ELECTRICAL NON DESTRUCTIVE TECHNIQUES

  Eddy Current Testing Basics 

Eddy current inspection is one of several NDT methods that use the principal of "electromagnetism" as the basis for conducting examinations. 

Eddy currents are created through a process called electromagnetic induction. When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. Eddy currents are induced electrical currents that flow in a circular path. They get their name from "eddies" that are formed when a liquid or gas flows in a circular path around obstacles when conditions are right.

In the proper circumstances, eddy currents can be used for:

Crack Detection, Material Thickness Measurements, Coating Thickness Measurements, Conductivity Measurements for Material Identification, Heat Damage Detection, Case Depth Determination, and Heat Treatment Monitoring.

Eddy current probes are used for crack detection, corrosion inspection, or conductivity testing. These types of instruments contain a simple bridge circuit, which compares a balancing load to that measured on the test specimen. If any changes in the test specimen occur which deviate from normal you will see a movement on the instruments meter.

  

SE-100 Surface Crack Indicator is an eddy current technique for detecting fatigue crack in ferrous weld or raw material. This instrument greatly reduces the cost and time in comparison to alternative techniques that require paint removal.

Eddy Current Inspection

Eddy current inspection uses alternating current flowing through a coil in a probe. This current has a resultant magnetic field around it that induces a magnetic field in a nearby conductive test piece. The magnetic field in the test piece in turn creates small currents ("eddies"). The nature of these eddy currents will be affected by the part conductivity and structure, and their presence is detected by the probe in a feedback loop. Variation in this electrical interaction is monitored on a test device. This method may be used to detect small surface flaws, and may be sometimes used to measure the thickness of a test sample or of a nonconductive coating on the piece. Eddy current testing is also used to sort materials based on their electrical conductivity.

Advantages of Eddy Current Inspection

• Sensitive to small cracks and other defects at the surface
• Inspection gives immediate results
• Equipment can be very portable
• Method can be used for more than flaw detection
• Minimum part preparation

Limitations of Eddy Current Inspection

• Only conductive materials can be inspected
• Ferromagnetic materials require special treatment to address permeability effects
• Skill and training required is more extensive than other techniques
• Surface finish and roughness problematic
• Reference standards needed for setup
• Depth of penetration is limited

Schematics of Eddy Current Usage

Crack Detection

Material Thickness Measurements

Coating Thickness Measurements