| Popis: |
It has been about a decade since one of the authors (HW) used a highly sensitive SQUID magnetic gradiometer to measure changes in one direction of magnetic field near a succession of 1018 steel specimens subjected to stress in a tensile-testing machine [1]. On every trial of every specimen a peak was observed in the magnetic field change observed as a function of stress, when the stress was about 0.6 of the elastic limit. A later study [2] using a conventional fluxgate magnetometer confirmed this behavior. While the cause of this sign reversal for the slope of the magnetoelastic response (dΦ/dσ) is not known — perhaps there is a connection to a similar sign reversal in the magnetostriction in iron (albeit at a field on the order of a tesla) — the fact that it always seemed to occur at a value of stress for which phase slip is initiated, gave hope that this magnetic behavior could be used as a marker for the onset of fatigue. Specifically, one could apply a vibrational stimulus, e.g., via an acoustic horn, to a steel structural element and observe the phase relation between the periodic stimulus and the magnetic response: if the two signals have the same phase, that section of the structure being probed is on the low stress side of the magnetoelastic peak; for a 180-degree phase relationship, the structure would be on the high stress side. |
