Eddy Current Testing
Eddy-current testing is one of many electromagnetic testing methods used in NDT making use of electromagnetic induction to detect and characterise surface and sub-surface flaws in conductive materials.In its most basic form — the single-element ECT probe — a coil of conductive wire is excited with an alternating electrical current. This wire coil produces an alternating magnetic field around itself. The magnetic field oscillates at the same frequency as the current running through the coil. When the coil approaches a conductive material, currents opposed to the ones in the coil are induced in the material — eddy currents.
Other Techniques
Pulsed eddy current (PEC) testing uses a step function voltage to excite the probe. The advantage of using a step function voltage is that such a voltage contains a range of frequencies. As a result, the electromagnetic response to several different frequencies can be measured with just a single step.
One of the advantages of this type of testing is that there is no need for direct contact with the tested object. Testing can be performed through coatings, sheathings, corrosion products and insulation materials. This way even high-temperature inspections are possible.
Surface Inspection
Surface inspection is used extensively in the aerospace industry, but also in the petrochemical industry. The technique is very sensitive and can detect tight cracks. Surface inspection can be performed both on ferromagnetic and non-ferromagnetic materials.
Tubing Inspection
Tubing inspection is generally limited to non-ferromagnetic tubing and is known as conventional eddy current testing. Conventional ECT is used for inspecting steam generator tubing in nuclear plants and heat exchangers tubing in power and petrochemical industries. The technique is very sensitive to detect and size pits. Wall loss or corrosion can be detected but sizing is not accurate.