Advanced Composites Handout (I): Laminates, Fiber Types and Applications
Laminated structure
Composites consist of a combination of materials that are blended together to achieve specific structural properties. The individual materials do not completely dissolve or fuse into the composite, but they will act together as a whole. Often, the interfaces between components can be physically recognized. The properties of a composite material are superior to the properties of the individual materials from which it is composed.
An advanced composite material is made of a fibrous material dissolved into a resin matrix, usually laminated by alternately oriented fibers to provide strength and stiffness to the material. Fibrous materials are not common; wood is the most common fibrous structural material known to man.
Applications of composites in aircraft include
-Deflector
-Flight control surfaces
-Landing gear doors
-Wing and stabilizer leading and trailing edge panels
-Internal components
-Floor beams and floor panels
-Vertical and horizontal stabilizer primary structures for large aircraft
-Principal wing and fuselage structures of the new generation of large airplanes
-Turbine engine fan blades
-Propeller
Main components of laminates
An isotropic material has uniform properties in all directions (meaning isotropic properties of the same material). The measured properties of isotropic materials are independent of the test axis. Aluminum and titanium, which are metallic materials, are used as examples to illustrate the illustration of isotropic materials.
Fibers are the main load-bearing elements of composites. Composites have strength and stiffness only in the direction of the fibers. Unidirectional composites have predominantly mechanical properties in one direction, known as anisotropy, where the mechanical or physical properties differ from the direction of the natural reference axis inherent in the material. Components made from fiber-reinforced composites can be designed so that the fiber orientation produces the best mechanical properties, but they can only approach the truly isotropic properties of metals, such as aluminum and titanium.
The composite matrix supports the fibers and bonds them in the composite. The matrix transfers any applied loads to the fibers, keeps the fibers in their position and chosen orientation, gives the environmental resistance of the composite, and determines the maximum service temperature of the composite.
Properties
The structural properties of composite laminates, such as stiffness, dimensional stability and strength, depend on the stacking order of the laminations. The stacking order describes the distribution of layup orientations in the thickness of the laminate. As the number of layers with selected orientations increases, more stacking orders are possible. For example, a symmetric eight-ply laminate with four different layup orientations has 24 different stacking orders.
Fiber direction
The strength and stiffness of a composite depends on the order in which the layers are oriented. The actual strength and stiffness of carbon fibers ranges from low to high values, such as those provided by glass fibers, to high values of strength and stiffness provided by titanium fibers. This range of values is determined by the orientation of the laminate to the applied load. In advanced composites, proper selection of the layup orientation is necessary to provide an efficient design of the structure. The part may require 0° ply reactive axial loads, ±45° ply reactive shear loads, and 90° ply reactive side loads. Because the strength design requirements are a function of the direction of applied loads, the ply orientation and ply sequence must be correct. During the repair process, it is critical to replace each damaged layer with a layer of the same material and orientation.
The fibers in a monolithic material move in one direction, with strength and stiffness only in the direction of the fibers. Prepreg (prepreg film) tapes are an example of unidirectional layup orientation.
Fibers in a bi-directional material flow in two directions, usually 90° apart. Plain structures are an example of bi-directional lay-up directions. These layup directions have strength in both directions, but not necessarily the same strength. As shown in Figure 1
The quasi-isotropic layups have layer sequences of 0°, -45°, 45° and 90° or 0°, -60° and 60°. These types of ply orientations simulate the properties of isotropic materials as shown in Figure 2. Many aerospace composite structures are made of quasi-isotropic materials.
Figure 1: Bidirectional and unidirectional paving material properties
Figure 2: Symmetric isotropic material layups
Warp direction
The warp direction refers to the longitudinal fibers of the fabric. Due to the straightness of the fibers, warp direction is the direction of high strength. Warp warp direction is used to describe the direction of the fibers on a chart, specification sheet, or manufacturer's sheet. If there is no warp direction on the fabric, the warp direction defaults to zero when the fabric comes off the roll. Therefore, 90° to zero is the width of the fabric. As shown in Figure 3
Figure 3: Twist Lock
Fiber configuration
All product forms usually start with a unidirectional line of raw fibers that are packed into continuous strands. An individual fiber is called a filament. The term "thread" is also used to denote an individual glass fiber. The bundled filaments can be categorized as spun yarns, yarns or rovings. Fiberglass yarns are twisted, whereas kevlar® yarns are not. Filament bundles and rovings do not have any twist. Most fibers are dry fibers and need to be impregnated with resin before use (pre-impregnation) or with pre-impregnated material where the resin has already been applied to the fibers.
Coarse fibers (yarn bundles)
A roving is a group of filaments or fiber ends, such as 20- or 60-end glass roving. All filaments are oriented in the same direction and are not twisted. Carbon fiber rovings are usually identified as 3K, 6K or 12K rovings, with K denoting 1000 filaments. Most roving product applications utilize a mandrel for fiber winding and then resin curing to the final configuration.
Unidirectional (with)
Unidirectional prepreg tapes have been the standard in the aerospace industry for many years, and the fibers are usually impregnated with a thermosetting resin. The most common preparation method involves pulling collimated raw (dry) strands into an impregnating machine, where the hot-melt resin is bonded to the strands by heat and pressure. The tape product has high strength in the direction of the fibers and almost no strength in the fibers. The fibers are held in place by the resin. Tapes are stronger than woven fabrics. As shown in Figure 4
Figure 4: Tapes and fabric products
Fabric
For complex shaped laminations, most fabric constructions offer more flexibility than straight unidirectional tapes. Fabrics offer the option of impregnating the resin through a solution or hot melt process. Typically, fabrics for structural applications use fibers or strands of the same weight or yield in both warp (longitudinal) and weft (transverse) directions. For aerospace structures, tightly woven fabrics are often the weight-saving choice, reducing the size of resin voids and maintaining fiber orientation during manufacturing.
The fabric structure usually consists of reinforced reinforcement bundles, strands or yarns that are interlaced during the weaving process. The more common fabric styles are plain weave or satin weave. Plain weave structures are formed by alternating fibers above and below each crossing strand (bundle, bunch or yarn). In common satin weave styles, such as 5- or 8-bundle, the fiber strands move back and forth in the warp direction and weft direction less frequently.
These satin fabrics are less crimped and more easily deformed than plain fabrics. In plain weaves and most 5 or 8 bunch fabrics, there are equal numbers of fiber strands in the warp and weft directions. For example:3K plain weaves usually have an additional name such as 12 x 12, which means 12 tows per inch in each direction. This count designation can be changed to increase or decrease the weight of the fabric, or to accommodate different fibers in different weights. As shown in Figure 5
Figure 5: Typical fabric weaving style
Non-woven fabrics (woven or sewn)
Woven or sewn fabrics can offer many of the mechanical advantages of unidirectional tape. Fiber placement can be straight or unidirectional, without the up-and-down turns of woven fabrics. After pre-selected orientation of one or more layers of drywall, the fibers are sewn together with fine yarns or threads to hold the fibers in place. These types of fabrics provide a wide range of multi-layer orientations. While some weight may be added or some of the final reinforcing fiber properties may be lost, some improvement in interlaminar shear and tenacity properties may be achieved. Some common sewing yarns are polyester, aramid or thermoplastic. As shown in Figure 6
Figure 6: Non-woven materials (stitching)
Types of fibers
Glass fiber
Fiberglass is commonly used in secondary structures of aircraft, such as fairings, radomes and wingtips. Glass fibers are also used in helicopter rotor blades. There are several types of glass fibers used in the aerospace industry. Electronic glass fiber, or E-glass, is recognized for such electronic applications. It has a high resistance to electrical currents. E-glass is made from borosilicate glass fibers.S-glass and S2-glass are structural glass fibers that have higher strength than E-glass.S-glass glass fibers are made from magnesium-aluminum silicates. The advantages of glass fibers are lower cost than other composites, chemical or electrical resistance, and electrical properties (glass fibers do not conduct electricity). Glass fibers are white in color and can be used as dry fiber fabrics or prepregs.
Aramid fiber
Kevlar is the name of DuPont's aramid fiber. Aramid fibers are lightweight, strong and tough. Two types of aramid fibers are used in the aerospace industry; Kevlar® 49 has high stiffness and Kevlar® 29 has low stiffness. An advantage of aramid fibers is that they are highly resistant to impact damage, so they are commonly used in areas susceptible to impact damage. The main disadvantage of aramid fibers is their general deficiencies in compressibility and moisture absorption. Service reports indicate that some parts made of kevlar® absorb up to 8% of their weight in water. Parts made from aramid fibers therefore need to be protected from the environment. Another drawback is that kevlar fibers are difficult to drill and cut. The fibers lint easily and require special scissors to cut them.
Kevlar is commonly used in military ballistic and body armor applications. It has a natural yellow color and is available as a dry fabric and prepreg. The size of an aramid fiber bundle does not depend on the number of fibers like carbon or glass fibers, but rather on the weight.
Carbon/Graphite Fiber
The first difference between this fiber is between carbon and graphite fibers, although the terms are often used interchangeably. Carbon and graphite fibers are based on a network of single graphite (hexagonal) layers in carbon. A material is defined as graphite if the single graphite layers or planes are stacked in a three-dimensional sequence. Extended time and temperature processing is usually required to form this order, making graphite fibers more expensive. Bonding between planes is weak. Disorder often occurs such that only a two-dimensional order exists in the layers. This material is defined as carbon fiber.
Carbon fiber is very tough and 3 to 10 times stiffer than glass fiber. Carbon fiber is used in aircraft structural applications such as bottom beams, stabilizers, flight controls, and main fuselage and wing structures. Advantages include high strength and corrosion resistance. Disadvantages include lower electrical conductivity than aluminum; therefore, for aircraft components that are susceptible to lightning strikes, a lightning grid or lightning-resistant coating must be installed. Another disadvantage of carbon fiber is its high cost. Carbon fiber is gray or black in color and is available as dry fabric and prepreg. When used with metal fasteners and structures, carbon fiber has a high potential to cause galvanic coupling corrosion.
Figure 7: Glass fibers (left), aramid fibers (middle), carbon fiber material (right)
Boron fiber
Boron fibers are very hard and have high tensile and compressive strength. The fibers are relatively large in diameter and do not bend well; therefore, they can only be used as prepreg tape products. Epoxy resin matrices are often used with boron fibers. Boron fibers are used to repair cracked aluminum aircraft casings because boron's thermal expansion is close to that of aluminum and has no galvanic coupling corrosion potential. Boron fibers are difficult to use if the substrate surface has a contoured shape. Boron fibers are very expensive and can be dangerous to personnel. Boron fibers are primarily used in military aviation.
Ceramic fiber
Ceramic fibers are used in high temperature applications such as turbine blades for gas turbine engines. Ceramic fibers can be used for temperatures up to 2200°F.
Lightning protection fiber
Aluminum planes are very conductive and can dissipate high currents from lightning strikes. Carbon fiber is 1,000 times more resistant to current than aluminum, and epoxy resin is 1,000,000 times more resistant (i.e., perpendicular to the skin). The surface of external composite components usually consists of a layer or layers of conductive material for lightning protection because composites are less conductive than aluminum. Many different types of conductive materials are used, ranging from nickel-plated graphite cloth to metal mesh to aluminized glass fibers to conductive coatings. The material can be used as a wet lay-up layer or prepreg.
In addition to normal structural repairs, technicians must recreate the design to the conductivity of the component. These types of repairs often require conductivity testing with a resistance meter to verify the minimum resistance of the entire structure. When repairing these types of structures, it is very important to use only approved materials from authorized suppliers, including such things as potting compounds, sealants, and adhesives. As shown in Figures 8 and 9
Figure 8: Copper mesh lightning protection material
Figure 9: Aluminum mesh lightning protection material