IB Design Technology Topic A3.2.6: Young’s Modulus and Structural Applications Notes

Choosing The Right Young's Modulus Matches A Material's Stiffness To The Job

1. Young's Modulus (E), read as the slope of the elastic line on the stress-strain graph in A3.2.5, measures stiffness: a high E resists deformation, a low E flexes easily.
2. This lesson is about using that number to select materials.

Stiff Or Flexible By Design

Neither high nor low stiffness is better in the abstract; each suits a different job.

1. A high Young's Modulus (stiff, resists bending) suits:
   1. Building frames, bridges, skyscrapers and aircraft wings, where the part must hold its shape.
   2. Steel (around 190 to 210 GPa) is the workhorse for load-bearing structures.
   3. Carbon-fibre-reinforced polymer (about 70 to 150 GPa) gives that stiffness with very low weight.
   4. Ceramics are stiffer still (300 to 450 GPa) but brittle, which limits their structural use.
2. A low Young's Modulus (flexible, absorbs energy) suits:
   1. Parts that must give, flex or absorb energy.
   2. Rubber (about 0.01 to 0.1 GPa) stretches and springs back, ideal for tyres, seals and shock absorbers.
   3. Polymers such as polyethylene and nylon (around 0.1 to 4 GPa) suit flexible joints, medical devices and products that bend without breaking.

Selecting A Material: Stiffness Against Weight

1. Stiffness is rarely the only thing that matters; real selection weighs Young's Modulus against weight.
2. A very stiff material is little use in an aircraft if it is also very heavy, so engineers compare a material's stiffness-to-weight ratio rather than its raw modulus.
   1. Steel and carbon fibre are both stiff, but carbon fibre delivers that stiffness at a fraction of the weight, so it wins in aerospace and racing while steel stays the economical choice for buildings.

Young's Modulus In Real Structures

Most real products combine materials of different modulus so each does what it is best at.

1. Aerospace: carbon fibre gives aircraft and spacecraft high stiffness with low weight.
2. Reinforced concrete: concrete, strong in compression, is paired with steel bars whose high modulus and tensile strength carry the tension that concrete cannot, so together they resist both squashing and bending.
3. Automotive: one product uses both extremes at once, a stiff high-modulus steel frame for rigidity and safety, and low-modulus rubber in the tyres and suspension that flexes to absorb road shocks.