Eddy Current: A Green Testing Solution

Manufacturers are striving to reduce their environmental impact by reducing the waste generated in their plants and moving to technologies that are clean in nature. Eddy current testing provides a green solution to manufacturers by not only reducing scrap, but also by eliminating the chemicals and dyes used in traditional testing methodologies. Eddy Current Testing is a proven non-destructive testing method that is used to verify proper heat treat and material structure, and to find flaws and cracks in metal components and raw materials.

Eddy Current Testing Reduces Scrap

Eddy current testing solutions allow manufacturers to perform in-line testing of 100% of their parts. This is possible because eddy current testing can be done at production line rates. Eddy current systems are able to instantly identify reject conditions, and use in-line material handling stations to move the part to a reject bin. This is much superior to sample testing where if a reject condition is found, the entire lot may have to be scrapped.
Eddy current systems are also able to identify when multiple rejects occur in a row. This typically indicates that there is a process issue upstream of the testing station. Manufacturing managers can quickly shut down the production line and remedy the production fault before an entire shift of bad product has been manufactured.

Eddy Current Testing Reduces Chemical Usage

Traditional non-destructive and sample testing including magnetic particle inspection, dye penetrant inspection, and inspection using nital etching techniques all involve the use of chemicals. These chemicals must be purchased, stored, used, and disposed of in accordance with government guidelines. Also, most of these inspections require that the component is cleaned before and after inspection, adding more chemicals to the process.

Eddy Current Testing Fundamentals

Eddy Current is a non-destructive electromagnetic testing method. An alternating current (AC) signal is passed through a coil of wire which generates a magnetic field. A metallic component placed in this field will have “eddy currents” flow inside of it as a response to the magnetic field. The eddy currents will flow differently in parts that have differing “material structure” due to varying heat treat conditions, alloy content, cracks, voids, and physical characteristics. Modern eddy current instruments compare the flow of eddy currents in a component-under-test to a “known-good” condition. Testing criteria is established to determine the “accept” and “reject” thresholds of the component. If a reject condition is identified, an industrial I/O signal is sent from the eddy current instrument to a material handling station which will move the reject component to a scrap bin.

Eddy Current Heat Treat Testing

Eddy current is well suited to determining differences in metallic structure between two different components. In Europe, eddy current testing is often referred to as material “structure” testing. These structural differences in materials can be due to changes in heat treat condition or alloy content. Manufacturers use eddy current testing to look for parts that have conditions such as shallow case, delayed quench, air cool, misplaced case, or missing heat treat.

For heat treat verification testing, the tests are run with encircling type coils. The component-under-test must pass though the coil. Heat treat testing is run in “static test mode” as the test must be activated as the part is centered in the coil or at the time a specific area is inside the coil. Figure 1 shows a typical eddy current encircling coil. This can accommodate a wide variety of component diameters and even be used to test raw materials such as bar, tubing and wire. Figure 2 shows a test fixture where a powder metal gear is being tested for proper material hardness. This system can test up to 60 parts per minute.

Figure 1 - Eddy Current Encircling Coil

Figure 2 - Eddy Current Powder Metal Gear Heat Treat Test


For testing complex components, multi-coil probes are custom designed and built to test specific areas on a part such as a bearing surface. Figure 3 shows a probe that was custom built to test an automotive wheel spindle. The copper wire from one of the eddy current coils can be seen on the bottom of the probe. Before using this test, the automotive manufacturer would take multiple samples of parts from each shift. The parts would be cut and nital etched to verify proper heat treat conditions. Now they only cut and etch one part per shift to validate the eddy current systems, saving hundreds of thousands of dollars.

Modern eddy current instruments are capable at testing components at more than one frequency simultaneously. This greatly increases the chance of finding a defect verses testing at a single frequency, and allows much tighter control over the accept/reject criteria. Figure 4 shows a screen shot of a typical heat treat test running at eight frequencies.

Figure 3 - Eddy Current Automotive Wheel Spindle Probe

Modern eddy current instruments are capable at testing components at more than one frequency simultaneously.  This greatly increases the chance of finding a defect verses testing at a single frequency, and allows much tighter control over the accept/reject criteria.  Figure 4 shows a screen shot of a typical heat treat test running at eight frequencies.

Figure 4 - Eddy Current Treat Testing Screen

Eddy Current Crack Testing

Eddy current crack testing is performed as a “dynamic” test where an eddy current coil must pass over the area under test.  When there is a crack or flaw in the component, the eddy currents move around the anomaly which is detected by the eddy current instrument.  For simple tests, a small “pencil probe” as shown in Figure 5 is used to find defects.  For more complex parts, a custom multi-coil probe like the one in Figure 6 is used in conjunction with a material handling station.

Figure 5 - Eddy Current Pencil Probe

Figure 6 - Multi-Coil Eddy Current Crack Testing Probe

Like eddy current heat treat testing, many eddy current crack tests are run at multiple frequencies.  Higher frequencies are able to find very small surface cracks while lower frequencies are able to penetrate further down into the material.  A typical eddy current crack test response can be seen in Figure 7.  The alarm box is shown as a small circle toward the center of this display which indicates that a large flaw has been found.

Figure 7 - Crack Test Flaw Responses on Eddy Current Instrument

An integrated eddy current crack test system can eliminate the need for off line testing with dye penetrant or magnetic particle techniques.  This will eliminate the labor to clean the parts and do the actual inspection.  It also eliminates the chemical consumption and disposal issues as well as the pre and post inspection washing processes.

Conclusion

Eddy current is a clean testing technology that can eliminate the need for chemical testing products and procedures.  For heat treat testing, it can reduce the number of parts to be testing with nital etching techniques.  For the detection of cracks and flaws, it can eliminate the need for magnetic particle or dye penetrant testing.