What is the difference between stiffness and strength?

Hint: Can you think of any common material applications?

Strength and stiffness may seem similar, but they are not interchangeable!

  • Strength: Measure of the maximum load that can be placed on a material before it permanently deforms or breaks (Ultimate Tensile Strength- Max point on stress strain curve)

  • Stiffness: measure of the amount of deflection that a load causes in a material (Young’s Modulus E-Linear slope of elastic region)

Take for example some flexible rubber tubing. A piece of rubber surgical tubing has very low stiffness because it deflects a lot under load, but it is relatively strong. A piece of glass filament is the opposite — it deflects very little under load but might not carry a huge load before it breaks.