Reinforcement components and manufacturing a composite material.
What is a composite?
A composite ("composite" or fiber-reinforced polymer FRP) is the result of combining two or more materials in order to obtain a unique combination of properties.
Composite materials have been widely used in the history in order to improve the properties of a material. Thus for centuries has been used the mud mixed with straw to build houses of adobe.
The fiber reinforced composite materials can be separated mechanically. The main feature of these materials is that a component forms a matrix that surrounds the rest of the material so they work as one, but both will maintained their original formats separately.
In the case of products for structural reinforcing fibers embedded in a polymer matrix used, the most common being the epoxy resin. This (the matrix) give it and stiffening environmental / chemical protection to the fibers.
Moreover, the most common fibers contain carbon, aramid and glass fibers, giving the "composite" high tensile strength and high modulus of elasticity.
The most important thing to consider is that fiber is the component that "absorbs" the tensile stresses in the axial direction thereof. Direction perpendicular to the direction of the fibers; the strength properties will be exclusively provided by the polymer matrix, being clearly inferior.
FUNCTIONS OF FIBER AND MATRIX
To understand the role of composite materials, it is important to know the function of each component in the assembly.
Main functions of the fibers:
1.Contribute required tensile strength against a tensile stress.
2.Contribute rigidity (high elastic modulus), tensile strength, and other parameters.
3.Conductividad or electrical insulation, depending on the fiber type.
The matrix provides vital properties to the composite improving performance:
1. Engage fibers to work together, and transfers them to the tensile stresses.
2. Separate fibers between them and separately working well. This avoids / slows down the propagation of cracks in the support.
La matriz actúa como un revestimiento protector de las fibras, la protección contra ataques mecánicos (golpes) y los productos químicos (medio ambiente, las sustancias agresivas, ...). Las fibras de carbono son conductores, mientras que la aramida y vidrio son aislantes.
SPECIAL FEATURES "COMPOSITES" VERSUS TRADITIONAL MATERIALS.
Composite materials have been designed and manufactured for applications requiring high performance with minimal dead load to the structure.
Some of the advantages offered over traditional composites reinforcements (typically metal-based solutions) are:
- 1. All metal parts can be replaced by a single equivalent composite section (or composite).
2.The composites have a high elastic modulus. They have a higher modulus than steel and weigh only a fifth of this .
3.The steel enters fatigue when subjected to 50 % of its tensile strength. The composites show no fatigue until at least 90 % of its tensile strength.
4.The composites do not rust . The steel and aluminum is oxidized in the presence of water and air, and require special care, being forced to use protective coatings . The polymer matrix of a composite protects the reinforcing fibers .
5.The thermal expansion coefficient of the composite is very close to zero . As a result, provide a high dimensional stability to the metal reinforcements.
6.The composites are manufactured in long lengths , allowing large spans without running boards , welding, machining parts , etc. . All this results in less time manufacturing and installation costs.
7.For the application of a composite , it requires light hand tools. Metal reinforcements must be installed using heavy equipment , props , welding, etc. . Installation costs of a composite reinforcement are very low and reduce the overall cost of a booster.
The fibers are the reinforcement of composite and give it stiffness and characteristic strength. The most common types of fiber are glass, aramid, carbon and boron. Aramid fiber is what gives a higher ratio of tensile strength to weight.
In our case we will focus on the carbon fibers being the most universal use.
• Fiber PAN (polyacrylonitrile) is a polymer textile material.
• PITCH fibers are obtained from coal tar or petroleum purified.
First, are the most widespread in the production of carbon fibers, the pitch fibers they provide greater rigidity while more fragile (and break elongation less).
During the manufacturing process, the raw materials are exposed to oxidation, employing extremely high temperatures.
Subsequently in carbonization and graphitization processes. During these processes, the original fibers undergo a series of chemical changes that bring higher level of stiffness to weight ratios and tensile strength-weight.
In order that the fibers wil responsible for absorbing the efforts , the matrix must be of low modulus and be more deformable than the reinforcement . This determines the temperature of the reinforcement system and the type of suitable manufacturing process for its production.
The carbon fiber laminates which are used in the structural reinforcement , are mainly composed of a matrix of thermosetting resin of epoxy type .
Thermosetting resins , have the particularity that once hardened can not re- melt or reshape because not return to the original state.
During this process in thermosetting resins, the dimensional chains are entangled with each other . As a result, the molecules are not flexible and therefore will not melt or remold to return to the original state of the resin. The greater the number of three dimensional intertwined chains , the greater the rigidity of the final product and the higher the glass transition temperature thereof.
For this reason, it is important to ensure proper temperatures when working with thermosetting resins manipulate . The rationale for the structural reinforcement of thermosetting resins is their high thermal and dimensional stability, good stiffness, as well as its high electrical resistance, chemical and dissolved , and offers a good impregnation of the fibers .
It is a very versatile type of resin as it has a wide range of properties. His great grip on all media makes it a widely used in a variety of product applications . To improve its performance , its properties are modified to suit the specific use as its glass transition temperature , curing time, viscosity , hardness, etc. .
The curing of an epoxy resin , start by mixing a hardener ( component II ) , creating a three-dimensional network of molecules and resulting in a solid epoxy that provides a high chemical and corrosion resistance . The glass transition temperature of such resins is higher , providing good performance at temperatures of 80-100 ° C.
The exposure of the epoxy resin UV (stiffeners exposed to sunlight ) can cause degradation. To avoid this we recommend applying a polyurethane coating .
The manufacturing process aims to provide the same in a given section . In the case of the structural reinforcement material is more usual rectangular section , but may be manufactured tubular sections ( vacuum tubes or solid ) , complex shapes such as automobile chassis , etc .
When processing carbon fiber laminates with thermosetting epoxy resin matrix is called pultrusion.
Pultrusion is a continuous manufacturing process , low cost , high-volume machine in which the fibers ( reinforcement ) impregnated with resin ( matrix ) are pulled at a constant to obtain a product of a pre- speed section . This process is similar to extruding metal through an orifice , stretching the same time pressing it through this . The first stage of the process, and channeled fibers impregnated in a resin bath . Subsequently, the material is passed impregnated by a mold at a temperature that ensures proper curing of the resin , controlling the content and gives the desired profile shape . Finally , cut to the desired length , the finished product is inspected and quality control tests are performed to certify that the technical product information is truthful.
By processing by pultrusion carbon fiber with epoxy matrix , we obtain the carbon fiber laminates . They are rigid facings of dark gray color.
Furthermore, there are sheets of carbon fiber. Filaments are formed by carbon fiber nonwovens, maintaining its orientation only by fiberglass yarns . Are unidirectional and flexible sheets , which allow the execution of rolled in situ by manual impregnation (sometimes using specific machinery ) with epoxy resin of formula adapted .
The fiber sheets are readily adaptable to irregular facing, while the composites manufactured in industry are rigid because they subministran pultruded and therefore can only be applied on flat surfaces .
Manufacturers of products facilitate quality control information of the supplied items, to certify the accuracy of the information detailed in the technical specifications of products.
Several types of tests to characterize the products forming the reinforcement system structure. They are contained in ASTM , UNE , JIS (Japan) , BS (British Standards ), NF (France ) , DIN (Germany ) standards. They normalize tackifying resins characterization and testing matrix and fiber reinforced plastic ( tensile strength, resin content in the matrix , the fiber volume , etc . ) . To test the properties , the rules require the test conditions (dimensions of the cylinder - type , ...) .
Different formats for carbon fiber :
Laminates 50 , 80, 100 and 120mm wide. Usually present in two commercial thicknesses: 1, 2 and 1.4 mm.
Fabrics of unidirectional carbon fiber :
Normal weight (200, 300, 400 g / m² ) for resin impregnation in their placing. In heavyweight530 g/m² para colocar por saturación de resina epoxi.