Fibre-reinforced plastic (FRP; also called fibre-reinforced polymer, or in American English fiber) is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass (in fibreglass), carbon (in carbon-fibre-reinforced polymer), aramid, or basalt. Rarely, other fibres such as paper, wood, boron, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.

FRPs are commonly used in the aerospace, automotive, marine, and construction industries. They are commonly found in ballistic armour and cylinders for self-contained breathing apparatuses.

History

Bakelite was the first fibre-reinforced plastic. Leo Baekeland had originally set out to find a replacement for shellac (made from the excretion of lac bugs). Chemists had begun to recognise that many natural resins and fibres were polymers, and Baekeland investigated the reactions of phenol and formaldehyde. He first produced a soluble phenol-formaldehyde shellac called "Novolak" that never became a market success, then turned to developing a binder for asbestos which, at that time, was moulded with rubber. By controlling the pressure and temperature applied to phenol and formaldehyde, he found in 1905 he could produce his dreamed of hard mouldable material (the world's first synthetic plastic): bakelite. He announced his invention at a meeting of the American Chemical Society on 5 February 1909.

The development of fibre-reinforced plastic for commercial use was being extensively researched in the 1930s. In the United Kingdom, considerable research was undertaken by pioneers such as Norman de Bruyne. It was particularly of interest to the aviation industry.

Mass production of glass strands was discovered in 1932, when Games Slayter, a researcher at Owens-Illinois accidentally directed a jet of compressed air at a stream of molten glass and produced fibres. A patent for this method of producing glass wool was first applied for in 1933.

Owens joined with the Corning company in 1935 and the method was adapted by Owens Corning to produce its patented "fibreglas" (one "s") in 1936. Originally, was a glass wool with fibres entrapping a great deal of gas, making it useful as an insulator, especially at high temperatures.

A suitable resin for combining the "fibreglas" with a plastic to produce a composite material, was developed in 1936 by du Pont. The first ancestor of modern polyester resins is Cyanamid's resin of 1942. Peroxide curing systems were used by then. With the combination of and resin the gas content of the material was replaced by plastic. This reduced the insulation properties to values typical of the plastic, but now for the first time the composite showed great strength and promise as a structural and building material. Confusingly, many glass fibre composites continued to be called "fibreglass" (as a generic name) and the name was also used for the low-density glass wool product containing gas instead of plastic.

<!-- WP:NFCC violation: thumb|right|Ford prototype plastic car -->

thumb|right|Fairchild F-46

Ray Greene of Owens Corning is credited with producing the first composite boat in 1937, but did not proceed further at the time due to the brittle nature of the plastic used. In 1939, Russia was reported to have constructed a passenger boat of plastic materials, and the United States a fuselage and wings of an aircraft. The first car to have a fibre-glass body was the 1946 Stout Scarab. Only one of this model was built. The Ford prototype of 1941 could have been the first plastic car, but there is some uncertainty around the materials used as it was destroyed shortly afterwards.

The first fibre-reinforced plastic plane was either the Fairchild F-46, first flown on 12 May 1937, or the Californian built Bennett Plastic Plane. A fibreglass fuselage was used on a modified Vultee BT-13A designated the XBT-16 based at Wright Field in late 1942. In 1943, further experiments were undertaken building structural aircraft parts from composite materials resulting in the first plane, a Vultee BT-15, with a GFRP fuselage, designated the XBT-19, being flown in 1944. A significant development in the tooling for GFRP components had been made by Republic Aviation Corporation in 1943.

Carbon fibre production began in the late 1950s and was used, though not widely in British industry until the early 1960s. Aramid fibres were being produced around this time also, appearing first under the trade name Nomex by DuPont. Today, each of these fibres is used widely in industry for any applications that require plastics with specific strength or elastic qualities. Glass fibres are the most common across all industries, although carbon-fibre and carbon-fibre-aramid composites are widely found in aerospace, automotive and sporting good applications. Reinforcement of the matrix occurs by definition when the FRP material exhibits increased strength or elasticity relative to the strength and elasticity of the matrix alone.

Process description

FRP involves two distinct processes, the first is the process whereby the fibrous material is manufactured and formed, the second is the process whereby fibrous materials are bonded with the matrix during moulding.

Forming processes

A rigid structure is usually used to establish the shape of FRP components. Parts can be laid up on a flat surface referred to as a "caul plate" or on a cylindrical structure referred to as a "mandrel". However, most fibre-reinforced plastic parts are created with a mould or "tool". Moulds can be concave female moulds, male moulds, or the mould can completely enclose the part with a top and bottom mould.

The moulding processes of FRP plastics begins by placing the fibre preform on or in the mould. The fibre preform can be dry fibre, or fibre that already contains a measured amount of resin called "prepreg". Dry fibres are "wetted" with resin either by hand or the resin is injected into a closed mould. The part is then cured, leaving the matrix and fibres in the shape created by the mould. Heat and/or pressure are sometimes used to cure the resin and improve the quality of the final part.

The different methods of forming are listed below.

Bladder moulding

Individual sheets of prepreg material are laid up and placed in a female-style mould along with a balloon-like bladder. The mould is closed and placed in a heated press. Finally, the bladder is pressurised forcing the layers of material against the mould walls.

Compression moulding

When the raw material (plastic block, rubber block, plastic sheet, or granules) contains reinforcing fibres, a compression moulded part qualifies as a fibre-reinforced plastic. More typically the plastic preform used in compression moulding does not contain reinforcing fibres. In compression moulding, a "preform" or "charge", of SMC, BMC is placed into mould cavity. The mould is closed and the material is formed and cured inside by pressure and heat. Compression moulding offers excellent detailing for geometric shapes ranging from pattern and relief detailing to complex curves and creative forms, to precision engineering all within a maximum curing time of 20 minutes.

Autoclave and vacuum bag

Individual sheets of prepreg material are laid-up and placed in an open mould. The material is covered with release film, bleeder/breather material and a vacuum bag. A vacuum is pulled on part and the entire mould is placed into an autoclave (heated pressure vessel). The part is cured with a continuous vacuum to extract entrapped gasses from laminate. This is a very common process in the aerospace industry because it affords precise control over moulding due to a long, slow cure cycle that is anywhere from one to several hours. This precise control creates the exact laminate geometric forms needed to ensure strength and safety in the aerospace industry, but it is also slow and labour-intensive, meaning costs often confine it to the aerospace industry. When forces are exerted perpendicular to the orientation of fibres, the strength and elasticity of the polymer is less than the matrix alone. In cast resin components made of glass reinforced polymers such as UP and EP, the orientation of fibres can be oriented in two-dimensional and three-dimensional weaves. This means that when forces are possibly perpendicular to one orientation, they are parallel to another orientation; this eliminates the potential for weak spots in the polymer.

Failure modes

Structural failure can occur in FRP materials when:

  • Tensile forces stretch the matrix more than the fibres, causing the material to shear at the interface between matrix and fibres.
  • Tensile forces near the end of the fibres exceed the tolerances of the matrix, separating the fibres from the matrix.
  • Tensile forces can also exceed the tolerances of the fibres causing the fibres themselves to fracture leading to material failure.

Macroscopic Damage

  • Transverse cracks that occur perpendicular to the direction of the reinforcing fibres.
  • Shear failure of the fibre bundles.
  • Cracks in the matrix.
  • Delamination between fibre bundles in a composite laminate.
  • Tensile failure of fibre bundles,
  • Final fracture.
  • Advantages over a traditional rudder made from sheet aluminium are:
  • 25% reduction in weight
  • 95% reduction in components by combining parts and forms into simpler moulded parts.
  • Overall reduction in production and operational costs, economy of parts results in lower production costs and the weight savings create fuel savings that lower the operational costs of flying the aeroplane.

Glass-fibre-reinforced polymers

Engine intake manifolds are made from glass-fibre-reinforced PA 66.

  • Advantages this has over cast aluminium manifolds are:
  • Up to a 60% reduction in weight
  • Improved surface quality and aerodynamics
  • Reduction in components by combining parts and forms into simpler moulded shapes.

Automotive gas and clutch pedals made from glass-fibre-reinforced PA 66 (DWP 12–13)

  • Advantages over stamped aluminium are:
  • Pedals can be moulded as single units combining both pedals and mechanical linkages simplifying the production and operation of the design.
  • Fibres can be oriented to reinforce against specific stresses, increasing the durability and safety.

Aluminium windows, doors and façades are thermally insulated by using thermal insulation plastics made of glass fibre reinforced polyamide. In 1977 Ensinger GmbH produced first insulation profile for window systems.

Structural applications

FRP can be applied to strengthen the beams, columns, and slabs of buildings and bridges. It is possible to increase the strength of structural members even after they have been severely damaged due to loading conditions. In the case of damaged reinforced concrete members, this would first require the repair of the member by removing loose debris and filling in cavities and cracks with mortar or epoxy resin. Once the member is repaired, strengthening can be achieved through wet, hand lay-up of fibre sheets impregnated with epoxy resin, applied to the cleaned and prepared surfaces of the member.

Two techniques are typically adopted for the strengthening of beams, depending on the strength enhancement desired: flexural strengthening or shear strengthening. In many cases it may be necessary to provide both strength enhancements. For the flexural strengthening of a beam, FRP sheets or plates are applied to the tension face of the member (the bottom face for a simply supported member with applied top loading or gravity loading). Principal tensile fibres are oriented parallel to the beam's longitudinal axis, similar to its internal flexural steel reinforcement. This increases the beam strength and its stiffness (load required to cause unit deflection), but decreases the deflection capacity and ductility.

For the shear strengthening of a beam, the FRP is applied on the web (sides) of a member with fibres oriented transverse to the beam's longitudinal axis. Resisting of shear forces is achieved in a similar manner as internal steel stirrups, by bridging shear cracks that form under applied loading. FRP can be applied in several configurations, depending on the exposed faces of the member and the degree of strengthening desired, this includes: side bonding, U-wraps (U-jackets), and closed wraps (complete wraps). Side bonding involves applying FRP to the sides of the beam only. It provides the least amount of shear strengthening due to failures caused by de-bonding from the concrete surface at the FRP free edges. For U-wraps, the FRP is applied continuously in a 'U' shape around the sides and bottom (tension) face of the beam. If all faces of a beam are accessible, the use of closed wraps is desirable as they provide the most strength enhancement. Closed wrapping involves applying FRP around the entire perimeter of the member, such that there are no free ends and the typical failure mode is rupture of the fibres. For all wrap configurations, the FRP can be applied along the length of the member as a continuous sheet or as discrete strips, having a predefined minimum width and spacing.

Slabs may be strengthened by applying FRP strips at their bottom (tension) face. This will result in better flexural performance, since the tensile resistance of the slabs is supplemented by the tensile strength of FRP. In the case of beams and slabs, the effectiveness of FRP strengthening depends on the performance of the resin chosen for bonding. This is particularly an issue for shear strengthening using side bonding or U-wraps. Columns are typically wrapped with FRP around their perimeter, as with closed or complete wrapping. This not only results in higher shear resistance, but more crucial for column design, it results in increased compressive strength under axial loading. The FRP wrap works by restraining the lateral expansion of the column, which can enhance confinement in a similar manner as spiral reinforcement does for the column core.

Elevator cable

In June 2013, KONE elevator company announced Ultrarope for use as a replacement for steel cables in elevators. It seals the carbon fibres in high-friction polymer. Unlike steel cable, Ultrarope was designed for buildings that require up to of lift. Steel elevators top out at . The company estimated that in a high building, an elevator would use 15% less electrical power than a steel-cabled version. As of June 2013, the product had passed all European Union and United States certification tests.

Design considerations

FRP is used in designs that require a measure of strength or modulus of elasticity for which non-reinforced plastics and other material choices are ill-suited, either mechanically or economically. The primary design consideration for using FRP is to ensure that the material is used economically and in a manner that takes advantage of its specific structural characteristics, but this is not always the case. The orientation of fibres creates a material weakness perpendicular to the fibres. Thus the use of fibre reinforcement and their orientation affects the strength, rigidity, elasticity and hence the functionality of the final product itself. Orienting the fibres either unidirectionally, 2-dimensionally, or 3-dimensionally during production affects the strength, flexibility, and elasticity of the final product. Fibres oriented in the direction of applied forces display greater resistance to distortion from these forces, thus areas of a product that must withstand forces will be reinforced with fibres oriented parallel to the forces, and areas that require flexibility, such as natural hinges, will have fibres oriented perpendicular to the forces.

Orienting the fibres in more dimensions avoids this either-or scenario and creates objects that seek to avoid any specific weakness due to the unidirectional orientation of fibres. The properties of strength, flexibility and elasticity can also be magnified or diminished through the geometric shape and design of the final product. For example, ensuring proper wall thickness and creating multifunctional geometric shapes that can be moulded as a single piece enhances the material and structural integrity of the product by reducing the requirements for joints, connections, and hardware. In addition to concerns regarding safe disposal, the fact that the fibres themselves are difficult to remove from the matrix and preserve for re-use means FRP's amplify these challenges. FRP's are inherently difficult to separate into base materials, that is into fibre and matrix, and the matrix is difficult to separate into usable plastics, polymers, and monomers. These are all concerns for environmentally-informed design today. Plastics do often offer savings in energy and economic savings in comparison to other materials. In addition, with the advent of new more environmentally friendly matrices such as bioplastics and UV-degradable plastics, FRP will gain environmental sensitivity.