R & D to electromagnetic NDT in the German nuclear safety research program — Material characterization of ageing phenomena and online monitoring of fatigue and fracture-mechanical tests

Obviously it is a fact: The nuclear energy technology worldwide was and still is an important driver for the development of NDT/NDE. This is true in Germany too. Concerning the most relevant task of NDT - detection, classification, and sizing of material irregularities (inhomogeneity, defects) the development of NDT technologies and methodologies like UT and imaging with phased array transducers or defect reconstruction algorithms like SAFT (synthetic aperture focusing technique) or ET of steam generator heat exchanger tubes with eddy current probes and multi-frequency approaches primarily were in the focus of R&D. However, in Germany there was also a strong demand to develop NDT for characterizing the materials of the nuclear components in their properties. The basic idea was to have NDT technology available for inservice inspection of primary circuit components which can characterize the microstructure as well as load-induced and residual stresses. The characterizing should be performed in terms of mechanical properties as hardness, strength like yield and tensile strength but also toughness properties like Charpy energy and fracture appearance transition temperature. Beginning 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 of the material under inspection and therefore 3MA can only be applied at ferromagnetic materials. The techniques collect information which is generated by interaction of Bloch walls with microstructural parameters (lattice defects as vacancies, dissolved atoms, dislocations, precipit- tions, grain and phase boundaries as well as stress fields). The magnetization processes utilized are reversible and irreversible. Therefore the information collected is divers and redundant which helps to enhance the statistical significance for prediction and tu suppress disturbance influences. 3MA was applied to characterize aging 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 characterizing material states of austenitic stainless steels it strongly depends on the chemical composition whether the material - for instance when exposed to mechanical static or cyclic loads - reacts localized with phase transformation to bcc α´ martensite or not. The phase-transformed microstructure is ferromagnetic and therefore 3MA techniques can be applied. In all other cases UT is applied and - for instance a time-of-flight-measurement - is the tool to characterize cyclic deformation. When the mechanical loading is at elevated temperatures (300°C) EMAT are applied. Monitoring of mechanical-technological destructive tests by NDT technology can significantly enhance the information content. Fatigue as well as fracture mechanical tests were on-line monitored.