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Homework Help: Advice needed for composite

  1. Feb 7, 2009 #1
    Advice needed for composite....

    Hello guys. Newbie here this is my first post ;-). I am a 2nd year Eng. Mech. Univ student.

    An assignment for one of the credits is to design a composite material (made up of 2 materials)of rectangular section width 30mm and thickness 20mm, length 400mm. The composite must NOT include a metal material while having good strength and rigidity with low density.

    The component will be tested by simply supporting at each end and a heavy load strapped at midpoint (say 60kg). Deflections must be small and within elastic range.

    Now for the life of me excluding carbon fiber, teflon and a few other exotic materials what 2 materials and bonding method could I use to make the section strong enough?? I have access to all kind of woods and many polymers (ABS, Polycarbonate, Polyethylene...etc). Any suggestions guys? Thanks for any help!
  2. jcsd
  3. Feb 7, 2009 #2
    Re: Advice needed for composite....

    Kind of a weird assignment because many of the exotics are considered strong because they flex (and distribute energy), not because they are stiff. Think about ballistics and US army helmets deflecting bullets.

    I can think of one case for an airline cockpit door which was too stiff and did not deflect the test blow of an axe. I'm fuzzy on the details but either the hinges failed from the shock or the axe penetrated the skin.

    Do you have access to an autoclave? F1 car body shells (baked/composite/kevlar) to protect the driver are very stiff, like cast iron (almost rings when knocked on). It's the things around this shell which act as the crush zone and absorb the impact protecting from sudden deceleration.
  4. Feb 7, 2009 #3


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    buddy13: Perhaps start by looking up the tensile strength or compressive strength of various materials you might consider using. Will this beam consist of two separated materials, or will it consist of two materials in a mixed substance?
  5. Feb 8, 2009 #4
    Re: Advice needed for composite....

    The composite will be made up of alternate layers of the 2 materials bonded togther (somehow?). I already took a look at the specs (mod of rigidity, density...etc) of different kinds of woods and some platics but am still very unsure :-(...what type of bonding material can I use for my composite?
  6. Feb 8, 2009 #5
    Re: Advice needed for composite....

    Thanks for your reply. Yeah I know what you mean by the exotics deforming. But under the load specified it is very unlikely that say carbon fiber will deform plastically. So I think it is a good choice albeit an expenisve and hard to source material.

    I have friends who got carbon fiber bonnets (maybe 4mm thick) you can stand up on!

    Hmm..what is an autoclave exactly?
  7. Feb 8, 2009 #6
    Re: Advice needed for composite....

    A fancy oven used to make certain resin/fabric combo's stronger.

    Don't bake anything in an oven which will ever be used to consume food in again.

  8. Feb 8, 2009 #7
    Re: Advice needed for composite....

    You can make your own 'carbon-fibre' parts with simple equipment - you need a vacuum pump, vacuum bag, an epoxy resin (can use other resins too, and buy them from the internet) and pairs of jeans (preferably black for effect). You need to cut strips of the jeans, brush them with epoxy, layer them up, pop 'em in the vacuum bag to de-gas, then stick them in an oven to cure (as previously said not one you intend to ever cook in again). That's quite a simplified description I know, but you can still get pretty immense samples with a bit of playing around. Once you have a block of the right thickness, you can always machine it down to be the right size.
  9. Feb 9, 2009 #8
    Re: Advice needed for composite....

    Sounds interesting. Might give it a try. One question though..is it intelligent to assume that "my" carbon fiber will only be one component of the binary composite I need to create? Now where's that old pair of jeans...;-) ?
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