Barkhausen Noise Study of Fatigue in AISI 4340 Steel

The purpose of the present work is to use magnetic non-destructive evaluation techniques to examine fatigue in AISI 4340 steel, a material widely used for mechanical construction. Cylindrical test pieces were machined following the ISO 1143 standard in material which had been heat treated to give a hardness of 38 HRC. Fatigue testing was carried out in a rotating-bending machine with the ratio of minimum to maximum stress R = -1. The curve of applied stress vs. number of cycles to rupture (S/N curve) was determined, as was the fatigue limit, and the results were in good agreement with the literature. During the fatigue testing, at least half of the samples underwent periodic measurements of the magnetic Barkhausen noise. Numerical analysis of the Barkhausen bursts furnished values for the root mean square (RMS) Barkhausen voltage. As the test pieces approached rupture, the RMS Barkhausen voltage gradually increased and then presented a complex behavior. Similar behavior has been observed in other steels.