Stress Strain Curve of a Mild Steel Rod
Generally, mild steel rods are highly preferred for many construction purposes.
As it has high tensile strength when used with concrete.
Knowing the behaviour of mild steel rod under loading
helps in choosing for better use.
This behaviour of a mild steel rod under loading can be analysed using stress strain curve of mild steel rod.
The stress strain curve consists of strain along the x axis and stress along the y axis.
Stress strain curve consist of various stages, they are proportional limit, elastic limit, upper yield, lower yield, ultimate stress and breaking point.
Now let us discuss one by one.
Proportional limit
If the stress is applied gradually to a mild steel rod, the strain also gradually increased that is represented in point A.
In this stage, when stress is removed from the rod the strain returns to zero.
Thus, the O to A points is called as proportional limit.
In this limit, the stress is directly proportional to strain.
Stress ∝ Strain
σ∝e
That means the steel rod obeys Hooke’s law in this stage.
The proportionality is expressed as E.
σ = E e
E =σ/e
Where, E is equal to Young's modulus or modulus of elasticity
Thus, Young's modulus is defined as the ratio of stress to a strain of the mild steel rod.
The unit of Young's modulus is Newton per millimetre square.
Now let us move on the elastic limit stage.
Elastic limit
The increase of stress to the point B, the strain also increased proportionally, it is called as elastic limit.
In this stage also, if the applied stress is removed, the strain returns to an original position that is rod gains to original shape.
The stress increased beyond the elastic limit, the material undergoes deformation.
Which means the permanent extension occurs, the mild steel rod does not regain its original shape even after the stress is removed.
This is expressed as C to D point in the graph which is called as yield point.
In point “C” at which maximum stress is required for a rod to undergo deformation. This is called as upper yield point.
In point “D” of at which minimum stress is required for a rod to undergo deformation. This is called as lower yield point.
The increase of stress beyond this limit, the strain is also increased gradually to a point.
This point is “E” which is called as ultimate stress or ultimate strength point.
Ultimate stress is the maximum stress the rod can withstand.
Thus, this portion is called as strain hardening.
Further increase of stress beyond the ultimate stress, the localised reduction occurs in the cross sectional area of the rod this is the weakest point of the material.
This is called as necking stage that means a breaking point stage.
At this stage, the mild steel rod breaks, thus the curve drops to the point F.
Thus, the curve obtained is the stress strain curve for the mild steel rod under the action of load.
The point A to B is elastic region and point B to F is the plastic region of mild steel rod.
Similarly, if you plot the stress strain curve for any material you can determine the yield strength, ultimate strength and the breaking point of the material.
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Generally, mild steel rods are highly preferred for many construction purposes.
As it has high tensile strength when used with concrete.
Knowing the behaviour of mild steel rod under loading
helps in choosing for better use.
This behaviour of a mild steel rod under loading can be analysed using stress strain curve of mild steel rod.
The stress strain curve consists of strain along the x axis and stress along the y axis.
Stress strain curve consist of various stages, they are proportional limit, elastic limit, upper yield, lower yield, ultimate stress and breaking point.
Now let us discuss one by one.
Proportional limit
If the stress is applied gradually to a mild steel rod, the strain also gradually increased that is represented in point A.
In this stage, when stress is removed from the rod the strain returns to zero.
Thus, the O to A points is called as proportional limit.
In this limit, the stress is directly proportional to strain.
Stress ∝ Strain
σ∝e
That means the steel rod obeys Hooke’s law in this stage.
The proportionality is expressed as E.
σ = E e
E =σ/e
Where, E is equal to Young's modulus or modulus of elasticity
Thus, Young's modulus is defined as the ratio of stress to a strain of the mild steel rod.
The unit of Young's modulus is Newton per millimetre square.
Now let us move on the elastic limit stage.
Elastic limit
The increase of stress to the point B, the strain also increased proportionally, it is called as elastic limit.
In this stage also, if the applied stress is removed, the strain returns to an original position that is rod gains to original shape.
The stress increased beyond the elastic limit, the material undergoes deformation.
Which means the permanent extension occurs, the mild steel rod does not regain its original shape even after the stress is removed.
This is expressed as C to D point in the graph which is called as yield point.
In point “C” at which maximum stress is required for a rod to undergo deformation. This is called as upper yield point.
In point “D” of at which minimum stress is required for a rod to undergo deformation. This is called as lower yield point.
The increase of stress beyond this limit, the strain is also increased gradually to a point.
This point is “E” which is called as ultimate stress or ultimate strength point.
Ultimate stress is the maximum stress the rod can withstand.
Thus, this portion is called as strain hardening.
Further increase of stress beyond the ultimate stress, the localised reduction occurs in the cross sectional area of the rod this is the weakest point of the material.
This is called as necking stage that means a breaking point stage.
At this stage, the mild steel rod breaks, thus the curve drops to the point F.
Thus, the curve obtained is the stress strain curve for the mild steel rod under the action of load.
The point A to B is elastic region and point B to F is the plastic region of mild steel rod.
Similarly, if you plot the stress strain curve for any material you can determine the yield strength, ultimate strength and the breaking point of the material.
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