Fiber-reinforced composite

A fiber composite material is generally from two main components ( a bed matrix and reinforcing fibers ) existing multi-phase or composite material. Through mutual interactions of the two components of this material receive higher properties than either component individually involved.

• 3.1 fibers
• 3.2 matrix

• 4.1 laminates
• 4.2 injection molded parts
• 4.3 molding parts
• 4.4 strand drawn parts
• 4.5 Sheet Molding Compounds ( SMC)
• 4.6 fiber concrete

• 8.1 compositor
• 8.2 ESAComp
• 8.3 LamiCens
• 8.4 Composite Star
• 8.6 R & G laminate computer

General

In contrast to the previous composites, such as reinforced concrete with the introduction of ultra- thin fibers (a few microns in diameter ) is used, among other things, the effect of the specific resistance. This relationship was discovered in the twenties by Griffith and is: has a material in fiber form in the fiber direction a much greater strength than the same material in a different form. The thinner the fiber, the greater its strength. The reason for this lies in an increasing rectification of the molecular chains with decreasing of available space. In addition, throughout the material at very large distances, so that the fibers are substantially free of defects that can cause a break to the break leading defects ( weakest link theory ). Since with equal strength, the heavy, solid component can be saved and replaced by a lighter, a material with a high specific strength arises (ratio of strength and weight). Also, an error in the material does not lead to failure of the entire component, but initially only to break a single fiber in the composite.

Since the fibers in their density (number per unit area ) can be adapted aligned depending on frequency and arise with the aid of appropriate manufacturing processes tailored parts. To influence the strength in different directions, woven web or scrim may be used instead of single fibers, which are made prior to contact with the matrix.

Operation

The high-order properties of a fiber composite material can be achieved only through the interaction of the two components. For two components, thus acting three phases in the material result: Very high tensile strength fibers, a relatively soft, they bedding matrix and both components connecting boundary layer.

Conditions for the reinforcing effect of fibers

Not all combinations of fiber and matrix materials, lead to an increase in strength and stiffness of the new composite. There must be met three conditions so that in fiber direction parallel to a reinforcing effect occurs:

Perpendicular to the fiber usually occurs on no increase in strength. Reason is the stretching magnification.

Tasks of the components

• The matrix gives the fiber composite material its appearance. Both the color and the texture are virtually no limits. In mechanical terms, they must keep in position and transfer tensions between them and distribute the reinforcing fibers. In terms of the durability, it has the function of protecting the fibers from external mechanical and chemical influences.

• The fibers give the fiber composite material the necessary strength. In addition to the tensile strength, if the material is subjected to compressive stress, and the bending strength to play a role.

• The boundary layer is of the voltage transfer between the two components. It transmits only shear and can take very abstract forms, when the thrust for example, via pure friction. In other cases, however, such as thrust over adhesive bond, it is manufacturing technology willed and physically. In the latter case, the fibers are coated before the first contact with the matrix with a coupling agent which chemically react with both components, and a continuous transition as possible is guaranteed.

An important factor in the design of fiber composites is the volume ratio ( fiber volume fraction ) between fibers and matrix. The higher the proportion of fibers, the stronger, more rigid and brittle, however, is the material. This can cause problems when certain strains are exceeded.

Principle of power transmission

As shown in Figure 1, it is impossible to have this effect directly on the fibers in the case of a concentrated applied tensile force, as they are always covered by a matrix layer. Thus, the tensile force acts only on the matrix in the form of concentrated stresses and is distributed by the latter to the nächstliegendsten fibers. The size of this " Ausbreitfeldes " ( the contributory length of fiber ) depends on the tension between fiber and matrix: a soft matrix combined with stiff fibers results in major contributory lengths, a rigid matrix with soft fibers provides small contributory lengths. However, voltages need not be applied strictly in concentrated form, a variant for the production of tensile stresses, for example, an applied torque. The principle does not change.

In the case of longitudinal position of the fiber flow acting pressure, as also occurs during bending, does the matrix, such as a ballast and the fiber ( the fiber bundle ), such as an elastically bedded beams, see figure 2, there are important material properties of the matrix rigidity k and the bending stiffness the fiber e · I ( stiffness multiplied by the area moment of inertia ). The calculation is now much more complex, because now except the sheer tensile strength of the fiber and the diameter plays a role because of the moment of inertia. The case pressure is being explored since the mid- sixties of the 20th century and is still a scientific challenge through use of computers and modern FEM programs is currently trying to prove and understand the theoretical approaches numerically. The problems are firstly the fact that there is a stability problem and thus even the smallest changes in the material composition can have a significant impact on the tolerable forces. On the other failed an advanced multi-phase material in many different ways and different mechanisms take turns during the failure and require partially each other. Pressure failure takes place very suddenly, quickly and partially upheld without warning. Thus, it is very difficult to observe, which makes the analysis more difficult.

Materials

In addition to the purely mechanical properties, ie the necessary calculated strength, especially Durability and price issues play a large role in the choice of materials. To ensure proper functioning, the stiffnesses of the two components should be coordinated so that a resultant force peaks can be well distributed in the material. Specifically, the following materials are used:

Fibers

• Glass fibers Glass fibers are mainly due to their relatively low price, the fiber most commonly used types. There are fiber types for different applications.
• Carbon fibers See there.
• Ceramic fibers Endless ceramic fibers of alumina, mullite ( mixed oxide of alumina and silica ), SiBCN, SiCN, SiC, etc., are expensive specialty fibers for high-temperature durable composites with a ceramic matrix. The non-oxide fibers are similar to carbon fibers produced from organic resins in which in addition to carbon and silicon is contained.
• Aramid fibers See there.
• Boron
• Basalt fibers Basalt fiber is a mineral fiber that is used primarily because of their good chemical resistance and temperature resistance in the container and vehicle.
• Steel fibers Steel fibers are used mainly in the construction of steel fiber concrete. This application is growing rapidly and has the most economic benefits.
• Natural fibers The fibers most commonly used for the production of fiber-reinforced composites are the domestic wood fibers, flax and hemp fibers as well as subtropical and tropical fibers such as jute, kenaf, ramie or sisal fibers.
• Nylon fibers Fibers with a high elongation at break is an advantage if the component has to absorb shocks and this property is decisive for the design.

Matrix

The choice of matrix divides the fiber composites into two groups: fiber - plastic composites ( reinforced plastic, fiber reinforced plastic), and Other.

• Fiber -plastic composite As a matrix, the following polymers are used: Thermosets ( other names: thermoset resins )
• Elastomers
• Thermoplastics

While the synthetic resins and elastomers are present up to their curing liquid, thermoplastics up to 150 ° C ( in some cases up to 340 ° C) firmly. The thermoset resins are glassspröde generally and do not deform plastically. Fibre reinforced plastics from thermoplastics can be subsequently transform under heat. The micro-and Makrotränkung the fibers is simpler than in the case of solid thermoplastic resins. Thermoplastics can be heated to the impregnation or dissolved in a solvent.

In recent years, research in the area of the biopolymers was greatly intensified. Through the use of thermoset and thermoplastic biodegradable bioplastics is permanent or composite materials based on renewable raw materials can be produced which often comparable properties as natural and glass fiber reinforced petroleum-based polymers.

• Other Cement and Concrete
• Metals
• Ceramics as a matrix for nichtspröde Ceramic Fiber Composites
• Carbon carbon fiber -reinforced carbon (CFC)

Types and methods of manufacture

Laminates

The group of laminates takes full advantage of the unique fiber orientation. They usually consist of several superimposed fiber semi-finished products (eg tissue, scrims, mats ) with different principal fiber directions. For their production, there are several methods:

• Hand laid process The fiber semi-finished (fabric / scrim / fiber mats) are inserted by hand into a mold and impregnated with resin. Subsequently, they are deaerated with the aid of a roll by pressing. This should not only present in the laminate structure of air, but also excess resin to be removed. This procedure is repeated until the desired thickness is present. This is also called a " wet on wet " method. After the application of all the layers of the component is cured by a chemical reaction of the resin with the hardener. The method provides no great demands on the tools and is also suitable for very large components. It is often used in series production, where, although lightweight components are desired, but should also be produced at low cost. Benefits are less tool and equipment expense, which is offset by the lower quality components ( lower fiber content ) and the high manual effort, the trained laminator requires. The open processing of the resin makes high demands on the labor protection.

• Laying on of hands with vacuum presses After the introduction of all reinforcing and sandwich materials, the mold with a release film, a bleeder and a vacuum bag is covered. A vacuum is generated between the vacuum foil and the mold. This means that the composite is compressed. Air may still present is removed by suction. Excess resin is taken from the bleeder. So can be achieved compared to the hand lay an even higher part quality.

• Prepreg Technology With pre-impregnated matrix material (that is already saturated ), the fiber mats are placed on the mold. The resin is no longer liquid, but has a slightly sticky solid consistency. The composite is then deaerated by vacuum bag and then, often cured in an autoclave under pressure and heat. The prepreg is (cooling systems, autoclave) and the demanding process control (temperature management) one of the most expensive manufacturing process due to the necessary operating equipment. However, it allows not only for filament winding and the injection and infusion processes, the highest quality of the parts. The method is primarily in the aerospace, motor sport, as well as for power sports equipment application.

• Vacuum infusion vacuum structure finished component In this method, the dry fiber material (rovings, mats, scrims, woven, ...) is inserted in a release-coated mold. In a separating tissue as well as a distribution medium is placed, to facilitate the smooth flow of the resin. By means of vacuum - sealing tape, the film is sealed against the mold and the component and then using a vacuum pump ( rotary vane pump usually ) evacuated. The air pressure compresses the inserted parts and fixed it. The temperature-controlled liquid resin is drawn through the vacuum applied in the fibrous material. To prevent excess resin from falling into the vacuum pump after passing through the fibers, a resin brake and / or the resin trap is mounted upstream of the pump. After the fiber is completely impregnated, the resin supply is stopped and the impregnated FRP can be removed from the mold after curing. The curing times are dependent on the selected matrix material (resin) and the temperature. Advantage of this process is the almost uniform and bubble-free impregnation of the fibers and thus the quality of the components produced, and the reproducibility. There are already manufactured components such as rotor blades for wind turbines with a length of more than 50 meters with this procedure. Further developments to the vacuum infusion process are the Differential Pressure Resin Transfer Moulding ( RTM DP ) and single-line injection method (SLI).

• Filament winding The filament winding process is a technique for laying continuous fiber strands (rovings ) to one ( at least substantially ) cylindrical shape. With this method, fibers are very tight and close- lying position together with a high dimensional accuracy. To the winding of the fibers body is necessary, which gives the part its subsequent form. This body is called as usual during prototyping core. Also in filament winding, a distinction between lost and reusable cores. Lost cores are usually made ​​of lightweight foam that remains either in part or chemically dissolved. In wound pressure vessels peculiarity is that the thin-walled core ( consisting, for example, HD Polylethylen ) as a gas-tight barrier remains inside. Are these so-called liner made of metal, they can also be load-bearing and, together with the matrix of composite material, a hybrid system. Here the core is even "lost", but at the same time a functional part of the structure. Reusable cores are usually made ​​of aluminum; they restrict naturally a design freedom in the design, since the core must be removed from the PCB. Examples of filament-wound parts are lighthouses, cases of tram cars and buses or silos. When impregnation processes are common: The continuous fiber or strand is first passed through an impregnating bath in which it is wetted with the matrix material and is then wound around a form.
• There are wound prepreg fiber webs which are cured only by heating.
• There are unimpregnated fibers wound that interact with a resin injection method (see above) are soaked.

• Fiber spraying The fiber spraying is not strictly lamination, since the material is not in the layers (Latin: lamina ) is applied. However, the result of the application and the material are similar to the laminated product, so this technique is listed here. Spraying the fiber continuous fibers (rovings ) to be cut by a cutter to the desired length and taken together with the resin and hardener by means of a spray gun in the form of fiber. In addition to use as a hand lay-up the laminating roller, to compress the laminate. The main disadvantage of this alternative is the much lower resistance to laminated fabric.

Injection molding parts

Most of the parts made ​​of fiber- reinforced plastics are produced inexpensively by injection molding. Typical glass fibers for reinforcing the possibility to eg 11 microns thick and 300 microns long. Fibers of more than one millimeter in length apply in the plastics processing industry already as "long". A common matrix material, for example polyamide 6.6, the incorporation of glass fibers generally lies between 20 and 50 % by weight. An appropriate material, which is filled to 35% by weight with glass fibers, is characterized by " PA66GF35 ". The plastic manufacturer supplies the material in the form of pellets in which the glass fibers are already embedded in the matrix material. During melting in the extruder and spraying this mixture into the mold to align the fibers according to the flow direction more or less, so that the strength of the finished component is not equal to all places, not in all directions. Glass fibers also have an abrasive effect, so that the processing of glass fiber reinforced thermoplastic leads to increased wear and tear of the most steel injection mold, compared to non-reinforced plastic.

Molding parts

When molding or resin transfer molding ( RTM) dry fibers can be inserted and then flows around with liquid resin under pressure into a mold. By heat, the resin is cured. The fiber orientation can be adapted by sewing and embroidery process in the preform by selectively depositing the load cases.

Strand drawn parts

Compact and hollow profiles having dimensions of 1 mm diameter up to about 250 mm x 500 mm and outer dimensions essentially constant cross-sections can be produced very efficiently in the extrusion process. In this case, all of the fibers are oriented in the same lengthwise direction, resulting in a very good reproducibility. The mechanical properties can be influenced by the supply of rovings, mats and webs to a limited extent.

Sheet Molding Compounds ( SMC)

For sheet molding compound, a kind of fiber-reinforced plastics, in a prefabrication of resins, curing agents, fillers, additives, etc., and glass fiber to pieces of 50 mm length, a so-called resin-impregnated mat is manufactured. After a maturation period ( storage time ) for several days at 30 - 40 ° C, the viscosity of the resin mat increased from honey-like to wake up firmly coriaceous. This defines determined viscosity, depending on the resin formulation Matt, the mat can be further processed.

Further processing is then heated tools in the pressing process. The resin mat, and geometry, cut, depending on part size in well-defined sizes and positions according to a defined schedule insert in the tool. When closing the press, the prepreg is distributed throughout the tool. Here, the previously reached during the ripening period increase in viscosity decreases almost to the level of semi-finished products.

This leads to two phenomena:

The advantage of this class of materials lies in the easy representation of three-dimensional geometries and differences in wall thickness in a single operation. The final shape of the component is passed through the cavity of an at least two-part tool and usually exhibits both sides smooth, aesthetically pleasing surfaces.

After a curing time of 30 seconds to several minutes at temperatures of 140 ° C to 160 ° C - duration and amount of the component thickness and the reaction system used depends - the finished component can be removed from the mold. However, the component must be carefully cooled evenly due to the still high component temperatures, so there is no micro-cracks in the component. SMC components are - due to the greater fiber length than at BMC - usually higher loads than BMC components. SMC components can be used with an appropriate design in lacquered view areas.

Fiber concrete

The strength ( train and pressure) of concrete or cement can be increased by the addition of fibers. The fibers have only a few centimeters in length (the high modulus of elasticity of the concrete makes long fibers nonsensical ) and are disoriented distributed in the matrix. The result is an isotropic material. The fibers are like normal supplement mixed with the concrete and cured together in a formwork ...

Safety precautions during processing

Safety goggles and protective gloves provide a minimum level of protection against contact her with the resin system. Resin and hardener and accelerator specially often contained substances that act also allergy -promoting in addition to their toxicity. In the cured state, however, is partly achieved even food safety.

Accelerator and hardener are never added together directly. Both components can react violently, it may cause injury. Therefore, the accelerator is usually added to the resin prior to mixing with the hardener.

When machining ( machining ) of fiber reinforced plastics produces very fine particles that may be of respirable size depending on the fiber type. Therefore, a mouth guard is mandatory.

Carbon fiber dust can damage its electrical conductivity electrical equipment. Therefore, the processing is performed under hazardous.

Calculation of the elastic properties

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

Be calculated.

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.

For the exact calculation procedure is referred to the literature and textbooks.

Calculation and detection

The strength data, in particular fiber plastic composites, is via fracture criteria. Due to the brittleness and strength anisotropy of most fiber composites specific failure criteria for fiber plastic composites are necessary.

There are a number of different criteria and hence breakage detection methods. Often, individual firms have (for example, in the military or large civil aircraft ) develops its own detection methods.

Calculation programs

Compositor

This Excel-based program was developed by the Institute of Plastics Processing (IKV ) at RWTH Aachen. It contains - in addition to the calculation of the film stress and the engineering constants according to the classical laminate theory - a module in which the Puck'schen action level criteria (see: failure criteria for fiber plastic composites ) are implemented for strength analysis. In addition to the layered tensions and failure loads are therefore predictable.

ESAComp

ESAComp was developed on behalf of the European Space Agency ESA. It provides an interface to FE programs, but it can also be used without FE program. In addition to the layer-wise voltage analysis can be performed using different failure criteria failure loads.

ESAComp was developed at the Institute for Lightweight Structures at Helsinki University of Technology.

LamiCens

A freely available, easy -to-use Excel application for the determination of important properties of fiber-reinforced plastic laminates was developed by H. Funke. This semi-finished products should be selected with menu-driven and stack, as in the lamination. LamiCens determined production-specific characteristics such as laminate thickness and weight, fiber weight, resin consumption and cost variables. The engineering constants for the homogeneous disk load ( elastic moduli and shear moduli, and Querdehnzahlen ) are calculated using the classical laminate theory. A stress analysis is not possible.

Composite Star

This software was developed by the Belgian company Material SA, Brussels. In particular, it is to be used (same company ) in compounds with wound components made from fiber -plastic composite and the corresponding simulation software CADWIND.

ELamX is a freely usable, written in Java laminate calculation program, which was developed at the Department of Aircraft Engineering of the Technical University of Dresden. The calculations are based on the classical laminate theory. The software is modular and is constantly expanding. Currently ( January 2014 ) are modules for laminate calculation including stress analysis, engineering constants and hygrothermalen effects to stability, deformation and natural frequency tests any bearing fiber composite panels with and without stiffening elements, and for comparison of different strength criteria ( 3D representation of the failure body, The World -Wide Failure Exercise) available. Enhancements in the field of stability and deformation considerations and new modules, for example with respect to the micromechanics of laminated layers, the non-linear calculation according to VDI guideline VDI 2014 and the stability calculation using methods of aviation technical manual, are in development. eLamX 2.1 is available and still free to use since April 2014.

R & G laminate computer

Free Online laminate calculator that can quickly and easily calculate parameters such as thicknesses and resin consumption as well as the fiber content of laminates. Depending on the type of fiber reinforcement textiles and processing methods practical fiber volume fractions are proposed. The choice of parameters is menu- based. A reverse function is integrated, starting from a predetermined number of layers the laminate thickness can be determined. The input of own values ​​is possible.

Natural fiber composite material

Wood in its natural form is often grown template for the design of fiber - reinforced polymer composites. The reason for this is that wood fibers are just like other natural fibers composed of different " individual components ". Rigid cellulose fibrils are embedded in a matrix of hemicellulose and lignin, and serve as a strengthening element in the cell wall. Even in his artificially created forms pressboard or MDF at least the natural fibers are introduced as a component.

Bone is a composite fiber material in two respects: in the nanometer range, the collagen fibers are immersed in hydroxyapatite, cortical bone in the micrometer range osteon additionally act as fibers.

Areas of application

Fiber composites surround us in all walks of life, mostly without us being aware of it. The spectrum ranges from clothing, furniture, household appliances, to multi-storey buildings, bridges, boats, and the aerospace industry. The main application area for natural fiber reinforced plastics is the automotive industry.

Economic Importance

The fiber-reinforced composites with the greatest economic importance are the glass fiber-reinforced plastics (GRP ) with a share of over 90%. 2009 were produced in Europe, 815,000 tonnes of glass fiber reinforced plastics. The largest producers in the European market are Spain, Italy, Germany, Great Britain and France. Due to the economic crisis, the production amount in all application industries is equally shrunk by about a third compared to 2007. The strongest open processing methods such as hand lay-up or spray-up are affected by this market development. This general trend oppose alone the bio-based fiber composites. A comparison of economic development in the different sub-sectors showed that only the natural fiber-reinforced plastics are understood in growth - with a significant economic growth of over 20%.