Probe Types and Their Usage
Pencil Surface Probes
These are the probes normally used for surface crack detection, also known as High Frequency Eddy Current probes (HFEC). They have a small coil that can be made shielded or unshielded. Most are absolute types, although they can be made with the balance coil built into the probe body to ensure good balance and increased frequency range. There are many types available, both in straight and angled versions, to match any requirements. They are also available with flexible shafts that can be adjusted to different shapes.
Pencil probes can be designed to operate at various frequencies, depending mostly on the material to be tested. For aluminum, 100 kHz is the most popular, allowing use of up to 200 kHz or more, depending on the balance coil and the instrument used. The higher frequencies will give better angle to lift-off, although as the probe approaches 500 kHz it becomes more lift-off sensitive and will not penetrate as much material. Because of this, it is normally preferable to stay at lower frequencies.
It has become common to use pencil probes below 100 kHz when looking for first layer cracks that originate in the opposite side and are growing, but have not broken the surface yet (even more so with clad skins). A frequency between 20 kHz and 50 kHz will penetrate the clad and detect a defect that is only 50% through the thickness. Some standard 100 kHz probes can be run at 50 kHz provided that we compensate for this by using higher gains; however, it is better to use probes designed for the lower frequencies, even if we have to accept a slightly larger diameter.
For low-conductivity materials, such as titanium or stainless steel, it is necessary to choose a frequency of 1 MHz to 2 MHz to improve sensitivity and phase angle to surface breaking cracks. Magnetic steels are not very critical as far as frequency is concerned, although good results are often obtained at 1 MHz or 2 MHz to minimize permeability variations. When the material is cadmium-plated, lower frequencies are needed to minimize its effect and sometimes a frequency of 25 kHz to 50 kHz is the best, although a bigger probe diameter is required.
Surface Spot Probes
Also known as Low-Frequency Eddy Current probes (LFEC), spot probes are used at low frequencies for subsurface detection of cracks and/or corrosion. They are available from 100 Hz and up (to penetrate the thicker structures), in both shielded and unshielded versions. Shielded probes are more popular as they concentrate the magnetic field under the probe and avoid interference from edges and other structures; however, they are more sensitive to small defects. Reflection types are also widely used due to the lower drift and often higher gain in the more demanding applications. Spring-loaded bodies are useful to maintain a constant pressure when needed, such as when spot testing for conductivity differences.
These are similar to the surface spot probes, except that the center has been enlarged (and made into a hole) to accept the diameter of the fastener head/hole to be inspected. They provide greater sensitivity to cracks, as the fastener/hole interface aids the penetration. This is even more noticeable with ferrous fasteners, but permeability variations can also give problems. The probe internal diameter (ID) is the more important dimension, and should be chosen to be slightly bigger than the fastener head. The outside diameter (OD) is not normally critical but it should not overlap other fastener heads. The probe height is not critical; however, in cases of limited access, special low-profile types are available where the test coil and balance coil sections of the probe are separated to further reduce the height of the probe.
Bolt Hole Probes
Bolt hole probes are designed to inspect the bore of holes after the fastener is removed. They can be divided into two groups:
Manually operated with adjustable collar. The probe is indexed to the right depth and rotated manually. Typical coil configuration used with the manual bolt hole probes is absolute, bridge and bridge differential.
Rotating scanner. These are manufactured to mate with the various scanners in use, and provide the best coverage and high inspection speeds. Rotating scanner probes typically contain reflection-differential coil configurations as the differential coils are less sensitive to interface and provide better detection of defects. Reflection mode is used to maximize gain, provides a wider frequency range and minimizes drift which could be caused by heat build up in the probe as it rotates at high RPM's.
Other Hole Inspection Probes
Low frequency bolt hole probes. Used inspect holes through bushings, low frequency coils are incorporated into the design of the probes. These probes use coils similar to those in the surface spot probes and typically are limited to larger diameter hole due to the larger coil size.
Countersink probes. These are made to fit specific fastener head shapes to inspect the open hole entrance. They can be made for manual or rotating scanner operation, with the same coil configurations used in the standard bolt hole inspections. If a large number of holes need inspection, the rotating scanner type provides a much faster coverage.
Large Diameter Rotating Scanner Probes
For many years, large diameter holes have been inspected using manual bolt hole probes. The reason for this was that the existing probe designs were too heavy and unbalanced to rotate freely for use with standard hand-held rotating scanners. Manual scanning and indexing is not only a slow process, but it is also difficult to ensure complete coverage. In addition, large holes are often in thick parts, and that means that a large number of scans are required to cover the complete thickness.
The new large diameter probes have been designed to minimize weight and optimize mechanical balance. In this way, the comparatively small power rotating guns can drive them without excessive speed loss and shaking. Diameters in excess of 2 in. (50 mm) have been successfully tested. The adjustable diameter probe types allow for the probe to be set at the correct diameter to prevent too much friction and not lose sensitivity to small defects.
- Not all hand-held scanners have the same power and the larger diameter probes need more power or the inspection results will be unreliable. If in doubt about your rotating scanner, give us a call and we will advise you.
When testing large diameter holes, the coil is traveling faster over the defect. This changes the duration of the signal and means that the filter settings in the instrument may need to be reset to higher values. The high-pass filter (HPF) that normally reduces the effect of slow changing variables, such as ovality (lift-off changes), will not be as effective and the setting will need to be increased, for example, from 100 Hz to 200 Hz or more. The low-pass filter (LPF) may cut part of the defect signal. Again try increasing the setting to avoid this, for example, from 200 Hz to 500 Hz or more. Band-pass filters (BP) are a combination of both and are available in some instruments. They also need resetting to a higher value. Always adjust the filters for the best signal-to-noise ratio. Some instruments may not have enough filter settings to take full advantage of the large diameter probes.
There are many probe types that are made for specific customer requirements. Please send us a drawing or sketch of your application, and we will quote a special eddy current probe to fit your part.
When experiencing difficulty in operating a probe, it is advisable to do some simple tests.
- Check that the operating frequency is within the probe's range. If the probe is not balancing properly, the instrument may have entered into "saturation." This can be verified easily. If the signals produced by lift-off and defect (or an edge) superimpose on each other, there is no phase angle and saturation has occurred. The frequency may be too high, or the probe coil and the balance coil are not of the same value. Try lowering the probe drive voltage. Please note that some instruments have the capability of very high output values that may be excessive for some probes.
Try moving the cable, particularly where it joins the connector or the probe body as these are the weaker points. If it shows intermittent operation, the cable needs replacement. Also, it may be necessary to clean connector contacts. Silicon spray or an electrical contact cleaner will often help.
If the dot appears dead or the signals are small and/or distorted, look at the filter settings. Many instruments now offer a range of "high pass" and "low pass" filters. These are very useful, but if set incorrectly will cause various effects.
High-pass filters (HPF) will always bring the dot to the balance point and, at high settings, (as used for rotating scanners) will make the dot appears as static at the balance point. For hand operation, set the high pass filter to OFF (or 0 Hz).
Low-pass filters (LPF) will make the display speed dependent. The best setting for manual use is typically 100 Hz, but if the signal is too noisy it may be necessary to reduce this setting. If so, the scanning speed will need to be kept low enough as not to reduce the size of the signals.
- Examine the probe test surface. It may be damaged or worn. Watch for exposed wires or other damage. Use Teflon tape at the probe face whenever possible. This reduces probe wear and also prevents possible contact with the ferrite that will often produce noise.
When confronted with high signal-to-noise ratios typically seen when using rotating scanner probes it is a good practice to insert a small piece of sponge or foam rubber to enhance the coil's contact with the inner surface of the hole. This technique will greatly reduce noise and increase sensitivity.