Young's Modulus - Cost

General Information

• Young's modulus measures stiffness and is a material constant, i.e. it is the same whatever the size of the test-piece.
• Many applications require stiff materials, e.g. roof beams, bicycle frames - these materials lie at the top of the chart
• Many applications require low cost materials, e.g. packaging foams - these materials lie to the left of the chart.
• Cheap stiff materials lie towards the top left of the chart – mostly metals and ceramics.

Physical Insights

• Polymers don't seem like a good choice for stiff, cheap products - but they can be reinforced by incorporating stiffeners into the design (for instance look inside a plug).
• Diamond is the stiffest material because of its full covalent bonding - but its price means it is not used for engineering applications.
• Polyethylene has a wide range for Young’s modulus because the ‘bubble’ include both LDPE and HDPE.

Example Uses

• Concrete is cheap and stiff - ideal for structures.
• Cast iron is ideal for a machine-tool bed as it provides high stiffness at low cost.

Simple Questions

• What applications might require low stiffness materials?
• Paper has the same Young’s modulus as oak – so why do people think it’s not as stiff?
• Since wood is not that cheap, why was it used so much until recently?

Further Questions

• Since composites don’t appear to provide stiffness at low cost - why are they used?
• Why is lead used as ‘flashing’ to keep water out of buildings (e.g. roof/chimney joins).
• What is different about urea formaldehyde compared to the other polymers on the chart that makes it the stiffest?
• Why are ceramics, such as silicon carbide, used for machine tool tips?
• Foams are stiffer than rubbers (why?), but not as strong as rubbers (why?). What are the implications for suitable applications?

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