Electromagnetic NDT for lifetime management by monitoring of ageing phenomena
Starting in 1976, the 3MA-methodology (Micromagnetic, Multiparameter, Microstructure and Stress Analysis) was developed. 3MA has its basis in micromagnetic NDT techniques which are 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. All of these techniques ask for a local magnetization in a hysteresis loop and therefore 3MA can only be applied at ferromagnetic materials. 3MA was used to characterize 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 localized with phase transformation to bcc ' martensite - 3MA techniques can be applied. In all other cases UT is utilized and - for instance a time-of-flight-measurement - can characterize cyclic deformation. When the mechanical loading is performed at elevated temperatures (300°C) EMAT are applied.