Residual magnetic field sensing for stress measurement

The evaluation of both applied and residual stresses in engineering structures to provide early indications of stress status and eventual failure is a fast growing area in non-destructive testing. Much work has been done in recent years in the development of magnetic stress measurement techniques for ferromagnetic materials using applied magnetic fields to monitor changes in the magnetic properties of materials, such as variations in the hysteresis curve or Barkhausen emission. But the area of passive field measurement is relatively unexplored. When magnetic metals are strained, they irreversibly transformed from a non-magnetic state to a magnetic state, this is referred to as metal magnetic memory (MMM) or the residual magnetic field (RMF). This paper investigates the phenomena under different circumstances and applies the residual magnetic field technique to stress measurement. A three axis magneto-resistive magnetic field sensor is used to measure the residual magnetic fields parallel to the applied stress and the material surface (Bx) and perpendicular to the material surface (Bz), generated by the magneto-mechanical effect without the application of an external field, using steel samples exposed to stresses. The test results show that without using an applied field, the stresses in a sample can be measured using magnetic field sensing, with Bx showing particularly good correlation. The work concludes that the novel passive field technique including analysis of the magnetic field pattern and magnetic field variation rate, would prove advantageous in certain circumstances, for example in-service inspection of structures with complex geometries. Further research directions are also highlighted.