A fiber -plastic composite ( FRP ) (also fiber-reinforced plastic, or FRP, FRP) is a material consisting of reinforcing fibers and a polymer matrix. The matrix surrounding the fibers that are bonded by adhesive or cohesive forces to the matrix. The use of fiber -based materials have fiber - plastic composites a directional elasticity behavior (see elasticity law ).
Without matrix material, the high specific strength and stiffness of the reinforcing fiber are not usable. Only through the appropriate combination of fiber and matrix material, a new construction material.
Fiber - plastic composites typically have high specific stiffness and strength. This makes them suitable materials in lightweight applications. For fiber - reinforced polymer composites mainly flat structures are produced.
The mechanical and thermal properties of fiber - reinforced polymer composites can be adjusted over a wide range of parameters. In addition to the fiber-matrix combination, for example, the fiber angle, fiber volume fraction, the layer order and more can be varied.
Fiber - plastic composites belong to the class of fiber-reinforced materials ( composite materials ), which in turn belong to the class of composite materials.
- 2.1 Micromechanics
- 2.2 macromechanics
- 3.1 Inorganic reinforcing fibers 3.1.1 metallic reinforcement fibers
- 3.2.1 natural fibers
- 3.4.1 Short fibers L = 0.1 to 1 mm
- 3.4.2 Long fibers L = 1-50 mm
- 3.4.3 continuous fibers L> 50 mm
- 4.1 Influence of moisture
- 4.2 Influence of temperature 4.2.1 mechanical effects
- 4.2.2 high temperatures
- 4.2.3 low temperatures
- 5.1 Thermoplastic Matrix
- 5.2 Thermosetting matrix
- 5.3 elastomer matrix
- 5.4 Choosing a matrix system
- 6.1 Thermoplastic semi-finished products
- 6.2 Thermoset semi-finished products
- 8.1 Procedure for pre-impregnated semi-finished products
- 8.2 Process for dry semi-finished products
- 9.1 stiffness
- 9.2 Fatigue analysis
Principle of operation
A fiber plastic composite can be considered as a construction. Its elements must be combined to adjust the desired properties. Through the interaction of the specific properties of the fiber material and matrix material, a new material.
Division of labor
The fibers transmit the force. Due to their high stiffness compared to the matrix they pull the burden. Since the fiber has a higher stiffness than the matrix, the load is directed along the fibers. Transverse to the fiber matrix and fiber often have similar elastic moduli. In addition, the forces have to be passed through adhesive forces of the fiber-matrix interface. Therefore, across the grain usually takes place no reinforcing effect. This is due to the stretching magnification.
The matrix embeds the fibers. Beds in this context means that it fixes the fibers in space and the load introduction and Lastausleitung possible. In addition, the matrix supports the fibers, for instance against buckling in faserparallelem pressure. The load is transferred via the adhesion between fiber and matrix. It can be about normal or shear forces. Composites in which no fiber - matrix adhesion is, can be loaded only in special cases. The matrix also has the task of protecting the fibers against environmental influences.
Not every fiber-matrix combination is a meaningful construction material. Three criteria must be met in order to increase stiffness and strength in the fiber direction in the composite.
Effectiveness of the fiber form
The fiber is superior to the compact material. This concerns both the strength and the elastic moduli. The following effects make the fiber the compact form superior:
Size effect: In a fiber, the maximum size of a defect is limited. A spherical air entrapment can have greater fiber diameter, for example, any diameter. Means, there are no major errors. Additionally, an effect occurs, which is based on the statistical distribution of the errors. Thus, the error-free length of fiber is growing strongly. Therefore, this results in very thin fibers long distances before a vacancy occurs. These effects only increase the strength of the fiber, not its rigidity.
Orientation: In the manufacture of fibers, the crystal or molecular planes are oriented. Suitable methods are spinning and drawing. Figuratively speaking, is made of a resilient ball of wool, a strand stiff wool threads. Materials with long-chain molecules (polyethylene fibers, aramid fibers, carbon fibers), are particularly suitable to produce a high orientation. As a rule, is accompanied by an anisotropy of the fiber with the growing orientation. With natural fibers such as hair, wool, hemp, sisal, etc. the orientation arises during growth. The orientation mainly increases stiffness.
Some types of fiber, such as glass fiber or basalt fiber have no orientation. These fibers are amorphous. The advantage of the fiber form is so alone in the use of the size effect and the reduction of defects. Amorphous fibers, which have to be deducted from the melt, a further advantage: On its surface occur during cooling compressive stresses. The compressive residual stresses may prevent cracks of the fiber.
Mechanical observation levels
Fiber plastic composites are considered at different mechanical levels. The observation plane depends on whether the global values of the composite or of the individual reinforcing fibers are of interest.
The micro-mechanics regards the individual, embedded in the matrix fiber. There is a two-phase mixture. With the help of Micromechanics the stresses and strains in the fiber and matrix can be calculated. Micromechanics allows the calculation of the elastic properties of the fiber-plastic composite of the properties of the fiber and the matrix (see classical laminate theory).
In the macro mechanics of fiber plastic composites, the composite is considered to be homogeneous. This means that its properties are independent of the location. However, its properties are still directional. With the help of the macro mechanics are obtained global stress and strain values . They can be considered as the average sizes over the fiber and matrix.
Macro mechanism is used to describe the behavior of components.
Inorganic reinforcing fibers
Inorganic fibers have an amorphous structure. Their advantages are the high temperature strength and mostly low price. Just the raw materials for glass and basalt fiber are almost fully available.
- Basalt fibers
- Glass fibers
- Ceramic fibers
- Silica fibers
Metallic reinforcing fibers
- Steel fibers
Organic reinforcing fibers
Organic fibers have a high degree of orientation. Your module along and across the fiber is significantly different. Due to high temperatures, organic fibers or melt decompose. However, this temperature limit may vary widely.
- Aramid fibers
- Carbon fibers
- Polyester fibers
- Nylon fibers
- Polyethylene fibers
- Plexiglas fibers
Main article: Natural fiber reinforced plastic
Have renewable reinforcing fibers based on other reinforcing fibers predominantly a low density. Since their mechanical properties are low, they are not used in structural components. Their main area of application they have, in combination with thermoplastic matrix materials, in trim components. A short-cut, they are used as an inexpensive diluent.
- Flax fibers
- Hemp fiber
- Wood fibers
- Sisal fibers
Appointment of reinforcing fiber bundles
Following the designation of yarns, bundles of reinforcing fibers, so-called roving, named tex yarn count. The greater the number of tex is, the higher the weight of the length of the fiber bundle. A roving 4800 tex for example, weighs 4.8 g per meter.
Especially with carbon fibers, the name by the number of individual filaments has prevailed. A 12k roving accordingly consists of 12,000 individual filaments. About the density of the fiber can be converted into the number of filaments, the Tex- number.
Classification according to fiber length
Short fibers of L = 0.1 to 1 mm
Short fibers are used in the injection molding art and can be processed directly with an extruder. There are thermoplastic granules which were already provided with a certain fiber volume fraction and fiber mass fraction of short fibers.
L = fiber length 1 to 50 mm
Long fibers can be processed in extruders also still. You will find a large scale during fiber spraying. Long fibers are often mixed with thermosets as a filler.
Continuous fibers L> 50 mm
Continuous fibers are used as rovings or fabric in the fiber-reinforced plastics. Components with continuous fibers have the highest stiffness and strength.
Since the individual fiber filaments are difficult to handle, one summarizes the dry fibers together into semi-finished. The preparation processes are taken in much of the textile industry such as weaving, braiding or embroidery.
- Tissue Fabric formed by the interweaving of endless fibers, such as rovings. The weaving of fibers inevitably goes hand in hand with the undulation of the fibers. The undulation effect, in particular a reduction in the parallel fiber compressive strength. Therefore, clutches are used for mechanically high-quality fiber - plastic composites.
- Scrim In a nest of the fibers are ideally situated parallel and stretched. It will only find endless fibers use. Clutches are paper or by a thread stitching together.
- Multiaxial If the fibers are oriented not only in the plane, then one speaks of multiaxial fabrics. In most cases the additional fibers are oriented perpendicular to the plane of the laminate to improve the delamination and impact behavior.
- Gesticke If you want individual rovings not muster stretched in the plane, but on any tracks, you use Gesticke. The rovings are prepared on a substrate (eg a fleece) embroidered and so fixed. Gesticke are frequently used in the area of load application, since often a complex fiber orientation is desired. Gesticke be used as preforms for RTM ( Resin Transfer Moulding).
- Braids In braiding hoses are braided from rovings mainly serving the manufacture of pipes, tanks or generally hollow components.
- Matting When equipment is to be made with quasi-isotropic properties to offer fiber mats. The mats are usually made of short and long fibers that are loosely connected to each other via a binder. Through the use of short and long fibers, the mechanical properties of components made of mats of woven fabrics which are inferior.
- Fleeces Nonwovens produced by the needling of long fibers. They are applied as a thin layer, the surface protection or enhancement of the surface waviness. The mechanical properties are inferior to those of quasi-isotropic and tissues.
- Fine-cut Sipes are mainly used as filler use. You can increase the mechanical properties of neat resin areas and possibly reduced lower the density.
- Spacer fabric Spacer fabrics are used for the production of sandwich structures.
In the processing of fibers, for example weaving, is on the fibers, a protective coating - the coating - applied. This is necessary especially for notch- sensitive fibers such as fiberglass. Such sizing is called Webschlichte. It is usually removed after weaving.
The size may also serve as a bonding agent between fiber and matrix. To this end, however, the size must be matched to the corresponding matrix system. Fibers with a Epoxydschlichte ( Silane ) are of limited use in thermoplastics. An adhesion promoting job can increase the fiber - matrix adhesion significantly.
The assessment of environmental effects on fiber - plastic composites is carried differentiated. As the material on micro-mechanical level is not homogeneous, the environmental influences have different effects on the fiber and matrix material. In addition to the impact on the individual components always include the resulting consequences for the composite must be considered.
Influence of moisture
The influence of moisture is primarily concerned with the matrix material, since the majority of the fiber materials does not absorb moisture. An exception to aramid and natural fibers. The polymeric matrix materials absorb moisture, this relates to both the thermal and the thermosets. The moisture absorption is carried out by diffusion and is thus dependent to a great extent on the time and the concentration gradient. This makes a computational detection difficult. The following phenomena occur at moisture absorption:
- Weight gain
- Decrease the glass transition temperature
- Decrease of the elastic modulus of the matrix material
- Emergence of source residual stresses
- Decrease of the fiber - matrix adhesion
- Decrease in the strength of the matrix material
- Climbing the material damping
- Increase of the elongation at break of the matrix material
- Osmosis damage (with corresponding concentration gradients in the laminate )
Weight gain. Especially when the aircraft weight gain of the structure by moisture is not negligible. The more fiber - plastic composites are installed in an aircraft, the more water it absorbs. Most fiber - plastic composites are ideal dry after manufacture. Only after a conditioning or storage in a humid atmosphere they reach, by the absorption of moisture, their final weight.
Glass transition temperature. The glass transition temperature decreases substantially with increasing moisture content of the composite. This can cause the glass transition temperature of a fiber-reinforced plastic composite is below the operating temperature. Thus, the matrix and the component softens failed. This effect is relevant climate especially in a hot and humid ( hot- wet). When choosing the operating temperature limits of the fiber-plastic composite therefore always the expected humidity must be considered. A conservative hedging can be done by boiling test ( boiltest ). In this test, the device will be stored for several hours in boiling water and then tested in the hot - wet condition.
Influence of temperature
The influence of temperature is primarily concerned with the matrix material. The fiber material is also affected by temperature, but the effects in comparison to the matrix are often low. Therefore, the matrix material dominates the temperature characteristics. Therefore, the following effects do not occur with any fiber-matrix combination.
Temperature differences have micro-mechanical stresses result when the fiber and matrix materials have different thermal expansion coefficients. These voltages occur between fiber and matrix, and shall be assessed as negative because they are requesting the fiber - matrix interface. This can lead to premature failure of the composite.
Macro mechanical cause temperature differences on layered fiber - reinforced polymer composites to tensions between the layers of the composite. In this case, the stresses are higher, the greater the angular difference of the fiber angle in the network. Reason is the difference in thermal expansion parallel and perpendicular to the fiber direction. This is regardless of whether the isotropic fiber material used or expands transversalisotrop.
Higher temperatures may cause the modulus of the matrix material to fall. The softening results in a decrease of the matrix- dominated modulus of the composite result. Since the fiber material is often much later softened as the Martixwerkstoff changes the parallel fiber module only very slightly under the influence of temperature. Usually also decrease the strength of the composite, in particular the parallel fiber compressive strength. Depending on the type of matrix, the glass transition temperature, melting temperature or the decomposition temperature of the matrix material is the temperature limit.
Particularly unpleasant properties of fiber - reinforced polymer composites are greatly accelerated creep and relaxation at high temperatures. This applies in particular measure stresses transverse to fiber direction, since the loads are transmitted through the matrix material. Creep and relaxation can be minimized if the fiber -plastic composite is designed according to the network theory. If the fiber material itself affected by creep and relaxation is the interpretation according to the network theory, with respect to the temperature behavior, largely ineffective.
With respect to the unreinforced matrix material, the creep and relaxation behavior of the composite is much cheaper.
In addition to the effects mentioned above occur at high temperatures to further effects. The degree to which they occur depends primarily on the matrix material.
- Moisture absorption increases
- Diffusion rate increases
- Attack by the media is accelerated
- Accelerated aging
- Attenuation increases
- Schlagzähig rises
- Elongation at break increases
- Decrease of the fiber - matrix adhesion
The thermal expansion coefficients no longer behave constant at low temperature, but reduced. In addition, the stiffness increase in both the fiber and the matrix.
Influence of radiation
High-energy radiation ( ultraviolet, infrared, X-ray, and cosmic radiation, radioactive ) first results in epoxy resins at a low dose in a shorter exposure of a post-curing to improve the mechanical properties. However, stronger doses and / or longer exposure times deplete the origin strength. Polyester resins are decomposed even under stronger influence of radiation.
Influence of corrosive media
Fiber - plastic composites are also used in areas with high corrosion, such as in the wastewater sector. In the strong alkalies it comes accompanied with polyesters to saponification with embrittlement and degradation reactions. The fibers, especially E-glass fibers are attacked in strong alkalis and strong acids. Remedy higher quality resins such as vinyl ester and epoxy resin, higher quality fibers and resin-rich chemical protective layers which reduce the penetration of media. In addition to the stability of the materials used and the diffusion behavior of the matrix and the void-free processing and the fiber - matrix connectivity plays a crucial role for the durability.
Basically, there are fiber-reinforced plastics with thermoplastic (thermoplastic) and thermoset ( thermoset ) matrix.
As matrix are basically all common thermoplastics used. Fibre-reinforced plastics with a thermoplastic matrix can be transformed or weld later. After cooling, the matrix fiber-reinforced plastics with a thermoplastic matrix is ready for use. However, they soften at elevated temperature. With increasing fiber content decreases their tendency to creep. As thermoplastic materials at high temperatures, for example:
- Polyetheretherketone (PEEK)
- Polyphenylene sulfide ( PPS)
- Polysulfone (PSU)
- Polyetherimide (PEI)
- Polytetrafluoroethylene (PTFE)
Fibre-reinforced plastics with thermoset matrix can no longer be transformed after the curing or cross-linking of the matrix. However, they have a high temperature range. This is particularly true for heat-curing systems, which are cured at high temperatures. The temperature limit is determined by the position of the glass transition temperature. Fibre-reinforced plastics with thermoset matrix usually have the highest strengths.
As the matrix, the following resins may be used. Percentage is the mass fraction of the production in Europe in 2005:
- Epoxy resin ( EP ) 2%
- Unsaturated polyester resin (UP ) 8%
- Vinyl ester resin ( VE)
- Phenol-formaldehyde resin (PF) 38%
- Diallyl phthalate (DAP )
- Methacrylate (MMA)
- Polyurethane (PUR)
- Amino Melamine resin (MF / MP ) 7%
- Urea resin (UF ) 45%
The most extensive deployment of so resin systems, which are used for the production of fiber-matrix semi-finished products in mass production. Not all resins listed above are processed entirely in fiber composite technology. Find partly as an adhesive or castables application.
Typical representatives of elastomers as matrix in fiber-reinforced plastics, rubber and polyurethane ( PUR) are mentioned. Elastomers, because of their low stiffness, structural components not used. An exception is loop-shaped components such as wedge or toothed belt.
Choice of a matrix system
The choice of the matrix system, decides on the limits of use of the fiber- reinforced plastic. In addition to the mechanical properties of the matrix, such as modulus of elasticity, there are a number of other criteria:
- Operating temperature range (melting point, glass transition temperature)
- Chemical resistance (acid, alkaline)
- Resistance to radiation (UV radiation )
- Long-term behavior (creep, relaxation)
- Moisture absorption
- Impact strength
Pre-impregnated semi-finished products
In addition to pure semi-finished fiber ( woven fabrics, nonwovens, etc.) there are a number of pre-impregnated fiber matrix semifinished products. These semi-finished products are generally present in the disk, tape or rope form.
Thermoplastic semi-finished products
UTC is the abbreviation for glass mat reinforced thermoplastics. In the production of glass fiber fabric or glass fiber mats in conjunction with thermoplastics (mostly PP) processed into semi-finished products. This semi-finished products can be further processed after heating by pressing. GMT mats are available with different fiber lengths. On the assumption that a GMT component has a higher strength with continuous fibers, however, usually not true. Thus have portions having a small cross -section and are mixed with short fibers to a greater strength. One reason for this is that the continuous fibers are compressed and buckled by the pressing. Under load, the negative effect on the strength.
The combination with other reinforcing fibers except glass fiber is possible.
LFT is the abbreviation of long fiber reinforced thermoplastics. The G- LFT process long fibers are in the form of granules ( PP matrix ) from an open extruder directly into a mold and shaped. The D- LFT process the matrix in an extruder (generally PP) is plasticized and mixed in a mixer with a shortened length to continuous filaments. The fibrous extrudate is then pressed into the form.
Thermoset semi-finished products
SMC (Sheet Molding Compound ) consists of short and long fibers. It is available in sheets and are processed in the hot pressing process. Aggregates prevent sticking of the matrix of tools and so make the semi-finished handle. The matrix is frequently an unsaturated polyester resin (UP) application. Is when the component has high impact resistance required, even vinyl ester resins are used ( VE). Other matrix systems also exist. Curing the fiber reinforced plastic material is carried out by increased temperature and optionally additional pressure.
BMC ( Bulk Molding Compound ) consists of short and long fibers. It is available as a pasty, shapeless mass. The composition is similar to the SMC. Curing takes place as in SMC.
Prepregs ( preimpregnated Fibers ) consist of continuous fibers ( filament ). Prepregs are usually wound up as a band-shaped goods are delivered. The continuous fibers can be present as unidirectional tapes (UD - tapes), Braided copper tapes or multiaxial prepreg. Curing takes place as in SMC and BMC in normal industrial applications. In the high- line area with carbon fibers as a reinforcing prepreg are processed in an autoclave to form components.
The way, as a fiber -plastic composite can be re-used depends on the matrix system. However, applies to all composites that complete material recycling, as in metals, is not possible.
A special position specific matrix systems with a natural fiber. These are part of fully biodegradable. Such composites have low strength and stiffness and are therefore only low mechanical components subject to application.
Fiber-plastic composites with such a matrix systems are very limited recyclable. The chemical extraction of the fibers prohibits in most cases for environmental and cost reasons. One possibility is in the grinding of the components. The powder so obtained can be used as an extender, for example, SMC and BMC.
A material recycling of thermoplastic fiber - reinforced polymer composites is partly possible. For this purpose, the device will be shredded and used as a short-fiber- reinforced plastic. By the use of time and re- melting, however, degrade the properties of the plastic. Such recycled material granules are therefore used only for minor applications. Furthermore, long or continuous fibers are not preserved. The mechanical quality of the Recyklats decreases significantly.
The method for manufacturing components made of fiber - reinforced polymer composites depend primarily on the type of semi-finished products used. Some methods are applicable to both impregnated and dry semi-finished products.
The choice of method depends on to the number of pieces to be produced as well as the geometrical dimensions of the component. Since many structures can alternatively be made with other semi-finished products and processes, economic criteria play an important role in the selection.
Procedures for pre-impregnated semi-finished products
- Fiber spraying
- Winding process
Method for dry semi-finished products
- Hand lay-up
- Resin transfer molding
- Winding process
Design and calculation
The design and calculation of fiber - reinforced polymer composites is described in VDI 2014. Older guidelines, such as the VDI 2013 have been withdrawn and are no longer valid.
The elastic properties of fiber-reinforced composites are based on the properties of elementary monolayers calculated ( unidirectional layers). This calculation method is known as the classical laminate theory. Tissues are given as the two imaged at an angle of 90 ° turned, unidirectional layers. Influences by the undulation of the fibers in the fabric are accounted for by reduction factors. A design method for optimal weight laminates is the network theory.
Result of the classical theory, the so-called laminate engineer constant of the composite material and the disk plates, the stiffness matrix. Said matrix consists of the following elements:
- Disc stiffness matrix
- Plate stiffness matrix
- Coupling matrix
Based on these matrices, the reactions of the composite can to
- Wheel loads: normal stresses and shear in the plane
- Plate stresses: bending moments and moment Drill
The switching matrix coupled while the disc loads the disc deformation and vice versa. Is of interest for practical use that an occupied switching matrix causes thermal distortion. As well as thermal strains are coupled, composite parts to warp, their coupling matrix is busy. Goal of many research projects is to use the couplings in the disk disk rigidity matrix deliberately constructive.
A rough interpretation is possible with the network theory. It neglects the interaction of the matrix and thus goes from the worst case. The network theory is, among other components in applications where it must be expected that the matrix softens or melts.
The strength analysis is carried out using failure criteria for fiber plastic composites. These can be differentiating, so distinguish the types of fracture, or a flat rate. A standard proof says nothing about the mode of failure. In the VDI 2014 a differentiating criterion is used ( inter-fiber failure criterion by Puck ).
For parts made of fiber-reinforced plastic in the strength verification tests play an important role. Since the liability between fiber and matrix are not known, may be on an experimental verification rarely waived. Furthermore, combined environmental influences such as the media attack and high temperatures can be judged only by quasi a try.
- Carbon fiber reinforced plastic (CFRP, coll: "Carbon" ) is in the aerospace, industrial components and also often used in sports. It can be found for example in Formula 1 cars, racing bicycles and inline speed skating shoes.
- The use of prepregs for the manufacture of printed circuit boards (PCBs)
- Micarta with vegetable fibers and epoxy resin
- Hard fabric with vegetable fibers and phenolic resin
- FFU synthetic wood for railway sleepers