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Thursday, 22 October 2015

STRENGTH

STRENGTH:

                     The strength of metal is its ability to withstand various forces to which it is subjected during a test or in service. It is usually defined as tensile strength, compressive strength, proof stress; shear strength etc. strength of material is a general expression for the measure of capacity of resistance processes by solid masses or pieces of various kinds to any cause tending to produce in them a permanent and disabling change of form or positive fracture.
Accordingly, one may broadly classify strength into following two categories:

        a) depending upon the value of stress, the strengths of a metal may be Elastic or Plastic.
       b) Depending upon the nature of stress, the strength of a metal may be tensile, compressive, shear, bending and torsional. Now, we study all these types of strengths.

            Ultimate strength: It is the load required to fracture a unit cross-section of material.

           Elastic Strength: It is the value of strength corresponding to transition from elastic to plastic range, i.e., when material changes its behavior from elastic range to plastic range. One cannot measure it accurately. This is known as ideal stress value and used to define elastic strength of a material. 

           Plastic strength:  It is the value of strength of the material which corresponds to plastic range and rupture. It is also termed as ultimate strength. In actual practice, a specimen is subjected to a stress which is always less than the working stress. The ratio of ultimate stress to the working stress of a metal is termed as factor of safety or factor of ignorance. This greatly depends upon the nature of loads or stresses. Usually, the following values of factor of safety are generally kept for various loads.

Dead load
4 or 5
Live load
6
Alternating kind of load
8 to 12
Shock loading
12 to 15

                  One adopts the lower value of factor of safety by ensuring the metal to be without any defect, which is done through 'non-destructive' tests. In determining the reliability of the design, the factor of safety is of great importance.

              Tensile Strength:  It is the maximum tensile stress which a material is capable of developing when subjected to loading up to rupture. Mathematically,


                       The tensile stress is expresses in N/mm2  or MN/m2 . In actual practice, a given specimen is always subjected to a tensile stress less than the working tensile stress. Tensile strength is obtained from the above relation.

                     Tensile strength is the ultimate strength in tension and corresponds to the maximum load in a tension test. It is measured by the highest point on the conventional stress-strain curve. This strength provides the basic design information on the material's acceptance in engineering tests. In ductile materials the load drops after the ultimate load because of necking and this indicates the plastic instability. For working stresses, the ultimate tensile strength is a logical basis in brittle materials. We may note that like yield strength, tensile strength is used with a factor of safety. 

Compressive Strength:

                     For a metal, the compressive strength is the value of load applied to break it off by crushing. Mathematically,

             Compressive stress is also expressed in N/mm2  or MN/m2. In actual practice, a given specimen is also subjected to a compressive stress less than the working compressive stress.

Shear Strength:

                 The shear strength of a metal is the value of load applied tangentially to shear it off across the resisting section. Mathematically, we can express,

              It is also expressed in N/mm2  or MN/m2. In actual practice, a given specimen is also subjected to a shear stress less than the working shear stress.

Bending Strength:

For a metal, it is that value of load which can break the metal off by bending it across the resisting section. Mathematically,


Torsional Strength:

For a metal, the torsional strength is that value of load applied to break the metal by twisting across the resisting section. Mathematically,

               This is also expressed in N/mm2  or MN/m2. In actual practice, a given specimen is also subjected to a shear stress less than the working shear stress. 



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