Fibre-reinforced plastic (FRP), also called fibre-reinforced polymer or fibre, is a composite material of a polymer matrix reinforced with fibres.
Rarely, other fibres, such as paper, wood, boron, or asbestos, have been used. The polymer is usually epoxy, vinyl ester, or polyester thermosetting plastic, though phenol-formaldehyde resins are still used.
FRPs are commonly used in the aerospace, automotive, marine, and construction industries. They are widely found in ballistic armour and cylinders for self-contained breathing apparatuses.
FRP composites are different from traditional construction materials like Steel and aluminium. FRP composites are anisotropic, whereas Steel and aluminium are isotropic.
Therefore, their properties are directional, meaning that the best mechanical properties are in the direction of fibre placement.
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Table of Contents
How is Fibre-Reinforced Plastic (FRP) Manufactured?
The following two processes are used in the manufacturing of Fibre-Reinforced Plastic (FRP):
- The first step is the process of manufacturing and forming the fibre material. The fibres are obtained from glass, carbon, basalt, and at times from paper, wood, or asbestos.
- The second step is the process of binding and moulding the fibrous materials with a matrix of polymer or plastic, such as epoxy, vinyl ester, or polyester matrix, to form the Fibre-Reinforced Plastic (FRP).
The nature of fibre and plastic depends on the target properties achieved in the composite output material.
The fibre is used to provide stiffness, and the polymer provides superior support and flexibility between fibres, making the material resistant to exposure to dynamic environmental conditions like heat, cold, rain, and dust.
Reasons to Consider FRP
There are many reasons to consider using FRP composites in civil engineering applications. The most relevant of these reasons, as applied to engineering, are discussed below.
The primary criteria for engineers to select a material for a job are durability, corrosion resistance, cost, weight, material properties, and ease of construction.
FRP composites are attractive alternatives to conventional construction materials for these and several other compelling reasons, as follows:
Structural Considerations
The items presented in this section are a brief presentation of structural considerations.
1. Tensile strength
FRP composites provide several structural properties, making them an attractive alternative to many conventional engineering materials.
Their tensile strength can range from about the strength of mild reinforcing steel to stronger than that of prestressing steel.
As such, they offer a good incentive for use in situations where high tensile strength is an asset.
FRP composites generally exhibit linear tensile stress-strain behaviour throughout their load-carrying range and, as such, do not change their modulus over their loading history.
Since FRP composites are materials composed of structural fibres in a plastic matrix, the fibres can be custom-oriented to suit individual needs.
2. Fatigue
Research indicates that FRP composites exhibit good fatigue resistance in tension cycling (American Concrete Institute, State-of-the-Art Report on Fibre Reinforced Plastic (FRP) for Concrete Structures).
Research has yet to document the effects of temperature, moisture, reverse loading, long-term and compression load cycling, and holes on fatigue resistance.
Long-fibre composites generally retain a high proportion of their short-term strength after 107 cycles. Carbon-fibre composites exhibit the highest fatigue resistance, followed by aramid and glass.
3. Low mass
Excessive structural mass is often a reason to consider alternate materials that provide high load-carrying capacity and low density.
FRP composites have densities of 1,200 to 2,600 kg/m3, which makes them attractive alternatives to structural materials such as steel, with a density of around 7,850 kg/m3.
4. Specific strength
The specific strength of materials, defined as the yield strength divided by the density, is often used to make comparisons between materials based on strength and mass.
Because of their high strength and very low density, FRP composites have specific strengths up to 60 times that of high-strength steels.
The high specific strengths associated with FRP composites are very useful in applications such as structural cladding panels, low-density framing materials, and vehicle components.
Their low weight makes the assembly and disassembly of temporary structures much easier and less time-consuming than similar structures made of wood or steel.
The cost of many of the FRP composites, although higher than conventional construction materials on a pound-per-pound basis, is competitive when the specific strength of the materials is taken into consideration.
Final construction costs can even be lower than conventional materials if such factors as more efficient design, transportation costs, and lifting equipment costs are taken into account.
5. Vibration damping
The specific modulus of FRP composites, defined as the modulus of elasticity divided by the density, is also high and provides characteristics such as low vibration in situations where vibration may be a problem.
Steel has high density, high modulus, and low damping characteristics, whereas composites have low densities, moderate moduli, and high damping characteristics.
The use of composites in floors and bearing pads, where damping of vibration is of concern, can reduce these problems.
6. Repair using composites
Structural repairs of conventional materials using FRP composites can be advantageous from the standpoint of ease of installation and reduced maintenance costs.
Conventional techniques for externally strengthening cracked concrete structures call for steel plates or bars to be installed across the crack to carry the structural loads no longer carried by the concrete.
FRP plates can be structurally bonded across cracks to replace the steel repair components. The low mass of these materials makes their handling more convenient, and their non-corrosive nature eliminates the need to protect them from rusting deterioration.
Production Options On FRP
1. Fabrication
The variety of fabrication techniques that are available with FRPs provides for many custom properties.
Multiple types of fibres can be combined to produce materials with the advantages of each component; fibres can be oriented in specified directions to suit specialised loading conditions better; and material properties such as strength and stiffness can be controlled to meet the user’s needs.
Special moulding techniques allow complicated pieces to be fabricated as one unit, eliminating joint conditions that can be a source of weakness.
One method of producing FRP composites is by a technique known as pultrusion, a process much like extrusion.
In the pultrusion process, the FRP materials are pulled through dies while the matrix is being cured and is in a moldable condition. These dies can be in the form of an I-beam, a channel section, or any custom cross-section.
2. Colour and coating
Since the matrix of FRP composites consists of resins that begin in the liquid state, many architectural treatments can be added before they harden.
For example, custom colouring can be added to the resins in the manufacturing process, thereby eliminating the need for and cost of painting or other colour applications after the fact.
Since the colour is integrally mixed in the matrix, it cannot be scraped off or abraded during its lifetime.
It is also possible to embed sand or other nonslip surface treatments as a secondary operation, making the treatment part of the component. Nonslip gratings and walkways are an example of this type of application.
Economic Considerations
1. Life-cycle costs
While the initial cost of composite materials is usually higher than that of alternative construction materials, there are a number of economic considerations which make their use feasible and economic.
Corrosion protection was mentioned as an area where composites are beneficial in terms of the cost of maintenance. Many life-cycle costs could be eliminated or drastically reduced with the use of FRP composites.
The costs associated with periodically repainting steel to protect it against corrosion are maintenance costs that would be eliminated if materials that did not require such coatings were used.
The costs of rehabilitating structures damaged by corrosion, such as blast cleaning of steel to remove corrosion products, would be eliminated with noncorrosive composite materials.
In general, periodic maintenance of structures would be reduced, and replacement costs would be delayed through greater use of FRP composites.
Some FRPs could require coating protection for aesthetic reasons or for exceptionally harsh environments.
2. Construction and transportation costs
Construction and transportation costs can be reduced by using low-density composites. Since many charges for freight are based on weight, the low densities of FRP composite components reduce shipping costs and require less heavy construction handling equipment at sites.
Fabrication costs will be reduced in two areas. Through increased ease of handling of components, smaller crews can be utilised to handle components assembled in the field.
Furthermore, the preassembly of some components can reduce field assembly costs. In addition to reduced costs, faster construction times can be realised through improved handling capabilities.
Environmental Considerations
1. Reduced environmental toxicity
Many of the building materials that we presently use are harmful to our environment in some way or another.
Examples of such materials are lead-based paints, creosote, and other petroleum products used in piling to kill or ward off marine borers and shipworms.
The components of FRP materials are, for the most part, inert and will not leach into the environment. The use of conventional maintenance coatings on structures can be toxic to the environment.
The use of FRPs eliminates some of these hazardous chemicals. Piling made from FRP materials does not rot, nor are they attacked by marine organisms, so there is no need to heat pilings with harmful chemicals such as creosote.
2. Recycling
Many of the plastic materials used as food containers and composite components of automobiles can be recycled when no longer needed.
These recycled plastics and glass fibres can be reused to make FRP composite components, thereby reducing the volume of waste we put in our landfills.
Material Property Considerations
1. Magnetic properties
FRP composites possess some properties that are not available from more conventional materials. Because their components are plastic resins coupled with glass, carbon, and aramid fibres, they are immune to magnetic forces.
FRP materials are used in several designs for vehicles and guideways of magnetically levitated transportation systems to eliminate any adverse forces that would be induced through proximity to the magnets used for levitation and locomotion.
Components of vehicles where magnetic compasses are employed often use composites in the vicinity of the compasses to eliminate any magnetic influence in the guidance systems.
Special facilities that employ electromagnetic technology are often built entirely using composites.
2. Conductivity
Electrical conductivity is a hazard in many construction environments. High voltages passing through metallic construction materials acting as conductors can cause injury or even death.
Most FRP composites (including glass and aramid fibre composites) are electrically non-conductive.
This makes them good candidates for construction materials where the threat of electrocution is a consideration.
For many years, stepladders have been made from fibreglass composites due to their non-conductive properties.
Electric cable trays, walkways in the vicinity of exposed electric conductors (such as at power plants), and booms of bucket trucks are all examples of FRP composites used to eliminate electrocution hazards and other electrical problems.
Repair of FRP Composites
As with any construction material, FRP composites are subject to damage. This damage may be intentional or unintentional.
Intentional damage can occur when the composite components or structures are cut, drilled, or otherwise manipulated during installation or fabrication of the structure.
Unintentional damage can be caused by accidental impact, unexpected excessive loading, or long-term environmental exposure.
It is important to note that any damage or alteration to the fibres and/or the resin matrix may alter the performance properties (e.g., corrosion resistance and mechanical strength) of the composite component.
Routine Maintenance On FRP
The following addresses the routine maintenance of FRP:
- A properly designed and fabricated composite system will generally not require much in the way of routine maintenance.
- Composites intended for direct exposure to weathering and ultraviolet radiation generally have a surface coating to improve corrosion and ultraviolet resistance.
Repair during installation on FRP
Sawing, drilling, grinding, routing, and other such procedures may be necessary to accomplish the installation or fabrication of the composite structure.
Any such procedures that cut through the resin surface sealant or otherwise expose the reinforcement fibres can significantly reduce the corrosion resistance of the composite system.
The exposed new surface must be appropriately sealed. To help ensure a proper repair, residual dust or other debris resulting from the installation operations must be thoroughly removed before the repair procedures.
Final Conclusion on FRP
FRP is the material of calculated compromise. An engineer chooses it not because it is the cheapest or easiest to work with, but because its unique set of properties is the optimal solution for a specific, demanding set of requirements where weight, strength, and corrosion are paramount.
It is a testament to human ingenuity—a material we engineered from the molecular level up to defy the inherent limitations of its constituent parts.
While it faces serious challenges, particularly regarding its environmental lifecycle, the relentless pace of innovation suggests that FRP will not only remain a cornerstone of advanced engineering but will evolve to become smarter, more sustainable, and even more integral to the technology of the future.
It is a material that has earned its place and is poised to redefine it.
That’s all.
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