Eddy Current Inspection: Sliding Probes
High and low frequency eddy current testing has been used as an NDT tool for many years, since the development of pencil, ring and spot probes in the 1960s and 1970s. It was not until the early 1980s that the development of sliding probes came into play. The first sliding probes were used in low frequency, and then in the late 1980s, mid frequency was used for inspecting only in the first layer. These LFEC and MFEC sliding probes were developed to slide over countersink fasteners and used to detect surface and subsurface flaws. Countersink fasteners are generally installed on the outside of the aircraft skins along lap joints, lap seams tear straps or areas on the exterior of the aircraft so as to not cause any drag or affect aerodynamics.
Typically, cracks occur in the faying surfaces between the skin and second layer, or in some cases, in the third-layer structures inside the fastener holes. From there, they work their way along the grain boundary until they come to the next fastener hole, weakening the structure until failure occurs.
One other technological improvement-in eddy current instruments themselves-was needed to permit the development and the use of sliding probes as a viable eddy current inspection method,. With the introduction of impedance-plane instruments, operators were able to adjust horizontal and vertical gain, phase angle, filters and a host of other critical instrument parameters. Until the introduction of the impedance-plane instruments, most eddy current inspection was being done using meter/analog instruments. NDT technicians in the field had to watch the needles swing back and forth, guessing what was a defect and what was just a geometry change affecting the eddy currents. Later, with both impedance-plane instrument and sliding probe technology available, the time was right for inspection improvements and time savings, enabling aircraft to return to service in less amount of time.
Rohmann and Staveley (now Olympus NDT) pioneered the first sliding probe technology in the 1980s. Major advancement in this area came while supporting McDonnell Douglas DC-9 and DC-10 SID programs at Long Beach, and with Boeing in Seattle. In 1988, an Aloha Airlines 737-200 experienced an explosive decompression and structural failure at about 24,000 feet due to metal fatigue in an upper cabin area, with about an 18-foot-long section of the fuselage separating from the aircraft. The crew was able to execute a successful emergency landing with a significant portion of the upper fuselage missing. This event made metal fatigue in aging aircraft a major area of concern for civil aviation authorities.
Sliding probes are so called because they move over fasteners in a sliding motion.
Fixed Sliding ProbesFixed sliding probes are typically used to detect cracks in the second layer, like lap splices. Lap spice thickness may be only .04 to 0.12 inches, depending on aircraft size and configuration. The frequency of these probes ranges from 1 kHz to 50 kHz. Fixed probes are typically used to detect cracks that are running along the fastener row along the 9 o’clock and 3 o’clock positions, in the same direction as the row of fasteners. Care must be taken to keep the sliding probe centered over the fasteners to get maximum flaw signal indications.
When a sliding probe passes over the countersink fasteners, an indication similar to Figure 1 is obtained. The liftoff is normally adjusted to conventional horizontal left direction on the eddy current instrument. The liftoff signal for sliding probes is not a straight line, like with other probes, but curves downward. It is important to remember that this test requires a reasonable comparison to a reference standard (Reference standards are samples simulating the area being inspected, with known defects, and are used to calibrate the instrument).
Conditions like paint, fastener location and head size will have an effect on instrument signals and adjustment may be needed. As the sliding probe passes over fasteners, the screen signal movement begins from the null point (right side of the screen) and moves left in the direction to X point. This will cause an arch-like indication, a mainly horizontal shift of the display along that distance as the eddy currents pass over the fastener and then move back to the null point.
When the sliding probe passes over a fastener where there is a crack in the second layer, there will be an indication similar to Figure 2. The crack disrupts the eddy current field and affects signal response. There is still an arch, but in addition, a hump or loop indication passes along the screen from the null point to X point and back to null point again.
Fixed sliding probes can be used to detect cracks that are oriented at 90 degrees from the inspection direction in the second layer. Special care is required when doing this test, and it requires two passes over each row of fasteners with differently-oriented probes for the 12/6 o’clock and 3/9 o’clock orientations to complete the inspection. Adjustable sliding probes are better suited for these applications.
Adjustable Sliding ProbesThese probes can be adjusted by adding or removing spacers between the driver coil and the pick-up coil based on the fastener head size. As the fastener head size gets larger, the structure tends to get thicker, which will affect the depth of penetration of the generated eddy current field. Adjustable sliding probes come with clear acrylic spacers that allow the operator to see the probe center as it passes over the fasteners. The number of spacers used will vary as the structural thickness changes.
Adjustable sliding probes are used for finding subsurface cracks in thick multi-layer structures, like under wing skins where the thickness can be as much as 0.75 inches. This is about the maximum penetration depth for an eddy current field in these types of structures.
The frequency range for these sliding probes is from 100 Hz to 50 kHz. Probes can be scanned along fasteners looking for cracks that are at the 12 and 6 or 9 and 3 o’clock positions in the countersink fastener hole. Typically when you scan at 90 degrees to the cracks, you will not get the loop like indications as fixed sliding probes, but a sharp narrow indication on the screen. A good fastener indication, as the sliding probe passes over the fasteners, is slightly similar to fixed probes.
When comparing a good fastener to fasteners with crack indication, you will only see an increase in the vertical direction separation between good fasteners to ones with crack indication on the screen. The crack indication is similar in appearance to the indication of the good fasteners; this is do to the sliding probe scanning for cracks that are 90 degrees to the fastener and 90 degrees to the generated eddy current field. See Figure 3.
Note: Operating at 500 Hz to detect cracks in the third layer in a total stake up of .2 inches.
Note: Operating at 1 KHz inspecting for cracks that are under a total stack-up of .125 inch.
Aluminum rivets are the primary fastener used in manufacturing aircraft. Rivets are a simple and inexpensive means to fasten aircraft skin to stringers, lap joints and other structures. Rivets are bucked or cold worked into the fastener hole and fill the holes they are fastening. Rivets installed on commercial aircraft from the 1960s to the 1980s had an anodized finish that created an electrical barrier between the rivet and the countersink holes in which they were installed. This helped the eddy currents flow around the fastener and detect cracks in the rivet holes.
From the late 1980s to 1995, alodine rivets were used as a replacement for anodized rivets. The primary reason for replacement was that the electrical barrier that was so beneficial for eddy current testing had negative effects as well. When lighten strikes an aircraft, it generally follows the rows of rivets. When it comes across the electrical barrier between the rivet and the countersink holes, the result can be blown holes or blown rivet heads. The alodine finish on the rivets allows it to have intimate contact with the aluminum skin. This contact becomes homogeneous and no electrical barrier occurs between the rivet and countersink hole, if installed correctly. This does not offer any area to allow the eddy current to flow, and it becomes difficult to use sliding probes to inspect rows of countersink rivets.
The solution is to use Dual Frequency Eddy Current testing technique (DFEC). This offers the ability to still use sliding probes to scan over countersink rivets and detect cracks, while filtering out noise from the varying rivet conditions caused by the alodine rivets. This requires the NDT technician to use an instrument with dual frequency capability and adjust the instrument to mix fastener signal variations and increase the sensitivity to the defect signal. Dual frequency is a technique that relies on adjusting the two frequencies, frequency 1 (F1) and 2 (F2) to see the rivet signals and the lower frequency, frequency 2 (F2) is set to see and detects the defects in the second layer.
As the probe slides over the row of fasteners, the signal for either an alodine or anodized fastener, your signals will arch down off of the screen. Fasteners that are not making good contact in the countersink holes with defects will have a large loop indication. These large loop indications will trigger the instrument’s alarm gate.
If the rivet is installed correctly, with good intimate contact between fastener and countersink hole, you will get a straight vertical indication.
NDT technicians armed with an eddy current instrument and a sliding probe will be able to inspect most aluminum skinned aircraft. Even under the most difficult conditions, with proper training and a good NDT background to aid in their interpretation, they can consistently get the desired results to ensure that the aircraft structure does not have critical defects that will compromise the structures strength and integrity. j