Aircraft Materials Construction and Repair PDF

NON-METALS AENG 213 - Aircraft Materials Construction and Repair  WOODS Although wood was used for the first airplanes because of its favorable strength-to-weight ratio, it is primarily the cost of the additional hand labor needed for wood constr...

NON-METALS AENG 213 - Aircraft Materials Construction and Repair  WOODS Although wood was used for the first airplanes because of its favorable strength-to-weight ratio, it is primarily the cost of the additional hand labor needed for wood construction and maintenance that has caused wood aircraft to become almost entirely superseded by those of all-metal construction. However, there are still many home-built airplanes that feature wood construction, and occasionally, commercial designs intended for low-volume production appear using some degree of wood in their structures. Orville Wright's famous first airplane flight.  Quality of Materials Sitka spruce is the reference wood used for aircraft structures because of its uniformity, strength, and excellent shock-resistance qualities. Wood Substitution Other types of wood are also approved for use in aircraft structures. However, the wood species used to repair a part should be the same as the original wood whenever possible. If using a wood substitute, it is the responsibility of the person making the repair to ensure that the wood meets all of the requirements for that repair. AC 43.13-lB outlines information regarding acceptable wood species substitutions. Woods Only certain species of wood are suitable for aircraft structures.  Woods Wood Assessment The cut of the wood, slope of the grain, and the number of growth rings are factors to examine when determining quality. The way wood is cut affects its shrinkage characteristics and strength qualities. The slope of the grain is another factor to consider when assessing wood.The maximum slope of the grain for aviation-grade lumber is 1:15. The slope of the grain is the amount of grain rise over the grain length. In other words, the grain may not rise more than one inch in a 15-inch section of wood. According to FAA standards, a grain slope of 1:15 is the maximum allowable slope allowed in aviation-grade wood.  Woods Following are several wood defects the technician must be able to identify to properly assess wood qualit.y Checks - A lengthwise separation or crack of the wood that extends along the woodgrain. Compression failure- Characterized by a buckling of fibers that appear as streaks on the surface of the wood that are at right angles to the grain. Decay - The destruction and eventual reduction of wood to its component sugars and base elements through attack by organisms such as fungi and certain insects such as termites Dry rot - A term loosely applied to any dry, crumbly rot but especially a wood easily crushed to dry powder in its advanced stage. Split - Longitudinal cracks produced by artificially Heartwood - The inner core of a woody stem or log, induced stress. extending from the pith to the sap, which is usually Shakes A separation or crack along the grain, darker in color. This part of the wood contains dead the greater part of which may occur at the cells that no longer participate in the life processes of common boundary of two rings or within growth the tree. rings. Knot - That portion of a branch or limb that is embedded in the wood of a tree trunk, or that has been surrounded by subsequent stem growth.  Woods Effect of Shrinkage When the moisture content of a wooden part is lowered, the part shrinks. Since the shrinkage is not equal in all directions, the mechanic should consider the effect that the repair may have on the completed structure. The shrinkage is greatest in a tangential direction (across the fibers and parallel to the growth rings), somewhat less in a radial direction (across the fibers and perpendicular to the growth rings), and is negligible in a longitudinal direction (parallel to the fibers).  Woods Types of Adhesives Most older airplanes were glued with casein glue, which was a powdered glue made from milk. Casein glue deteriorates over the years after it is exposed to moisture in the air and to wide variations in temperature. Plastic resin glue is a urea-formaldehyde resin that is water-, insect-, and mold-proof. Plastic resin glue rapidly deteriorates in hot, moist environments, and under cyclic stresses, making it obsolete for all aircraft structural repairs Resorcinol glue is a two-part synthetic resin glue consisting of a resin and a hardener and is the most water-resistant of the glues used. Resorcinol adhesive meets the strength and durability requirements of the FAA, making it one of the most common types of glue used in aircraft wood- structure repair. Phenol-formaldehyde glue is most commonly used in the manufacturing of aircraft-grade plywood. Phenol-formaldehyde glue requires high curing temperatures and pressures making it impractical for use in the field. Epoxy resins are two-part synthetic resins that generally consist of a resin and a hardener mixed together in specific quantities. Epoxies have excel-lent working properties and usually require less attention to joint quality or clamping pressures as compared to other aircraft adhesives Woods Bonding Process Following are the three most important requirements for a strong and durable structural bond. Preparation of the wood surface prior to applying the adhesive. Utilization of a good quality aircraft-standard adhesive that is properly prepared. Performing a good bonding technique consistent with the manufacturer's instructions.  Woods Applying The Adhesive It is important to observe the orientation of the wood grain to avoid applying glue to the end grain. End grain is wood that is cut at a 90° angle to the direction of the grain. An acceptable cut of wood has been cut nearly parallel to the direction of grain. Avoid end-grain joints when gluing wood scarf joints. Make sure the wood is cut with the grain of both pieces as close to parallel as possible. Using end-grain joints increases the chance of future warping. Woods Almost all types of adhesives have four time periods that are critical to the bonding process. Pot life is the useable life of the glue from the time it is mixed until the time it must be used. Discard the glue once the pot life has expired. The open-assembly time is the allowable time between the application of the glue and the time the joint is assembled. If the open-assembly time is too long, the glue will begin to set up on the joint surfaces and the glue line will weaken. The closed-assembly time is the allowable length of time between the assembling of the joint and the application of the clamping pressure. Closed-assembly time allows for the movement of parts to place them in the proper alignment. The pressing time is the period during which the parts are pressed or clamped together and is essentially the adhesive curing period. Pressing time must be sufficient to ensure that the joint is strong enough to withstand manipulation or the machining process. Assignment Create an infographic timeline of events relating the use of woods in Aviation Innovation.  Woods SOLID WOOD  Woods LAMINATED WOOD  Woods Quality of Wood o Knots ▪ Knots are the part where the tree branch sprouted from the trunk. There are different types of shapes of knots depending on the cut from the wood e.g. round, oval, or spiked. ▪ Hard knots are allowed up to 3/8 inch of its dimension but with some restrictions. ▪ Small pin-knots will only be allowed if it does not cause any grain imperfections to the wood. o Pitch Pockets ▪ It is a type of wood imperfection caused by small holes in the annual rings of a tree. Woods with pitch pockets are only allowed to be used if they are 14 inches apart and with a volume of 11/2-inches x 1/8-inch x 1/8-inch deep. o Compression Wood ▪ NEVER a good wood for aircraft construction and repair. ▪ Wood taken from a tilted tree as it grows that lead to its wood having a denser and weaker wood structure than a normal grown tree. o Compression Failure ▪ Should not be mistaken for compression wood ▪ Usually can be identified by its irregular and thread-like line on the grains ▪ Also, NEVER a good wood for aircraft construction and repair.  Woods Quality of Wood o Checks, Shakes, and Splits ▪ A check is a crack running through or across the annual grain ▪ A shake is a crack or separation in which it can be seen from a detached two annual rings along its boundaries o Stains and Decay ▪ Stains are caused by a decay on the wood usually appearing as streaks in the grains. ▪ An evidence of a decay is the stain that uniformly discolored the annual rings. ▪ Decay on woods varies in color from red to white stains. ▪ Decay on woods no matter what its stage will lessen the toughness of the wood until it gets brittle with little to no strength at all. ▪ Decay on wood is caused by fungi growing in damp woods that eats its fiber. ▪ Decay can be minimized by properly drying the wood up to 20% and the application of wood varnishes for the wood to be protected from the elements.  PLASTICS Plastics are an important component in aircraft construction. Its application to aircraft construction ranges from a thermoset plastic reinforced fiberglass to thermoplastic material for windows.  COMPOSITES Composites are combinations of two or more materials that differ in composition or form. Advantages reduces weight, that means if weight can be saved, more cargo, fuel or passengers can be carried high strength to weight ratio reducing of parts and fasteners reducing wear corrosion resistance Disadvantages generally expensive not easy to repair; that means you need well trained staff, tools, equipment and facilities to repair composite components Composites Elements of Composite Structure In aircraft construction, most currently produced composites consist of a reinforcing material to provide the structural strength, joined with a matrix material to serve as the bonding substance. The three main parts of a fiber-reinforced composite are the fiber, matrix, and interface or boundary between the individual elements of the composite. 1. Reinforcing Fiber Reinforcing fibers provide the primary structural strength to the composite structure when combined with a matrix. Reinforcing fibers can be used in conjunction with one another (hybrids), woven into specific patterns (fiber science), combined with other materials such as rigid foams (sandwich structures), or simply used in combination with various matrix materials. Composites Fiberglass (Glass Cloth) Fiberglass is made from small strands of molten silica glass that are spun together and woven into cloth. One of the disadvantages of fiberglass is that it weighs more and has less strength than most other composite fibers. However, with newly developed matrix formulas, fiberglass is an excellent reinforcing fiber currently used in advanced composite applications. The two most common types of fiberglass are S- glass and E-glass. E-glass, otherwise known as "electric glass" because of its high resistivity to current flow, is produced from borosilicate glass and is the most common type of fiberglass used for reinforcement S-glass is produced from magnesia-alumina-silicate, and is used where a very high tensile strength fiberglass is needed. Composites Aramid In the early 1970s, DuPont introduced aramid, an organic aromatic polyamide polymer, commercially known as Kevlar. Aramid exhibits high tensile strength, exceptional flexibility, high tensile stiffness, low compressive properties, and excellent toughness. Aramid fibers are non-conductive and produce no galvanic reaction with metals. Another important advantage is its strength-to-weight ratio; it is very light compared to other composite materials. Aramid-reinforced composites also demonstrate excellent vibration-damping characteristics in addition to a high degree of shatter and fatigue resistance. Composites Carbon/Graphite Advantages to carbon/graphite materials are in their high compressive strength and degree of stiffness. However, carbon fiber is cathodic while aluminum and steel are anodic. Thus, carbon pro-motes galvanic corrosion when bonded to aluminum or steel, and special corrosion control techniques are needed to prevent this occurrence. Carbon/graphite materials are kept separate from aluminum components when sealants and corrosion barriers, such as fiberglass, are placed at the interfaces between composites and metals Carbon/graphite is a black fiber that is very strong, stiff, and used primarily for its rigid strength characteristics. Fiber composites are used to fabricate primary structural components such as the ribs and skin surfaces of the wings Composites Boron Boron fibers are made by depositing the element boron onto a thin filament of tungsten. The resulting fiber is approximately.004 inch in diameter, has excellent compressive strength and stiffness, and is extremely hard. However, boron is not commonly used in civil aviation because it can be hazardous to work with, and is extremely expensive. Ceramic Ceramic fibers are used where a high-temperature application is needed. This form of composite will retain most of its strength and flexibility at temperatures up to 2,20°0F. Tiles on the Space Shuttle are made of a special ceramic composite that dissipates heat quickly. Some firewalls are also made of ceramic-fiber composites. Composites 2. Core Material Core materials are the central members of an assembly and are used extensively in advanced composite construction. When bonded between two thin face sheets, a component can be made rigid and lightweight. Composite structures manufactured in this manner are sometimes referred to as sandwich construction. Honeycomb Cores Honeycomb core materials consist of the six-sided shape of a natural honeycomb, which provides a core with a very high strength-to-weight ratio. The ribbon direction of a honeycomb core is the direction in which the honeycomb can be pulled apart. It is important to line up the ribbon direction of the replacement honeycomb core with that of the original when performing a repair honeycomb core repair to ensure consistent structural strength along with uniform compressive strength Honeycomb Sandwich Construction Composites Foam Cores Foam core materials offer different densities and temperature characteristics for high-heat applications and fire resistance. When using foams in a repair operation, it is important to use the proper type and density. Styrofoam, urethane foam, poly vinyl chloride (PVC), andstrux are several common types of foam cores used in aircraft composite construction. ▪ Styrofoam (Polystyrene)is commonly used on home-built aircraft and should only be used with an epoxy resin.Polyester resins dissolve Styrofoam. Styrofoam is comprised of smaller cells, which pro-duce a much stronger core material. ▪ Urethane foam can be used with epoxy or polyester resins. However,urethane cannot be cut with a hot wire. Subjecting urethane foam to high heat pro-duces a hazardous gas. ▪ Other foam core materials include poly vinyl chloride (PVC),and strux (cellular, cellulose acetate) foam. PVC foam can be used with either polyester or epoxy resins and cut with a hot wire. Strux foam is commonly used to build up ribs or other structural supports. Composites ▪ Phenolic has very good fire resistant properties and can have very low density, but relatively low mechanical properties. ▪ Polypropylene is used to make airfoil shapes; can be cut with a hot wire; compatible with most adhesives and epoxy resins;not for use with polyester resins, dissolves in fuels and solvents. Wood Cores Balsa wood or laminations of hard wood which are bonded to laminates of high-strength materials are occasionally used for other types of composite sandwich construction. Wood core materials provide high compressive strength to composite structures. Composites 3. Matrix Material The function of the matrix in a composite is to hold the reinforcing fibers in a desired position. It also gives the composite strength and transfers external stresses to the fibers. The ability of the matrix to transfer stress is the key to the strength of a composite structure. Resin is an organic polymer used as a matrix to contain the reinforcing fibers in a composite material. Polyester resin, an example of an earlier matrix, used in conjunction with fiberglass has been used in many non-structural applications such as fairings and spinner.s Resin matrix systems are a type of plastic and include two general categories: thermoplastic and thermosetting. Thermoplastic and thermosetting resins by themselves do not have sufficient strength for use in structural applications. However, plastic matrixes reinforced with other materials form high-strength, lightweight structural composites. Composites Thermosetting Resins Thermosetting resins use heat to form and irreversibly set the shape of the part. Thermosetting plastics, once cured, cannot be reformed even if they are reheated. At this time, most structural airframe applications are constructed with thermosetting resins. ▪ Polyester resin, an early thermosetting matrix formula, is mainly used with fiberglass composites to create non structural applications such as fairings, spinners, and aircraft trim. ▪ Epoxy resin matrices are two-part systems consisting of a resin and a catalyst. The catalyst acts as a curing agent by initiating the chemical reaction of the hardening epoxy. Epoxy resin systems are well known for their outstanding adhesion, strength, and resistance to moisture and chemicals. They are also useful for bonding nonporous and dissimilar materials, such as metal parts to composite components. ▪ Vinyl Ester resin, the corrosion resistance and mechanical properties are much improved over standard polyester resin composites. ▪ Phenolic (Phenol-formaldehyde) resin are used for interior components because of their low smoke and flammability characteristics. ▪ Polyimide resins excel in high-temperature environments where their thermal resistance, oxidative stability, low coefficient of thermal expansion, and solvent resistance benefit the design. Composites Thermoplastic Resins Thermoplastic resins use heat to form the part into the desired shape. However, this shape is not necessarily permanent. If a thermoplastic resin is re-heated, it will soften and could easily change shape. Types of Thermoplastic Material used for Aircraft Windshield and Side Window:s 1. Cellulose Acetate – transparent and lightweight. It has a tendency to shrink and turn yellow. When applied with acetone it softens. 2. Acrylic – identified by trade names as Lucite or Plexiglass or in Britain Perspex. It is stiffer than cellulose acetate. More transparent and for all purpose is colorless. It burns with a clear flame and produces a fairly pleasant odor. If acetone is applied to acrylic it leaves a white residue but remains hard. Composites Curing Stages of Resins Thermosetting resins use a chemical reaction to cure.There are three curing stages, which are called A, B, and C A stage: The components of the resin (base material and hardener) have been mixed but the chemical reaction has not started. The resin is in the A stage during a wet layup procedure. B stage: The components of the resin have been mixed and the chemical reaction has started. The material has thickened and is tacky. The resins of prepreg materials are in the B stage. To prevent further curing the resin is placed in a freezer at 0 °F. In the frozen state, the resin of the prepreg material stays in the B stage. The Curing starts when the material is removed from the freezer and warmed again. C stage: The resin is fully cured. Some resins cure at room temperature and others need an elevated temperature cure cycle to fully cure. Composites Working with Resins and Catalyst pot life - is the amount of time a catalyzed resin remains in a workable state. shelf life - is the time that the product is still good in an unopened container. resin rich - if too much resin is applied to thepart. resin starved - is one where not enough resin was applied, which weakens thepart. 60:40 fiber to resin ratio- for advanced composite lay-ups is generally consid-ered the best for strength characteristics The resin and catalyst are divided into separate containers that are attached on one end. When ready for use, the partition, which separates the resin from the cata-lyst, is broken to allow the two to mix Composites Fiber Science The strength of a reinforcing material within a composite is dependent upon the weave of the material, the wetting process (how the matrix is applied), filament tensile strength, and the design of the part. The strength and stiffness of a composite build up depends upon the orientation of the plies relative to the load direction while a sheet metal component will have the same strength no matter in which direction it is tested. Fabric Orientation When working with composite fibers, it is important to understand the construction and orientation of the fabric because all design, manufacturing, and repair work begins with the orientation of the fabric.  Composites Warp The warp of threads in a section of fabric run the length of the fabric as it comes off the roll or bolt. Warp direction is designated as 0°. There are typically more threads woven into the warp direction than the fill direction, making it stronger in the warp direction. Composites Weft/Fill Weft or fill threads of the fabric are those that run perpendicular (90°) to the warp fibers. The weft threads interweave with the warp threads to create the reinforcing cloth Selvage Edge The selvage edge of the fabric is the tightly woven edge parallel to the warp direction, which prevents edges from unraveling. The selvage edge is removed before the fabric is utilized. Bias The bias is the fiber orientation that runs at a 45° angle (diagonal) to the warp threads. The bias allows for manipulation of the fabric to form contoured shapes. Fabrics can often be stretched along the bias but seldom along the warp or fill.  Composites Fabric Style Fabrics used in composite construction are manufactured in several different styles: unidirectional, bi- directional, multidirectional, and mat. Component designers can use any or all of these fabric styles, depending on the strength and flexibility requirements of the component part. Unidirectional Unidirectional fiber orientation is one in which all of the major fibers run in one direction, giving the majority of its strength in a single direction Bi-directional/Multi-directional Bi-directional or multi-directional fabric orientation calls for the fibers to run in two or more directions. Bi- directional fabrics are woven with the warp threads usually outnumbering the weft, so there is usually more strength in the warp direction than the fill. Mats Mat fabrics consist of chopped fibers compressed together and typically used in combination with woven or unidirectional fabrics. A mat is not as strong as a unidirectional or bi-directional fabric, and is therefore is no commonly used alone in repair work.  Composites Unidirectional Fabrics are not woven together. Warp Fiberglass mat provides the high strength of glass to fibers run parallel to each other and are kept in place by reinforce thermosetting resins without the expense of small cross threads. woven cloth. Composites Pre-Impregnated Materials Pre-impregnated fabrics, commonly known as "prepregs," are fabrics that have the resin system already saturated into the fabric. Fabrics are pre-impregnated with the proper amount and weight of a resin matrix to eliminate the mixing and application details such as proper mix ratios and application procedures. Manufacturers often prepare these fabrics by dipping the woven fabric into a resin solution containing the proper amount of resin and catalyst, weighed and mixed together. A catalyst is a substance that changes the rate of a chemical reaction. Composites Fillers Fillers, also known as thixotropic agents, are materials added to resins to control viscosity and weight, to increase pot life and cured strength, and to make the application of the resin easier. Fillers increase the volume of the resin, making it less dense and less susceptible to cracking, as well as lowering the weight of the material. Microballoons are small spheres manufactured from plastic or glass.Plastic microballoons must be mixed with a compatible resin system that will not dissolve the plastic. Glass microballoons, on the other hand, are not affected by resin mixtures, making them the primary thixotropic agent used in composite construction. Microballoons reduces the overall weight, and they provide lower stress concentrations throughout the structure but do not add strength to the composite structure. Chopped fibers are made from any type of fiber cut into certain lengths, commonly 1/4 to 1/2-inch lengths. Flox is the fuzzy fiber taken from the fabric strands. Both chopped fibers and flox may be used when added strength is desired. Composites Adhesives Film Adhesives Structural adhesives for aerospace applications are generally supplied as thin films supported on a release paper and stored under refrigerated conditions (–18 °C, or 0 °F). Paste Adhesives Paste adhesives are used as an alternative to film adhesive. These are often used to secondary bond repair patches to damaged parts and also used in places where film adhesive is difficult to apply. Paste adhesives for structural bonding are made mostly from epoxy. Foaming Adhesives Foaming adhesives are used to splice pieces of honeycomb together in a sandwich construction and to bond repair plugs to theexisting core during a prepreg repair. Composites Upper Left: Film Adhesives Upper Right: Paste Adhesives Bottom: Foam Adhesives Composites Manufacturing Process In general, most composite manufacturers augment the strength of the finished product by applying heat and pressure to the matrix/fiber mix as it cures, which accomplishes several things: 1. The heat and pressure facilitates the complete saturation of the fiber material. 2. The heat serves to accelerate the curing process of the matrix. In some instances, a high temperatures required to effect a proper cure of the matrix formula. 3. The pressure squeezes out excess resin and air pockets from the reinforcing fibers, which helps to produce a more even blend of fiber and matrix. Compression Molding Compression molding is a manufacturing process that uses a male and female mold to form the part. It is normally used to manufacture a large number of precision-formed parts Vacuum Bagging Vacuum bagging is the most commonly used method to apply pressure to composite repairs. With this technique, the assembly is placed into a plastic bag and the air is then withdrawn by the use of a vacuum source. Composites Filament Winding Another manufacturing method that produces incredibly strong structures is the filament winding method. A continuous thread of reinforcing fiber is wound around a mandrel in the same shape of the desired part. Wet Lay-up This technique simply involves the mixing of the fiber reinforcement with the matrix, then laying the wet fabric over a surface forcuring. Although this technique is less precise than other manufacturing methods, it is the most flexible procedure available.  Composites Vacuum Bagging Compression Molding Wet Lay-up Filament Winding