Introduction to Composites An Aerospace Manufacturing Perspective

Course Overview Composite Material Structure Composite Material Components Aluminum versus Composites Advantages and Disadvantages in Aerospace Composite Applications Composite Manufacturing Techniques Subsequent Composite Modules copyright J. Anderson, 2008 2

Composites in Aviation What are composites? Combinations of different materials which yield a product with superior properties Composite armor used by the Greeks in antiquitiy (http://www.youtube.com/watch?v=Aznz9mj5grA) Modern composites, or advanced composites are typically fiber reinforced plastics. copyright J. Anderson, 2008

Fiber Reinforced Plastic (FRP) Composites Consists of at least two materials Plastic which binds the fibers together, also called the matrix Fibers, typically small in diameter and long in length Fibers may also be short in length to facilitate processing – e.g., injection molded nylon with glass fibers In general the matrix imparts toughness, or crack resistance, and the fiber imparts ultimate strength copyright J. Anderson, 2008

Fiber Reinforced Plastic Composites, contd. Fibers Plastic Matrix copyright J. Anderson, 2008

Function of the Fiber Carry the load 70 to 90% of load carried by fibers Provide structural properties to the composite Stiffness Strength Thermal stability Provide electrical conductivity or insulation copyright J. Anderson, 2008 6

Function of the Matrix Binds the fibers together Provides rigidity and shape to the structure Isolates fibers to slow crack propagation Surface quality Corrosion and wear protection for fibers copyright J. Anderson, 2008 7

Relative Strength of Fiber and Matrix Note that for the same level of stress, the fiber deforms much less than the resin. This leads to the composite material being much stronger in the direction of the fiber. If the fibers are unidirectional (all in the same direction) the composite material is strong in the direction of the fibers, but weak in the directions perpendicular to the fibers. We can alleviate this by adding multiple plies laid with the fiber direction different. copyright J. Anderson, 2008

Varying Fiber Direction in Plies Varying fiber direction in plies builds a laminate structure with strength in more than one direction copyright J. Anderson, 2008

Commercial Fiber Fibers are available as Yarn – a bundle of fibers twisted together Tow - Large bundles (Carbon Fiber), several thousand fibers Roving - Large bundles (Fiber Glass) Uni-directional tape Woven fabric or mat copyright J. Anderson, 2008

Material Configurations copyright J. Anderson, 2008 courtesy Ten Cate Avdanced Composites 11

Composite Fiber Materials Common Fibers Used in Composites Glass, or fiberglass Starts as a silica sand Carbon Starts as a polyacrylonitrile fiber copyright J. Anderson, 2008

Types of Plastics used in Composites Plastics are polymer materials, that is to say that they are made up of long chain molecules. There are two types of plastics based on how these molecules are bonded together. Thermoplastics Thermoplastics can be melted and re-solidified when cooled. Thermosets Start out as liquids or paste-like solids and become rigid when cured. Thermosets can’t be re-melted once cured. copyright J. Anderson, 2008

Common Thermoset Plastics used in High Performance Composites Thermosets Epoxy Polyester Phenolics Cyanate Esters Bismaleimide (BMI) Polyimide Thermoplastics Nylon Polyetheretherketone (PEEK) copyright J. Anderson, 2008

Aluminum vs. Composites Aluminum is an “isotropic material”, which means it has the same properties in all directions. Composites are “anisotropic” which means they have different properties depending on the direction of the fibers vs. the direction of the applied loading. Composites are built in layers called ply’s that are stacked “laid-up” to form a laminate. Each layer has fibers that run in defined directions. Because of the layers the properties are different “in-plane” vs. “through the thickness” copyright J. Anderson, 2008

Advantages of Composite Materials over Metals for Aerospace Light weight Resistance to corrosion High resistance to fatigue damage Reduced machining Tapered sections and compound contours easily accomplished Can orientate fibers in direction of strength/stiffness needed Possible reduced number of assemblies and reduced fastener count when co-cure or co- consolidation is used Absorb radar microwaves (stealth capability) Thermal expansion close to zero reduces thermal problems in outer space applications copyright J. Anderson, 2008

Disadvantages of Composite Materials over Metals for Aerospace Corrosion problems can result from improper coupling with metals, especially when carbon or graphite is used (sealing is essential) Degradation of structural properties under temperature extremes and wet conditions Poor energy absorption and impact damage May require lightning strike protection Expensive and complicated inspection methods Reliable detection of substandard bonds is difficult copyright J. Anderson, 2008

Design Comparison Studies for Lockheed L-1011 Aircraft Inboard Aileron   Aluminum Composite Weight (lbs) 141 104 # of Ribs 18 10 # of Parts 398 205 # of Fasteners 5253 2574 Vertical Fin Box   Aluminum Composite Weight (lbs) 858 623 # of Assemblies 21 15 # of Parts 714 229 # of Fasteners 40800 10150 copyright J. Anderson, 2008 From “Composite Airframe Structures”, Niu

Composite Usage in Boeing 777 copyright J. Anderson, 2008

Composite Component Content copyright J. Anderson, 2008 Chart courtesy of Composites Market Reports

Building Composite Parts Composite parts are built by laying up multiple plies (layers) using molds (or tools) then cured under heat and pressure. copyright J. Anderson, 2008

Combining the Fibers with Matrix There are several methods for arranging the fibers and plastic in the desired shape. We can arrange the fibers, usually as a fabric, in the mold and then pour on the liquid matrix material. For one part we might hand cut the fabric and fit it into the mold . copyright J. Anderson, 2008 22

Ply Cutting and Kitting For a production system we wish to make the same part many times, in the most efficient manner, and have the same process every time. In this case we use a CNC cutting machine to cut the patterns out, then assemble a “kit” of raw materials to make a part. copyright J. Anderson, 2008 Photo courtesy Accudyne Systems, Inc

Wet Lay Up We can arrange the fibers, usually as a fabric, in the mold and then pour on the resin. Typically the resin is a two part formulation that, once mixed reacts in a fixed time. In order to make the lightest part with the necessary strength, we must control the amount of resin we use on the part. The process includes; Laying the fabric in the mold Saturating the fabric with mixed liquid resin Working the resin into the fabric so that it conforms to the mold Adding another ply of fabric Repeat the application of resin and working as above Continue until all the plys are in place, excess resin has been worked to the edges, and the composite conforms to the mold copyright J. Anderson, 2008 24

Wet Lay Up, contd. copyright J. Anderson, 2008 25

PrePreg Lay Up In wet layup it is very hard to control the amount of resin.This problem may be addressed by impregnating fabric with a pre-mixed resin. This “prepreg” material is held at low temperatures to retard the curing process. The prepreg sheets or tape are laid into the mold, and heated to cure. copyright J. Anderson, 2008 26

Debulking the Part copyright J. Anderson, 2008 27

Oven Cure Once the layup is accomplished and the part is debulked, we can put it into a furnace to cure the resin. Typically the parts are instrumented with a thermocouple to track the temperature of the part in the oven. The temperature of the oven is increased until the thermocouple registers the correct curing temperature and then the part is “soaked” at temperature until it is cured. copyright J. Anderson, 2008 28

Autoclave Cure copyright J. Anderson, 2008 29 Photo courtesy WSF Ind & ASC Process Sys. copyright J. Anderson, 2008 29

Typical Autoclave Cycle copyright J. Anderson, 2008 30

Vacuum Resin Infusion Vacuum resin infusion is similar to wet lay up except that the fabric is laid out in the mold, the part is vacuum bagged, and resin is pulled into the bag and through the fabric by a vacuum pump. Photos courtesy Airtech Adv. Materials copyright J. Anderson, 2008 31

Automated Lay Up copyright J. Anderson, 2008 32

Tow Placement copyright J. Anderson, 2008 Photo courtesy Accudyne Systems, Inc & Cincinnati Machine

High Dexterity Tape Placement copyright J. Anderson, 2008 Photo courtesy Accudyne Systems, Inc

Variable Angle Ply Lamination copyright J. Anderson, 2008 Photo courtesy Accudyne Systems, Inc

Large Parts copyright J. Anderson, 2008 courtesy ATK

Future Directions Embedded sensors and actuators More Automation Embedded sensors and actuators “Out of Autoclave” high performance materials copyright J. Anderson, 2008

Subsequent Composites Modules Composite Specifications in Drawings Manufacturing Techniques Process Control and Tooling You Have Just Completed The Introduction To Composites copyright J. Anderson, 2008