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Superconductors under dynamic mechanical stress
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This study investigated the training phenomenon in superconducting magnets by measuring stress-strain curves of NbTi commercial superconductors under applied magnetic fields and transport currents, finding that normal transitions occurred at successively higher stress levels — a training behavior in short samples. Acoustic emission monitoring revealed distinct microplastic behavior at low strains, with irreversible signals indicating permanent deformation mechanisms at strain levels well below the macroscopic elastic limit.
Experiments were performed to study the cause for the training phenomenon in superconducting magnets. Stress-strain curves of commercial superconductors were taken with applied field and transport current. In experiments where the samples were strained in a constant field and with a transport current below I c , the normal transition occured at successively higher stress levels. That means, a training behaviour of short samples was found. In the low strain region a distinct microplastic behaviour of NbTi wires was found by monitoring acoustic emission. The irreversibility of the detected signals indicates that mechanisms of permanent deformation exist at strain levels well below the macroscopic elastic limit. These mechanisms may be responsible for the training effect.
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