Stress & Strain

Applied Science: BTEC Level 3 National Extended Diploma, topic 4.10.2

Strain and Stress

Summary values of materials are useful for engineers to compare the qualities of different materials. Two examples of summary values are stress and strain.

Effect of forces

Forces can change an object's shape.
The study of stress and strain is a study of how forces change an object's shape.
Some forces stretch the object.
- These forces are called tensile forces.
Some forces squash the object.
- These forces are called compressive forces.

Stress

Stress can be defined as:
Stress = $\frac{force\space along\space the\space axis\space of\space the\space object}{cross-sectional\space area \space of \space the \space object}$
$\sigma = \frac{F}{A}$
If the force is tensile, then the stress is positive.
If the force is compressive, then the stress is negative.
The units are newtons per metre squared (N/m²) or pascals (Pa).

Strain

Strain can be defined as:
$strain = \frac{extension}{original\space length}$
$\epsilon = \frac{\Delta L}{L}$
If the force is tensile, then the strain is positive.
If the force is compressive, then the strain is negative.
There are no units for strain because strain is a ratio.

Breaking stress

Breaking stress is the stress required to break the material.
The maximum tensile stress an object can withstand is called the ultimate tensile stress.
Some materials can undergo some strain beyond the point of ultimate tensile stress before breaking.

Stress-Strain Curves

Materials have a wide variety of properties that are best illustrated on a stress-strain curve.

Ductility

A material is ductile if it can be stretched out into a wire and remain that way.
Ductile materials like metals show a gradual decrease in stress with the increasing strain.
This means they become easier to deform as their stress-strain values approach the fracture point.

Stress-strain curves

The stress-strain curve for a ductile material is shown.
The yield point is the point where the material extends without much (or any) increase in stress applied to the material.
The fracture point is the point at which the material breaks.

Brittleness

Brittle materials fracture and break before they experience a large extension.
The fracture point and yield point of a brittle material are the same.

Test yourself on Stress & Strain

Which statements about stress and strain are true?
Which statments about forces are true?
Which of the following is a unit of stress?
The graph shows the stress-strain curve for a ductile material.
What is point A called?
The graph shows the stress-strain curve for a ductile material.
What is point B called?

Practise this topic for free