The physics of magnetic particle inspection

Abstract

This thesis describes the results of an investigation into the Physics of Magnetic Particle Inspection. This is a non destructive technique used to detect the presence of surface breaking cracks in ferrous metals. On applying a magnetic ink to the surface of the part being magnetised, particles will be attracted towards any regions of flux discontinuity. This allows the defect to be visible to the eye. The main areas of study were the magnetic properties of the inks used, a computer simulation of the indication process and work of a more practical nature concerning MPI. From the magnetic measurements it was found that field induced agglomeration of the particles imparts the ink with a coercivity and remanence which varies with concentration. Low field time decay measurements show the existence of two reversal mechanisms within the magnetic ink, i.e. Brownian and Neel. The effect interparticle interactions have on a single aggregate has been studied by the use of remanence curves and this was found to lower the aggregates' magnetisation. This therefore lowers the magnetic force on a particle during the inspection process. The spin reversal mechanism has also been determined. The computer simulation of the indication mechanism originally written by McCoy has been improved to accommodate time varying waveforms. Results from this investigation indicate that the rms of the waveform is the important parameter when specifying field values for magnetisation levels for low remanence steels. This is contrary to the British Standard BS 6072 but in agreement with the draft European Standard. It was also found that increasing the paint layer thickness decreased the number of particles at a crack. This variation was found to be similar to the variation of the magnetic leakage field from the crack with increasing paint thickness. From a study of the effects of viscosity and radii on the number of particles arriving at a crack, we find that for particles whose radius is greater than 10 microns no change is observed as the viscosity is altered. Thyristor controlled power supplies are increasingly being used to magnetise inspection pieces and for this reason the relationship between the mean and rms of the waveform has been determined. It was found that the relationship between the two is non linear. This has implications if the user wishes to remain within BS 6072. Experimental verification of the simulation results is also presented. An offshoot of this is a possible method of determining the crack width using MPI, this is also presented. Finally it was noted that upon increasing the applied magnetic field, particles present at the crack rotated through 180 degrees. This also occurred upon decreasing the magnetic field. Investigation of this unusual effect are reported as well as a possible mechanism for it.