Title: Composite Materials for Aerospace Application
1 Composite Materials for Aerospace Application Composite materials are created and applied to take advantage of the high strength and/or stiffness of fibers. Fibers are combined with a matrix material in order to create a useful structure. The matrix
Binds the fibers together
Transfers loads to the fibers
Protects fibers from damage
Specific matrix materials are usually selected according to the manufacturing processes to be used. 2 Fiber and Matrix Material Selection Selection of materials for a composite structure requires consideration of much more than properties in a table. Selection of the fiber the form in which the fiber is to be used and the matrix material involves many factors.
Environmental conditions e.g. fatigue temperature humidity corrosives
Number to be produced
3 Fiber Materials For many materials much higher strength can be achieved in fiber form than in bulk form. Several of these materials are suitable for use in fiber composites.
Aramid e.g. Kevlar (DuPont)
Other materials such as SiC and Al2O3 used in metal or ceramic matrix
Lets compare some properties of these materials. 4 Representative Fiber Properties 5 Fiber (Pre-Preg) Forms Tape Unidirectional fibers Fabric Simple and complex weaves that are selected depending on the application. Plain 5-harness satin 8-harness satin 6 Matrix Materials
Although some fiber composite structures have been produced with thermoplastic matrix most to date have utilized thermosets.
Many lower cost composite structures have used polyester resins.
For higher performance requirements such as in aerospace applications epoxies are most commonly used.
Epoxies have reasonably high modulus and strength.
Epoxies exhibit low shrinkage on cure (good for adhesives).
Epoxy service temperature to 125-175ºC.
7 Potential Benefits of Thermoplastic Composites
Thermoplastics bring to fiber composites some advantages over thermosets
Potentially simplified joining and repair
Faster turn over time in production
Thermoplastics also offer the potential for lower cost fabrication.
However the manufacturing processes for thermoplastic composites are different from those for thermoset composites and require different equipment.
8 Representative Matrix Material Properties 9 Unidirectional Composite Material Coordinates The basic element of a unidirectional composite is a thin sheet (ply). Material axes are defined as follows Longitudinal direction (1) parallel to fibers Transverse direction (2) perpendicular to fibers in plane Normal direction (3) out of plane 10 Modeling Unidirectional Composite Behavior
Fibers are assumed to be all the same (perhaps with circular cross section) and uniformly distributed throughout the matrix.
The quantity of fibers present is expressed as the fiber volume fraction Vf.
The matrix volume fraction is given by Vm 1 - Vf.
Composite density rc rfVf rmVm.
11 Composite Modulus using Rule of Mixtures Longitudinal Modulus E1 Vf Ef Vm Em Transverse Modulus Shear Modulus 12 Example Composite Modulus Calculate the density and longitudinal modulus for a graphite/epoxy composite material with a fiber volume fraction of 70. The properties for the carbon fibers and epoxy matrix is as follows Carbon Fibers Density 1.77 gm/cm3 Modulus 241 GPa Epoxy Matrix Density 1.2 gm/cm3 Modulus 3.12 GPa 13 Unidirectional Composite Properties 14 Composite Laminates A useful structure will require that fibers be oriented in more than one direction. A common approach to creating such a structure is by stacking layers or plies to form a laminate. Ply orientations are defined using the angle (1-2 axis relative to the x-y axis). 15 Laminate Stacking Sequence Definition
Sequence of plies (laminae) within lay-up
Ply orientations are defined using the angle q (1-axis relative to the x-axis).
Individual ply angles are separated by slashes e.g 45/0/-45
Plies are ordered top to bottom (positive z-direction)
The total stacking sequence is enclosed in square brackets
s symmetric with respect to middle surface
n repeated n times
Center layer uses overbar in symmetric lay-up with odd number of plies.
An engineer must understand the capabilities and limitations of manufacturing processes to create cost-effective designs regardless of the materials to be used.
In order to design any composite structure it is essential that the processes be understood.
Selection of processes depends on
the type and form of material
the shape of the parts to be made (some shapes cannot be made)
the quantity to be produced
the quality e.g. tolerances required
the allowable cost of manufacturing
19 Manufacturing Processes
Resin transfer molding
20 Wet Lay-up Wet lay-up is the simplest and most widely used process. Layers of dry fabric are placed on a mold and resin is brushed or sprayed on. Advantages Large parts Low tooling cost Disadvantages Labor intensive Uniformity difficult to maintain 21 Prepreg Lay-up Prepreg is preimpregnated fiber-reinforced material. Fiber can be unidirectional (tape) or woven fabric. Advantages High fiber volume fractions Unidirecti onal material not possible with wet lay-up Uniform fiber distribution -- higher quality Simplified manufacturing Disadvantages Labor intensive (like wet lay-up) More expensive curing equipment Added cost of prepreg Thermoset prepregs have limited shelf life 22 Vacuum Bag Molding Application of uniform pressure before curing improves consolidation and facilitates removal of excess resin and volatiles. The stack of prepreg plies built up on the tool surface is covered with a film of flexible heat-resistant material. 23 Vacuum Bag Molding 24 Autoclave Processing An autoclave is a pressure vessel that permits application of a higher pressure than a vacuum bag alone. 25 Autoclave Systems Aircraft parts stacked in an autoclave Laboratory-size autoclave 26 Typical Cure Cycle for Carbon/Epoxy Composite 27 Filament Winding Filament winding wraps a continuous reinforcement of resin-impregnated fibers onto a mandrel. The combination of mandrel rotation and axial motion of fiber source produce a helical pattern. 28 Filament Winding Advantages Automated process low labor cost Capable of making large parts Disadvantages Limited to axisymmetric parts Void content may be high without the use of an autoclave. In winding thick parts the process may have to be stopped to allow partial cure of initial layers as the pressure of additional layers may squeeze out resin. 29 Filament-Wound Products 30 Resin Transfer Molding (RTM) In RTM resin injected into a mold that contains a fiber preform. The mold is heated to cure the resin. Advantages High quality Good surface finish Large parts Disadvantages Expensive mold Limited Vf 31 Vacuum Assisted Resin Transfer Molding (VARTM) 32 Manufacturing Thermoplastic Composites
Processes adapted from thermoset processes
Provision must be made for melting the matrix material prior to creating shape.
Equipment must allow for higher temperature
Matched die forming as used for sheet metal is also used for thermoplastic composites.
33 Rubber Forming With external heating the dies can be maintained at a lower temperature. 34 Diaphragm Forming 35 AV-8B HarrierComposite Wing Box 36 V-22 OspreyComposite Primary Structure (Carbon Glass) 37 B-2 SpiritAll Composite Skin and Much of Other Structure 38 Use of Composites in Military Aircraft R. Martin D. Evans JOM 52 2000 pp. 24-28 39 Commercial Transport Aircraft The Boeing 787 has 50 of its primary structure made of composites.
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