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# Eddy Current Testing (Physical Aspects)

In this article, we will discuss Eddy Current Testing (Physical Aspects). So, let’s get started.

Physics aspects of ECT

Conductivity

• Conductivity is defined as the ability of a material to conduct electric current.
• It is denoted by σ. The unit of conductivity is mho per meter.
• The conductivity of a conductor decreases with the increase in the temperature.
• Each element has a unique value of conductivity.
• Copper, silver and gold have high conductivities where as, carbon has a very low conductivity.
• An eddy current is a flow of electrons. The amount of electron flow through an electrically conductive material is directly related to the conductivity of the material.
• If the conductivity increases, the flow of eddy current increases.

Resistivity

• Resistivity is reciprocal of the conductivity.
• Therefore the materials that have high resistivity have poor conductivity and vice versa.
• The resistivity is denoted by ρ and is defined as the ‘ratio of electrical intensity (emf) to the current per unit cross-section area.’
• Mathematically it can be written as ρ = E / I /A or ρ = EA/ I
• ρ = resistivity, E = emf, A = area
• The unit of resistivity is Ohm-meter.
• Resistivity of a material changes with the change in temperature.

Magnetic Permeability

• Magnetic permeability is an intrinsic property of a material.
• It is the ability of a material to concentrate magnetic lines.
• It is denoted by the Greek letter µ.
• Any material that is easily magnetized, such as soft iron,
concentrate the magnetic flux.
• This is the main feature separating magnetic materials from nonmagnetic materials.
• The magnetic permeability is equal to the induced magnetic flux density B divided by external magnetic field intensity (magnetizing force) H.
• μ = B/H ,
• Where μ = magnetic permeability, B = flux density (tesla), H = magnetizing force (amperes/metre)
•  For air, vacuum, and non-magnetic materials the μ is constant.
• The numerical values of μ for different materials are assigned in comparison with air or vacuum.
• This is called the relative permeability and is defined as μr = μ/ μ0
• Where μr = relative permeability, μ = permeability, μ0 = permeability in vacuum

Inductance

• Inductance is the property of a conductor by which a change in current flowing through it induces a voltage (electromotive force) in both the conductor itself (self-inductance) and in any nearby conductors (mutual inductance).
• It is customary to use the symbol L for inductance.
• In the SI system the measurement unit for inductance is the henry (H).
• Inductive Reactance (XL), is the property in an AC circuit which opposes the change in the current.

Impedance

• Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied.
• In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current (AC) circuit.