Stress and Strain

STRESS – STRAIN

What is stress?

Stress is the resistance offered by the body when the load is applied to the body.

Stress = Force / Area

Stress is the resistance force offered per unit area of the body when external force is applied to it.

Every substance will try to resist the force which is applied to it according to its ability. If the substance has more strength then it will withstand high force and resist highly giving more stress without fail (deformation, crack, etc.).

What is strain?

Strain is the deformation produced in the body when the load is applied to the body.

Linear Strain = Deformed length / Original length

Volumetric Strain = Deformed Volume / Original Volume

Strain is the deformation caused in the body per unit length or per unit volume according to the load applied to it.

When a load is applied to a body it will try to oppose the load (stress) and when it cannot withstand the load it starts to deform (strain) and finally it fails (cracks or breaks).

So the study of relationship between stress and strain is very important for engineers to choose a material for a particular application.

For this reason stress strain curve is established for different kind of materials like ductile and brittle and it is used for the selection of materials.

Stress – Strain Curve:

Stress – Strain curve is the plot of the curve in XY Coordinate taking stress as Y axis and strain as X axis. It is the curve of the stress and strain of the body from zero to until the failure of the body.

To plot this graph engineers will need the data of the stress and strain of the body when the load is applied.

How can we obtain this data?

 Engineering graduate will know this as they did tensile test in their engineering practical lab classes.

What is tensile test?

The desired material will be taken as per the standard (ISO 1608 - 1972 for steel product) as specimen and the tensile load will be applied in many increments. The deformation will be measured for each increment until the fracture occurs. This test is called tensile test.

 In the above figure, di = initial diameter, li = initial length = 5di    for circular cross section. We can also use circular or rectangular cross sections.

The specimen is fitted in the tensile load applying equipment and the initial diameter and length is measured. The tensile load is applied at a certain increment and the deformation of the length is measured using extensometer. Again the load is applied and deformation is noted until the material fractured.

Stress is calculated by dividing the load by the initial cross sectional area.

Here,

The data for every incremental load and the deformation are noted and the calculated stress and strain are also noted in the tabular form. Using the stress and strain the graph is plotted taking stress in Y axis and strain in X axis. Thus the stress and strain curve is obtained.

To learn more about Mechanical Properties of Engineering Materials follow the link below

Mechanical Properties of Engineering Materials