Hello everyone we have learnt that the stress is directly proportional to strain (within elastic limit). This statement indicates that when substance undergoes deformation due to deforming forces it can break at certain point. Then it becomes very essential to understand the maximum load that can be bare by the material. These studies helps us to select the appropriate material and its dimensions required for specific purposes like construction of bridges, buildings, cables of elevator etc.
Let’s understand this in more detail……!
Consider the uniform metallic wire suspended from rigid support and subject to load at its through hanger attached to it. Just take a look on following points!
1) The load applied to the wire is increased continuously in steps of 1/2 kg until finally the wire breaks. During each step, the extension in the wire is measured & a graph of stress against strain is plotted. As shown below.
2) The initial part [OE] of the graph is straight line, which indicates that stress is directly proportional to strain i.e. Hooke’s law is obeyed.
3) Further addition of weight in the hanger beyond the point E, the graph is gets curved, showing that the strain increase more rapidly. This region indicates that now the elastic limit is comes to an end.
4) When we add further load, we get region E’Y up to yield point Y at which the tangent drawn to the curve is parallel to the strain axis, i.e. At this point stress. The plastic deformation begins at this stage.
5) Some more addition of weight in the wire during plastic deformation, we reach a point N, at which the wire forms a thin at its weakest part & begins to increase in strain at reduced stress breaking finally at point B, which represents the breaking stress.
By knowing the value of breaking stress, we can design the material with the factor of safety taken in consideration.
Just go through the following terms……..!
Elastic limit: (E)
The maximum stress to which a body can be subjected without permanent deformation is called its elastic limit.
Yield stress:
The stress in the wire at which strain goes on increasing even without any increase in the stress is called as yield stress.
Breaking stress: –
The maximum stress to which a wire can be subjected is called its breaking stress.
Yield point: – [Y]
At this point strain goes on increasing even without any increase in the stress.
Breaking Point : – [N]
The point on the stress-strain curve at which the wire breaks is called as the breaking point.