Materials: Stress-Strain Graphs

Introduction to Stress-Strain Graphs

Stress-strain graphs are used to illustrate the physical properties of a material under different levels of strain.
Stress represents the load or force applied per unit area on a material and is measured in Nm^-2 or Pascals (Pa).
Strain is the deformation experienced by the material when subjected to stress; it is a dimensionless quantity.

Understanding the Stress-Strain Graph

A typical stress-strain graph has strain on the x-axis and stress on the y-axis.
The line’s gradient represents the material’s modulus of elasticity. For most materials, this gradient will be linear at first, indicating that stress is directly proportional to strain.

Stages on the Stress-Strain Graph

The initial linear part of the graph represents the material’s elastic behaviour; it will return to its original shape once the stress is removed.
The end point of this linear section is known as the limit of proportionality; beyond this, the graph is no longer linear.
The maximum point on the graph is the ultimate tensile strength (UTS). This represents the maximum stress the material can sustain before it fractures.
The point at which it starts to deform permanently is known as the yield point. Beyond this point, even if the stress is removed, the material will be permanently deformed.
The part of the graph beyond the yield point represents the material’s plastic behaviour; it will not return to its original shape when stress is removed.

Factors Affecting the Stress-Strain Graph

The shape of the stress-strain graph can heavily depend on the material’s properties such as its temperature, molecular structure and rate of strain.
Metals generally show a clear yield point, while materials like rubber and glass do not.
Different materials exhibit different stress-strain properties, which is why the selection of materials in engineering applications is critical.

Material Property Determination via the Stress-Strain Graph

The area under the graph prior to the yield point indicates the material’s elastic strain energy.
The entire area under the stress-strain graph represents the material’s toughness, or the total energy absorbed before fracture.
The elastic modulus of a material can be determined from the initial, linear part of the stress-strain graph. High modulus indicates a stiff material.

** Different Types of Material Behaviour**

Ductile materials have a large plastic region on the stress-strain graph.
Brittle materials fracture with little to no plastic deformation and have a small plastic region on the graph.
Elastic materials regain their original shape after the force is removed while plastic materials permanently deform under stress.

By understanding the stress-strain graph, properties of different materials can be compared and appropriate selections can be made for various applications. This is a crucial component in materials science and engineering fields.