Limit of proportionality (P)
Materials - OCR A-Level Physics
- Limit of proportionality (P)The point on a stress–strain graph beyond which stress and strain are no longer directly proportional.: the point up to which stressThe force applied per unit cross-sectional area of a material. Measured in pascals (Pa). is proportional to strainThe fractional change in length of a material under stress. It is dimensionless (no units). (Hooke's lawThe extension of a spring is directly proportional to the applied force, provided the limit of proportionality is not exceeded. applies). The graph is linear up to this point
- Elastic limit (E)The maximum stress a material can withstand and still return to its original shape when the load is removed.: the point beyond which the material no longer returns to its original shape when stressThe force applied per unit cross-sectional area of a material. Measured in pascals (Pa). is removed. Permanent deformation begins
- Yield point (Y)The point on a stress–strain graph where the material begins to deform plastically — a small increase in stress causes a large increase in strain.: a large increase in strainThe fractional change in length of a material under stress. It is dimensionless (no units). occurs with little or no increase in stressThe force applied per unit cross-sectional area of a material. Measured in pascals (Pa).. The material begins to 'flow'
- Ultimate tensile stress (UTS)The maximum stress a material can withstand before breaking.: the maximum stress a material can withstand. Beyond this, the material 'necks' (localised thinning)
- Fracture point (F)The point on a stress–strain graph where the material breaks.: where the material breaks
- The area under the stress-strainThe fractional change in length of a material under stress. It is dimensionless (no units). graph gives the energy stored per unit volume of the material
Examiner Tips and Tricks
- When labelling a stress-strain graph, place the letters P, E, Y, UTS and F in the correct order along the curve.
- Examiners penalise swapping the order.
- Remember: P before E before Y, then UTS at the peak, then F.