Magnetomechanical behavior of steel via SQUID magnetometry

Steel bars were subjected to stress-strain hysteresis cycles in a horizontal tensile-testing machine while changes in the vertical component of the magnetic field above these bars was recorded by a DC SQUID second-order gradiometer. Magnetic Barkhausen noise was observed after repeated cycling and especially at stress levels for which the magnetic field was at an extremum value. Of greatest practical interest is the fact that the area of the magnetomechanical hysteresis loop (/spl Delta//spl Phi//sub z/-/spl sigma/) increased dramatically-by orders of magnitude-over a large number (up to 10/sup 5/) of fatigue cycles. This may have important implications for the in situ nondestructive evaluation of steel.<
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