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What is FRP?

Fibre-reinforced plastic (FRP) (also fibre-reinforced polymer) are composite materials made of a polymer matrix reinforced with fibres. The fibers are usually fiberglass, carbon, or aramid, while the polymer is usually an epoxy, vinylester or polyester thermosetting plastic. FRPs are commonly used in the aerospace, automotive, marine, and construction industries.

FRP process definition

A polymer is generally manufactured by polycondensation, Polymerization or polyaddition, when combined with various agents to enhance or in any way alter the material properties of polymers the result is referred to as a plastic. Composite plastics refer to those types of plastics that result from bonding two or more homogeneous materials with different material properties to derive a final product with certain desired material and mechanical properties. Fiber reinforced plastics are a category of composite plastics that specifically use fibrous materials to mechanically enhance the strength and elasticity of plastics. The original plastic material without fiber reinforcement is known as the matrix. The matrix is a tough but relatively weak plastic that is reinforced by stronger stiffer reinforcing filaments or fibers. The extent that strength and elasticity are enhanced in a fiber reinforced plastic depends on the mechanical properties of both the fiber and matrix, their volume relative to one another, and the fiber length and orientation within the matrix.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.

FRP 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 the molding process.

Fiber process

The manufacture of fibre fabric

Reinforcing Fiber is manufactured in both two dimensional and three dimensional orientations

  1. Two Dimensional Fiber Reinforced Polymer are characterized by a laminated structure in which the fibers are only aligned along the plane in x-direction and y-direction of the material. This means that no fibers are aligned in the through thickness or the z-direction, this lack of alignment in the through thickness can create a disadvantage in cost and processing. Costs and labor increase because conventional processing techniques used to fabricate composites, such as wet hand lay-up, autoclave and resin transfer molding, require a high amount of skilled labor to cut, stack and consolidate into a preformed component.
  2. Three-dimensional Fiber Reinforced Polymer composites are materials with three dimensional fiber structures that incorporate fibers in the x-direction, y-direction and z-direction. The development of three-dimensional orientations arose from industry's need to reduce fabrication costs, to increase through-thickness mechanical properties, and to improve impact damage tolerance; all were problems associated with two dimensional fiber reinforced polymers.

The manufacture of fiber preforms

Fiber preforms are how the fibers are manufactured before being bonded to the matrix, fiber performs are often manufactured in sheets, continuous mats, or as continuous filaments for spray applications. The four major ways to manufacture the fiber preform is though the textile processing techniques of Weaving, knitting, braiding and stitching.

  1. Weaving can be done in a conventional manner to produce two-dimensional fibers as well in a multilayer weaving that can create three-dimensional fibers. However multilayer weaving is required to have multiple layers of warp yarns to create fibers in the z- direction creating a few disadvantages in manufacturing,namely the time to set up all the warp yarns on the loom. Therefore most multilayer weaving is currently used to produce relatively narrow width products, or high value products where the cost of the preform production is acceptable. Another one of the main problems facing the use of multilayer woven fabrics is the difficulty in producing a fabric that contains fibers oriented with angles other than 0" and 90" to each other respectively.
  2. The second major way of manufacturing fiber preforms is Braiding. Braiding is suited to the manufacture of narrow width flat or tubular fabric and is not as capable as weaving in the production of large volumes of wide fabrics. Braiding is done over top of mandrels that vary in cross-sectional shape or dimension along their length. Braiding is limited to objects about a brick in size. Unlike the standard weaving process, braiding can produce fabric that contains fibers at 45 degrees angles to one another. Braiding three-dimensional fibers can be done using four step, two-step or Multilayer Interlock Braiding.Four step or row and column braiding utilizes a flat bed containing rows and columns of yarn carriers that form the shape of the desired preform. Additional carriers are added to the outside of the array, the precise location and quantity of which depends upon the exact preform shape and structure required. There are four separate sequences of row and column motion, which act to interlock the yarns and produce the braided preform. The yarns are mechanically forced into the structure between each step to consolidate the structure in a similar process to the use of a reed in weaving.Two-step braiding is unlike the four step process because the two-step includes a large number of yarns fixed in the axial direction and a fewer number of braiding yarns. The process consists of two steps in which the braiding carriers move completely through the structure between the axial carriers. This relatively simple sequence of motions is capable of forming preforms of essentially any shape, including circular and hollow shapes. Unlike the four step process the two step process does not require mechanical compaction the motions involved in the process allows the braid to be pulled tight by yarn tension alone. The last type of braiding is multi-layer interlocking braiding that consists of a number of standard circular braiders being joined together to form a cylindrical braiding frame. This frame has a number of parallel braiding tracks around the circumference of the cylinder but the mechanism allows the transfer of yarn carriers between adjacent tracks forming a multilayer braided fabric with yarns interlocking to adjacent layers. The multilayer interlock braid differs from both the four step and two-step braids in that the interlocking yarns are primarily in the plane of the structure and thus do not significantly reduce the in-plane properties of the preform. The four step and two step processes produce a greater degree of interlinking as the braiding yarns travel through the thickness of the preform, but therefore contribute less to the in-plane performance of the preform. A disadvantage of the multilayer interlock equipment is that due to the conventional sinusoidal movement of the yarn carriers to form the preform, the equipment is not able to have the density of yarn carriers that is possible with the two step and four step machines.
  3. Knitting fiber preforms can be done with the traditional methods of Warp and [Weft] Knitting, and the fabric produced is often regarded by many as two-dimensional fabric, but machines with two or more needle beds are capable of producing multilayer fabrics with yams that traverse between the layers. Developments in electronic controls for needle selection and knit loop transfer, and in the sophisticated mechanisms that allow specific areas of the fabric to be held and their movement controlled. This has allowed the fabric to form itself into the required three-dimensional preform shape with a minimum of material wastage.
  4. Stitching is arguably the simplest of the four main textile manufacturing techniques and one that can be performed with the smallest investment in specialized machinery. Basically the stitching process consists of inserting a needle, carrying the stitch thread, through a stack of fabric layers to form a 3D structure. The advantages of stitching are that it is possible to stitch both dry and prepreg fabric, although the tackiness of the prepreg makes the process difficult and generally creates more damage within the prepreg material than in the dry fabric. Stitching also utilizes the standard two-dimensional fabrics that are commonly in use within the composite industry therefore there is a sense of familiarity concerning the material systems. The use of standard fabric also allows a greater degree of flexibility in the fabric lay-up of the component than is possible with the other textile processes, which have restrictions on the fiber orientations that can be produced.

Molding processes

There are two distinct categories of molding processes using FRP plastics; this includes composite molding and wet molding. Composite molding uses Prepreg FRP, meaning the plastics are fiber reinforced before being put through further the molding process. Sheets of Prepreg FRP are heated or compressed in different ways to create geometric shapes. Wet molding combines fiber reinforcement and the matrix or resist during the molding process. The different forms of composite and wet molding, are listed below.

Composite molding

Bladder molding

Individual sheets of prepreg material are laid -up and placed in a female-style mold along with a balloon-like bladder. The mold is closed and placed in a heated press. Finally, the bladder is pressurized forcing the layers of material against the mold walls. The part is cured and removed from the hot mold. Bladder molding is a closed molding process with a relatively short cure cycle between 15 and 60 minutes making it ideal for making complex hollow geometric shapes at competitive costs.

Compression molding

A "preform" or "charge", of SMC, BMC or sometimes prepreg fabric, is placed into mold cavity. The mold is closed and the material is compacted & cured inside by pressure and heat. Compression molding 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 / vacuum bag

Individual sheets of prepreg material are laid-up and placed in an open mold. The material is covered with release film, bleeder/breather material and a vacuum bag. A vacuum is pulled on part and the entire mold 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 the molding process due to a long slow cure cycle that is anywhere from one to two 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 labor intensive, meaning costs often confine it to the aerospace industry.

Mandrel wrapping

Sheets of prepreg material are wrapped around a steel or aluminum mandrel. The prepreg material is compacted by nylon or polypropylene cello tape. Parts are typically batch cured by hanging in an oven. After cure the cello and mandrel are removed leaving a hollow carbon tube. This process creates strong and robust hollow carbon tubes.

Wet layup

Fiber reinforcing fabric is placed in an open mold and then saturated with a wet [resin] by pouring it over the fabric and working it into the fabric and mold. The mold is then left so that the resin will cure, usually at room temperature, though heat is sometimes used to ensure a proper curing process. Glass fibers are most commonly used for this process, the results are widely known as fiberglass, and is used to make common products like skis, canoes, kayaks and surf boards.

Chopper gun

Continuous strand of fiberglass are pushed through a hand-held gun that both chops the strands and combines them with a catalyzed resin such as polyester. The impregnated chopped glass is shot onto the mold surface in whatever thickness the design and human operator think is appropriate. This process is good for large production runs at economical cost, but produces geometric shapes with less strength then other molding processes and has poor dimensional tolerance.

Filament winding

Machines pull fiber bundles through a wet bath of resin and wound over a rotating steel mandrel in specific orientations Parts are cured either room temperature or elevated temperatures. Mandrel is extracted, leaving a final geometric shape but can be left in some cases.

Pultrusion

Fiber bundles and slit fabrics are pulled through a wet bath of resin and formed into the rough part shape. Saturated material is extruded from a heated closed die curing while being continuously pulled through die. Process is could for any extruded material and geometric shape such as roadside reflector poles and ladder rails.

RTM & VARTM

Fabrics are placed into a mold which wet resin is then injected into. Resin is typically pressurized and forced into a cavity which is under vacuum in the RTM process. Resin is entirely pulled into cavity under vacuum in the VARTM process. This molding process allows precise tolerances and detailed shaping but can sometimes fail to fully saturate the fabric leading to weak spots in the final shape.

Advantages and limitations

FRP allows the alignment the glass fibers of thermoplastics to suite specific design programs. Specifying the orientation of reinforcing fibers can increase the strength and resistance to deformation of the polymer. Glass reinforced polymers are strongest and most resistive to deforming forces when the polymers fibers are parallel to the force being exerted, and are weakest when the fibers are perpendicular. Thus this ability is at once both an advantage or a limitation depending on the context of use. Weak spots of perpendicular fibers can be used for natural hinges and connections, but can also lead to material failure when production processes fail to properly orient the fibers parallel to expected forces. When forces are exerted perpendicular to the orientation of fibers the strength and elasticity of the polymer is less then the matrix alone. In cast resin components made of glass reinforced polymers such as UP and EP, the orientation of fibers 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 fibers, causing the material to shear at the interface between matrix and fibers.
  • Tensile forces near the end of the fibers exceed the tolerances of the matrix, separating the fibers from the matrix.
  • Tensile forces can also exceed the tolerances of the fibers causing the fibers themselves to fracture leading to material failure.

Material requirements

The matrix must also meet certain requirements in order to first be suitable for the FRP process and ensure a successful reinforcement of itself. The matrix must be able to properly saturate, and bond with the fibers within a suitable curing period. The matrix should preferably bond chemically with the fiber reinforcement for maximum adhesion. The matrix must also completely envelope the fibers to protect them from cuts and notches that would reduce their strength, and to transfer forces to the fibers. The fibers must also be kept separate from each other so that if failure occurs it is localized as much as possible, and if failure occurs the matrix must also debond from the fiber for similar reasons. Finally the matrix should be of a plastic that remains chemically and physically stable during and after reinforcement and molding processes. To be suitable for reinforcement material fiber additives must increase the tensile strength and modulus of elasticity of the matrix and meet the following conditions; fibres must exceed critical fiber content; the strength and rigidity of fibers itself must exceed the strength and rigidity of the matrix alone; and there must be optimum bonding between fibers and matrix

Glass fiber material

FRP plastics use textile glass fibers; textile fibers are different from other forms of glass fibers used for insulating applications. Textile glass fibers begin as varying combinations of SiO2, Al2O3, B2O3, CaO, or MgO in powder form. These mixtures are then heated through a direct melt process to temperatures around 1300 degrees Celsius, after which dies are used to extrude filaments of glass fiber in diameter ranging from 9 to 17 µm. These filaments are then wound into larger threads and spun onto bobbins for transportation and further processing. Glass fiber is by far the most popular means to reinforce plastic and thus enjoys a wealth of production processes, some of which are applicable to aramid and carbon fibers as well owing to their shared fibrous qualities.

Glass fiber material processes

Roving

Process where filaments are spun into larger diameter threads. These threads are then commonly used for woven reinforcing glass fabrics and mats, and in spray applications.

Fiber fabric

Web-form fabric reinforcing material that has both warp and weft directions

Fiber mats

Web-form non-woven mats of glass fibers. Mats are manufactured in cut dimensions with chopped fibers, or in continuous mats using continuous fibers.

Chopped fiber glass

Processes where lengths of glass threads are cut between 3 and 26 mm, threads are then used in plastics most commonly intended for molding processes.

Glass fiber short strands

Short 0.2–0.3 mm strands of glass fibers that are used to reinforce thermoplastics most commonly for injection molding.

Carbon fiber

Carbon fibers are created when polyacrylonitrile fibers (PAN), Pitch resins, or Rayon are carbonized (through oxidation and thermal pyrolysis) at high temperatures. Through further processes of graphitizing or stretching the fibers strength or elasticity can be enhance respectively. Carbon fibers are manufactured in diameters analogous to glass fibers with diameters ranging from 9 to 17 µm. These fibers wound into larger threads for transportation and further production processes. Further production processes include weaving or braiding into carbon fabrics, cloths and mats analogous to those described for glass that can then be used in actual reinforcement processes.

Aramid fiber material process

Aramid fibers are most commonly known Kevlar, Nomex and Technora. Aramids are generally prepared by the reaction between an amine group and a carboxylic acid halide group (aramid); commonly this occurs when an aromatic polyamide is spun from a liquid concentration of sulfuric acid into a crystallized fiber.Fibers are then spun into larger threads in order to weave into large ropes or woven fabrics (Aramid).Aramid fibers are manufactured with varying grades to based on varying qualities for strength and rigidity, so that the material can be somewhat tailored to specific design needs concerns, such as cutting the tough material during manufacture.

Applications

Fiber-reinforced plastics are best suited for any design program that demands weight savings, precision engineering, finite tolerances, and the simplification of parts in both production and operation. A molded polymer artifact is cheaper, faster, and easier to manufacture than cast aluminum or steel artifact, and maintains similar and sometimes better tolerances and material strengths.

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Structural Applications of FRP

FRP can be applied to strengthen the beams, columns and slabs in buildings. It is possible to increase strength of these structural members even after these have been severely damaged due to loading conditions.

For strengthening beams, two techniques are adopted. First one is to paste FRP plates to the bottom (generally the tension face)of a beam. This increases the strength of beam, deflection capacity of beam and stiffness (load required to make unit deflection). Alternately, FRP strips can be pasted in 'U' shape around the sides and bottom of a beam, resulting in higher shear resistance.

Columns in building can be wrapped with FRP for achieving higher strength. This is called wrapping of columns. The technique works by restraining the lateral expansion of the column.

Slabs may be strengthened by pasting FRP strips at their bottom (tension face). This will result in better performance, since the tensile resistance of 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.

 

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