Friday, December 7, 2012

Stress and Strain

Introduction: There are so many things around us and we do work with them to accomplish different goals.
We construct our houses and live in them because they provide us the protection from the harmful environment. For making houses we use different materials which are strong and durable to withstand their own weight and  surrounding environment.
For example we use the beams and columns to support the weight of the slab and the upper elements of the building. When the weight of the upper stories or the  slab is put on the beams and columns,they must not break or fail.When the external loads are applied on the column or beam the internal resistance is induced.
This internal resistance depends upon the physical and chemical composition of the material used in the manufacturing/construction of the beams and columns.
This internal resistance is denoted by a term 'stress'. Stress is a contact contact force, measured per unit area.
Stress = Contact force/Cross section area
The value of the internal resistance/stress is finite for every material. This finite limit of the internal resistance is known as the strength of the material.
The most important thing for a material to remain un-failed is that value of the internal resistance in the material must not reach the strength of the material when, they are applied with the external loads.

Now to  make it sure that the internal resistance do not reach the strength limit we have to calculate the magnitude of the internal stresses which gets induced in the structure due to the external loads.
After that we can decide the amount of the external load which can be put on the structural elements, safely. So there are three things which we must know to become a structure Engineer :
(1) strength of materials
(2) Theories of structure
(3) The design philosophy to design the structural elements for the various load requirements(Design) with different materials.

Stress :
When the external loads are applied on a structural elements the internal resisting stresses are developed in the elements.
When the axial loads are applied on the element (i.e. tensile or compressive) the axial/normal(tensile or compressive) stresses are induced.
So when the tensile force is applied tensile stresses gets induced and similarly when the compressive forces are applied the compressive stresses are induced.
These stresses are common for the  structural elements like columns.
Now when transverse loading is applied the induced stresses are called bending stresses. These stress are also of two types i.e. tensile and compressive.
Beams are the structural elements which are subjected to the transverse loading.
There is one more type of stress which is known as the shear stress. When the load is applied along the surface of the area of consideration such that it tries to shear the element into two parts along the surface then the resisting stresses are known as the shear stresses.

The simple formula to calculate the normal stress on a plane is = Applied Load/ Area of cross section

The units for the stress is N/mm2, KN/cm2 etc.

To be continued.......

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