Basic
Magnetism
Poles repel and attract
Magnetic field
Bar magnet
Electric Current
introduction to magnetism
Basic properties of MagnetsEvery magnet is made up of a north and south pole. If two north pole are set near each other they will repel each other and the same goes for two south poles. If you place a north pole near a south pole they will attract each other.
If you cut a magnet in half it will become two magnets both with a north and south pole. This is the key feature to magnetic particle testing.
A permanent magnet is a piece of metal that has been magnetized and retains is magnetic properties. The dipoles creates a magnetic field which exert forces on moving charged particles such as tiny pieces of iron.
Electric current through a loop generates the same magnetic field as a permanent bar magnetic. This is what is actually used to do magnetic particle testing.
If there is a break in the magnet it creates a flux leakage and creates a new set of dipoles that attract each other and magnetic particles.
Magnetic Particle Testing
Magnetic particle testing uses electric current to create a magnetic field through a material. Once the magnetic field is established tiny iron particles are dusted over the area. If the material has a crack within the magnetic field it acts the same as a break in a magnet and creates a new dipole point which attracts the iron flakes. This creates a deviation in the normally observed magnetic field making the crack visible to the naked eye, even if it is slightly below the surface of the material.
This process of testing materials is extremely useful for detecting imperfections in welds.