If a defect or crack exists in the material then the stress cannot be transferred through this region and a localised build up of stress occurs. This accumulation of stress around the crack tip is known as a stress concentration. If the stress in this region builds up to a sufficient extent to be above the yield strength of the material then localised plastic deformation occurs and the crack increases in size.

There are a number of means by which engineers reduce the likelihood of stress concentrations occurring through careful design of components. These include:

• Avoiding sharp corners and only using rounded corners with maximum radii. eg plane windows

• Sanding and polishing surfaces to remove any notches or defects that occur during forming and processing.

• Placing notches and threads in low-stress areas. eg. holes in centre of beams