A stress concentration is a phenomenon encounterered in mechanical engineering where an object under load has higher than average local stresses due to its shape.

The types of shape that cause these concentrations are: cracks, sharp corners, holes and narrowing of the object. High local stresses can cause the object to fail more easily than its overall size suggests. A task for the engineer is to design the shape of the object to reduce stress concentrations.

A counter intuitive method of reducing one of the worst types of stress concentration, a crack, is to drill a large hole at the end of the crack. The drilled hole, with its relatively large diameter, causes less stress concentration than the sharp end of a crack.

Classic cases of metal failures provoked by stress concentrations include metal fatigue in the windows of the De Havilland Comet aircraft and brittle fractures at the corners of hatches in Liberty ships in cold and stressful conditions in winter storms in the Atlantic Ocean.