It’s Not Just for Detecting Surface Flaws

February 5, 2003
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The most widespread used of an eddy current is to detect surface flaws in parts. Either the operator moves the probe over the part, or the part is moved across a stationary probe. Photo: Zetec Inc.

Eddy current testing has been used for more than 30 years in testing steam generator tubing and bar, tube and wire stock. More recently, it has been applied to the testing of individual components on production lines. Eddy current offers rapid testing that makes 100% inspection a reality, thus reducing both scrap and warranty costs.

Current systems are used to detect anomalies in metallic objects. Manufacturers are interested in whether cracks and flaws are present in a part, whether the part has been correctly heat treated, whether parts have been made from the correct material and whether physical attributes such as threads or splines have been accurately machined. Modern eddy current systems can rapidly detect these conditions and enable assembly line integration.

The instruments

Eddy current testing instruments function by energizing a set of electrical current driver coils and processing the returned signals from the pick-up coils. The instrument amplifies and processes the returned signals and displays the results. Displays and user interfaces for crack and flaw testing differ from those used when eddy current is used for hardness testing. An eddy current coil passing over a flaw creates a Lissajous type pattern while hardness test data tends to be grouped within alarm boxes on the screen. Modern instruments have the ability to drive coils simultaneously at multiple frequencies. This is critical for hardness testing applications where multiple failure conditions may occur.

Instrument setup includes setting the drive amplitude and frequencies, and the receive gain, filter settings and display controls. Both "passed" and "failed" components are analyzed and alarm box labels are arranged around clusters of passing data points. During actual testing, if a test part exceeds the established alarm or acceptance limits, the part is rejected and the instrument’s industrial I/O is triggered. Individual signal conditioning and I/O configurations can be stored and recalled at any time. Increased throughput and performance are achieved by high instrument sampling rates. In addition, with data logging, it is possible to store the test data as inspection occurs.

Finding flaws

Eddy current technology is most often used in applications where the manufacturer wants to find surface flaws. Typical depth of penetrations for aluminum parts are 6 millimeters while surface inspections of ferromagnetic materials are limited to the surface layers only. To correctly detect cracks and flaws, an eddy current probe must pass over the flaw. This can be accomplished by either moving the probe across the part, or moving the part in relation to the probe. A standard pencil probe, scanned across the top of a cylinder liner, for example, can be automated by either rotating the cylinder liner while keeping the probe fixed, or keeping the cylinder liner fixed and moving the probe around the part.

For parts with complex geometries, custom probes are used to reach in critical areas. Inspection of a wheel spindle for cracks and flaws, a common application, can be done using an eight-coil probe. Each of the coils is positioned to inspect a critical area on the spindle. The uppermost coil is used to inspect the underside of the keeper groove near the top of the shaft. Cracks as small as 0.004 by 0.005 inch are the minimum flaw criteria called for. To detect such minute flaws, the wheel spindle is gripped and rotated at 120 rpm during the test. The eddy current probe moves over to the part and the test is run while the part spins. The actual coils ride in near proximity to the wheel spindle. Wear-resistant ceramic wheels keep the probe correctly positioned with respect to the part. Spindles can be tested at a rate of one every 6 seconds, as compared to more than 3 minutes for each spindle using magnetic particle inspection. Industrial I/Os on the eddy current test instrument drive programmable logic controllers that route parts out of the assembly line, keeping track of the number of passes and failures.

Passing the 'bar'

The bar, tube and wire industries have been long-time users of eddy current technology for flaw testing. Many of these products are made in a continual feed fabrication process—sometimes at several thousand feet per minute—and tested at the same time. The test system coils are often covered with stainless steel or ceramic probe guides for protection of the electronics. Encircling coils offer the advantage of fast, simple detection but have lower flaw detectability. They detect flaws around the entire area that the coils cover. For improved flaw detection, rotating probe systems using spot type probes, or array probes that offer alternatives to encircling coils. Flaw detection in ferromagnetic tubes and pipe is made difficult because of the eddy current "noise" caused by the permeability variations within the steel. By introducing a strong magnetic field in the area of inspection, these permeability variations are reduced, increasing the signal-to-noise ratio and allowing for better defect detection.

Take the heat

Correct hardness and case depth treatments are critical to many manufactured components. Incorrectly hardened automotive bearing surface may fail after 5,000 miles instead of 250,000 miles. As most hardness and case depth treatments are invisible to the naked eye, instrument testing becomes critical. Traditional static indentation testing, such as Rockwell and Vickers hardness tests, are time consuming, require that the surface be perpendicular to the direction of the force applied by the tester and damage the part to some degree.

Eddy current heat treat testing is a relative test. While it will not display an absolute Rockwell hardness number, it will indicate whether a part under test is within a few Rockwell points of the desired hardness. The acceptability of that level of accuracy is manufacturer and design dependent.

In many cases, heat treating of bearing races is done by induction heating equipment, in contrast to batch heating in an oven. Induction heating allows bearing races to be hardened while retaining the strength of the underlying metal structure. However, variations within the induction heating process can affect the quality of the heat treating. Conditions such as a misplaced case, shallow case or inadequate quench can occur, all of which need to be identified. Eddy current probes are designed to position test coils over the exact location of the hardened bearing races. Using multiple coils allows several points to be tested simultaneously. These probes can be integrated in production line material handling systems, where testing and positioning time fit in the product cycle time. Stainless steel jackets over the probe protect the coils from wear caused by continuous use.

Sorting and detection

Symmetrical parts, such as ball bearings and small gears, can be passed through standard shaped encircling probes. A single-coil, single-frequency tester will work for such an application. Automation can be achieved by connecting the instrument to a simple sorting chute or to a complex sorting mechanism. Used in small part testing applications, the eddy current system is often integrated in the production line. For larger parts such as auto bumpers, spot probes may be used, either in a benchtop application or with a portable handheld probe for spot checks. These applications can be done on the production line, but can also be used in the field.

Eddy currents are also sensitive to physical configuration of metals. While this can be a detriment, because of lift-off variations in testing, it can be used to verify attributes such as thread installation and assembly verification. Probes can be used to detect missing, undersized and oversized threads, as well as double-threaded parts. Probes can also be configured to look for taps that have broken off in threaded holes. The correct broaching of features such as splines can be verified as well.

As for the ensuring the correct mechanical assembly of components, eddy current technology is used for verifying that all ball bearings have been installed in a bearing race. A single coil is held briefly over the race, and a missing ball will trigger a failure. Airbag manufacturers use eddy current technology to verify that a single diffuser has been installed. Conditions such as no diffuser or multiple diffusers are detected, noted and trigger alarms.

Material handling demands

To achieve 100% in-line inspection, the component under test must be consistently positioned with respect to the eddy current probes. Components may have to be moved so that a probe can be inserted. Fitting the probe to the component is a fundamental consideration in the test process to avoid the negative effects of lift off. Lift off occurs if the probe is tilted or raised away from the component. In an operation that runs 24 hours per day and 7 days per week, the eddy current probes must be robust. In many instances, probes are covered with stainless steel or ceramic covers to prevent damage to the coils.

Eddy current testing times, in the order of a 100 milliseconds, are possible. Even handling and positioning of the component, with respect to the probe, adds a few seconds of time. Such needs are consistent with most production line processes. As the probes are environmentally sealed, they work well in the manufacturing environment.

The material handling system must be able to handle rejects. When the eddy current system detects a flaw, an I/O response is activated. This can drive industry standard industrial controllers that direct the component to a reject chute, mark the part or stop the line while alerting an operator that a reject has occurred. All of these responses can be used to trigger a check on the manufacturing process up-stream. Such a verification procedure can reduce part variations and enable corrective action or adjustment to be taken.

Because numerous components may be manufactured on a single assembly line, it is important that the material handling system handle different sizes and styles of probes. Modern eddy current instruments have the ability to store numerous configuration files in the instrument. This allows an operator to quickly call up the correct test for the manufactured part.

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