, and exceptional toughness and impact resistance. Moreover, aramid fibres are also very durable as moisture at ambient temperature has negligible influence on tensile strength and also exhibit excellent thermal and dimensional stability. On the other hand, no acceptable surface treatment has been developed to (b) (a) (c) CIE5128 - FRP Structures Reading 1.1 - Fibres, resins and cores M. Pavlovic, F. Csillag September 2018 © All Rights Reserved3
aramid fibres - which are known to be vulnerable to UV radiation - for this reason they are often combined with other fibres. Another drawback is the lack of adhesion to the matrix, which results in a poor transverse tension, longitudinal compression and interlaminar shear strengths. Aramid fibres are also sensitive to short-term creep, even though they perform better in long-term creep than glass fibres, thus stress rupture failure mode is less dominant . Polyester fibresare low density, high tenacity fibre with very high impact resistance, excellent chemical and abrasion resistance but low modulus. Polyester fibres are mostly used in surface tissue for laminates or as fibre mats and surfacing veil in resin transfer moulding fabrication processes of FRP structures. Natural fibres such as jute and sisal are readily available, cheap fibres, whereas their long-term environmental performance is unknown and their mechanical properties are usually low. Flax fibres have relatively higher strength and stiffness, nevertheless they are prone to moisture absorption and rotting . As can be seen in Figure 5 all the fibres behave linear-elastically until breaking, whereas they demonstrate different stiffness and strain to failure values. Polymer Resins In composites, fibres are surrounded by the polymer resin, which is also referred to as the ‘matrix’. The matrix has four major roles: 1) fixing the fibres in desired geometrical arrangement, 2) transferring the forces to and between the fibres, 3) preventing buckling of the fibres and 4) protecting the fibres from environmental effects.
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- Summer '19
- Tensile strength, Composite material, Fiberglass, FRP Structures, M. Pavlovic