CONTEST. new applications and innovative uses for product design. e by Olympus-FRP

Transcription

1 CARBON FIBER D E S I G N CONTEST new applications and innovative uses for product design technical c sheet e by Olympus-FRP

2 1. Composite materials and their properties Composite materials represent the evolution of science and technologies of materials, since they combine their best features that determine their extremely high physical and mechanical properties. The study of composites is a design philosophy that aims to optimize both the composition of material and the structural project within a converging and interactive process. In a historical perspective the idea of reinforcement with fi bers is a very old one. Even in the Bible there are references to the reinforcement of bricks with straw in ancient Egypt. Iron bars were used to reinforce the walls in the 19th century and this led to the development of reinforced concrete. Phenolic resins reinforced with asbestos were introduced during the 20th century. The fi rst fi berglass boat was built in 1942 and the reinforced plastics employed for aeronautics and electrical components date back to the same period. Wrapped items were invented in 1946 and used in the fi eld of rockets during the 50s. By the end of the 70s the application of composites widely expanded to the fi leds of aeronautics, automotive, sporting goods and biomedical industry. The 80s led to a signifi cant development in the use of fi bers with high modulus of elasticity. Today the emphasis is on developing modern mortar matrix composites and hybrid matrices with mortars and epoxy resin for high temperature applications. They have a number of different applications: underground pipes, containers, ships, ground vehicles, aircraft and space structures, applications in civil building, automotive components, sport equipments, biomedical products and many other products designed for high mechanical performances and / or dimensional stability. 2. Definition and properties A composite material is defi ned as a system consisting of two or more phases, whose properties and performances are designed in such a way as to be higher than those of the constituent materials that act independently. Usually, one of the two phases is discontinuous, stiffer and stronger and is called reinforcement, while the less rigid and weaker one, is continuous and is called the matrix. Sometimes because of chemical interactions or other effects, there may also be an additional phase, called interphase, between reinforcement and matrix. The properties of a composite depend on the properties of its constituents, on geometry and on the distribution of phases. One of the most important parameters is the volume (or weight) of the reinforcement fraction or the volume ratio of fi bers. The distribution of the reinforcement determines the characteristics of the system. The less uniform the reinforcement, the more heterogeneous the material and the higher the probability of breaking in the weakest areas. The geometry and orientation of reinforcement, however, affect the anisotropy of the system. The phases of the composite play different roles depending on the type and implementation of the composite. In the case of low or medium performance composites, reinforcement usually with short fi bers or particles- provides some tightening but only locally strengthens the material. On the other hand matrix is the main component to bear the loads and determines the mechanical properties of the material. In the case of high- structural performance composites, reinforcement usually consists of continuous fi bers and forms the skeleton of the material, providing stiffness and strength towards the fi ber direction. The matrix phase provides protection, support to the fi bers and the transfer of local strains from one fi ber to another. The interphase, although of a small size, can play an important role in controlling the breaking mechanism, the breaking strength and, above all, the behavior strains/ deformation of the material. 3. The fibers Due to their small size fi bers show an unusual structural perfection, this feature, combined with the intrinsic properties of the constituent materials, assures them high mechanical strength, very high elastic modulus, very low specifi c gravity and linear elastic behavior up to the breaking point. The most important fi bers for their use in composites can be glass, carbon, organic and mineral. They can be found in the composites or in the form of continuous fi bers arranged parallel to each other in a fl at surface or in the form of chopped fi bers arranged in a fl at surface with a random orientation (MAT) or, fi nally, they can follow a confi guration warp/weft and arranged in a fl at surface.

3 4. The carbon fibers For a long time, the most used fi bers in structural applications of composites were the glass fi bers. Although they show a good strength and a low density, they have a relatively low modulus of elasticity. For this reason, some 25 years ago, experimentation began to convert organic composite fi bers and fi bers in carbon and graphite. The high mechanical properties of carbon fi bers derive from the particular crystal structure of graphite. The higher the crystal structure, the higher are the properties of the material. A crystal of graphite has a structure consisting of plane layers made out of carbon atoms. The bonds between the same atoms contained in the same layer are strong (covalent bonds), while those between atoms in different layers are relatively weak (van der Waals bonds); it is, thus, evident that the crystals are highly anisotropic structures and the manufacturing process will be responsible of arranging the crystalline structure in the desired direction. Of course this is not an easy task, since in practice we can never get perfect crystals and precision in orientation, so the resulting mechanical properties are lower than the theoretical ones. The carbon fi bers are obtained by graphitizing in an inert atmosphere, to over 2000 C, organic textile fi bers of rayon or polyacrylonitrile (PAN). The starting fi bers are called precursors. During the process of graphitization of the fi bers they are subject to tension, the greater is the traction stress exerted, the higher is the Young s modulus of the product. On the other hand the increase of the module is balanced by a decrease in resistance. So, available on the market there are both high modulus carbon fi bers, penalized in the resistance, and low modulus ones showing an higher resistance. The two types are referred to as C1 and C3 or with Anglo-Saxon terminology, HM (standing for High Modulus ) and HS (standing for High strength, high tensile strength). Compared to glass fi bers, the carbon ones show three major advantages: a very high elastic modulus, a low density and a very low coeffi cient of thermal expansion. For this reason they are replacing glass fi bers in all those fi elds where alongside with a low weight, high stiffness or high dimensional stability to varying temperatures are required. The production costs of carbon fi bers are substantially higher than those of glass fi bers, but their wide diffusion is justifi ed by their strong mechanical properties.

4 5. Manufacturing Technologies There is a great number of manufacturing technologies used for the construction of items made out of composite materials; materials with extremely high physical and mechanical properties and a high volumetric percentage of fi bers can be obtained as well as items with lower properties and with considerably reduced production costs. The manufacturing technologies for components made out of composite materials vary according to the shape, size and properties required to the fi nished item. Depending on the desired characteristics of the composite element, whether or not it should be reproducible or required for a continuous production process, the existing technologies can be divided into technologies using closed or open molds or they can also be divided into continuous or discontinuous and manual or automated. The term open mold indicates a mold able to obtain a single controlled well-defi ned surface. In the case of reinforcement systems in civil engineering molds consist in the same structural elements that are reinforced. The open-mold processes are commonly fi t for the manufacture of voluminous parts: in these cases it would be almost impossible to use closed molds for their diffi cult handling due to overweight. In the fi eld of civil engineering, when using dry fi bers, impregnation is performed using a brush during the creation of the detail and at the same time the layers are modelled on the surface of the mold that in this case consists in the masonry bearing or reinforced concrete. The inevitable air bubbles between the layers are removed by performing a rolling and possibly, when better results are required, using a vacuum bag. This series of operations, however, has some drawbacks compared to more advanced manufacturing technologies: fi rst, when impregnating the fi bers by hand a higher quantity of resin than that strictly needed is used and it is very diffi cult, even using vacuum forming, to remove the surplus; consequently the composite will be of poorer quality since highly mobile fi ber layers increase the diffi culties of running a good vacuum bag. However the manufacturing processes of composite materials are different. The most common are: a. manual lay-up; b. resin transfer molding (RTM); c. fi lament winding; d. pultrusion; e. Vacuum infusion (RIFT); f. production in an autoclave. Manual impregnation without application of pressure or vacuum It is a process still widely used for works on large surfaces such as swimming pools or boat hulls, where production typically involves small lots and is the most widely used manufacturing process in civil engineering. Reinforcements in the form of mat or fabric as a percentage of project, are applied inside the mold that, in the case of civil engineering applications, consists in the masonry bearing or reinforced concrete, then, fi bers are impregnated with catalyzed resin and successively manually consolidated using metal or plastic rolls in order to remove excess resin. The polymerization generally takes place at room temperature. In some cases, in order to improve the quality of the pressed laminate, the impregnation of fabrics is performed prior to their installation with a special equipment so to use the adequate amount of resin for each laminate. Filament Winding The process consists essentially in winding resin-impregnated continuous fi laments around a rotating body, called chuck, whose shape is identifi ed in the geometry of the workpiece to be produced. The hardening of resin is obtained by putting the component in the oven or in the autoclave. The major factors involved in this production technology and having a decisive infl uence on the features of the fi nal composite product obtained are: - The type of winding; - The type of impregnation; - The type of chuck; - The type of machine; - The type of polymerization process.

5 Pultrusion The meaning of the term pultrusion is very clear if you think of the technologic model behind the process. In fact, while the extrusion of aluminum or thermoplastics is accomplished by exerting a pressure on the material to force it to pass through the mold, in the case of reinforced plastics, the same shape can be obtained by exerting a pulling force (pull) on the fi bers forcing them to pass, after being wetted with the resin, into the mold. So, the pressure typical of the extrusion is replaced by the action of pulling (pull), hence the term pultrusion, become in Italian pultrusione. The pultrusion technology is characterized by the continuity of production. When the system is equipped with an automatic fl ying saw the production takes place with a minimum human intervention, limited to the startup and the check for any interruptions of power supply of reinforcement well as of the level of resin into the impregnation tank. The high tensile strength and the high percentage of reinforcement obtained, combined with other important properties such as electrical insulation, resistance to corrosion and low weight, have broadened the range of pultruded products for applications such as rods for insulators, walkways, platforms and railings, ladders, pipes for isolators and fuses, highway barriers, structural beams and many more. The process requires a fi brous reinforcement, essentially a continuous one, and a low viscosity resin, usually a liquid thermosetting. The reinforcement most widely used are the glass rovings. The basic process is: 1. reinforcement s power supply; 2. impregnation; 3. preforming; 4. forming and polymerization; 5. pulling; 6. cutting; 7. postforming. Resin injection molding (RTM resin transfer molding) It is a technology used for the production of polymer matrix composites by injecting the catalysed resin into a cavity having the same shape as the piece to be produced and where a dry reinforcement was previously laid up The cavity is obtained by leaning mold and countermold one against the other. These can be of different types, most commonly fi berglass or metal molds are used. The RTM manufacturing steps can be summarized in the following phases: 1. mold cleaning 2. application of release agent 3. application of gel-coat 4. layup of reinforcement 5. closing and locking of the mold 6. injection and polymerization of the resin 7. mold opening and ejection 8. fi nishing operations The technology of resin injection molding is a very interesting one, for it offers the possibility of automation and thus allows the production of components even in large series. Moreover, even items of considerable size can be printed. Resin Infusion Under Flexible Tooling (RIFT) The RIFT (Resin Infusion Under Flexible Tooling) process, a variant of RTM, is performed using the polymer fl exible bag instead of using one of the sides of the rigid mold. It is more cost effective and, since vacuum is created inside, the resin is pushed inside the dry reinforcement so to reduce the contact of the operator with the resin in a liquid state and with all its volatile components released during the implementation. For low volume production alternative resins are used, low styrene content epoxy or polyester resins. In the short term this may be an effective solution, but in the long run the process must be redesigned so to exploit the benefi ts of the more cost-effective systems and to reduce safety risks. The RIFT, as mentioned above, mainly differs from the RTM for the use of a fl exible polymer fi lm instead of a rigid mold. Vacuum is created through specifi c outlet points in order to compact reinforcement, while resin is let into through other points and then, under the action of the external atmospheric pressure, is conveyed on the reinforcement through some distribution channels. The impregnation obtained with the fl ow can be of two different kinds: the fi rst one provides a normal direction of the fl ow towards the plane of position of the reinforcement, a second way involves an impregnation with two mixed fl ows, a normal one and a fl ow parallel to the plane of position of the reinforcement, similar to that characterizing the RTM.

6 Forming in an autoclave The technology for the production of composite elements using the autoclave, allows the production of laminates with much higher mechanical properties compared to the more traditional and cost effective technologies, so far analyzed. The use of the autoclave allows an intensifi cation of the compaction action with an increase in pressure during the curing cycle up to about 7-10 atm and with temperatures up to 2000 C. The latest autoclaves offer the possibility to vary pressure and temperature during polymerization cycle, according to the best suitable laws for each specifi c type of resin used. A higher pressure avoids vacuum between the layers and allows to reach percentage of up to 65% which is the upper limit in the manufacture of composites. This kind of manufacturing is usually chosen when producing details with very special mechanical features, such as those required for aeronautics, space and medical applications. by Olympus-FRP