Stress-Strain Graphs (Edexcel International A Level Physics)

Stress-Strain Graphs

• Stress-strain curves give an indication of the properties of materials such as

  • Up to what stress and strain they obey Hooke's Law

  • Whether they exhibit elastic and/or plastic behaviour

  • The value of their Young Modulus

  • The value of their breaking stress

• Each material has a unique stress-strain curve

Stress-strain graph for different materials up to their breaking stress

Comparing Force-Extension to Stress-Strain Graphs

The key features of the graph which are also on the force-extension graph are:

• Limit of proportionality, beyond which Hooke's law no longer applies

• The elastic limit, before which a material returns to its original length or shape when the deforming force is removed

• The yield point beyond which the material continues to stretch (more strain is seen) even though no extra force is being applied to it (without additional stress)

• Elastic deformation where the material will return to its original shape when the load is removed

• Plastic deformation where the material will not return to its original shape when the load is removed

The important points shown on a stress-strain graph

The stress-strain graph is also used to find;

• The Young Modulus is found from the gradient of the straight part of the graph

• Breaking stress (also called fracture stress) is the stress at the point where the material breaks

  • At the yield point the atoms in the material had started to move relative to each other, at the breaking stress they separate completely

  • Breaking stress is not the same as ultimate tensile stress which is marked on many graphs