The Basics of Non-Destructive Material Testing – Part 2
Hailey Kupiec posted on June 22, 2016 |
Dye penetrant inspection, eddy current testing and thermographic inspection.

Continuing the three-part series on non-destructive testing we discuss dye penetrant inspection, eddy current testing and thermographic inspection.

4. DPI - Dye Penetrant Inspection

Detection of a crack by dye penetrant inspection

Detection of a crack by dye penetrant inspection

Dye penetrant inspection is often used to detect casting, forging and welding surface defects. This includes hairline fractures, leaks and fatigue cracks.

This method is based on the concept of capillary action, which is caused by intermolecular forces. In this case the capillary action occurs when the combination of the surface tension of the liquid penetrant and the adhesive forces of the walls of the crack work together to cause the liquid to climb and color the developer.

The basic steps of dye penetrant inspection are as follows:

  1. Clean the part to be inspected thoroughly, removing anything that would prevent the penetrant from entering a defect. Ensure the surface is dry and free of contaminations that could lead to false results.
  2. Apply the penetrant and leave it for the time it takes to sink into any defects (the dwell time).
  3. Thoroughly remove any excess penetrant.
  4. Apply the developer, this draws the penetrant out of any defects, coloring the area.
  5. Visually inspect the part for remaining penetrant, which reveals the defects. 

5. ECT – Eddy Current Testing

Eddy current testing is used to detect surface and subsurface defects in parts composed of conductive material. It is based on the concept of electromagnetic induction.  A current running through a wire coil creates a magnetic field. If an alternating voltage is applied to the coil, a counter voltage from self-induction by the alternating magnetic field causes a decrease in current consumption.

A crack is detected in the part. Screenshot courtesy of Hochschule Karlsruhe - University of Applied Sciences
A crack is detected in the part. Screenshot courtesy of 
Hochschule Karlsruhe - University of Applied Sciences

When the coil is run over a conductive material, the current increases because the alternating magnetic field induces a current in the material, called the eddy current. The eddy current creates its own magnetic field which counteracts that of the coil. This decreases the self-induction in the coil and increases the current consumption.

If a defect is present in the material and is in the path of the eddy current, the current flows around the defect, lengthening its path and increasing the resistance in the eddy current circuit. The resistance increase causes a corresponding decrease in the strength of the magnetic field. Thus, the self-induction of the coil increases and its current consumption decreases.

A decrease in the coil current is therefore indicative of a defect in the material. Monitoring the change in current shows the location of defects.

6. IR - Thermographic Inspection

An overheating terminal in an industrial fuse block. By Hotflashhome (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

An overheating terminal in an industrial fuse block. By Hotflashhome (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

Thermographic inspection is used to find cracks, voids, cavities and other defects in shafts, pipes and other metal or plastic parts.

Every object gives off radiation. The more energy an object has, the more radiation it emits. Thermographic inspection uses infrared video or still cameras to see radiation in the infrared spectrum and create an image of thermal patterns on an object’s surface.


There are two approaches to thermographic inspection:

  1. Passive: features of interest are naturally at higher or lower temperature than the background
  2. Active: energy source is required to produce thermal contrast between the features of interest and the background

In part 1, we discuss radiographic testing, ultrasonic testing and magnetic particle inspection. In part 3 we cover leak testing, guided wave testing and visual inspection.

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