The Metal Magnetic Memory method 

MMMThe Metal Magnetic Memory method is an inspection technique for ferromagnetic materials based on the magnetomechanical effect. When a load is applied to a ferromagnetic component, the load and the presence of the earth’s magnetic field generate a magnetostrictive effect that irreversibly reorients the magnetic domains around stress concentrations. These changes to the magnetic domains remain even after the load is removed. The magnitude of the changes relates to the maximum load applied. By measuring the strength of the magnetic field leakage perpendicular to the component surface, the MMM technique provides information on stress concentrations near to the surface of the component.

The MMM method is a surface technique and can be used for defect detection, detection of regions of high stress and assessment of the stress history of a component.

For further information on MMM see:
BS ISO 24497-2:2007 ‘Non-destructive testing – Metal magnetic memory – Part 2: General requirements’

For 2018 papers on MMM see:

Haihong Huang et al, ‘Evaluating the wear state of ferromagnetic material using magnetic memory signals based on tribo-magnetisation’, Insight: Non-Destructive Testing and Condition Monitoring, Vol 6, No 6, pp 335-342, June 2018.

What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: