Stress, strain and the Young modulus · Year 13
Stress and strain let us compare how different materials stretch, whatever the size of the sample. The Young modulus, stress divided by strain, is a property of the material itself: it tells engineers how stiff steel, copper or nylon is when choosing cables, bridges and wires.
Key facts
- Tensile stress σ = F/A (Pa, or N/mm² = MPa).
- Tensile strain ε = (ΔL)/L (no unit).
- Young modulus E = stress ÷ strain = (FL)/(AΔL); steel ≈ 200 GPa.
- Elastic limit: beyond it the material does not return to its original length. Ultimate tensile stress: the maximum stress before breaking.
- Brittle materials (glass) snap with little plastic deformation; ductile materials (copper) stretch a lot plastically.
- Elastic strain energy = ½FΔL; energy per unit volume = ½ × stress × strain.