Designing a Strong Composite Material: Advice Needed

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Discussion Overview

The discussion revolves around designing a composite material for an engineering assignment, specifically focusing on creating a rectangular section composite that excludes metals while ensuring good strength, rigidity, and low density. Participants explore material options, bonding methods, and the implications of stiffness and flexibility in composite design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant seeks advice on materials and bonding methods for a composite that must not include metals, with specific dimensions and load requirements.
  • Another participant notes that many exotic materials are strong due to their ability to flex and distribute energy rather than being purely stiff, referencing ballistic applications.
  • There is a question about whether the composite will consist of two separated materials or a mixed substance, with a clarification that it will be made of alternate layers of two bonded materials.
  • Concerns are raised about the stiffness of certain materials, with a participant mentioning the potential for carbon fiber to not deform plastically under specified loads.
  • Discussion includes the use of an autoclave to enhance the properties of composite materials through pressure and temperature, with a participant seeking clarification on what an autoclave is.
  • One participant describes a DIY method for creating carbon fiber parts using common materials, suggesting that this could be a feasible approach for the assignment.
  • There is a playful inquiry about whether carbon fiber can be considered one component of the required binary composite.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding material choices and the implications of stiffness versus flexibility in composite design. No consensus is reached on the best approach or materials to use.

Contextual Notes

Participants mention specific materials and bonding methods but do not resolve the uncertainties regarding the optimal choices for the composite design. There are also references to the performance characteristics of different materials without definitive conclusions.

buddy13
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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!
 
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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.
 
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?
 


nvn said:
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?

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?
 


kach22i said:
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.


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?
 
buddy13 said:
Hmm..what is an autoclave exactly?
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.

http://www.netcomposites.com/education.asp?sequence=62
Maximising performance of thermoset composite materials, requires, amongst other things, an increase in the fibre to resin ratio and removal of all air voids. This can be achieved by subjecting the material to elevated pressures and temperatures.
 


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.
 


timmay said:
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.


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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