A section from a photoelastic model of turbine disc with a single blade viewed in polarised light to reveal the stress distribution.

Risk is defined as the possibility of something happening multiplied by the consequences when it does happen.  The public understanding of risk sometimes only extends to the first half of this definition.  Engineers seek to reduce the risks associated with component failure.  This means accepting a non-zero probability of failure happening and then designing for least catastrophic consequences.  So for instance in a jet engine, this implies designing so that if a crack develops it is in a blade rather than the disc to which all of the blades are attached.  The engine casing can be designed to contain a single blade breaking off and thus protect the rest of the plane from flying debris, but not to contain the rupture of an entire disc and set of blades.

For more information on the photoelastic stress analysis techniques used to generate the image, see http://www.experimentalstress.com