Use of Barkhausen Noise in Fatigue

Abstract

changes are known to be affected by residual and/or applied stresses /2, 3/. Monitoring the Barkhausen noise under controlled conditions then provides a means of evaluating the stress state of the material /4/. The relation between stress and Barkhausen noise level is illustrated in Figure 1: the lower the compressive stress or the higher the tensile stress, the higher the Barkhausen noise level. Uniaxial calibration curves have been most commonly used in the magnetoelastic Barkhausen noise stress measurements. Such curves are obtained by measuring the level of Barkhausen noise while stressing the test piece in compression and tension in one direction only and assuming that the stresses in other directions are zero. Recent experiments have, however, indicated that by applying biaxial calibration, the accuracy of stress measurement by this method can greatly be improved /5/. It is also known that the intensity of the Barkhausen noise is sensitive to the density and distribution of dislocations /6, 7 I. An abrupt increase of 20% in noise level has been observed under tensile loading just at the yield strength /8/. This abrupt increase corresponds entirely to the effect of new dislocations since the external stress remains constant at the yield strength during Luder's straining. The effect of applied unidirectional tensile stress on Barkhausen noise level is illustrated in Figure 2 /9/. The test piece was strained up to a certain stress level and unloaded. The Barkhausen noise was measured on the unloaded sample both parallel (Bp) and transverse (Bt) to the axis of loading. The sample was then reloaded to a higher stress level, unloaded and measured once again. When the loading was below the macroscropic yield strength (282 N/mm2), a slight change in Barkhausen noise was observed indicating local micro-yielding. A drastic change of Barkhausen noise level takes place just at the yield stress. Bp decreased 40% at the yield and Bp-Bt decreased 200%, whereas hardness increase was only 12%. The conclusion is that in addition to being sensitive to elastic stress/strain, Barkhausen noise is also sensitive to plastic stress/strain whether it is caused by micro-or macro-yielding.