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Advanced NDT of Aerospace Components: Eddy Current and Linear Phased Array Testing


Introduction


Aerospace components draw from a broad range of materials and geometries and typically require advanced nondestructive testing techniques to inspect the various aluminum, composite and other advanced material components. Additionally, Federal Aviation Administration (FAA) non-destructive testing oversight is very structured and does not necessary transfer or complement NDT personnel and procedures established in other industries like oil and gas, civil, and construction industries. This article presents aerospace industry NDT personnel qualifications and a few unique NDT applications to the aerospace industry: Eddy current testing of fastener holes and linear phased array testing of composite skins.



Figure 1: Eddy current inspection in accordance with Piper Aircraft, Inc., Service Bulletin No. 1345 requires that the two lower outboard bolt holes on the lower main wing spar cap were inspected for fatigue cracks.


NDT Personnel Qualification and Certification for Aerospace


NAS 410 Certification and Qualification of Non-destructive Test Personnel establishes the NDT training and on-the-job (OTJ) requirements for personnel aspiring to perform eddy current (ET) and phased array testing (PAUT) in the aerospace industry. The standard is generally comparable to SNT-TC-1A and CP-189 in terms of NDT training hours and OTJ NDT hours. For example, both standards require 40 hours of NDT Level 1 and 2 classroom training for ultrasonic and eddy current testing. The minimum experience requirements for Level 2 ultrasonic and eddy current testing are 200 and 600 hours for Level 1 and 2, respectively. In contrast, the requirements of CP-189 Level 2 ultrasonic and eddy current testing are 210 and 630 hours in the method and 1200 hours in total NDT experience. These subtleties must be considered as NDT personnel migrate between industrial sectors.


Eddy Current Testing of Fastener Holes


Fastener hole inspection using eddy current testing can be accomplished cost-effectively with generic ET equipment like an Olympus NORTEC 600 and a rotating or manual fastener hole ET probe. The rotating probes are configured with auxiliary equipment that rotate the probe as it travels through the thickness of the fastener hole. For low quantity eddy current inspections, a manual ET fastener probe may be used. Manual fastener hole probes have coils that are positioned perpendicularly to the probe’s shaft length. The ET probes are rotated manually inside the fastener hole searching for fatigue cracks oriented radially and axially. Most ET probes are differential coils that operating between 100 kHz to 2 MHz. An example fastener hole inspection application is shown in Figure 1. The Piper Aircraft, Inc., Service Bulletin No. 1345 required that the two lower outboard bolt holes on the lower main wing spar cap are inspected for fatigue cracks after exceeded a certain amount of flight time. The fastener holes are accessed from the bottom side of the spar box. In the figure shown below, a NORTEC 600 is being used with a manual bolt hole ET probe to inspect for fatigue cracks in the spar box and lower spar flange.


Calibration of the ET probe and NORTEC 600 settings using a USAF general purpose eddy current standard. The standard is generally fabricated using 7075-T6 aluminum and contains twenty fastener holes ranging in size from 0.156” to 0.750” diameter. The holes contain a variety of corner, bore, surface, and tapered notches.


Linear Phased Array Ultrasonic Testing of Composite Skins


Composite skin inspection of wing and fuselage components often present complex geometries to the NDT inspector. Olympus’ Rollerform probe provides a semi-automated method to inspect large areas of composite structure efficiently. Rotating scanner probes are available in a variety of diameters and styles to work with Olympus rotating scanners. Composites also present a challenge to conventional and phased array ultrasonic testing due to the thinness of the material, its anisotropy, and attenuative nature since resolution and penetration depth are competing with one another.


Linear phased array testing of composites most commonly uses a 0-degree longitudinal wave to inspect through the thickness of the composite skin. The composite is inspected for delamination or damage using A-scan, B-scan, and C-scan data as shown in Figure 2. Detection of internal flaws is monitored through observation of an A-scan or B-scan reflection between the interface and backwall echoes. Alternatively, the linear PAUT backwall reflection may be monitored for a prescribed amplitude drop implying and obstruction or damage has formed between the interface and back wall reflection echoes.


Both PAUT interpretation scenarios are shown in the Figure 2 B-scan. The width of the B-scan in the top left is equal to the active aperture of the roller form probe. The high amplitude red interface echo is observed across the entire width of the probe. The linear PAUT back wall reflection is observed almost across the entirety of the probe except where the lamination was present between the interface and backwall reflections. This simple data analysis example shows that defects in composites may be detected using the true positive and true negative detection strategies.




Figure 2: Linear phased array testing of composite skins.


Summary


Eddy current testing and phased array ultrasonic testing (PAUT) are two advanced non-destructive testing techniques that are applied to advanced materials used in the aerospace industry. Eddy current testing is commonly used for fatigue crack detection in aluminum fastener holes using rotating and manual ET probes. Eddy current calibration must be performed using industry standard blocks like the USAF general purpose plate. Composite aircraft wing and fuselage structures may be inspected for damage and delaminations using the true positive and/or true negative detection approaches. PAUT A-scan, B-scan and C-scans are used to map out the damage in 3-D in conjunction with an encoded scan.


Aerospace NDT personnel are generally certified to Level 1, 2, and 3 inspectors using NAS 410 Certification and Qualification of Non-destructive Test Personnel published by the Aerospace Industries of America, Inc. (AIA). NDT personnel training requirements for eddy current and ultrasonic testing personnel are identical while equivalent experience differs slightly.


References


  1. 2020 NAS 410 Certification and Qualification of Non-destructive Test Personnel

  2. 2020 ANSI/ASNT CP-189 ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel

  3. SNT-TC-1A: Personnel Qualification and Certification in Nondestructive Testing (2020)

  4. Piper Aircraft, Inc., Service Bulletin No. 1345 March 27, 2020

  5. SAE ARP4402 - Eddy Current Inspection of Open Fastener Holes in Aluminum Aircraft Structure ARP4402A

  6. SAE AS4787 Eddy Current Inspection of Circular Holes in Nonferrous Metallic Aircraft Engine Hardware


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