The Possibility of Exploiting Magnetic Phenomena in the Testing of Steel

THE known phenomenon that a test-piece of ferro-magnetic steel is magnetic after it has been broken in tension indicates that, while it is being loaded, the steel acquires magnetic properties. In the present investigation of the influence of the stress on the magnetic flux, repeated pulsating stresses of increasing amplitude, varying from zero to the maximum tensile value, i.e., pull-pull loading, were employed. The specimens were prepared from two types of carbon steel and two of alloy steel. The magnitude of the magnetic flux was determined indirectly. The voltage and mode of variation of the electromotive force induced in a coil surrounding the stressed steel specimen were measured as an indication of the varying magnetic flux within the specimen. Curves representing the electromotive force induced by the varying magnetic flux show that as the tensile load increased so also the magnetic flux increased, with a corresponding increase in measured e.m.f. At a certain stress there is a sharp bend in the voltage curve (point Win the diagram, Figure 5); this critical stress corresponds very closely with the yield stress (0.2 per cent permanent strain). Beyond this point, slight increases in the stress produce large increases in the voltage. The wave form of the e.m.f. was registered on an oscillograph. Analysis of the curves so obtained demonstrated the necessity of continuing these experiments, in order to determine the type of induced wave form associated with various types and amplitudes of stresses. In these experiments, the shape of the stress wave was sinusoidal, although the shape of the induced e.m.f. was sinusoidal only up to a certain load and became more and more complex above this load.