Here we are using an assumption that the total force or total load carried by the rectangular bar is
uniformly distributed over its cross – section.
But the stress distributions may be for from uniform, with local regions of high stress known as stress
If the force carried by a component is not uniformly distributed over its cross – sectional area, A, we
must consider a small area, ‘ A' which carries a small load P, of the total force ‘P', Then definition
of stress is
As a particular stress generally holds true only at a point, therefore it is defined mathematically as
The basic units of stress in S.I units i.e. (International system) are N / m2 (or Pa)
MPa = 106 Pa
GPa = 109 Pa
KPa = 103 Pa
Some times N / mm2 units are also used, because this is an equivalent to MPa. While US customary
unit is pound per square inch psi.
TYPES OF STRESSES :
only two basic stresses exists : (1) normal stress and (2) shear shear stress. Other stresses either are
similar to these basic stresses or are a combination of these e.g. bending stress is a combination
tensile, compressive and shear stresses. Torsional stress, as encountered in twisting of a shaft is a
Let us define the normal stresses and shear stresses in the following sections.
Normal stresses : We have defined stress as force per unit area. If the stresses are normal to the areas
concerned, then these are termed as normal stresses. The normal stresses are generally denoted by a
Greek letter ( )