STRESSES

In static and dynamic bodies are considered as rigid the bodies do not undergo any change in shape under the action of force in contrast strength of materials deals with the behavior of the bodies under the action of external loads. Moreover the deformation (bodies are no longer assumed to be rigid) under the load is the major interest and an attempt is made to find out whether or not a given member fail under the load. The basic concepts of strength of materials are discussed in this chapter.

Loads:

Any external forces acting on a component are called loads. The loads may be classified as given below.

a)   According to the manner in which they are applied.

1)   Point or concentrated loads

2)   Distributed loads

Point or concentrated load acts over a very small area which is considered as a point it is expressed in N or KN.

Load distributed over an area or along a length is called distributed load a load may be uniformly or non-uniformly distributed along a length or an area it is expressed in N/m² or N/m.

b)   According to the nature of action:

     I.        Static load

   II.        Dynamic load

 III.        Fluctuation or cyclic load.

Static load gradually increases from zero to a certain value and them remains constant during entire time of its application. Dynamic load refers to the impact or falling load which time duration is a small fraction of second. Fluctuation or cyclic load refers to the load in which its magnitude and direction varies continuously. 

C)   According to the effect produced:

1. Axial load                         
2. Tensile load
3. Compressive load
4. Shear load
5. Bending load

Axial loads are applied along the longitudinal axis of the structural member if the action of load is to increase the length of the member the member is said to be in tension and the applied load is tensile load. A load which tends to shorten a member and place the member in compression is known as a compressive load. Shear loads act perpendicular to the axis of a structural member it tends to slides the surface with respect to other or to cut the part. Bending load is transverse load tending to bend the structural member. 

Stress:

When load acts on a body the internal resistive force (strength) resists deformation. This internal force is equal and opposite to the external load. The total resistance per unit area of cross-section is called induced stress or stress. Stress is usually denoted by σ and if a is the cross-sectional area normal to the load then

Stress = load/area of cross-section

          σ = P/A N/mm²

In SI units the stress is expressed in N/m² or N/mm² it may be noted that

Type of stresses:

Depending on the direction of load the following types of simple stresses are induced in a component.

·      Tensile stress

·      Compressive stress

·      Shear stress

Tensile stress:

The load acting along the axis and perpendicular to the area of cross-section is called normal or axial load the axial load tending to stretch the bar is called tensile load and corresponding stress induced is called tensile stress.

Tensile stress σ₁ = tensile load/area of cross-section

          σ₁ =P/A

Compressive stress:

The axial load tending to compress the block is called compressive load and the stress induced is called compressive stress.

Compressive stress σc = compressive load/ area of cross-section

Shear stress:

The tangential load tending to cut or slide the part is called is shear load and the corresponding stress induced is called shear stress in this case load tangential to the area over which it acts.

Shear stress τ= shear load/area of cross-section

τ = P/A