Stress and Strain

### Stress

• 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:
(1)
\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

• 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:
(2)
\begin{align} \varepsilon = \frac{e}{l} \end{align}

Where:

• $\varepsilon$ is the strain
• $e$ is the extension (m) (This is found by $l_{final} - l_{original}$)
• $l$ is the original length (m)
page revision: 3, last edited: 04 Jul 2011 06:32