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STRESS CONCENTRATION
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 a localized increase in stress at that 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, changes in the cross-sectional area of the object. High local stresses can cause the object to fail more quickly than if it wasn't there. Engineers must design the geometry to minimize stress concentrations.
A counter-intuitive method of reducing one of the worst types of stress concentrations, a crack, is to drill a large hole at the end of the crack. The drilled hole, with its relatively large diameter, causes a smaller stress concentration than the sharp end of a crack. This is however, a temporary solution that must be corrected at the first opportune time.
Examples
This orthosis is implanted to support the femur after a fracture, but the concentration of stress at its bend increases the possibility that it may break under force.
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 due to stress concentrations include metal fatigue at the corners of 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 of the gross cross-section. These factors can be found in typical engineering reference materials to predict the stresses that could otherwise not be analyzed using strength of materials approaches. This is not to be confused with 'Stress Intensity Factor'.
References
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