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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.