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stitched, and bonded.
•Inserts/ribs/etc can easily be put into the mold before it is closed.
•most resins can be used, but low viscosities are useful.
•Advantages,
-Very large and complex shapes can be made efficiently and inexpensively
-production times are very short compared to layup
-low clamping pressures
-better surface definition than layup
-inserts and special reinforcements are easily added
-operators may be unskilled
-A large number of mold materials may be used
-part consistency is good
-worker exposure to toxic chemicals is reduced
•Disadvantages,
-Mold design is complex
-material properties are good, but not optimal
-resin to fibre ratio is hard to control, and will vary in areas such as corners
-reinforcement may move during injection, causing problems
52.2.9 GENERAL INFORMATION
•Resin curing is best done through slow heating, rapid heating will reduce final strength of the part.
•The composite sheets may be strong, but in thin layers they are less capable of resisting bending moments. To overcome this a honeycomb core can be used inside to increase bending resistance. Typical core materials are,
-PVC foams
-aluminum honeycombs
-paper honeycombs
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• Joining of composites may be done using adhesives,
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Single Lap
Double Lap
Stepped Lap
Scarf
Single Overlay
Double Overlay
•There are a wide variety of techniques for joining composites, beyond those shown here. Most attempt to maximize contact areas by using tongues, oblique planes, etc.
•Composites may also be joined with mechanical fasteners, (NOTE: use drilled holes, instead of trying to warp fiber about hole - this leads to resin rich areas)
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Straight Lap
Offset Lap
52.2.10 REFERENCES
Mallick, P. R., Fiber-Reinforced Composites; materials, manufacturing and design, Mercel-Dek- ker Inc., New York, 1988.
Mallick, P. K., and Newman, S., Composite Materials Technology, Hanser Publishers, New York, 1990.
Schwartz, M. M., Fabrication of Composite Materials, American Society for Metals, Metals Park, Ohio, 1985.
Strong, A. B., Fundamentals of Composite Manufacturing, Society of Manufacturing Engineers, Dearborn Michigan, 1989.
52.2.11 PRACTICE PROBLEMS
1.
a)List at least 5 advantages of composite materials.
b)List at least 3 disadvantages of composite materials.
2.For thermoset polymers, what effects does cross-linking generally have on the material properties?
3.Which type of glass is good for applications that require,
a)Low cost?
b)Operate at high temperatures?
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c) Are resistant to corrosion?
4.
a)List 6 different forms (other than single filaments) that composite fibres may be purchased in.
b)What form of composite fibres are best used for pultrusion?
5.If you were making boat hulls with pre-preg composite fibre and large moulds, what steps would be followed?
6.Indicate if the following parts are best made with pultrusion/filament winding/resin transfer moulding.
rocket engine tanks car body panels airplane fuselage
a mast for a sail boat
7.A composite section has a honeycomb core 1” thick and can withstand a maximum bending moment of 10KN. How much thicker/thinner would the honeycomb have to be to withstand 1KN?
8.TRUE / FALSE - Multi-directional fibres can be used with stereolithography to increase part strength.
9.What are the major factors that weaken composites? Explain the effect of each.
10.Describe the difference between alloys and composites.
11.Describe the properties of the matrix and fiber materials, and then describe why their combinations is so desirable.
12.What properties does a honeycomb core contribute to a composite part?
13.List 10 products that you have purchased or used that are made of composite materials.
14.What are the advantages and disadvantages of composite materials. What design considerations can be used to overcome the disadvantages?
15.A composite has more than one type of fiber. Why would this be desirable?
16.A part is made of a composite material that is 40% fibers (by area) with a Young’s modulus of 300 GPa, and a matrix of 60 Gpa. The UTS of the fibers is 2000 GPa and 100 MPa for the matrix. If the total cross sectional area of the part is 2cm by 0.2cm, what is the effective stiffness and failure load?
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17.Calculate the percentage increase in strength of nylon when e-glass fibers are added.
18.List 5 parts that benefit from the anisotropic properties of composites. Explain why.
19.Corrugated cardboard and composite honeycomb have similar construction. What are the similarities and differences in behavior?
20.List 8 different types of composite manufacturing processes and give an example of a part they are well suited to.
21.Composite materials typically cost more than metals. why are they preferred?
22.List 10 factors determine the strength of a composite materials and parts?