Every material we come across undergoes some change in shape and dimensions when loaded by external forces. We can classify materials into elastic, plastic and rigid materials based upon their deformation under loading.
Elastic material when subjected to external loads undergoes a deformation but regain it’s original shape and dimensions after removal of loading. On the other hand, the plastic material undergoes a continuous deformation when subjected to external loads and does not regain it’s original shape and size after removal of loads.
The deformation is temporary in case of elastic while permanent in case of plastic materials.
A rigid material does not undergo any deformation under the action of external loads. Practically, no body is perfectly rigid in nature but can be assumed to be rigid when the deformation is very very small compared to dimensions of the body.
What is ‘Stress’?
When material is subjected to external forces, it undergoes a deformation and the resisting force is generated internally from the body against this deformation, this is called as stress. This internal resisting force is generated because of the strength within the body.
So, Stress can be defined as the internal resisting force developed against deformation per unit cross-section area of the body. It is denoted by letter ‘σ‘ (sigma- greek letter). When we consider the equilibrium of the body, the resisting force developed internally is always equal to externally applied force.
Stress (σ) = F/A, where
F = Internal resisting force / applied force.
A = Cross-sectional area
SI unit of stress is Pascal (N/m2).
Types of Stress
Different types of stress are produced due to different types of loads applied to the body. Following diagram represents how stress is classified-
1) Direct stress is also called as simple stress and it develops under direct loading conditions. The load is acting directly in the direction of
i) Normal stress– The stresses acting perpendicular to the plane on which the load acts axially are called
- a) Tensile stress– When two equal and opposite pulling loads are acting on the body, the body is subjected to tensile stress.
- b) Compressive stress– When two equal and opposite pushing loads are acting on the body, the body is subjected to compressive stress.
ii) Shear stress– When equal and opposite forces act tangentially on the resisting section of the body in such a way that the body tends to shear off the section then the stress developed in the body is called as shear stress.
2) Indirect stress – In
i) Bending stress– When body is subjected to two or more loads in such a way that it tends to bend then the stress induced in the body is called as bending stress. In this case, body is subjected to both tension and compression.
ii) Torsional stress– When two equal loads act like a couple on the circumference of the body due to which it tends to twist, then the stress induced in the body is called as torsional stress.
3) Combined stress – When the body is subjected to more than one type of stresses mentioned above, then it is called as combined stress. Generally, all machine parts undergo combined stresses always.