Deformation of simple bars under axial load Deformation of bodies
Concept of strain : if a bar is subjected to a direct load, and hence a stress the bar will change in length. If the bar has an original length L and changes by an amount dL, the strain produce is defined as follows:
Strain is thus, a measure of the deformation of the material and is a nondimensional Quantity i.e. it has no units. It is simply a ratio of two quantities with the same unit.
Shear strain: As we know that the shear stresses acts along the surface. The action of the stresses is to produce or being about the deformation in the body consider the distortion produced b shear sheer stress on an element or rectangular block This shear strain or slide is f and can be defined as the change in right angle. or The angle of deformation g is then termed as the shear strain. Shear strain is measured in radians & hence is non - dimensional i.e. it has no unit.So we have two types of strain i.e. normal stress & shear stresses.
Hook's Law :
A material is said to be elastic if it returns to its original, unloaded dimensions when load is removed.
Hook's law therefore states that Stress ( s ) a strain( Î )
Modulus of elasticity : Within the elastic limits of materials i.e. within the limits in which Hook's law applies, it has been shown that
Stress / strain = constant
This constant is given by the symbol E and is termed as the modulus of elasticity or Young's modulus of elasticity Thus ,The value of Young's modulus E is generally assumed to be the same in tension or compression and for most engineering material has high, numerical value of the order of 200 GPa
Poisson's ratio: If a bar is subjected to a longitudinal stress there will be a strain in this direction equal to s / E . There will also be a strain in all directions at right angles to s . The final shape being shown by the dotted lines.
It has been observed that for an elastic materials, the lateral strain is proportional to the longitudinal strain. The ratio of the lateral strain to longitudinal strain is known as the poison's ratio .
Poison's ratio ( m ) = - lateral strain / longitudinal strain
For most engineering materials the value of m his between 0.25 and 0.33.
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