Stress and Strain


  • Stress can be defined as a measure of the internal reaction to an externally applied force.
  • Stress is measured in Pascals (Pa) and can be calculated by using:
\begin{align} \sigma = \frac{L}{A} \end{align}
  • Where:
    • $\sigma$ is the stress (Pa)
    • $L$ is the external force/"Load" (N)
    • $A$ is the effective area1 ($m^{2}$)
  • There are 3 main types of stresses:
    • Tensile stress:
      • It is an Axial Stress (The force is along the axis of the member)
      • It is a force that "EXTENDS" the member. (Pulling it apart)
    • Compressive stress:
      • It is also an Axial Stress
      • It is a force that "CONTRACTS" the member. (Crushing it inwards)
    • Shear stress
      • This stress is NON-AXIAL.
      • It is a force that is one member trying to "SLIDE OVER" another member
      • Covered in detail later.


  • Strain is a measure of how much a material/member will deform under a certain force.
  • It can be defined as the proportional change in length caused when a specimen is under an axial load and has no units. (It is a ratio)
  • It can be found by:
\begin{align} \varepsilon = \frac{e}{l} \end{align}


  • $\varepsilon$ is the strain
  • $e$ is the extension (m) (This is found by $l_{final} - l_{original}$)
  • $l$ is the original length (m)