Eddy current testing (ET) method

This method is widely used to detect surface defects, grade materials, to measure thin walls from a single face, to measure thin coatings, and in a few other applications to measure penetration depth. This method is applicable only to conductive materials. Here an eddy current is generated in the test object by bringing it close to an a.c. current inductor.

Figure 4.5(a) – The process of generating eddy currents in the test object.
Figure 4.5(b) – Sample has a defect that causes the eddy current to be distorted.

The alternating magnetic field of the inductor is changed due to the magnetic field of the eddy current. This change depends on the condition of the part located near the inductor, which is represented as a galvanometer or signal representation on the machine screen. Figure 4.5 shows the basic principles of the eddy current test method.

There are three types of probes (Figure 4.6) used in eddy current testing. Internal probes are often used to test heat exchanger tubes. Surround probes are commonly used to inspect rods and tubes during fabrication. The use of surface probes to locate cracks, classify materials, measure wall and coating thicknesses, and measure penetration depth.

Figure 4.6 – Types of probes used in eddy current testing.

This method is used to:

(1) Detection of defects in pipe materials.

(2) Classification of materials.

(3) Measure the thickness of the thin wall from only one side.

(4) Measure the thickness of the thin coating.

(5) Measure the depth of the permeable layer.

Some advantages of the eddy current test method:

(1) For immediate response.

(2) Easy to automate.

(3) This method is versatile.

(4) There is no need for direct contact between the probe and the test object.

(5) Equipment is made easy to move.

Some limitations of the eddy current test method:

(1) The operator should have a lot of experience.

(2) Only for conductive materials.

(3) Limited penetration ability.

(4) Difficult to apply on ferromagnetic materials.