What is stress concentration? State its significance in design of machine elements

Whenever a machine component changes the shape of its cross section, the simple stress distribution no longer holds good. This irregularity in the stress distribution caused by abrupt changes of form is called as stress concentration

In most machine elements have some forms of discontinuity, namely sudden change in cross section, grooves ,holes, keyways and other changes in sections. these continuity in machine element alter the stress distribution in the neighborhood so that the elementary stress equations no longer described the actual state of stress in the part, such discontinuity is called stress raisers and in the region in which these occur is called the area of stress concentration. Internal cracks and flaws, cavities in welds, blowholes, and pressure in certain points are the common examples of stress raisers.