ET THEORY

Eddy currents are

1) Constantly circulating (closed loops)

2) electrical currents induced into a conductive material by varying magnetic field

Depth of Penetration is limited to

¼ inch

What is skin effect?

Tendency for eddy currents to remain near the surface of a test object (AC); phenomenon that occurs when alternating current (AC) flows along the outer edge of a conductor

Depth of Penetration is affected inversely by Three Primary Variables:

Frequency, Permeability and Conductivity

What is the single most important parameter that an operator has control over?

Frequency

When frequency increases what happens to Depth of Penetration?

It decreases

What happens to Depth of Penetration when frequency is decreased?

Depth of Penetration is Increased

ET can be used to measure

• Conductivity changes

• Material sorting

• Thicknesses of nonconductive material

• Thickness of thin layers of metal

Eddy currents travel__________ to the winding of a coil and _________ the initial current

Parallel, always oppose

Flaws in test article cause distortion in the flow of the eddy currents which shows as

Change of impedance

Limitations of ET inspection

• Materials must be conductive

• Reference standards required for setup and defect verification

• Specific nature of defect may not always be clearly identified

• Depth penetration is limited

• Testing of ferromagnetic material is difficult

• Large number of variables affect ET inspection

Conductors

Composed of material which allows an electrical current to flow through it. Free moving electrons

What are the 3 best conductors

• Copper ( Cu)

• Silver (Ag)

• Gold (Au)

What is IACS

International Annealed Copper Standard

IACS is a scale that ______

Expresses material conductivity as a percentage of pure annealed copper

Examples of IACS

• Copper 100%

• Silver 105%

• Titanium 3.1%

• Aluminum 61%

Wavelength

Distance for one cycle to complete; measured from the same point on the wave.

Wavelength symbol is

Lamda

In Phase

Two or more sine waves that reach their maximum and minimum at the same point in time (independent of amplitude) are in phase.

Resistance will always be

In phase

What is Inductive Reactance formula?

XL= 2PiesFor Lunch

What are electrons?

Negativity charged particles that are part of the building blocks of any material

Resistance opposes flow of electrons _______

Through coil wire

Resistance does not cause

Current to lag voltage

Resistance is not caused by

Interaction with magnetic fields

Out of Phase

Two or more sine waves that reach their maximum and minimum at different points in time are out of phase

Inductive reactance will always be

Out of phase

Magnetism

Property possessed by certain materials by which these materials can exert a mechanical force on other like materials

Magnetic domain

Smallest region of magnetic activity in a material; is a region within a magnetic material where the magnetic moments of atoms are aligned in a uniform direction

Movement of magnetic domain generates

Heat

Magnetic domain means

That the magnetic fields of a atom are grouped together and point in the same direction

What are the three magnetic classifications?

• Diamagnetic

• Paramagnetic

• Ferromagnetic

What are the 2 non magnetic classifications?

• Diamagnetic

• Paramagnetic

Diamagnetic

Slightly repelled by a magnetic field

Paramagnetic

Slightly attracted by a magnetic field

Ferromagnetic

Materials that can be magnetized and are strongly attracted by a magnetic field

Magnetic field

The space outside a magnet where it’s poles have a force of attraction

• Fields exists in all directions around a magnet

What is Magnetizing Force symbol (H)?

The ability of a magnetic field to produce magnetism by induction(non-contacting); often referred to as field intensity

Reluctance

Materials resistance to being magnetized

Reluctance is opposite of

Permeability

Low permeability materials have

High reluctance

3 factors that affect Impedance

• Conductivity

• Permeability

• Geometry (Dimension)

What is Conductivity unit (Ohm)?

A measure of the ability of a material to conduct electrical current; the reciprocal of resistivity.

If Conductivity goes up then

There is a greater flow of electrons for a given period of time

What is Resistivity?

A measure of how strongly a material opposes the flow of electric current

What are the Test Material Variables which affect Conductivity?

• Chemical Composition

• Impurity Content

• Heat Treatment- excessive heat causes a change in grain structure (change in Conductivity)

• Quenching- cooling process that affects microstructure of material

• Hardness- harder material = lower Conductivity

• Residual Stresses

• Lattice Distortion/Dislocation - Cold working resulting in change of microstructure(change in Conductivity)

• Discontinuities- Surface Inspection and Tubing Inspection

Inductance and resistance both create

An opposition to current flow in a coil

Current lags the voltage by 90 degrees

Voltage leads current by 90 degrees

What is voltage unit (volts)?

Amount of force required to move electrons between two points

What is EMF electromotive Force?

Electrical energy derived from mechanical, Chemical or other forms of energy that must be applied across the material to force electrons to move.

Materials can be classified into two main groups when it comes to their ability to conduct electricity:

• Conductors

• Insulators

At a minimum, three basic pieces of equipment are required for ET inspection

• Eddy current Instrument

• Probes

• Reference Standards

What is Standard Depth of Penetration (SDP)?

Depth at which eddy current density has decreased to approximately 37 percent of the surface density.

Optimal to calibrate ET equipment to

1-2 SDP's cuz below this density is less reliable for accurately detecting discontinuities

Article undergoing inspection is coupled to the test coil by the coils

Magnetic Field

What is lift-off?

The distance between the probe and the test part

Lift-off is used when and Fill-Factor is used when

Working with surface probes working with inside diameter(bobbin) or outside diameter (encircling) probes

What is Faraday's Law of Electromagnetic Induction?

Whenever a conductor is placed in varying magnetic field EMF is induced