Composites+Failure

Composites+Failure - COMPOSITE MATERIALS Topic Notes...

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Unformatted text preview: COMPOSITE MATERIALS Topic Notes Introduction Rule of Mixtures for uni-directional properties Single ply (lamina) on-axis and off-axis properties Multiply laminates Failure Theories 1. Introduction Carbon fibre (or carbon fiber - US spelling, or graphite fiber) is combined with resins to form composite laminates and structures. Carbon fibre has high specific stiffness (E/ ) and strength ( tu / ) and can be molded into smooth shapes of almost any form. The properties of the laminate depend on the orientation of the fibres that leads to an extra dimension in the design process. The material as well as the structure needs to be designed. Carbon fibre laminates can fail in a brittle fashion. An aluminium structure that has been overloaded will be plastically deformed but usually a load path remains that can carry some of the design load. A carbon fibre component that is overloaded will often break into two or more pieces and there is no load path residual load path. As a raw material carbon is expensive (aircraft grade aluminium costs approximately $2-$5 per kg depending on alloy and grade. Carbon fibre costs approximately $20 kg again depending on fibre, resin type and demand. Recently the cost has risen due to the demand caused by the B787 manufacture). The focus has been on developing new manufacturing processes to make the final carbon fibre component competitive. Fuselage sections on the Boeing 787 are large single piece construction removing the need for riveted assembly of skin, stiffeners and frames. The expensive assembly process requires up to 50,000 fasteners. The final product is also claimed to have excellent fatigue and corrosion resistant properties reducing the cost of maintenance over the operational life of the airframe. To learn more about carbon fibre you should visit Wikipedia on the following topics Carbon fiber Carbon nanotubes Carbon fiber reinforced plastics CFRP Boeing 787 2. Rule of Mixtures for uni-directional properties f m m f f m m m f f f m m f m f m m f f G v G G G G v E v E E E E v E v E E + = + = + = + = 12 12 2 1 Volume ) matrix ( sin re Volume v sin re of fraction Volume Volume fibre Volume v fibre of fraction Volume m f = = For two different fibres and resin m m f f f f v E v E v E E + + = 2 2 1 1 1 2. Single ply on-axis and off-axis properties Apply uniaxial stress 1 1 1 12 2 1 1 1 E E - = = Apply uniaxial stress 2 2 2 2 2 2 21 1 E E = - = Hence, - - = 6 2 1 12 2 1 12 2 21 1 6 2 1 1 1 1 G E E E E = 6 2 1 66 22 12 12 11 6 2 1 Q Q Q Q Q 12 66 21 12 2 12 21 12 1 21 12 21 12 2 22 21 12 1 11 1 1 1 1 G Q E E Q E Q E Q =...
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Composites+Failure - COMPOSITE MATERIALS Topic Notes...

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