Stress: What is Stress?

 

Stress

When some external system of forces or loads acts on a body, the internal forces (equal and opposite) are set up at various sections of the body, which resists the external forces, this internal force per unit area at any section of the body is known as unit stress or simply stress.

The stress action on any surface can be resolved into two components one normal to the surface and the other tangential to surface. These are two basic stresses, namely normal stress and shearing stress. All other stress are either similar to these or a combination of these.

        i.            The axial stresses resulting from axial forces which tend to elongate a member are called tensile stress.

      ii.            Member which are subjected to predominantly axial tensile stress are known as ties.

    iii.            The axial stress which tends to shorten or compress a member is called compressive stresses.

    iv.            Members which are subjected to predominantly axial compressive stresses.

      v.            If a force is parallel to the surface on which it acts, it is called a shear force. The direction of shear stress on any will be parallel to the plane of shear.

    vi.            Ultimate strength is the greatest stress the material can with stand without rupture . For obvious reasons of safety, the full strength is never utilized.

  vii.            Allowable stress is that part of the ultimate strength which will be used when maximum permissible is applied.

viii.            In the straight line or elastic theory, the factor of safety is the stress at which the material yields (called as yield stress) divided by allowable stress.

    ix.            The selection of factor of safely will be based on “degree of safety “required economy , reliability of the material, required life of the structure, loading conditions, method of assessment of qualities of the material, method of assessment of loads, access into the structure for maintenance and environmental conditions.

Mathematically,

Stress (σ) =   P/A

Where, P = Force or load action on the body.

               A= Cross-sectional area of the body

In S.I. units , the stress is usually expressed in Pascal (Pa) , such that 1Pa=1 N/m² . In actual practice, we used bigger unit of stress i.e., magapascal (MPa) and gigapascal (GPa) such that ;

1MPa= 1 x 10⁶ N/m²= 1 N/mm²

1GPa= 1 x 10⁹ N/m²= 1 kN/mm²

 

Broadly, stress can be divided into two groups:

i)                    Direct stress

a)      Normal stress

It acts perpendicular to the cross-sectional area on which load is subjected.

1. Tensile stress

Tensile stress developed due to tensile force, which tends to elongate the body .

2. Compressive stress

Compressive stress develops due to compressive force, which tends to shorten the body.

b)     Shearing stress

·         It develops due to shear force.

·         It acts tangential to the surface.

c)      Bearing stress

·         It develops at the state of load transfer.

·         It defines the load bearing capacity of a body.

ii)                  Indirect stress

·         Bending stress: It develops due to bending of a body.

·         Torsional stress: It develops due to torsion action on a body.