A stress concentration or stress riser is a location in an object where stress is concentrated. An object is strongest when force is evenly distributed among it, but an area of stress concentration results in an increased chance of breakage at its location.

Causes

Geometric discontinuities cause an object to experience a local increase in the intensity of a stress field. The examples of shapes 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, so engineers must design the shapes of support devices 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.

Examples

The term "stress riser" is used in orthopedics; a focus point of stress on an implanted orthosis is very likely to be its point of failure.

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.

Concentration factor

A stress concentration factor is the ratio of the highest stress to a reference stress calculable from simple theory. These factors can be found in typical engineering reference materials to predict the stress in structures that could otherwise not be analyzed using strength of materials approaches. This is not to be confused with 'Stress Intensity Factor'.

References

• ESDU64001: Guide to stress concentration data (ISBN 1862462798)

External links

• When Metal Lets Us Down
• EBI - Medical Glossary

Physics | Engineering | Orthopedics