Ndt and Eddy Current Testing

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NDT AND EDDY CURRENT TESTING

After reading this section you will be able to do the following:

Explain how eddy currents are used in NDT. Explain why NDT is so important to our society.

Nondestructive testing (NDT) means exactly what the words say. NDT

literally means testing materials without destroying them. In other words,

we can look for defects in a variety of metallic materials using eddy currents

and never harm the material that we are testing. This is important because

if we destroy the material we are testing, it does not do much good to test

it in the first place. NDT is very important because often the defects that

we are looking for are not visible because paint or some other coating maycover them. There might also be defects that are so small they cannot be

seen with our eyes or any other visual method of inspection. Therefore,

inspection methods such as eddy current inspection have been developed to

detect the defects.

Try an Eddy Current Experiment

In the experiment below you will use eddy current testing to detect cracks

in a block of metal. You will notice that you are using a coil of wire wrapped

around a piece of iron to generate the magnetic field that caused the eddycurrents to form in the metal. In the field of NDT the coil is called the

inspection probe. The magnetic field that is generated by the eddy currents

can be detected using this same probe. We can monitor the magnetic field

being produced by these eddy currents with an instrument called an

eddyscope. If there is a change in the magnetic field from the eddy

currents, we can tell that we have found some sort of defect in the material

that we are testing. When the instrument sees a change in the magnetic

field generated by the eddy currents, it displays a change in the signal on

the screen.


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As long as the material being tested is very uniform in every way, the eddy

currents will be uniform and consistent. If there is some defect in the

material such as a crack, the eddy currents will be disturbed from their

normal circular shapes. NDT technicians use many different types of eddy

current testing equipment. Some are simple coils that are held on a piece ofmetal. Others use special probes, like the one shown above, that are pushed

inside of tube such as those in heat exchanger units.

The technicians on the right are

performing an eddy current inspection on

the tube of a heat exchanger. Heat

exchangers are used in places like nuclear

power plants. Radioactive water from the

reactor is circulated through the tubes andcooling water that will be returned to a

river or lake is circulated on the outside of

the tubes. It is very important that the

radioactive water and the cooling water do

not mix. Therefore, technicians perform

eddy current inspections on the tubes to


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find and defects that may be present before they become leaks in the

tubing.

Review

1. NDTstands for "Nondestructive Testing."

2. NDT methods are used to test materials and parts without

harming them.

3. Eddy current testing is just one of the methods used by

technicians to find defects before they cause problems.

EDDY CURRENTS


Explain what an Eddy Current is.

Discuss the one requirement necessary for a current to be

induced into an object.

In the discussion on the previous page you learned learned about

electromagnetic induction. You learned that anytime a conductor was placed

in a changing magnetic field that electrical current was generated in the

conductor. We talked about the conductor being a piece of wire that is

often wrapped into a coil, but the conductor does not need to be in the

shape of a coil and does not even need to be wire. It could be a piece of flat

steel, aluminum plate, or any other conductive object. The only requirement

is that the object must be able to conduct electrical current.


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When current is induced in a conductor such as the square piece of metal

shown above, the induced current often flows in small circles that are

strongest at the surface and penetrate a short distance into the material.

These current flow patterns are thought to resemble eddies in a stream,

which are the tornado looking swirls of the water that we sometimes see.Because of this presumed resemblance, the electrical currents were named

eddy currents.

Uses of eddy currents

Just like in our transformer experiment, these induced eddy currents

generate their own magnetic field. After all, this is an actual electrical

current and any current flowing in a conductor produces a magnetic field,

right? The detection and measurements of the strength of the magneticfields produced by the eddy currents makes it possible for us to learn things

about conductive materials without even contacting them. For example, the

electrical conductivity of a material can be determined by the strength of

the eddy currents that form. Also since cracks and other breaks in the

surface of a material will prevent eddy currents from forming in that region

of the surface, eddy currents can be used to detect cracks in materials.


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This is referred to as eddy current testing in the field of nondestructive

testing (NDT). NDT technicians and engineers use eddy current testing to

find cracks and other flaws in part of airplanes and other systems where

bad things can happen if the part breaks. On the next page you will learn

more about eddy current testing and be able to try an inspection yourself.

Review

1. Any electrically conductive object will conduct an induced

current if it is placed in a changing magnetic field.

2. Eddy currents are circular induced currents.

3. Eddy currents generate their own magnetic fields.

ALTERNATING CURRENT


Define what AC stands for and what it means.

Explain how AC is created and delivered to different places.

Discuss the differences between AC and DC.

AC is short for alternating current. This means that the direction of

current flowing in a circuit is constantly being reversed back and forth. This

is done with any type of AC current/voltage source.

The electrical current in your house is alternating current. This comes from

power plants that are operated by the electric company. Those big wires you

see stretching across the countryside are carrying AC current from the

power plants to the loads, which are in our homes and businesses. The

direction of current is switching back and forth 60 times each second.


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This is a series circuit using an AC source of electricity. Notice that the

light bulb still lights but the electron current is constantly reversingdirections. The change in direction of the current flow happens so fast that

the light bulb does not have a chance to stop glowing. The light bulb does not

care if it is using DC or AC current. The circuit is delivering energy to the

light bulb from the source, which, in this case, is a power plant.

Review

1. AC, or alternating current means the electrical current is

alternating directions in a repetitive pattern.2. AC is created by generators in power plants, and other

sources. This AC current is delivered to our homes and

businesses by the power lines we see everywhere.

3. The frequency of repetition of this current is 60 Hertz. This

means the direction of the current changes sixty times every

second.

DIRECT CURRENT


Explain what DC stands for and what it means.

Define what a good source of DC would be.


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Now that we have a fairly good understanding of basic electricity terms and

concepts, let's take a closer look at some more details of the electrical

current itself.

The battery we have been using for a current/voltage source generatesdirect current, which simply means the current flows in only one direction.

As long as electrons are flowing through the atoms of the circuit, work is

being done. We can see that work is being done in this circuit because it

lights the light bulb. The actual amount of electrons that are flowing is

determined by the type and size of the battery as well as by the size and

type of the light bulb. We could reverse the polarity of the battery by

switching the contacts (wires), and the current would flow in the opposite

direction and the bulb would still light.

Either way the battery is connected to the circuit, current can only flow in

one direction. Direct current (DC) can also be generated by means other

than batteries. Solar cells, fuel cells, and even some types of generators can

provide DC current.

Review

1. DC, or direct current means the electrical current is flowing in

only one direction in a circuit.

2. Batteries are a good source of direct current (DC).


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Ndt and Eddy Current Testing

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