Eddy Current testing
Eddy Current testing uses the principals of electromagnetism as the basis for conducting inspections. When alternating, current is applied to a conductor a magnetic field develops in and around the conductor. When a second conductor is introduced into the magnetic field an induced current flow is created. Eddy currents are a type of induced current. In the case of eddy current inspection, eddy currents are created using a probe. Inside the probe is a length of electrical conductor which is formed into a coil. AC flows in the coil at a chosen frequency and a dynamic expanding and collapsing magnetic field forms around the coil as the ac flows through the coil. When electrically conductive material is placed in the coil's magnetic field, electromagnetic induction occurs and eddy currents are induced. These eddy currents flowing in the material generate their own "secondary" magnetic field that opposes the coil's primary magnetic field. The strength of the generated currents, known as the standard depth of penetration vary depending on probe frequency, material conductivity and permeability.