TRANSPARENT PLASTIC
STRUCTURES
INTRODUCTION
Since the time the Wright brothers built their first airplane of wood and fabric, there have been major advances in aircraft construction. Metals, first as steel tubing, and later as aluminum in monocoque-type construction, were a quantum leap forward in terms of the ability to manufacture aircraft quickly and economically. However, wood was used on early aircraft because of its availability and relatively high strength-to-weight ratio. Because wood is a resilient material when properly maintained, many older wooden aircraft still exist, while a few mod-ern designs continue to use wood in select components. This information is only presented as an overview, and as such, information on wood or composite repairs for a specific aircraft should be referenced from the applicable aircraft's structural repair manual. In the event such manuals do not exist, consult Advisory Circular 43.13-1B, Acceptable Methods, Techniques, and Practices/Aircraft Inspection and Repair, and approve all major
AIRCRAFT WOOD STRUCTURES
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hand labor needed for wood construction and maintenance that has caused wood aircraft to become almost entirely superseded by those of all-metal construc-tion. However, there are still many home-built air-planes that feature wood construction, and occa-sionally, commercial designs intended for low-vol-ume production appear using some degree of wood in their structures. [Figure 3-1]
Figure 3-1. This Bellanca Viking incorporates wooden spars in its airframe structure.
This section will provide information on the mate-rials, inspection, and repair of wood structures. For a detailed description of the components and func-tion of aircraft structures, refer to Chapter One of this textbook, Aircraft Structures and Assembly & Rigging.
QUALITY MATERIALS
Wood and adhesive materials used in aircraft repair should meet aircraft (AN) quality standards and be purchased from reputable distributors to ensure such quality. Strict adherence to the specifications in the aircraft structural-repair manual will ensure that the structure will be as strong as the original. WOOD
Sitka spruce is the reference wood used for aircraft structures because of its uniformity, strength, and
excellent shock-resistance qualities. Reputable companies that sell wood for use in aircraft repairs, stringently inspect and verify that the wood product meets the appropriate FAA specifications. To meet the "Aircraft Sitka Spruce" grade specification, the lumber must be kiln-dried to a government specifi-cation known as AN-W-2. This specifispecifi-cation requires that the specific gravity shall not be less than .36, the slope of the grain shall not be steeper than 1 to 15, the wood must be sawn vertical-grain (sometimes called edge-grained), and shall have no fewer than six annular rings per inch. Each of these specification characteristics is discussed in detail later in this section. Most Sitka spruce now comes from British Columbia and Alaska due to the deple-tion of old growth spruce forests in the United States, thus making quality spruce valuable and occasionally, limited in supply.
WOOD SUBSTITUTION
Other types of wood are also approved for use in air-craft 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. If a substitute wood product meets the same quality standards as the original wood, it is considered an acceptable alternative. For example, you may substitute laminated wood spars for solid-rectangular wood spars as long as they are manufactured from the same quality wood and they are produced under aviation standards.
AC 43.13-lB outlines information regarding accept-able wood species substitutions. If there is any question about the suitability of a specific piece or type of wood for a repair, it would be wise to get the approval of the aircraft manufacturer or local FAA inspector before using it on the aircraft. [Figure 3-2] PLYWOOD
Structural aircraft-grade plywood is more com-monly manufactured from African mahogany or American birch veneers that are bonded together in a hot press over hardwood cores of basswood or
Species of Wood Strength Properties as Compared to Spruce Maximum Permissible Grain Deviation (Slope og Grain) Remarks
Spruce 100% 1:15 Excellent for all causes. Considered as standard for this
table.
Douglas Fir Exceeds spruce 1:15 May be used as substitute for spruce in same sizes or in slightly reduced sizes providing reductions are
substantiated. Difficult to work with hand tools. Some tendency to split and splinter during fabrication. Large solid pieces should be avoided due to inspection difficulties. Gluing satisfactory.
Noble Fir Slightly exceeds
spruce except 8 percent deficient in shear
1:15 Satisfactory characteristics with respect to workability, warping and splitting. May be used as direct substitute for spruce in same sizes providing shear does not become critical. Hardness somewhat less than spruce. Gluing satifactory.
Western Hemlock Slightly exceeds spruce
1:15 Less uniform in texture than spruce. May be used as direct substitute for spruce. Gluing satisfactory. Pine, Northern White Properties
between 85 percent and 96 percent those of spruce
1:15 Excellent working qualities and uniforn in properties but somewhat low in hardness and shock-resisting capacity. Cannot be used as substitute for spruce without increase in sizes to compensate for lesser strength. Gluing satisfactory.
White Cedar, Port Orford Exceeds spruce 1:15 May be used as substitute for spruce in same sizes or in slightly reduced sizes providing reductions are substantiated. Easy to work with hand tools. Gluing difficult but satisfacory joints can be obtained if suitable precautions are taken.
Poplar, Yellow Slightly less than spruce except in compression (crushing) and shear
1:15 Excellent working qualities. Should not be used as a direct substitute for spruce without carefully accounting for slightly reduced strength properties. Somewhat low in shock-resisting capacity. Gluing satisfactory.
Figure 3-2. Only certain species of wood are suitable for aircraft structures. This figure outlines the different types of wood approved for aircraft structural repair along with the characteristics and properties of each type in comparison to the standard, Sitka spruce.
poplar. Basswood plywood is another type of avia-tion-grade plywood that is lighter and more flexible than mahogany and birch plywood but has slightly less structural strength. All aviation-grade plywood is manufactured to specifications outlined in MIL-P-6070, which calls for shear testing after immersion in boiling water for three hours to verify the adhesive qualities between the plies meets specifications.
LAMINATED WOOD
Laminated wood is constructed of two or more lay-ers of solid wood that are bonded together. The lam-ination process differs from the plywood process in that each layer of laminated wood is bonded with the grain running parallel with each other. Plywood, on the other hand, is constructed of wood layers that are bonded with the grain direction at a 90 angle to the previous layer.
Laminated wood is stronger but less flexible than a piece of solid wood of the same type and size. However, laminated wood is much more resistant to warping than solid wood, making it a good substi-tute for solid wood components such as laminated spars in place of solid spars. Laminated wood is most commonly utilized for components that require curved shapes such as wing-tip bows and fuselage formers.
WOOD ASSESSMENT
Aircraft technicians who take on a wooden struc-tural repair must be able to properly assess the wood used. Familiarity with the quality and condi-tion of the wood along with the types of defects inherent to wood products is essential to competent wood assessment. The technician must make cer-tain that the quality of wood meets the original specifications outlined in the aircraft's repair
3-4 Wood, Composite, and Transparent Plastic Structures
manual. The following information describes wood characteristics that the maintenance technician must consider for proper wood assessment, not only for the initial use of a wood product, but also in the inspection phase of wooden structures.
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. Aviation-quality wood is usually quarter-sawed to reduce the amount of shrinkage over the life of the component. Quarter-sawn wood is cut from quar-tered logs so that the annual growth rings are at 90 angles to the wide face.
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. [Figure 3-3]
which stresses the importance of accurate identifi-cation of wood defects. Following are several wood defects the technician must be able to identify to properly assess wood quality. [Figure 3-4]
Figure 3-4. This figure illustrates several wood defects that a technician must be able to identify when evaluating wood condition and quality.
Figure 3-3. According to FAA standards, a grain slope of 1:15 is the maximum allowable slope allowed in aviation-grade wood.
Another factor to consider when assessing wood is the number of growth rings per inch. To accurately calculate the number of rings, look at the end of the board and count the number of growth rings in one inch. The minimum grain count for softwoods is six rings per inch. Port Oxford white cedar and Douglas fir are exceptions. The minimum grain count for these woods is eight rings per inch.
Certain defects are allowed and others disallowed when choosing the appropriate species of wood,
Brown rot - Any decay in wood that produces a light to dark brown, easily crumbled residue. An advanced stage of brown is referred to as "cubical rot" that splits the wood along rectangular planes.
Checks - A lengthwise separation or crack of the wood that extends along the wood grain. It devel-ops during drying and is commonly caused by differences in radial and tangential shrinkage or because of uneven shrinkage of the tissues in adjacent portions of the wood.
Compression failure - Characterized by a buck-ling of fibers that appear as streaks on the surface of the wood that are at right angles to the grain. Compression failures vary from pronounced fail-ures to very fine hairlines that require close inspection. This defect is caused from the wood being overstressed in compression due to: natural forces during the growth of the tree, felling trees on rough or irregular ground, or rough handling of logs or lumber.
Compression wood - Characterized by high spe-cific gravity, it has the appearance of an excessive growth of summerwood. Compression shows lit-tle or no contrast in color between springwood and summerwood, making it a difficult defect to identify. If you have any doubt whether a piece of wood is compression wood or not, reject it. Cross grain Wood in which the direction of the fibers or grain deviate from a line parallel to the sides. Crossed grain may look like diagonal grain, spiral grain, or a combination of the two.
Curly grain - Wood with distorted fibers result-ing in a curly appearance as in bird's-eye wood. The area covered by each curl may vary up to several inches in diameter.
Decay - The destruction and eventual reduction of wood to its component sugars and base ele-ments through attack by organisms such as fungi and certain insects such as termites; may also be referred to as "dote." Red heart and purple heart are also forms of decay.
Dry rot - A term loosely applied to any dry, crumbly rot but especially a wood easily crushed to dry powder in its advanced stage.
Hard knots - A knot that is solid across the sur-face, at least as hard as the surrounding wood, and shows no indication of decay.
Heartwood - The inner core of a woody stem or log, extending from the pith to the sap, which is usually darker in color. This part of the wood contains dead cells that no longer participate in the life processes of the tree.
Interlocked grain - Grain in which the direction of the fibers first follow a left- then a right-handed spiral, then alternate in a spiral direction every few years. Such wood is very difficult to split radially, although it may split easily in the tangential direction.
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. Mineral streaks - An olive to greenish-black or brown discoloration believed to show regions of abnormal concentrations of mineral matter in some hardwoods. Mineral staining is common in hard maple, hickory, and basswood.
Pin knot clusters Pin knots are knots with diameters less than or equal to 1/2 inch. Several pin knots in close proximity to each other make up a cluster.
Pitch pocket - Lens-shaped opening extending parallel to the annual growth rings in certain coniferous woods. May be empty or may contain liquid or solid resin.
Spike knots - Knots that run completely through the depth of the wood perpendicular to the annual rings. Spike knots appear most frequently in quarter-sawn lumber.
Split - Longitudinal cracks produced by artifi-cially induced stress.
Spiral grain - Wood in which the fibers follow a regular spiral direction (right-handed or left-handed) around the trunk of the tree instead of the normal vertical course. Spiral grain is a form of a cross grain.
• Shakes A separation or crack along the grain, the greater part of which may occur at the com mon boundary of two rings or within growth rings
• Wavy grain - Wood in which the collective
appearance of the fibers presents a regular form of waves and undulations.
ACCEPTABLE DEFECTS
Certain types of wood defects are permitted in avia-tion-grade lumber. The following list of permissible wood defects applies to the species of wood listed in figure 3-2.
• Cross grain. Spiral grain, diagonal grain, or a
combination of the two is acceptable providing the grain does not diverge from the longitudinal axis of the material more than specified in col umn 3 of figure 3-2.
• Wavy, curly, and interlocked grain. Acceptable if local irregularities do not exceed limitations
specified for spiral and diagonal grain.
• Hard knots. Sound, hard knots up to 3/8 inch in diameter are acceptable providing: (1) they are not projecting portions of I-beams, along the
edges of rectangular or beveled un-routed beams, or along the edges of flanges of box beams except in lowly stressed portions; 2) they do not cause grain divergence at the edges of the board or in the flanges of beams more than specified in col umn 3 of figure 3-2; and 3) they are in the center third of the beam and are not closer than 20 inches to another knot or other defect.
• Pin knot clusters. Small clusters are acceptable providing they produce only a small deviation of grain direction.
• Pitch pockets. Acceptable in the center portion of a beam providing they are at least 14 inches apart when they lie in the same growth ring and do not exceed 1-1/2 inches in length by 1/8 inch in depth.
• Mineral streaks. Acceptable providing that there is no decay indicated anywhere on the wood. NON-ACCEPTABLE DEFECTS
While there are certain defects that are allowed in wooden aircraft structures, there are more that are not acceptable. Choosing a section of wood with non-acceptable defects increases the chance of future structural failure. The following is a list of non-acceptable wood defects.
• Cross grain. Not acceptable unless they are
within the limitations specified in the descrip tion of acceptable cross-grain defects listed pre viously.
3-6 Wood, Composite, and Transparent Plastic Structures
• Wavy, curly, and interlocked grain. Not accept able unless they are within the limitations speci fied in the description of acceptable defects
listed previously.
• Hard knots. Not acceptable unless they are
within the limitations specified in the descrip tion of acceptable defects listed previously. • Pin knot clusters. Not acceptable if they produce
a large effect on the direction of the grain. • Spike knots. Reject wood that contains this type
of defect.
• Pitch pockets. Not acceptable unless they are within the limitations specified in the descrip tion of acceptable defects listed previously. • Mineral streaks. Not acceptable if any decay is
found.
• Checks, shakes, and splits. Reject wood contain ing these defects.
• Compression wood. Reject 'wood that indicates compression wood.
• Compression failures. Reject wood that contains an obvious compression failure. If there is a ques tion as to whether wood indicates compression failure, perform a microscopic inspection or
toughness test.
• Decay. Reject wood that indicates any form of decay or rot including indications of red heart or purple heart.
AIRCRAFT ADHESIVES/GLUES
The adhesive used in aircraft structural repair plays a critical role in the overall finished strength of the structure. The maintenance technician must only use those types of adhesives that meet the perfor-mance requirements necessary for use in aircraft structures. Not every type of glue is appropriate for use in all aircraft repair situations. Because of its importance, use each type of glue in strict accor-dance with the aircraft and adhesive manufacturer's instructions.
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 tem-perature. Many of the more modern adhesives are incompatible with casein glue. If a joint that has
been glued with casein is to be re-bonded with a dif-ferent type of glue, scrape all traces of the casein away before applying the new glue. The alkaline nature of casein glue may prevent the new glue from curing properly, thereby compromising the struc-tural integrity. The performance of casein glue is considered inferior to other available products and should be considered obsolete for all aircraft repairs.
Plastic resin glue is a urea-formaldehyde resin that is water-, insect-, and mold-proof. This type of glue usually comes in a powdered form. Mix it with water and apply it to one side of the joint. Apply a hardener to the other side of the joint, clamp the two sides together and the adhesive will begin to set. Mix plastic resin glue in the exact proportions specified by the manufacturer, otherwise the adhe-sive properties may be impaired.
Plastic resin glue rapidly deteriorates in hot, moist environments, and under cyclic stresses, making it obsolete for all aircraft structural repairs. Any use of this type of glue for aircraft repair should be dis-cussed with the appropriate FAA representative prior to use on certificated aircraft.
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. The glue is ready for use as soon as the appropriate amount of hard-ener and resin has been thoroughly mixed. Resorcinol adhesive meets the strength and durabil-ity requirements of the FAA, making it one of the most common types of glue used in aircraft wood-structure repair. Again, follow the aircraft manufacturer's recommendations when choosing the type of glue for any structural 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 gen-erally 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. They pene-trate evenly and completely into wood and plywood structures. However, varying degrees of humidity and temperature affects the joint durability in dif-ferent epoxies. Only use the recommended epoxy as outlined in the aircraft's repair manual.
THE BONDING PROCESS
The bonding process is critical to the structural strength of an aircraft wooden structure. To ensure the structural integrity of a wood joint, the bonding process must be carefully controlled. It is impera-tive to follow the manufacturer's repair procedures in detail when producing a wood joint.
Following are the three most important require-ments for a strong and durable structural bond. • Preparation of the wood surface prior to apply
ing 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.
WOOD PREPARATION
It is imperative to properly prepare the wood face prior to applying any adhesive. The wood sur-face must be clean, dry, and free of any oil, grease, or wax, otherwise the adhesive will not penetrate the wood evenly. Without proper adhesive penetra-tion, you will not gain a proper glue line, which "will weaken the joint. The glue provides the strength of a properly prepared wood splice-joint. In addition, wood changes in dimension according to its mois-ture content, therefore, the pieces of wood to be joined should be kept in the same room for a mini-mum of 24 hours to equalize the moisture content. Cut the wood to the required bevel with a fine-toothed saw, then plane or scrape the surface until it is smooth and true. Planer marks, chipped or loosened grain, and other surface irregularities are not permissible. The wood joint must join evenly over the entire bonded surface to produce a strong and durable bond. Do not use sandpaper to smooth the surface. Sanding may round corners and change the flatness of the wood surface resulting in a joint that does not properly meet. Sanding also produces dust that fills the ■wood pores and causes a weak glue line. Roughening the wood surface is also not recommended because it will prevent uniform contact of the wood surface, which is necessary for strong and durable glue joints. Before applying adhesive to the joint surfaces, vacuum them to remove anything that remains which may prevent glue penetration.
When wood surfaces cannot be freshly machined before bonding, such as plywood or inaccessible members, sand them lightly using a very fine grit such as 220. Very light sanding improves the
pene-tration quotient of the adhesive in these cases only. However, heavy sanding will change the flatness of the wood and deposit sawdust in its pores. Again, make sure the surfaces are clean and dry before applying adhesive.
APPLYING THE ADHESIVE
When the wood surfaces are prepared and ready to be glued, apply a smooth, even coat of glue to each surface. Then, following the adhesive manufac-turer's recommended procedures, join the surfaces together. 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. [Figure 3-5]
Figure 3-5. 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.
Almost all types of adhesives have four time-peri-ods 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. Using glue after the pot life has expired or adding thinners to the adhesive will not extend its life.
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 sur-faces and the glue line will weaken. Different types of adhesives have varying open-assembly times. Follow the adhesive manufacturer's procedures explicitly when bonding a structure.
The closed-assembly time is the allowable length of time between the assembling of the joint and the
3-8 Wood, Composite, and Transparent Plastic Structures
application of the clamping pressure. Closed-assem-bly 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 essen-tially the adhesive curing period. Pressing time must be sufficient to ensure that the joint is strong enough to withstand manipulation or the machin-ing process. The temperature of the bond line also affects the cure rate of the glue. Each type of glue requires a specific temperature during the curing cycle. [Figure 3-6]
Figure 3-6.This chart outlines an example of a manufacturer's recommended bonding times, pressure period, and assembly temperature for a specific type of plastic resin glue. Each type of adhesive has specific time-periods and procedures to be followed absolutely. If you waver from the manufacturer's pro-cedures, you must either discard the wood parts or remove the adhesive, clean the bond line, and start over. The struc-tural integrity of the joint will be compromised if the manu-facturer's procedures are not followed to the letter.
CLAMPING PRESSURE
When the joint is connected and properly aligned, apply pressure to spread the adhesive into a thin, continuous film between the wood layers. The strength of a glue line is partially dependent upon the correct pressure applied during the curing process. Clamping forces air out of the joint and brings the wood surfaces together evenly. Too little clamping pressure results in thick glue lines and weak glue joints. Too much clamping pressure can squeeze out too much glue weakening the joint. Clamping pressure is accomplished using clamps, presses, or by other mechanical means. Each type of adhesive requires a specific amount of clamping pressure. Therefore, follow the adhesive manufac-turer's gluing procedures in detail. For example, the recommended clamping pressure for soft-woods is between 125 and 150 psi and between 150 and 200 psi for hardwoods when using resorcinol glue.
METHODS OF APPLYING PRESSURE
In addition to the amount of clamping pressure, the method used to apply pressure is also important. Different methods range from the use of brads, nails, small screws, and clamps, to the use of hydraulic and electrical presses. The choice of clamping method is important to achieving a strong and durable joint.
Hand nailing is one method of applying pressure using small nails or screws in the bonding of ribs, attachment of plywood skins to the wing, control surfaces, and fuselage frames. However, both nails and screws can produce adverse effects such as splitting small parts and creating points where moisture may enter the wood structure causing decay. If you decide to utilize the hand nailing method, nailing strips are often used to spread the pressure over a larger area and to help in the removal of the nails after the glue has cured. To pre-vent the nailing strip from sticking to the wood structure, place a piece of waxed paper between the strip and the structure.
The nails or screws used in the hand nailing method may or may not be removed after the adhesive has cured. Nails used for clamping pressure are not intended to hold the structure together for strength purposes. When using nails, be careful not to crush the wood with heavy hammer blows and do not pen-etrate all the way through the wood structure. [Figure 3-7]
Figure 3-7. Nail strips may be used for clamping pressure on plywood skin during the bonding process. Remove nail strips once the glue is cured. Before applying the finish, fill the nail holes with a manufacturer's recommended wood filler to prevent any areas at which moisture may enter the structure.
Another common method is the use of screw clamps or "C" clamps in conjunction with pressure blocks. Pressure blocks distribute the clamping pressure and protect the members from local crushing. Clamps and pressure blocks apply pressure evenly over the entire glue joint to form a thin, even glue line, which produces a strong joint. [Figure 3-8]
Figure 3-8. Apply pressure evenly over the entire joint to avoid gaps between the mating surfaces. An even clamping pressure ensures that the adhesive is squeezed out of the glue-joint uniformly. Insufficient or uneven pressure usually results in thick bond lines that weaken the joint.
Apply pressure to a joint for the time recommended by the glue manufacturer. When the clamping pres-sure is removed, clean and inspect the joint and remove any glue that has been squeezed from the joint.
INSPECTION OF WOOD
STRUCTURES
To effectively inspect wood structures, be familiar with methods of inspection and the equipment used to examine them. Also, be able to identify the types of defects that are common to wood structures, as well as the failure modes that are unique to them. Most wood damage is caused by conditions such as moisture, temperature, and sunlight. Because wood is an organic material, it is subject to mildew and rot unless protected from moisture. Keep wood air-planes in well-ventilated hangars and take special care to ensure that all of the drain and ventilation holes remain open. If a ventilation hole becomes obstructed, changes in air temperature will cause moisture to condense inside the structure, which will cause the wood to deteriorate.
TYPES OF DETERIORATION
The maintenance technician must be able to iden-tify wood deterioration to determine the airworthi-ness of a wood structure. Along with the list under Wood Defects discussed earlier in this section, the following are several of the more common types of wood deterioration.
Wood decay results from the attack and growth of fungus upon wood products. Decay is indicated by softness, swelling when wet, excessive shrink-age when dry, cracking, and discoloration. Musty or moldy odors also indicate wood decay.
• Splitting or cracking of a wood member may occur due to the varying shrinkage rates of
bonded wood members, or due to an outside force applied to the structure. Wood splits often result when different types of woods are bonded
together. For example, bonding a mahogany ply wood doubler to a spruce member may produce a split. As the spruce dries, it tries to shrink.
However, the mahogany plywood, which shrinks at a lower rate, holds the spruce firmly in place. The induced stress in the spruce member exceeds its cross-grain strength, thus resulting in a split. • Bond failure is most commonly due
to an
improper bonding process or prolonged exposure to moisture. Using the wrong type of glue, not fol lowing the manufacturer's bonding procedures or improper wood preparation can all lead to bond failure of the wood joint.
• Finish failure is the breakdown of the protective finish applied to the wood structure to prevent decay. Finish failure results from long-term expo sure to water, wood splitting, ultra-violet light, and surface abrasion.
• Stress damage is caused by excessive impact,
mechanical, or aerodynamic loads imposed upon the wood structure. Over-tightening of fittings can also cause wood crush and possible bending of the metal fittings. Some applications use steel bush ings to prevent the bolts from being tightened to a point where the wood is crushed. Such bushings also add bearing strength to the assembly.
INSPECTION METHODS
When inspecting a wood structure aircraft, move it into a dry, well-ventilated hangar. Before beginning the inspection, remove all of the inspection and access panels to facilitate the drying of the wooden struc-tures. One of the first steps is to check the moisture content of the wood using a moisture meter. If the moisture content is high, dry the wood structures before inspecting further. Wooden structures of the air-craft need to be dry to be able to effectively determine the condition of the bonded joints. The following are several inspection methods and associated equipment employed for inspecting wooden structures.
MOISTURE METERING
Use moisture meters to determine the moisture con-tent of the wood structure. The moisture concon-tent of any wooden member is an important factor in its structural integrity. Wood that is too wet or too dry may compromise the strength and integrity of the structure. A moisture meter reads the moisture content through a probe that is inserted into a wooden member. Use a correction card to correct for temperature and the type of wood being tested.
3-70 Wood, Composite, and Transparent Plastic Structures
TAPPING
The wood structure may be inspected for structural integrity by tapping the suspect area with a light plastic hammer or screwdriver handle. Tapping should produce a sharp, solid noise from a solid piece of wood. If the wood area sounds hollow or feels soft, inspect further.
PROBING
If soft, hollow wood is found during the tap test, probe the suspect area with a sharp metal tool to determine whether the wood is solid. Ideally, the wood structure should feel firm and solid when probed. If the area feels soft and mushy, wood has rotted and disassembly of the structure is necessary to repair or replace the damaged area.
PRYING
Use prying to determine whether a bonded joint shows signs of separation. When prying a joint, be cautious not to use too much force, otherwise you may forcibly separate it. Light prying is sufficient to check the integrity of a joint. If there is any move-ment between the wood members of the joint, a fail-ure of the bond is confirmed. Repair or replace the bonded structure if a failure has occurred.
SMELLING
Smell is a good indicator of musty or moldy areas. When removing the inspection panels, be aware of any odors that may indicate damage to the wood structure. Odor is an essential indicator of possible wood deterioration. Musty and moldy odors reveal the existence of moisture and possible wood rot. VISUAL INSPECTION
Visual inspection techniques are used to determine any visible signs of damage. Both internal and exter-nal visual examinations are imperative to a com-plete inspection of the wood structure.
External Visual Inspection
Many airplanes that have an external skin made of thin mahogany plywood are covered with light-weight cotton or polyester fabric to increase both the strength and smoothness of its surface. A thor-ough inspection is required to ensure that the fabric covering has not pulled loose or torn away from the wood. A split or tear in the fabric could be an indi-cation of internal damage to the wooden structure. Subsequently, any known surface damage requires a careful inspection of the internal structure.
Minor bulging in the panels of a very light plywood structure may be acceptable. Refer to the aircraft manufacturer's repair manual for detailed specifica-tions. However, large bulges or any indication of the skin loosening requires careful examination to
deter-mine the source and extent of the damage. It is pos-sible for the layers of plywood to separate, or delam-inate, which is indicated by a slight hump in an otherwise smooth skin. Tap the suspected area with a coin. If the tapping produces a dull thudding noise rather than a solid ringing sound, it is possible that the plywood has delaminated. Determine the extent of the damage and repair or replace the skin. Internal Visual Inspection
The most likely place for wood deterioration to begin is the lowest point inside an aircraft's struc-ture while the airplane is in its normal ground atti-tude. Dirt collects at these low points and holds the moisture against the wood until the protective coat-ing is penetrated, wettcoat-ing the wood fibers. Since wood is an organic material, it is subject to mildew and rot unless it is adequately protected from mois-ture. For the best protection, treat wood structures with a rot-inhibiting sealer, then, after the sealant has dried for a specified length of time, cover the entire structure with good quality varnish.
Open and examine the internal structure if there is any reason to suspect glue failure or wood rot. This may entail creating inspection openings or even removing part of the skin. If any opening must be made, use procedures that are approved by the air-craft manufacturer or by a local FAA inspector. When the area of suspected damage is accessible, carefully scrape away all of the protective coating and examine the wood and glue lines. Be suspicious of any stains in the wood. Stains usually accompany decay and wood rot. Perform a probe test in the sus-pected area with a sharp point, dental probe, or other similar tool. If the wood pulls up in a chunk, it is rotten. However, if the wood splinters, it is usu-ally an indication that the wood is sound. Remove and replace any wood that shows signs of decay. Carefully check all of the glue lines for any indica-tion of separaindica-tion. Inspect glue lines with a magnify-ing glass, and then try to slip a thin feeler-gauge blade into any portion of the glue line that seems to be separated. If the blade inserts into the crack, the joint is not sound and must be repaired using the methods recommended by the aircraft manufacturer. To determine whether the glue failed or if the joint was forced apart, examine the surfaces of the dam-aged joint. If the joint separated and the glue surface showed an imprint of the wood but no wood fibers attached to the glue, the adhesive failed. However, if something physically forced the joint apart, pieces of wood would be attached to the glue surface.
If there are any wood screws in the area where decay is suspected, remove them and check to see if they
show any signs of corrosion or water stains. Replace the screw if the old one shows no indication of cor-rosion and the wood shows no sign of decay in or around the screw hole. Replace it with a screw of the same length but of the next larger size. Be sure that the replacement screw is made of the material spec-ified in the aircraft's illustrated-parts manual.
Wood spars utilize reinforcement plates made of birch plywood that are glued to the ends of any splices, under the butt-end fittings, and the strut attachment fittings. Carefully inspect these plates to ensure that they have not separated from the spar. If a glue line failure is indicated between the spar and the plate, remove the plate and all traces of the glue then install a new plate.
Shake the wing to detect any looseness between the struts and the wing spar. Any movement indicates possible elongation or wear at the bolthole. In this case, remove the bolts and carefully examine them and the boltholes for wear, cracking, or elongation. An elongated bolthole or any cracks near them require that you splice in a new section of spar or replace the entire spar. The manufacturer's repair manual outlines acceptable tolerances.
If a wooden structure has been subjected to any unusual strain or extreme loads, carefully inspect the main load-carrying members for any indication of compression failure on the side that carried the compressive load. A compression failure usually appears as a fine line across the grain, indicating that
the fibers in the wood have actually been ruptured. Replace any wood that shows this type of failure.
WOOD STRUCTURE REPAIR
The basic criterion for any aircraft repair is that the repaired structure must not only be as strong as the original structure, but the rigidity of the structure and the aerodynamic shape must also be equivalent. Materials used for the repair of a wooden structure should be the same as the original unless they have become obsolete. If substitutions are made, they must produce a repair that meets the basic require-ments of the manufacturer and the FAA.
WING SPAR REPAIRS
There are several types of wooden spars that are likely to be encountered in aircraft construction. Each type of spar is unique in design and requires specific repair procedures. Reference the aircraft manufacturer's repair manual for specific repair requirements. Some of the most common wood spars include solid spars, laminated spars with rec-tangular cross sections, and externally routed spars with cross sections resembling I-beams. The I-beam spar is routed to reduce weight while still providing adequate strength requirements. You may also encounter built-up box spars that utilize upper and lower flanges of solid spruce with webs of plywood. Other types include built-up I-beam spars with spruce webs and flanges, as well as internally routed box-spars made of two rectangular pieces of spruce glued together then routed to reduce weight. [Figure 3-9]
3-72 Wood, Composite, and Transparent Plastic Structures
SOLID WOOD SPAR REPAIRS
If an inspection reveals a longitudinal crack in a solid wood spar, repair it by carefully scraping away the finish on both sides of the spar and gluing reinforcing plates of spruce or plywood on each side of it. Reinforcing plates should be one-fourth as thick as the spar and extend beyond each end of the crack for at least three times the thickness of the spar. Bevel the ends of the reinforcing plates with a 5:1 taper to within 1/8-inch of the thickness of the plate, and attach the plates with glue, using no nails. Nails compromise the structure and produce moisture collection points, thus increasing the chance of wood decay. [Figure 3-10]
Splice or reinforce a wing spar at any point except under the attachment fittings for the wing root, landing gear, engine-mount, lift, or inter-plane struts. None of these fittings may overlap any part of a splice. If a splice will interfere with any of these fittings, you will have to change the design of the repair so that the spar can be repaired without inter-fering with the fittings. Regardless of the spar type, allow no more than two splices on a single spar. Attachments for minor fittings, such as those for drag or anti-drag wires or compression members, are allowed to pass through a spar splice with cer-tain restrictions. One restriction is that the rein-forcement plates for the splice are not allowed to interfere with the proper attachment or alignment of fittings. These fittings include pulley support
brack-ets, bellcrank support brackbrack-ets, or control surface support brackets. Do not alter the location of these fittings in any way. A second restriction dictates that the reinforcement plates may overlap drag or anti-drag wire or compression member fittings if the reinforcement plates are located on the front face of the front spar or on the rear face of the rear spar. In these situations, use new, longer length bolts to attach the components.
If a solid, laminated, or internally routed spar is damaged on either its top or bottom edge, repair them, providing that all of the damage can be removed without exceeding certain limits. Clean out the damaged material to a depth of no more than one-fourth of the spar thickness. Once the damage is removed, taper the ends of the area to a 5:1 slope then insert and glue a spruce block. Finally, glue spruce or plywood reinforcing-plates to each side of the spar, making them one-fourth the thickness of the spar and tapered to a 5:1 slope. [Figure 3-11] Splice solid or rectangular wood spars using a scarf repair that requires a taper of 1:10 or 1:12. Glue reinforcement plates to the end of the splice. If you decide to splice the spar without completely disas-sembling the wing, take special care to prepare the spar and the repair material. Cut the spar and the new material to the proper scarf angle.
Once the cut is prepared, put the two scarfed ends together and clamp them to a back-up board that is
Figure 3-10. A lack of strength in a solid wood spar caused by a longitudinal split can be restored by the addition of reinforcement plates on each side of the spar.
Figure 3-11. Repair of a damaged edge of a solid wood wing-spar requires replacement of the damaged material. After the plug is placed in the damaged area, glue reinforcement plates to both sides of the spar to increase its strength.
the same width as the spar and thick enough to give good, solid support. Be sure that the new material is perfectly straight and aligned with the original spar, then clamp it securely with cabinetmaker's parallel clamps or "C" clamps. Once secured, pass a fine-toothed crosscut saw through the scarf joint to remove material that does not match properly. Blow out all of the sawdust, loosen the clamps on the new material, and then tap the ends of the spar pieces to butt the two tightly together. Tighten the clamps again and make another cut to straighten both sides of the scarfed joint so the two pieces of wood match exactly. The strength of a scarf joint depends heavily on making sure the bevel cuts match precisely. To ensure a tight glue joint, use a very sharp plane or chisel to make a perfectly smooth surface with open pores. Do not use sandpaper to smooth the surface because sawdust will clog the pores and not allow the glue to properly adhere to the wood, weakening the joint. [Figure 3-12]
Spread the properly mixed glue on each prepared surface, join the pieces, and then apply even pres-sure, being sure that the spar is in correct edge alignment. When the glue has cured for the proper time, remove the clamps and pressure blocks and inspect the glue line, carefully cutting away any glue that squeezed from the joint. Once inspected
Figure 3-12. Clamping the two scarfed spar pieces together and making another scarf cut ensures a perfectly matched joint. If the initial cut produces rough or chipped wood, use a planer to smooth the surface; never sandpaper.
and cleaned, glue reinforcement plates over each end of the scarf. Make these plates one-fourth the thickness of the spar from solid spruce or plywood. Extend the reinforcement plates across the spar at six times the spar thickness on each side of the scarf line. Taper the ends of the plates to prevent an abrupt change in the cross sectional area of the repaired spar. Because the alignment of the fittings and attachments to a spar are critical, it is important
3-74 Wood, Composite, and Transparent Plastic Structures
Figure 3-13. A splice for a solid wood wing-spar requires reinforcement plates on each end of the splice.
not to drill the new boltholes in the spar until the splice is completed. [Figure 3-13]
Routed I-beam spars are spliced in much the same manner as solid rectangular spars. The exception is that the reinforcement plates installed on a routed I-beam spar must be one-half the thickness of the spar web and contoured to fit into the routed portion of the spar.
A built-up I-beam spar repair requires a 10:1 to 12:1 scarf joint between the original spar and the new material. This type of repair requires that you place solid spruce filler-blocks in between the spar flanges for added support. It also requires plywood reinforcement plates, one-half the spar web thick-ness, to be glued to the spar flanges and filler blocks to make a box-type repair at the splice. [Figure 3-14]
Figure 3-15. The splice for a built-up wood-box spar is critical. This type of spar carries the heaviest loads of all wood spars.
Built-up box spars carry the greatest loads of any of the wooden spars. For this reason, the built-up box spar repair is the most critical, and therefore requires the use of approved drawings as proper guidelines. The typical built-up box spar repair consists of removing portions of the webs from both sides of the spar and cutting the flanges to a 10:1 to 12:1 taper. Then spliced in new flanges and spruce reinforce-ment plates, one-half the thickness of the flanges, and install them on the inside of the spar. [Figure 3-15] When replacing the webs on repaired box-spars, install spruce filler blocks that are the same thick-ness as the flanges between the flanges. Scarf the undamaged portion of the web to a taper of 10:1 and install a filler-block at a point centered under each scarf joint. Stagger the scarf cuts in the two webs along the spar rather than directly across from each other to improve the strength of the webs. At this point, glue and nail the filler-block in place. Cut the new web section to an exact fit, then glue and nail it in place. When the glue has cured and all of the excess glue is removed, glue and nail a plywood cover strip over the end of the splice. [Figure 3-16] WING RIB REPAIRS
Wood wing ribs are usually made of spruce strips that have a cross section of approximately 1/4 to 5/16-inch. These small strips of wood accept the air loads from the covering of the wing and transmit them into the spars.
Figure 3-16. The splice for the web of a built-up wood box spar incorporates both filler blocks and plywood cover strips.
When manufacturing wing ribs, soften the upper and lower cap strips with steam before bending them over a form. Cap strips are the upper and lower surfaces that attach to the supporting web. These strips carry the bending loads of the wing and provide a surface for attaching the wing skin. When dry, place them in a jig and cut all of the cross members to fit between them. Cover each
3-76 Wood, Composite, and Transparent Plastic Structures intersection between a vertical member and a cap
strip with a gusset made of mahogany plywood. Glue the gussets to the strips and secure them with brads to provide the pressure needed to make strong glue joints. Slip the completed ribs over the spars and assemble and square up the wing truss with the drag and anti-drag wires adjusted to the proper tension.
CAP STRIP REPAIRS
If a cap strip is broken between two of the upright members, cut the strip to a taper 10 to 12 times its thickness. Then cut a new piece of the same type material with a matching taper. Cut a reinforcing block of spruce the same size as the cap strip and 16 times as long as its width, and glue it to the inside of the cap strip. Then cover both the cap strip and the reinforcement with plywood faceplates that are glued to the strip and held with brads. [Figure 3-17]
Figure 3-17. Use a rib cap-strip splice to repair a cap strip broken between two upright members.
If the damage is located above one of the upright members, cut the cap strip with a 10:1 to 12:1 taper with the center of the cut over the upright member. Then splice a new piece of cap-strip material into the structure. The upright member serves as the reinforcement so no block is needed under the splice. Put splice plates of thin plywood on each side of the splice so that none of the joints depends upon end-grain gluing. [Figure 3-18]
It is sometimes necessary to replace only the leading or trailing edge portion of a rib, so cap strips may be cut at a spar. When the cap strips are cut over a spar, use a 10:1 to 12:1 taper, and glue gussets of plywood the same size as the original to each side of the rib. [Figure 3-19]
Figure 3-18. Add splice plates to repair a cap strip broken over an upright member. When cutting splice plates, ensure that the grain is parallel to the grain of the cap strips to reduce the chance of warpage.
Figure 3-19. Use original size gussets on a rib-cap splice that is located over a spar.
TRAILING EDGE RIB REPAIR
The trailing edge of a wing is the area most likely to be damaged by moisture collecting and causing the wood to rot. All wings must incorporate drainage grommets at the lowest part of each rib bay to drain accumulated moisture. Drainage grommets also ven-tilate the compartment to prevent condensation. Occasionally, grommets will clog with dirt and not allow adequate drainage. Subsequently, moisture will collect around the wood structures producing an environment ripe for decay. If there is any move-ment when you flex the trailing edge, cut away the trailing edge fabric and examine the edge structure. If the rear end of the rib has rotted, cut it away and cut a spruce block to fit the removed rib section. Then cut reinforcing plates of plywood, glue them into place, and fasten new trailing edge finishing materials to the repaired rib. [Figure 3-20]
Figure 3-20. Repair to the trailing edge of a wood wing rib
requires replacement of the damaged portion with a wood block. Install a larger gusset to reinforce the structure.
Be sure to treat the wood repair with a rot-resistant sealer before re-covering the structure. When replacing corroded metal sections of the support structure, protect the new metal parts with a corro-sion inhibiting primer such as zinc chromate. Most wooden wings utilize metal compression members. However, wooden compression members are used on certain aircraft. When it is necessary to repair wooden compression members, use a 10:1 to 12:1 scarf joint. Glue reinforcing plates to each side of the splice that are made of the same material as the strip and 12 times its thickness. Then cover the entire repair with plywood to form a boxed rib. [Figure 3-21]
Figure 3-21. Compression rib repairs require reinforcement
of the entire splice with plywood to restore the original strength.
PLYWOOD SKIN REPAIRS
Aircraft that incorporate plywood skins normally carry a large amount of stress from the flight loads. Therefore, make repairs to plywood skins in strict accordance with the recommendations of the air-craft manufacturer. If you repair plywood skin exactly as described by the manufacturer or by Advisory Circular 43.13-lB, Acceptable Methods, Techniques, and Practices, Aircraft Inspection and Repair, the FAA will most likely approve the repair. If the repair cannot be made according to the approved data, contact the district agent of the FAA for approval of the proposed method before begin-ning the work.
3-18 Wood, Composite, and Transparent Plastic Structures
Figure 3-22. Use a splayed patch to repair small holes in thin plywood skin.
Use circular or elliptical plywood patches in ply-wood skin repair to avoid the stress concentrations developed by abrupt changes in the cross-sectional areas of square or rectangular patches. Following are several types of plywood patches approved for aircraft applications.
SPLAYED PATCH
Small holes in thin plywood skin may be repaired by a splayed patch. Use this type of patch if the skin is less than or equal to 1/10-inch thick and the hole can be cleaned out to a diameter of less than 15 thicknesses (15T). [Figure 3-22]
To fabricate a splayed patch, tape a small piece of scrap plywood over the center of the damage. Use it as a rest for the point of a drafting compass, and draw two circles. Draw one circle to form the trim size of the hole, which can be no more than 15T. For the other circle, the size of the outside of the patch can be no more than 5T beyond the edge of the hole. To produce the patch, remove the inner circle with a sharp knife, and then, using a chisel, taper the edges evenly from the outer circle to the edge of the hole. Cut the patch plug from the same material as the original skin and taper it to fit the hole exactly. Apply glue to the tapered edges of the hole and to the taper cut on the patch. Put the patch in place,
aligning the face grain of the patch with the face grain of the skin. Once installed, place a piece of vinyl plastic or waxed paper over the patch. With a pressure plate cut from scrap plywood that is just slightly larger than the patch, apply pressure and allow the glue to cure. After the glue has cured, remove the pressure plate, fill, sand, and finish the repair to match the rest of the surface.
SURFACE PATCH
If an airplane's plywood skin is damaged, repair it with a surface patch covered with aircraft fabric and finish it to match the rest of the airplane. This does not produce the best looking repair, but its simplic-ity and economy of time and labor make it a suitable repair for most working-type airplanes. [Figure 3-23] PLUG PATCH
Make a perfectly flush patch in a section of plywood skin by trimming the damage to a round or oval shape. Put a doubler inside the structure for support, then glue the plug patch to the doubler. [Figure 3-24] SCARFED PATCH
The most difficult type of patch to make on ply-wood skin is the scarfed patch. However, because it makes the least change in skin thickness or rigidity, it is preferred for most stressed wood skin repairs. [Figure 3-25]
Figure 3-23. Use surface patches to repair larger holes and damage in plywood skins. Make the patch of the same material as the damaged skin and run its face grain in the same direction as that of the skin.
3-20 Wood, Composite, and Transparent Plastic Structures
GRAIN DIRECTION OF SKIN, PATCH, AND DOUBLER
SAW CUT IN DOUBLER
BUTT JOINT OF PATCH TO SKIN
INNER EDGE OF DOUBLER
L- NAIL HOLES SCREW HOLES O BE FILLED BEFORE FINISHING
DIMENSIONS
A B C
SMALL CIRCULAR PLU G
PATCH 2-5/8 2 1-3/8 LARGE CIRCULAR PLU
G
PATCH 3-7/8 3 2-1/8
(TWO ROWS OF SCREWS AND NAILS REQUIRED FOR LARGE PATCH)
Figure 3-24. Use a plug patch to make a flush repair to plywood skin. Once the plug patch has been produced, fill the nail and screw holes with wood filler, as well as any space between the patch and skin for protection against wood decay.
COMPOSITE STRUCTURES
Composites are combinations of two or more materi-als that differ in composition or form. The con-stituents or elements that make up the composite retain their individual identities. In other words, the individual elements do not dissolve or otherwise merge into each other. Each can be physically iden-tified, and exhibits a boundary between each other. Composite structures differ from metallic structures in several ways: excellent elastic properties, ability to be customized in strength and stiffness, damage tolerance characteristics, and sensitivity to environ-mental factors. Consequently, composites require a vastly different approach from metals with regard to their design, fabrication and assembly, quality con-trol, and maintenance.
One main advantage to using a composite over a metal structure is its high strength-to-weight ratio. Weight reduction is a primary objective when designing structures using composite materials. In addition, the use of composites allows the forma-tion of complex, aerodynamically contoured shapes, reducing drag and significantly extending the range of the aircraft. Composite strength depends upon the type of fibers and bonding mate-rials used, and how the part is engineered to dis-tribute and withstand specific stresses.
COMPOSITE ELEMENTS
In aircraft construction, most currently produced composites consist of a reinforcing material to pro-vide the structural strength, joined with a matrix material to serve as the bonding substance. In addi-tion, adding core material saves overall weight and gives shape to the structure. The three main parts of a fiber-reinforced composite are the fiber, matrix, and interface or boundary between the individual elements of the composite.
REINFORCING FIBERS
Reinforcing fibers provide the primary structural strength to the composite structure when combined with a matrix. Reinforcing fibers can be used in con-junction with one another (hybrids), woven into specific patterns (fiber science), combined with
other materials such as rigid foams (sandwich struc-tures), or simply used in combination with various matrix materials. Each type of composite combina-tion provides specific advantages. Following are the five most common types of reinforcing fibers used in aircraft composites.
FIBERGLASS (GLASS CLOTH)
Fiberglass is made from small strands of molten sil-ica glass that are spun together and woven into cloth. Many different weaves of fiberglass are avail-able, depending on a particular application.
One of the disadvantages of fiberglass is that it weighs more and has less strength than most other composite fibers. In the past, fiberglass was limited to nonstructural applications. The weave was heavy and polyester resins were used, which made the part brittle. 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. [Figure 3-26]
Figure 3-26. Fiberglass is usually a white gleaming cloth. The widespread availability of fiberglass and its low cost make it one of the most common reinforcing fibers utilized in aircraft non-structural composites.
ARAMID
In the early 1970s, DuPont introduced aramid, an organic aromatic-polymide polymer, commercially known as Kevlar . Aramid exhibits high tensile strength, exceptional flexibility, high tensile stiff-ness, low compressive properties, and excellent toughness. The tensile strength of Kevlar composite material is approximately four times greater than alloyed aluminum. 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. [Figure 3-27]
Figure 3-27. Aramid fiber is usually characterized by its yel-low color, and as with most reinforcing fibers, comes in var-ious grades and weaves for different uses. Kevlar 49 is pre-dominantly used in aircraft composite reinforced plastics; both in thermoplastic and thermosetting resin systems.
Aramid is ideal for use in aircraft parts that are sub-ject to high stress and vibration. For example, some advanced helicopter designs have made use of aramid materials to fabricate main rotor blades and hub assemblies. Flexibility of the aramid fabric allows the blade to bend and twist in flight, absorb-ing much of the stress. In contrast, a blade made of metal develops fatigue and stress cracks more fre-quently under the same conditions.
A disadvantage to aramid is that it stretches, which can cause problems when it is cut. Drilling aramid can also be a problem if the drill bit grabs a fiber and pulls until it stretches to its breaking point. When cutting aramid fabrics, the material will look fuzzy if inappropriate tools are used. Fuzzy material left around fastener holes or seams may act as a wick and absorb moisture or other liquid contaminants such as oil, fuel, or hydraulic fluid. Liquid
contam-inates may deteriorate the resin materials in the composite structure, producing delamination. It is important to cut aramid cloth correctly because even a slight amount of moisture will prevent aramid from bonding properly. Fuzz around the drilled hole may also prevent a fastener from seating properly, which may cause joint failure.
CARBON/GRAPHITE
Carbon fibers are produced in an inert atmosphere by the pyrolysis of organic fibers such as rayon, poly-acrylonitrile, and pitch. The term carbon is often interchangeable with the term graphite. However, carbon fibers and graphite fibers differ in the temperature at which they are produced. Carbon fibers are typically carbonized at approximately 2400 F and composed of 93% to 95% carbon, while graphite fibers are produced at approximately 3450 to 5450 F and are more than 99% carbon. [Figure 3-28]
Figure 3-28. In general, Americans refer to carbon fibers as "graphite" fiber, while Europeans refer to it as carbon fiber. Carbon actually describes the fiber more correctly, because it contains no graphite structure. Carbon/graphite is a black fiber that is very strong, stiff, and used primarily for its rigid strength characteristics. Fiber composites are used to fab-ricate primary structural components such as the ribs and skin surfaces of the wings.
Advantages to carbon/graphite materials are in their high compressive strength and degree of stiff-ness. However, carbon fiber is cathodic while alu-minum and steel are anodic. Thus, carbon pro-motes galvanic corrosion when bonded to alu-minum or steel, and special corrosion control tech-niques are needed to prevent this occurrence. Carbon/graphite materials are kept separate from aluminum components when sealants and corro-sion barriers, such as fiberglass, are placed at the interfaces between composites and metals. To fur-ther resist galvanic corrosion, anodize, prime, and paint any aluminum surfaces prior to assembly with carbon/graphite material.
3-24 Wood, Composite, and Transparent Plastic Structures
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. In designing components that need both the strength and stiff-ness associated with boron, many civil aviation manufacturers are utilizing hybrid composite mate-rials of aramid and carbon/graphite instead of boron.
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 temper-atures up to 2,200 F. 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. The most common use of ceramic fibers in civilian aviation is in combination with a metal matrix for high-temperature applica-tions.
FIBER SCIENCE
The strength of a reinforcing material within a com-posite is dependent upon the weave of the material, the wetting process (how the matrix is applied), fil-ament tensile strength, and the design of the part. The tensile strength of fabrics as it is reported in many articles and books is usually the strength of the raw fabric only. However, aircraft composites incorporate a resin material. This decreases the overall strength, because resins tend to make the structure more brittle and lessen the tensile strength. Arranging the fibers in various orienta-tions helps to ensure adequate component strength partially corrects this reduction in strength.
The strength and stiffness of a composite buildup 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. For example, a helicopter rotor blade has high stress along its length because of the centripetal forces emanating from the rotating mass of the blade. If the blade is made of metal, its strength is the same in all directions. In the case of a composite blade, the majority of fibers run the length of the blade to give more strength in the direction of the greatest stress.
In another example, if a wing in flight bends as well as twists, the part can be manufactured so that fibers
will run the length of the wing to reduce its ten-dency to bend. By adding a layer of fibers that run at 45 and at 90 , twisting forces can also be limited. In this manner, each layer may have the major fibers running in a different direction. The strength of fibers runs parallel to the direction that the threads run, allowing designers to customize the strength objective for the type of stress that the part might encounter.
FABRIC ORIENTATION
When working with composite fibers, it is impor-tant to understand the construction and orientation of the fabric because all design, manufacturing, and repair work begins with the orientation of the fabric. Unlike metallic structures, the strength of a com-posite structure relies on the proper placement and use of the reinforcing fibers. Some of the terms used to describe fiber orientation are warp, weft, selvage edge and bias. [Figure 3-29]
Figure 3-29. All design, manufacturing, and repair work begins with the orientation of the fabric. Unlike metallic structures, the strength of a composite structure relies on the proper placement and use of the reinforcing fibers.
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 typi-cally more threads woven into the warp direction than the fill direction, making it stronger in the warp direction. Because warp is critical in fabricat-ing or repairfabricat-ing composites, insertion of another color or type of thread at periodic intervals identi-fies the warp direction. Marked plastic backings on the underside of pre-impregnated fabrics also iden-tify the orientation of the warp threads. Pre-impreg-nated fabrics are pre-impregPre-impreg-nated with resins by the manufacturer and later cured in the field.