Our choice of coil size and type will determine how well we can link our coil's energy into a test piece. Most of the coils that we use in aerospace component inspection would be addressed as "Probe" coils. They may contain one or more coils. Probe coils are typically moved across the surface of a material in the area of interest instead of just sitting in one position and taking a measurement. This scanning process can be done manually or it can be motorized or automated.
If the flaws that we are looking for are assumed to be near surface, then we will probably stay with the simple "pancake" or pencil probes shown in Figure 1. These typically have smaller diameters that would allow good sensitivity to short cracks. Their design parameters also dictate that they will have fairly high frequency ranges (50 to 500 kHz). This should put most of the coils' energy right at the coil/material interface. This will improve the sensitivity of the test.
• If we expect to have a problem with the geometry of the part (curved surfaces, changes in surface geometry, etc.) then maybe we should select a "spring-loaded" coil. These would normally be used on flat plates but the spring-loaded coil approach should help limit lift-off responses in other surface geometry situations also.
• If we expect to have difficulty in reaching the area of interest (through a hand-hole, on the backside of a bulkhead, in a tight radius) or encounter restrictions in the test area (bolt heads, seams, etc.) then we may need to try using a small diameter "pencil probe." Mechanically, based on how well we could control the position of the probe with respect to the surface, this would be less stable. We would then have to try to compensate for any geometry-related signals by using specific scanning techniques or applying electronic tools.
Signal Displays / Calibration
Eddy current test information can be displayed in many different ways. There are still a lot of meter-based eddy current systems in use but in most modern systems we would see a dot displayed on the screen of a "X-Y" system. A meter-based system only displays half of the information being generated as the probe responds to material changes. A trained technician using an X-Y system has a better chance of detecting small flaws while not being overly conservative.
Once we have decided which of the surface probe designs we need to accomplish this test the calibration process is going to be basically the same. We need to start with a sample (calibration block) that we know has regions that are undamaged, but which also contains representative flaws. This "cal" block will be used to establish our system settings of: frequency, gain, phase, and any display features that might enhance our detection potential.
A good thing to check on any system before starting the calibration process is to make sure that all special system features (mixers, filters, alarms, etc.) are off. Another key issue is to verify that the X:Y screen display sensitivity settings are in a nominal mode (1:1). Some systems allow the vertical and horizontal (V/H) screen sensitivity settings to be adjusted independently. This is an excellent tool for signal enhancement later, but it can be confusing if this feature is being used during the initial calibration process.
Check the probe to verify that it is in good condition. If this is a small pencil probe remember that over time the wear surface on the face of the coil can be removed just with normal use. You may need to add a protective layer of material to the face of the probe to protect the coil from damage. This might be a piece of Teflon® tape or a layer of something like nail polish or other hard material that can be easily applied.
1. If the system allows for multiple modes of coil drive operation (absolute, differential, driver/pick-up) then determine which is correct for this particular surface probe.
2. Select a frequency that is within the specified range of the coil.
3. Set the gain control to a setting that is in the low to medium range of its scale.
4. Place the probe on a good (non-flawed area) of the "cal" block.
5. Perform the electrical "balance" or "Null" function provided on the system.
At this time you should see a dot on the screen.
6. Rock the probe slightly from side to side to create a "lift-off" response. You do not have to completely remove the probe from contact with the surface. Note the direction of the lift-off response.
7. Rotate the "phase" adjustment on the system so that the lift-off response leaves the screen directly to the left. This would be the nine o'clock position on a clock. In ECT, we call that screen position "zero degrees" or we say that lift-off is now "on the horizon." Ninety degrees is at the 12 o'clock position and we continue through the range until we reach 360 degrees back at the 9 o'clock point.
8. Keeping the probe in a vertical position, move it slowly over the range of discontinuities on the standard. These will probably be EDM notches that represent cracks. The screen response should be a rapid movement of the dot into the upper left-hand quadrant of the screen.
Once you can verify that you have flaw detection capability then the only issues that remain are how to optimize the signal variation between flaw and non-flaw conditions. You may need to adjust gain to increase signal amplitude from the smaller EDM notches. You might want to alter the V/H ratio to improve the displayed signal-to-noise ratio. Keep in mind that filters can be used in some applications to improve flaw sensitivity but they should never be used with any manually controlled scanning process.
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