Introduction to Composites
Instructor Notes
This module, Introduction to Composites, is one of a series of modules designed to present subject matter of interest to engineering and technology students. It is designed to be used with college freshman and sophomore students in a variety of courses. This work was funded by a National Science Foundation grant entitled “Advanced Aerospace Manufacturing Education Project” (NSF Award #0603221). The content of this course uses examples in aerospace manufacturing to introduce students to some of the challenges in this industry segment.
Learning Objectives:
At the conclusion students will be able to
· Demonstrate an understanding of the construction of composites,
· Demonstrate the ability to search manufacturers literature for processing parameters
· Explain how composite parts are made
· Describe automation of the composite manufacturing process
· Explain how composites perform differently than metals, and why
Prerequisites – None. If used in a materials science course this material should be presented after metals for comparison purposes.
Slide notes:
1. Title Slide
2. Contents
3. Composites are used in many types of products. For example, decking materials used in home construction are frequently constructed of composite materials manufactured from wood and plastic. Composites are also not a new idea. The ancient Greeks used composite structures in their personal armor (see the referenced YouTube video). This module will focus on the segment of composites that is referred to as “high performance composites” and specifically fiber reinforced plastics. This includes most aerospace structural and non- structural applications, as well as things such as golf club shafts, and Corvette and Viper automobile bodies.
4. FRP’s (fiber reinforced plastics) consist of fibers encased in a plastic matrix. These fibers are typically 0.0002 inch to 0.0008 inch in diameter. Fibers can be separated into 2 groups by length. Short, or staple, fibers are fibers approximately ½” in length. These fibers are added to materials such as nylon. The fibers strengthen the plastic much as rebar strengthens concrete. This material is typically referred to as “filled” and may be processed by techniques such as injection molding.
Longer fibers offer greater strength in the final structure, but cannot be forced into a mold with techniques such as injection molding.
The plastic matrix holds the fibers in place.
5. This slide illustrates 2 layers of fibers in a matrix. Note that all the fibers are oriented in the same direction. The fibers are much stronger than the matrix material, so the response to a force applied parallel to the fiber length is much different than the response to a force applied perpendicular to the fiber length.
6. The most obvious property imparted on the composite structure by the fibers is strength. Thermal stability is also very important. The fibers tend to stabilize the plastic even as it approaches softening temperatures, extending the useful temperature range.
Electrical conductivity can be very important in aircraft construction. Addition of conductive materials in the composite helps dissipate static electricity and lightning strikes. Conversely, on the Cessna 350/400 models, non conductive materials are used in certain places on the fuselage to allow radio antennas to be placed inside the fuselage. This promotes a more streamlined outer fuselage.
7. The primary function of the matrix is to bind the fibers together and to separate the load. If an individual fiber is broken the load is not transferred directly to the adjacent fibers, but is instead partially taken up by the matrix. This is an important feature in slowing crack propagation.
The matrix material also provides some environmental protection to the fibers. Carbon or glass fibers are very strong, but also very brittle. This brittleness makes them very susceptible to wear and abrasion damage. Common matrix materials on the other hand have higher ductility and can tolerate abrasion or wear much better.
8. This slide illustrates the difference in tensile strength between the fibers and the matrix material. For a given force (stress is applied force divided by the area) the carbon fiber deforms (strain is the deformation divided by the original dimension in inches/inch) much less than the resin.
9. Combining the previous slides it is apparent that the composite strength is a result of the fibers, but the strength in the fiber direction is much different than the strength in a direction perpendicular to the fiber direction. In the case of the composite skin on a military jet fighter this would cause a serious problem if all the fibers ran in the same direction. The skin is designed to be a load bearing member but must withstand multi directional loading.
This problem can be alleviated by laminating multiple layers of fiber (usually in the form of woven cloth or tape) and change the direction of the fiber in the layers. This technique also gives us the opportunity to add additional strength only where it is needed without adding unnecessary material (and weight).
10. The terminology used with composites fibers comes from traditional terms used in the textile industry. Starting with individual fibers, a group is twisted together into a bundle called a yarn. The yarns are then combined into a tow which is a bundle containing parallel fibers, or a roving which is a bundle containing fibers twisted together. The term tow is most commonly used with carbon fibers and roving with glass fiber. On some automated layup machines a tow or roving is used directly to position fibers in the correct position and orientation.
In most cases, however, fibers are either woven together into a fabric, or held together in a partially cured matrix. This combination of fibers and matrix is called a prepreg. A prepreg material may have the fibers all running in the same direction (uni-directional tape) or may have a woven fabric structure.
11. This slide illustrates some of the different weaves available for woven sheets of fiber. The weave determines the thickness of the structure, the orientation of the fibers, and the density of the fibers. As a result selection of a particular weave impacts the weight and strength of the composite.
12. The most common fibers used in composites are glass, aramid and carbon.
The most common of these is glass. Glass fibers are available in three grades; E-glass, S- glass, and S2-glass. The major constituents in the glass used to extrude glass fibers is silica sand, limestone, fluorspar, boric acid and clay. More than half of the glass is silica. Varying the additives and raw materials changes the properties of the glass.
The E- glass has a tensile strength of around 3.45 GPa and costs approximately $1.00/ pound. S-glass has a tensile strength of 4.50 GPa and costs approximately $8.00/pound ( 2002 dollars).
The glass fibers are formed by mixing and melting the raw materials at approximately 3000°F. The molten glass is then forced through a die and cooled. The fiber is then pulled over a series of rollers that stretch it to form a fiber of 0.00027 to 0.00059 inches in diameter.
The term aramid comes from the molecular structure of the polymer. The molecular chain contains amide and aromatic groups, yielding a very strong material. The Dupont materials Kevlar® and Nomex® are aramids. Kevlar® has a tensile strength of 2.80 GPa and casts around $30.00/lb. The price of aramids, as a petroleum derivative, is tied to the price of oil.
Carbon fibers are formed by oxidizing and carbonizing PAN (polyacrylonitrile) fibers. Although base raw material costs are similar to S-glass, the conversion process is capital intensive and time consuming. Tensile strength is around 2.6 GPa, but fiber diameters are half that of glass, leading to a light, strong material.
As a comparison, tensile strength for steel is around 2.0 GPa and for aluminum it is 0.60 GPa.
13. The materials referred to as plastics belong to the family of polymers. Polymers have molecules that are long chains of atoms. The atoms in the chains are held together with very strong primary bonds. Plastics are further divided into two classes of plastic materials, thermoplastics and thermosets.
The long chain molecules in thermoplastic materials are held together with weaker secondary bonds. These secondary bonds break with relatively low levels of thermal energy. When the secondary bonds break, the molecular chains are free to move with respect to each other, i.e., the material goes from a solid to a liquid state. As the temperature is lowered the secondary bonds reform and the material transforms from a liquid state to a solid.
This property of thermoplastics allows it to be formed into the shape of a mold easily, but dictates a relatively low service temperature. Thermoset materials have molecules that are bonded together with primary bonds. This bonding between molecules is referred to as cross linking. The bonds between the atoms in the backbone of the chains are the same type of bond as the ones holding them together. This crosslinking between molecules means that the material does not melt into a liquid with the application of heat. Instead, when the temperature is increased to the point where the primary bonds are broken, the molecular chain is also destroyed and the material chars. Thermosets have much higher service temperatures, but cannot be reformed after they are cured.
14. For high performance composites such as those used in aerospace the matrix material is generally one of those listed on this slide. The selection of a matrix material is based on cost, strength, and service temperature. The service temperature is the maximum temperature at which the material retains structural rigidity. For example, an epoxy resin might have a tensile strength of 76 MPa at 23°C and only 26 MPa when the temperature is elevated to 149°C.
15. Aluminum is an isotropic material. This means that it’s properties, such as tensile strength are the same in all directions. Composite, on the other hand, derive their strength from the fibers and thus strength depends on the direction in which the fibers lie. This directional dependence is called anisotropic.
16. The picture in this slide is of a wing section for a Columbia Aircraft 350, a small general aviation airplane with a composite fuselage. It is shown supporting a Lincoln Navigator SUV. The major advantage for composites over aluminum is their anisotropic nature. This allows a composite structure to be designed to address only the design loading, saving excess material and weight.
17. Disadvantages of composites include cost. Many composite parts are manufactured in time and labor intensive processes as described in subsequent slides.
Another concern when using carbon fiber composites is galvanic corrosion of mating metal parts and fasteners.
18. The Lockheed L-1011 airliner was one of the first commercial aircraft designs to consider the use of composites. This slide illustrates two design studies that were done on the replacement of conventional metal parts with composite structures. While the composite aileron design shows some advantage over the conventional part, the vertical fin box design offers a much greater advantage. Since design changes such as these are expensive and time consuming, one would normally implement those changes that offer a greater advantage first.
19. The Boeing 777 was one of the first commercial aircraft to have a significant composite parts content.
20. This slide illustrates the increasing aircraft structural composite content. Note that the Boeing 777 had approximately 10% composites content by weight and the Boeing 787 design has approximately 50% composite content.
21. Composite parts are built as laminates. One sheet or grouping of fibers is added to another, or laid on each other. The term layup is derived from this process. In general all of the lamination processes are similar and follow the graphic on the right of the slide. The sheets are laid up into a laminate in a mold. The laminate (fiber and resin) is then pressed against the mold using a vacuum bag and, sometimes, pressure. The part is then cured at an elevated temperature.
The most time consuming part of the process is the layup and the machine shown is an automatic tape laying machine. This dispenses a prepreg tape and positions it in the part for the correct fiber orientation.
22. The most basic composite construction technique is the wet layup. It starts with placing the fibers in a mold. In most cases the fibers are in a woven sheet.
23. The sheets may be cut by hand, but for production parts an automated cutting table is frequently used. This is a CNC machine that cuts the fabric shapes to be laminated in a way that optimizes use of the raw material. These machines were initially developed to serve the garment industry.
Once all the individual sheet have been cut, they are assembled in a kit that is supplied to the workers who will do the layup. The cart shown on the slide is a kit of materials to make a specific part. Typically a drawing is included detailing how each component is to oriented and placed.
24. This slide describes the wet layup process. Airtech has produced a video describing the wet layup process at http://www.youtube.com/watch?v=rwam8Fq_U78
25. Fabric laid in mold is shown in this slide. The excess fabric is slit at the corners to minimize wrinkles. The mold is first coated with mold release and then coated with resin. The resin helps hold the first sheet of the fabric in place, aiding layup.