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The essence of the concept of composites is this: the bulk phase accepts the load over a large surface area, and transfers it to the reinforcement, which being stiffer, increases the strength of the composite. The significance here lies in that there are numerous matrix materials and as many fiber types, which can be combined in countless ways to produce just the desired properties.
A composite material consists of two or more physically or chemically distinct, suitably arranged or distributed phases, with an interface separating system.
In this Essay we will discuss about the reinforcement constructions for the wing skins, rocket motors, floor beams with service damage or lead to premature failures.
Reinforcements:- Reinforcing constituents in composites, as the word indicates, provide the strength that makes the composite what it is. But they also serve certain additional purposes of heat resistance or conduction, resistance to corrosion and provide rigidity. Reinforcement can be made to perform all or one of these functions as per the requirements.
Reinforcements are classified as
Classification of Composites:-
The reinforcement system in a composite material strongly determines the strengthening mechanism in a composite. It is thus convenient to classify composites according to the characteristics of the reinforcement, such as length, orientation etc.
A) Fibers: Reinforcements are not necessarily in the form of long fibers. They can be particles, whiskers, discontinuous fibers, sheets etc. A great majority of materials is stronger and stiffer in the fibrous form than in any other form. This explains the emphasis on using fibers in composite materials design.
There are many naturally occurring fibers: cotton, flax, jute, hemp, ramie, wood, straw, hair, wool, silk etc., but these have varying properties, and present many processing challenges.
Fibers used in advanced composites have very high strength and stiffness but low density.
They also should be very flexible (to allow a variety of methods for processing) and have high aspect ratio (length/diameter), that allows a large fraction of the applied to be transferred via the matrix to the fiber.
The most common and inexpensive fiber used is glass fiber, usually for the reinforcement of polymer matrices.
Typical composition is 50-60% SiO2, and other oxides of Al, Ca, Mg, Na, etc.
"Sizing" is a treatment applied to glass fibers to protect the strands from surface defects, bind the filaments into a strand, and reinforce the interface bond to the matrix. It is usually based on PVAc and silane coupling agents.
Glass fibers are available as:
Moisture decreases glass fiber strength.
Glass fibers are susceptible to static fatigue, i.e. they cannot withstand static loads for long periods of time.
Density is quite low (~2.55 g/cc)
Tensile strength is quite high (~1.8 GPa)
Stiffness is, however, low (70 GPa) (i.e., strains more easily)
Therefore, whereas strength/density is high, stiffness/density is low.
Good dimensional stability, resistant to heat, cold and corrosion
Strong covalent bonds in polymers, if aligned long the fiber axis of high molecular weight chains, can lead to impressive properties.
Two examples are UHMPE (ultra high molecular weight polethylene) called Spectra, made by Allied Corp., and Kevlar made by DuPont.
Spectra is a very light fiber (denisty ~0.97 g/cc) made from gels and solution. It has a stiffness of about 200 GPa. Its primary disadvantage is its low melting point (around 150 Celsius), but this may not be an issue in biomedical applications.
Kevlar is an aramid (aromatic polyamide) composed of oriented aromatic chains, which makes them rigid rod-like polymers. It has a very high Tg and poor solubility. Since very concentrated acids are used in its processing, this can be an issue in biomedical applications if all acid residues are not extracted. Although very strong in tension, Kevlar has very poor compression properties. Its stiffness can be as high as 125 GPa. The fibers are mostly used to increase toughness in otherwise brittle matrices.
B) Matrix materials
Polymers, metals and ceramic material structure and properties have been covered in previous lectures and will not be repeated here.
1) The interface is a bounding surface or zone where a discontinuity occurs, whether physical, mechanical, chemical etc.
2) More often than not, the interface between fiber and matrix is rather rough, instead of ideal planar.
3) The matrix material must "wet" the fiber. Coupling agents are frequently used to improve wettability. Well "wetted" fibers increase the interface surface area.
4) To obtain desirable properties in a composite, the applied load should be effectively transferred from the matrix to the fibers via the interface. This means that the interface must be large and exhibit strong adhesion between fibers and matrix. Failure at the interface (called debonding) may or may not be desirable. This will be explained later in fracture propagation modes.
5) Bonding with the matrix can be either weak van der Walls forces or strong covalent bonds.
6) The internal surface area of the interface can go as high as 3000 cm2/cm3.
7) Interfacial strength is measured by simple tests that induce adhesive failure between the fibers and the matrix. The most common is the Three-point bend test or ILSS (interlaminar shear stress test)
Component form and manufacture:
The fibre reinforcement is essentially a one dimensional strengthening process; a major function of the component-forming process is to orientate the fibers in the matrix in the appropriate directions and proportions to obtain the desired two-dimensional or 3-dimensional mechanical properties. The forming processes must not damage the fibres and must ensure that they are reasonably evenly distributed in a matrix, free from significant voiding or from large areas devoid of fibers.
There are several methods that can be used to arrage the fibers when forming the composite structure. They are
Laminating woven cloth or aligned fiber sheets
Filament winding onto a rotating mandrel
Braiding onto a rotating mandrel
Tow placement and
Coming into the reinforcement construction processes for the aerospace parts like wing skins, rocket motors and floor beams the above processes are used and the manufacturing processes are as follows;
Aeroplane wing skins:-
Manufacturing process: - In this process, sheets of reinforcement, pre-coated with resin(pre-preg) or with resin freshly applied, are forced against the surface of a mould under the required conditions of pressure, temperature, and time.
In the laminating process the manufacturing of aero space wings, the epoxy pre preg is useda as the primary material in the reinforcement which is considered as the harder material and is used in the process.
A pre-preg is formed by the reinforcement fabrics and fibre type. The material is woven Bi-directional cloth pre-preg is most commonly made from plain weave or satin weave fabrics. The method of pre-impregnation is to infuse the cloth with the matrix resin diluted with the solvent to lower its viscosity and the pre-preg passes through a heating tower to remove the solvent and stage resin. The hot melt method involves first continuously casting resin film on a non-stick backing film of coated paper or polymer. The blade is used to control the thickness of the resin film applied and the reinforcement is sandwiched between two of these films as the material passes through a pair of heated rollers. Unidirectional pre-preg is made by spreading and collimating many fibre tows into a uniform sheet of parallel fibres typically 0.1 to 0.2 thick and this is impregnated and provides laminates with best mechanical properties.
The pre-preg is kept at room temperature and it is moved into the cutting room as the lay up room is maintained as a clean room free of dust and specified temperatures. The pre-preg is unrolled onto the cutting table with its backing paper still in place. plies of the required size, shape and fibre orientation are then cut from the roll. Cutting may be done by oscillating blade as well as lasers and water jets.
Some aeroplane parts are still laid up by skilled workers. To reduce Lay-up times Automatic stacking procedures are used for the large shapes. As the wing skins are produced by tape laying
The lay-up is prepared for the curing for that an auto clave or vacuum bag is applied over the surface of the lay-up materials and sealed to the mold.so that a consolidated pressure can be applied during cure as the gases generated are removed from the system.
The wing skin with the thermosetting matrices are cured at elevated temperatures to ensure that service temperature of the composite is sufficiently high .As the component is cured in the oven under a vacuum bag has best result and the pressure is above one atmosphere. As the lay-up will be loaded into the auto claves and check sensor connections before closing the door. The viscosity of the resin falls with increasing temperature until the resin begin to chemically cross-link(gel).pressurization and heating will be started and target pressure is reached. So the gelation occurs to allow removal of entrapped gases and the removal of excess resins and the reduced autoclave leads to poor quality of the laminates.
After the curing the part is normally cooled to below 60ÂÂ°c and removed from the autoclave and then Finishing like trimming, drilling and the coatings has been done.
Failure or damage in laminate process:
As shown in the figure a unidirectional laminate loaded successively in two different manners the maximum normal stress has the same value denoted as ÃÆ’.
In the loading case the unidirectional specimen will rupture when,
Different modes of failure:
Manufacturing of Aerospace rocket motors: - For the Manufacturing the rocket motors the filament winding is the appropriate process. The filament winding is automated processes for creating parts of continuous resin impregnated fibers are wound over a rotating tool called mandrel. There are two types of processes in filament winding the first one is polar or planer methods and the second type is the high helix pattern winding.
The polar or planer method of winding utilizes a fixed mandrel and a shuttle that revolves around the longitudinal axis of the part to form longitudinal winding patterns.
In the high helical pattern winding, the mandrel rotates while the shuttle transverses back and forth. Both the mandrel rotation and shuttle movement are in the horizontal plane. By controlling the mandrel rotation and shuttle speed, the fibre angle can be controlled. Angles of 25ÂÂ°-85ÂÂ° to the mandrel rotation axis are possible.
Figure shows helical pattern winding
The filament winding is illustrated in detail in the figure. The fiber spools are mounted on a rack, called a creel, and then strands from many spools are gathered together and fed through a comb or similar alignment device so they make a band of fibers. The number of strands brought together determines the width of the band. The band then enters a resin bath where the fibers are soaked with resin. The resin is fully activated with initiator or hardener so that the only requirements to cure the part are heat and time. The fibers then go through a roller or wiper system to remove the excess resin and then through a ring or some other directing device called a payoff. The payoff directs the fibers onto a mandrel.
In the beginning, the fibers re manually fed through the system. The mandrel pulls them from the fiber spools through the system. Then the mandrel pulls the fiber spools, the payoff is mounted on a carriage, then it is synchronized with the turning of the mandrel to produce patterns.
The patterns are of many types the pattern depends on the angle of mandrel. The pattern depends on the angel of the mandrel. The patterns are Hoop or circumferential, helical, Longitudinal or axial.
Fig shows the different types of patterns
After winding the parts they are cooled at room temperature, or in ovens depending on the size and nature of resin. Then the resin is distributed evenly. The mandrel is removed and the material is ready, if in case the mandrel is too hard it is left and not removed. Thus the rocket motors casing are done by the filament winding or wound process.
The advantages of this process are excellent mechanical properties due to continuous fibers, High degree of design flexibility and also economical method. Resin content can be controlled by metering the resin onto each fiber tow through nips and dies.
Damage caused by filament winding:
A rocket motor case was fabricated by winding a finite width tape over the base layer which was mostly made of inplane wound layer as shown in Figure. Transverse cracks initiated from the surface would spread in the thickness direction to reach the interface between the hoop layer and the base layer as if the hoop layer was a unidirectional one, since the inclined angle of the fibers in the hoop layer was very small. Then, the full depth transverse crack could propagate either one of the two slight inclined fiber directions. This kind of damage can occur statistically in the hoop layer and possible damage state in the hoop layer. At the portion the stress in the hoop direction can be high.
Figure shows the winding structure of the motor case. The hoop layer wound on the modified inplane layer becomes an angle-ply laminate with very small angle.
In the above figure Initiation and propagation of the transverse cracks in the hoop layer is shown: (a) initial crack grows in the full depth of the hoop layer, (b) the crack with the full depth of the hoop-layer propagates parallel to one of the two fiber directions with accompanying some fiber breakages.
Manufacturing of Aerospace floor beams: In the manufacturing of the floor beams pultrusion process is used. Pultrusion is a continuous, high-volume manufacturing process used to make parts of constant cross-section. The materials are pulled through the machine wherein they are formed by a die as the extrusion process. The process begins with continuous fibers drawn from reels and formed into a shape that allows for movement into a resin bath. The fibers are then wetted by the resin and then further formed as they converge towards die. The mats or cloth is used in the reinforcement to include some fibers in a machine direction as an additional requirement. The formed parts enters the pulling system. The puller provides the force for movement of particles through the system. The process is as shown in the diagram.
Pultrusion is a continuous process with high material utilization. This pultrusion operation is used in many industries where composites are used in a wide variety. More than 90% of all pultruded products are fibre glass reinforced polyester. When better corrosion resistance is required, vinyl ester resins are used. When a combination of superior mechanical and electrical properties is required, epoxy resin is used. Higher temperature resistance and superior mechanical properties generally dictate the use of epoxy resins reinforced with aramid or carbon fibres.
Figure shows the pultrusion process
The major advantage that the process used in aerospace is the ability to produce consistent parts at very low cost in a short period of time. Pultruded composite parts exhibit all the features produced by other composite processes, such as, high strength to weight ratio, corrosion resistance, dimensional stability, etc.
Conclusion: Although, there are many process for the manufacturing of aerospace materials there are some service damages in the running time so if the material or fibers which are used in the manufacturing should be with high strength and also with greater mechanical properties then there can be a good aerospace design and manufacturing.