When engineers design parts for service, they do so with the mechanical properties of the material in mind. In other words, materials are not used in applications where they will be subject to stresses above their yield strength. So why then do materials often fail? The answer lies in stress concentrations.
When a force is applied to an object, the resulting stress is distributed throughout the material. However, 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: