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Eingeladener Vortrag
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The measurement of spontaneous magnetic stray fields of ferromagnetic materials is considered as a new method of non-destructive testing, the Metal Magnetic Memory method. Usually fluxgate sensors with spatial resolution in the order of a few millimeters are used as receivers. This contribution discusses the application of GMR (Giant Magneto Resistance) sensors with a spatial resolution in the micrometer range. Studies reveal that GMR sensors can detect clear demarcated plastic deformations in S235JR steel. Furthermore we present high spatial resolution magnetic measurements of deformation induced martensite formation of AISI 304 metastable austenitic steel for different deformation states.
The metal magnetic memory (MMM) technique relies on the measurement of stress-induced self-magnetic leakage fields (SMLFs) at the stress concentration zones (SCZs) of ferromagnetic materials during mechanical loading. However, there is an associated change in geometry of the specimen along with the stress due to plastic deformation. This paper presents a three-dimensional finite element (3D-FE) analysis of the stress-induced geometry effect on SMLFs in notched specimens during tensile deformation. The tangential (Hx) and normal (Hy) components of the SMLF signals have been predicted from the deformed specimens caused by different levels of tensile stress. Key parameters from the SMLF signals are determined for the possible estimation of damage in the specimen under tension. Studies reveal that the stress-induced geometry effect has a great influence (about 20%) on the SMLF signals, especially in the plastic deformation stage. The results show that the peak amplitude could be used for the estimation of different deformation stages under tension. The study also reveals that the SMLF signal is influenced by the thickness of the tensile specimen. The model-predicted thickness profile has also been experimentally validated.