NDT Innovations: Eddy Current Crack Testing Advances
February 3, 2008
At ibg NDT, a new eddy current instrument can detect multiple types of surface flaws simultaneously with no resolution loss for any of the types. Bill Buschur, general manager of ibg NDT Systems Corp. (Farmington Hills, MI), likened it to radio: listeners can clearly hear only one station at a time-or in the case of eddy current, only one flaw type at a time-because multiple radios tuned to multiple stations produce only unintelligible noise. This new instrument essentially allows people to listen to many stations at once and to hear all of them very clearly, or to detect many flaw types simultaneously, all of them with no loss of resolution.
With traditional eddy current instruments, “As long as you have only one type or closely similar types of cracks, you’re fine, because they will all be detectable at similar band pass frequencies and with similar phase angle characteristics,” Buschur says.
But if cracks are sufficiently different, for example, significantly different band pass filter and phase angle settings are required to detect them, traditional instruments run into problems. They have to use a single “wide open” band pass filter setting and a “compromise” phase angle setting, which can introduce too much unintelligible noise-think radios. The flaw types can get lost in this noise, so companies may have problems with some cracks getting past the test system. The new ibg method is different and allows technicians to find everything they need in one test with high resolution.
The eddyvisor C instrument solves the problem of detecting all types of flaws with high resolution, regardless of their frequency content or phase angle. It tests using a bank of 30 band pass filters simultaneously with a universal, closed tolerance field threshold within each filter band pass, so all flaws are automatically detected if their indication exceeds the threshold within any band pass filter and with any phase angle.
The 30 band passes are displayed in a column, and by clicking on any of them, the traditional x/y, x/t and y/t results for that band pass testing are graphically displayed. “It’s like testing the same part 30 times with 30 different traditional instruments each with a different setting,” Buschur says.
ResponseThe instrument was released about a year ago, and the response in its niche market has been encouraging.
“It’s been fantastically well received, particularly if they’ve worked with eddy current in the past,” Buschur says.
Traditional nondestructive testing (NDT) is associated with process industries, aerospace/aircraft maintenance and in-situ work, Buschur says. For example, a specially trained certified technician would monitor the wall thickness of pipe or tube sections and test individual particular parts in-situ.
“Where we’re different is that we test mass-produced metal components, so it has to be easy to use and maintain, high speed, highly reliable, highly repeatable and detect all specified flaws without false rejects.” Buschur says. “We definitely have a different perspective.”
Previously, finding different types of flaws required a solution that was difficult or expensive and required highly skilled personnel. Visual or magnetic particle testing is labor intensive, and human inspectors may miss a number of flaws. The development staff at ibg aimed to find an easier, more automatic way of testing. The instrument competes against other eddy current products, along with other NDT testing methods such as resonant testing or magnetic particle.
ibg NDT has been working primarily with suppliers of components to automotive companies and to bearing manufacturers.
TestingThe eddy current test instrument eddy-visor C detects surface-open and close-to-surface subsurface defects such as cracks and pores on electrically conductive parts. It tests components in a production environment, and can detect surface cracks with a depth of only 0.05 millimeter on plane or rotation-symmetrical parts with surface speeds up to 50 meters per second scan speed.
The path from calibration of eddyvisor C to test mode is operated via a 15-inch color touch screen with an ergonomically designed user interface. The eddyvisor C also is available in different versions: the basic version has two crack test channels, but up to 16 channels may be provided. The eddyvisor C is available as a desktop version eddyvisor DC (Desktop, Crack test) or as a divided version as eddyvisor C (Crack test) consisting of the operating unit eddyvisor HMl (Human Machine Interface) and the measuring unit eddy-visor MC.
The eddyvisor C (Crack test) may be combined with the structure test instrument eddyvisor S to eddyvisor SC. Thus crack detection and structure test are both in one instrument-simplifying integration in automatic test systems.
CalibrationOne or more good parts are required for calibration of the eddyvisor C. In calibration mode, the crack detection probe scans over flawless surface areas of good parts. Noise signals generated by the flawless surface finish are taken over a large number of band pass filter settings and are displayed within the individual tolerance fields for each band pass filter.
In test mode, all parts with signals exceeding the tolerance fields are sorted out as “NOK” (not okay). This guarantees detection of unexpected flaws as well as the specified flaws and avoids manual setting to a complex defect. Setting is possible by semi-skilled personnel. Traditional setup using a master crack also can be used.
Each crack test channel is independent. The test results of each channel are processed and displayed by the integrated PC (HMI). A variety of graphic display options of the test results are available, including bar graph, tolerance field view, x/y, x(t), y(t) or C-scan.
The test result per channel is displayed as good or bad. Historical test results are displayed as a histogram.
The crack test instrument eddyvisor C has lift-off compensation with the use of special probes. One lift-off probe requires two crack test channels. Lift-off compensation electronically balances changes of crack sensitivity that occur due to eccentricity or mechanical tolerances.
The eddyvisor C can internally store up to 50 part types with the relating settings, which can be recalled manually or by PLC. All internally stored data may be transferred to a PC or stored on a USB-stick. The instrument is equipped with three USB 2.0 interfaces (two of them on front) as well as one Ethernet interface. A standard printer (USB) can be used for printing the test result.
The product starts at about $30,000. While this setup may seem expensive, Buschur points out that significantly higher test resolution is provided while operation does not require skilled labor.
“While automatic eddy current crack detection requires capital expenditure up front,” Buschur says. “The automation eliminates ongoing labor cost of visual inspection methods like mag particle, and it is clean and very reliable.” ndt
Technology ContactFor more information on the eddyvisor C, contact
ibg NDT Systems Corp.
20793 Farmington Rd.
Farmington Hills, MI 48336