Introduction

The first ever reference to magnetism was as far back as the 6th century BC where a Greek philosopher named Thales discovered something called a lodestone.

Aristotle recorded that Thales believed that lodestones had souls, as iron is attracted to them.

The lodestone is a rare piece of magnetite (Fe3O4) that rather than just possessing a magnetising potential, it is actually found magnetised.

The Chinese later adapted the lodestone for use as a nautical compass in the 12th century.

The first use of magnetism to inspect objects took place in the 1860’s when cannon barrels were tested for defects by first magnetising the barrel and then running a compass down the length of the barrel. By monitoring the needle of the compass, defects within the barrel could be detected. This form of NDT became much more common post World War 1, in the 1920’s, when William Hoke discovered that flaws in magnetised materials created distortions in the magnetic field. When a fine ferromagnetic powder was applied to the parts, it was observed that they built up around the defects providing a visible indication.

During the 1930’s, magnetic testing began to replace the ‘Oil and Whiting’ method on ferromagnetic materials and much of the equipment being used then, is very similar to today, with the only advances coming in the form of fluorescent coatings to the particles which increases the visibility of indications, and more portable devices being used.

Magnetic Particle Inspection (MPI) is an NDT method which may only be used on ferromagnetic materials to detect surface breaking discontinuities and also, in certain cases, slight sub-surface discontinuities up to 2 mm or 3 mm below the material’s surface.

BS EN 1330-2 definition of discontinuity: Detectable change in the material produced inherently or artificially.