permeability μ0, measured at very lowfields at the toe of the hysteresis loop.It is a familiar phenomenon that someferromagnetic materials when magnetizedby an external field do not return to acompletely unmagnetized state whenremoved from that field. In fact, thesematerials must be subjected to a reversedfield of a certain intensity to demagnetizethem. Other ways to demagnetize are toheat the material to a characteristictemperature called the curie point(abovewhich ferromagnetic ordering of atomicmoments is thermally destroyed) or towork the material mechanically to reducethe magnetization. If an external fieldvaried in a controlled manner is appliedto a completely demagnetized (virgin)specimen and if the magnetic inductionin the specimen is measured, themagnetization curve of the material maybe determined. Figure 17 shows arepresentative hysteresis loop for aferromagnetic material.As shown in Fig. 17a, starting at theorigin O with the specimen in theunmagnetized condition and increasingthe magnetizing force Hin smallincrements, the flux density Bin thematerial increases quite rapidly at firstand then more slowly until it reaches apoint beyond which any increase in themagnetic field intensity does not increasethe flux density. This is shown by thedashed curve OA. In this condition thespecimen is said to be magneticallysaturated.When the magnetic field intensity isgradually reduced to zero, the curve ABresults (Fig. 17b). The amount ofmagnetism that the steel retains at pointB is called residual magnetismor remanenceand is represented by Br.When the magnetizing current isreversed and gradually increased inmagnitude, the flux continues todiminish. The flux does not become zerountil point C is reached, at which timethe magnetic field intensity is representedby OC (see Fig. 17c), which graphically337Electromagnetic Techniques for Material IdentificationPART4.Hysteresis Loop Characteristics1FIGURE15.Magnetization or BHcurveshowing relation between flux density Bandmagnetic field intensity Hin ferromagneticmaterials. The intensity of magnetization atsaturation is Is.Flux density B(relative scale)IsMagnetic field intensity H(relative scale)FIGURE16.Variation of magneticpermeability with flux density.Magnetic permeability μ(relative scale)Flux density B(relative scale)μmaxμ0
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designates the coercive force Hcin thematerial.As the reversed field is increasedbeyond C, point D is reached (Fig. 17d).At this point, the specimen is againsaturated but in the opposite polarity. Themagnetic field intensity is now decreasedto zero and the DE line is formed andretains reversed polarity residualmagnetism Brin the specimen. Againincreasing the magnetic field intensity inthe original direction completes the curveEFA. Now the cycle is complete and thehysteresis curve(ABCDEFA) is called thehysteresis loop. In alternating currentapplications, the ferromagnetic materialgoes through this cycle for every reversal
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