A residual stress phenomenon is used in recent developments of an industrial Structural Health Monitoring System. For fatigue loaded structures 'damage' means cracks and the detection and monitoring of crack length can be used as a measure of Structural Health. Fatigue acumulates due to repeated loading at a level less than that required to cause failure from a single load application. When nucleation or crack initiation occurs a plastic zone containing residual stresses forms ahead of the crack tip. The material in the plastic zone is also embrittled and sufficient energy must be put into the crack tip zone by fatigue loading for the crack tip to penetrate this zone. The crack growth is explosive on the relative scale and as a result propagates a stress wave into the structure from the seismic epi-centre or crack tip. Special sensors are used to detect the stress wave and it relative time of flight enables location of the origin by triangulation. From this data an assessment can be made as to the 'health' of the structure which can be used for directed maintenance and condition monitoring.