Micromagnetic and electromagnetic NDT for lifetime management by monitoring ageing of structural materials
Since 1976 the 3MA-methodology (Micromagnetic, Multiparameter, Microstructure and Stress Analysis) was developed in the German Nuclear Safety Research Program. The basis is in micromagnetic NDT techniques as the measurement of magnetic Barkhausen noise, of the magnetic incremental permeability, of the eddy current impedance, the harmonic analysis of the magnetic tangential field strength, and the measurement of the dynamic or also called incremental magnetostriction. 3MA can only be applied at ferromagnetic materials and was used to characterise ageing phenomena in pressure vessel and pipeline steels as thermal ageing and neutron degradation as well as material states when thermal ageing and low-cycle fatigue were superimposed. In the case of austenitic stainless steels when exposed to mechanical static or cyclic loads - the material reacts localised with phase transformation to bcc a'martensite - 3MA techniques can be applied. In all other cases UT is utilised and - for instance a time-of-flight-measurement - can characterise cyclicdeformation.