Mild Steel Shows a ductile nature and it has better capacity to resist tensile forces. It also has a good resistance to compressive forces, but we generally prefer it to resist tensile stresses because of following reasons:
1) It shows ductile nature.
2) It's is economic in comparisons to other materials which shows ductile behaviours.
3) It has higher modulus of rigidity, etc.
4) its durable in nature.
4) its durable in nature.
5) It shows rust resistance behaviour.
6) Steel can also be recycled easily,etc.
Experimentally, when a load is applied to the steel we can find a curve as shown in figure below.
when a load say 'P' is applied to the steel the steel gets deformed. On increasing the load, it deforms linearly up to the point say 'A'. The point 'A' here is known as proportional limit. Upto proportional limit, the steel shows its linear characteristics. Further, on increasing the load, it deforms in non-linear pattern, an reaches to point 'B'. The point B here is known as elastic limit. After that on further increasing the load the curve shows its non-linear behaviour in an increasing fashion and reaches to point 'C', which is known as Upper yield point, and afterward it reaches to lower yield point 'D' in decreasing fashion. The reason for decreasing fashion of curve is due to molecular dislocation of materials. Further, on increasing the load the curve tends to reach upward and it reaches to point 'E' on the curve known as ultimate strength point. After reaching ultimate strength point, the curve then reaches to facture point 'F' , and the materials fails.
let's detailly glance over Each important points on the curve:
1) Proportional limit (Point A): At first when the load is applied to mild steel, it deforms linearly i.e. when the load is removed it returns to it's original state restoring deformation wholly.
Mathematically, we can say that stress is directly proportional to the strain, which is popularly known as Hooke's law.
Mathematically,
Modulus of elasticity= Stress/strain.
The maximum stress upto which the curve shows it linear nature and returns back to original state after removal of load is called proportional limit (clearly indicated by point 'A').
2) Elastic limit (Point B): When the load is further increased after proportional limit, the materials starts to behave to show non-linear deformations for applied load but however after releasing the load material returns to original shape following the original curve, mild steel shows this behaviour upto point B as shown in figure.Thus, the maximum stress upto which the materials returns to original state and restore whole strain developed although by deforming non-linearly is called as elastic limit.
3) Upper Yielding point (Point C):After elastic limit, on increasing the load mild steel enters in elasto-plastic region and reaches to point 'C' called as upper yield point. The applied stress or load tends to break the molecular bonds as a results molecular dislocation start to occur in mild steel. The maximum stress which tends to form molecular dislocation or tends to break the molecular bonds in steel so that material enters from elasto-plastic region to complete plastic region is called Upper Yield point.
NOTE:When the material is in elasto-plastic range it doesn't wholly recover the strain developed in it, even after releasing the load from it. The material is in partial elastic and partial plastic situation. However the deformation set are small as compared to deformation set in plastic region.
4)Lower Yield Point (Point D): After molecular dislocation occurs in the materials its resistance to the applied force gets on decreasing so smaller force or stress can induce a plastic deformation on it.Thus, there is decreasing trend of curve after upper yield point to lower yield point.This decreasing trend goes on until it reaches to a point where numbers of dislocations tends to acts as a new resistance.And the minimum stress which can induce a plastic deformation on the material due to molecular dislocation in mild steel is known as Lower yield point.
5) Ultimate Strength Point (point E): Again after the lower yield point the curve tends to take it's peak value. This is because of the fact that due to molecular dislocation started from upper yield point to lower yield point the number of dislocations gets abundance on the materials and the numbers of new dislocation tends to act as new bond of resistance for plastic zone. Thus, due to new resistance developed the curve will show an increasing stress vs strain pattern but in non-linear and non-elastic fashion and after certain value of stress applied, the curve obtains a maximum value of strength in plastic zone called ultimate strength of material.
when the applied load is released during this plastic region the material will return following the dotted line as shown in figure. There it sets a permanent deformation on it, called permanent set.
6) Fracture or rupture point (Point F): When the material is in ultimate strength point, it's has maximum value of plastic resistance but after this point the material tends to have lower strength for the applied load, it show higher strain for smaller load and finally reaches to point F, where fracture occurs in materials.
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