What is Stiffness Of a Material ?

The resistance of a material to elastic deformation or deflection is called stiffness or rigidity. A material which suffers slight deformation under load has a high degree of stiffness or rigidity. For instance suspended beams of steel and aluminum may both be strong enough to carry the required load but the aluminum beam will "sag" or deflect further. In other words, the steel beam is stiffer and more rigid than aluminum beam.

If the materials follows Hook's law, i.e., has a linear stress-strain relation, its stiffness is measured by the Young's modulus E. The higher the value of the Young's modulus, the stiffer the material.

In tensile and compressive stress, it is called modulus of stiffness or "modulus of elasticity" ; In shear , the modulus of rigidity , and this is usually 40 per cent of the value of Young's Modulus for commonly used materials; in volumetric distortion , the bulk modulus.

The term flexibility is sometimes used as the opposite of stiffness. However, flexibility usually has to do with flexure and bending. Also it may imply ease of bending in the plastic range.

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