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# What is the Stress Strain Curve and How to Read it?

You’ve probably noticed that some objects stretch easily, but stretching an iron rod seems to be an impossible work. Here it will be explained that why some items are more malleable than. We’ll mostly be talking about stress-strain curves because they’re important for determining a material’s tensile strength. But before discussion of graph, it is compulsory to know that what is stress and strain and what is the stress strain curve.

### What is stress?

In mechanics, stress is defined as force per unit area. It is indicated by σ and its unit is newton per meter square.

σ = F / A

Stress is of three types: tensile stress, compressive stress and shear stress.

Tensile stress: This is the force per unit area which causes an increase in length of body. Objects under tensile stress become longer and thinner.

Compressive stress: This is force per unit area that causes decrease in length or area of body. Objects under compressive stress become shorter and thicker.

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### What is strain?

It is defined as the amount of deformation in the body that a body experiences due to applied stress. It is of three types, depending upon the type of stress; tensile strain, shear strain and volumetric strain. In case of tensile strain

ϵ = dl / l

where “dl” is change in length and “l” is initial length. Strain is a dimensionless quantity as it is the ratio of same quantity.

### What is stress and strain?

When load acts on a body, it causes stress as well as strain in that material. For example when you squeeze a football, it offers resistance and tries to retain its original shape.

### What is the stress strain curve?

In material science, characteristic and behavior of a material can be easily studied by its stress strain graph. Now the question arises, how is a stress strain curve plotted and read? Usually it is done by putting a rod of the test piece under tensile stress. The applied forces causes strain in that object. The values of stress and strain are recorded until a fracture occurs. Basically stress strain curve is a behavior of a material when it is subjected to an external load. This curve will be different for different materials that may be due to the difference in temperature and loading conditions.

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Here are the main points of curve.

• Proportional limit
• Elastic limit
• Yield point
• Ultimate stress point
• Fracture

### What is proportional limit?

This is the portion of stress strain curve that follows Hook’s law i.e. applied force is proportional to the strain produced. In this region, the ratio of stress to strain provides a proportionality constant, named as young’s modulus. The portion is a straight line, indicating that as more stress will be applied, material will show more deformation.

F ∝ – x, here F is applied force and “x” is displacement, this is the statement of hook’s law.

This point on curve is also known as limit of proportionality. After this point stress and strain don’t remain proportional.

### What is elastic limit?

Elasticity is a property of a material by which a material returns to its original shape when stress is removed. Elastic limit point is the limiting value of stress up to which a material is perfectly elastic. So elastic limit of stress train curve clearly indicates a portion of graph in which, if you remove stress at any point, material will regain its original shape. However, beyond this limit, material will not regain its shape and it will be plastically deformed.

### Yield point in stress strain curve

It is a point is stress strain curve at which material practically starts deformation. It can also be defined as the value of stress after which material starts to extend more quickly with even a little increase in load. After this point permanent deformation is done in material which is irreversible. The stress corresponding to yield point is yield point stress. At this value of stress, material has lost its elasticity.

### What is Ultimate Stress?

It is the maximum value of stress that a material can bear before being deformed or the maximum stress which a material have to bear before breaking. After this point, material finally breaks.

### Fracture

The point at which failure of material takes place. This is the point at which strength of material ends. This point is also termed as breaking point. The stress associated with this point is known as rupture stress. This region of graph is termed as necking region. A material in this region don’t needs any extra stress to plastically deform.

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