Crack Detection In Aircraft Fastener Holes Using FG RFEC Technique And SSEC System

1. Crack Detection In Aircraft Fastener Holes Using FG RFEC Technique And SSEC System Y. Sun, T. Ouyang, J. Xu and J. Long Innovative Materials Testing Technologies, Inc. 2501 N. Loop Drive, Ames, IA 50010, USA. Tel. 515296 5328, Fax. 515 296 9910 email. suny@imtt-usa.com.

2. Contents • Aging of aircraft & Aircraft NDI • Requirements to Aircraft NDI Techniques • Existing & Emerging Aircraft NDI Techniques • FG RFEC & SSEC Technique • Detecting Cross-Bolt Hole Crack Through Bushing • Fastener Hole Crack Detection: Raster Scan versus Rotational Scan • Detecting Cracks in Raised-Head Fastener holes • Detecting Cracks inFlush-Head Fastener • Summary

3. Aging of Aircraft & Aircraft NDI • Impact from Aging of In-Service Airplanes • Cracking – A Critical Issue Affecting Aircraft Life • Current Status of Aircraft Crack Detection • Period of Maintenance • Disassembly and Removal of Component • Cost Breakdown • Demand for Advanced NDI Techniques

4. Requirements to Future Aircraft NDI Technique • Multi-layered structures: up to 5 layers. • Deep penetration: total thickness can be up to 5-10 cm. • Materials: Al, Ti, Composite, etc. and Combinations . • Sensitivity to small-sized inner layer cracks. • Fasteners of different materials. • Reliability of detection without human factors involved. • High-speed and large-area inspection. • Discriminate noises, such as edge effect, Tapered thickness, etc. • Low cost. • Portability and convenience in use.

5. Existing & Emerging Aircraft NDI Techniques • do not meet the above requirements: • UT and Guide Wave UT do not penetrate through air-gap between layers. • X-Ray – heavy equipment and high cost. • Most ECTs and alternatives, limited by Skin-Depth effect, have little potential in increase of penetration depth. • SQUID is an exception, but with heavy and large equipment and high cost, too. • FG RFEC technique can be a good candidate of future Aircraft NDI Techniques

6. FG RFEC Technique Direct Coupling Path Pickup Unit Drive Unit Indirect Coupling Path • Probe blocks the direct coupling path. • Energy released is forced to go along the indirect coupling path. • Signal received by the pickup unit has passed the wall twice and carries the entire information about the wall condition. • Signal can be extremely weak, but is very clean without noise coming from the driving unit.

7. Monitor USB SSEC Board & Software Installed In a Book-Size PC Customer preferred Computer FG RFEC Probe SSEC System FG RFECProbe Test Panel SSEC System Current Version Next Version (Available Shortly) A high-gain and low-noise SSEC system amplifies the weak signal sensed by the pickup unit of an FG RFEC probe and bring the signal to a readable size on the PC screen.

8. Detecting Cross-Bolt Hole Crack Through Busing (1) • Experience high stress and cracking. • Cracks covered by bushing are difficult to be detected. • Currently three techniques are used for inspection of a single unit. • Undesired sensitivity, noise level and inspection speed. Cross-Bolt Holes in Boeing 767 Landing Gear

9. Slip Ring Probe Carriage Rotation Guide Probe Coils D = 5.84 mm Probe Carriage Modified NDT 636 Reference Standard Modified NDT 636 Reference Standard for Boeing 767 Detecting Cross-Bolt Hole Crack Through Busing (2)

10. Detecting Cross-Bolt Hole Crack Through Busing (3) 0.0 mm B A C 6.3 mm 1.25 mm  0.625 mm 12.5 mm 2.5 mm  1.25 mm 18.8 mm 25.4 mm EDM #1 EDM #2 EDM #3 360° 270° 180° 0° 90° A EDM #2 EDM #1 C B Zoom-in version of above curve

11. EDM L16 L15 L13 L14 EDM L26 L23 L25 L24 Fastener Hole Crack Detection (1): Raster Scan Inner-layer crack signals are submerged by the fastener signals/noises

12. Fastener HoleCrack Detection (2): Rotational FG RFEC Probe FG RFEC Probe Fastener Specimen Probe is rotating around fastener center

13. Fastener- head Fastener-head Fastener-shank Fastener-shank Crack Inner Layer Eddy Current Streamlines - - + + Differential Sensor Differential Sensor Excitation Coil Excitation Coil Fastener HoleCrack Detection (2): Underlying Physics No- Crack Case: Zero signal measured by the sensor; Crack Case : We see signal only when sensor passes a crack

14. Fastener HoleCrack Detection (2): First Prototype of Rotational Probe Rotation Guide & Suction System Probe Assembly Probe head

15. FG RFEC Probe A pocket closely matches fastener head Additional Guide for Probe Rotation Test Specimen Specimen Detecting Cracks in Raised-Head Fastener holes (1) A round probe head serves as a guide for probe rotation

16. Imaginary, Y Impedance Plane Real, X Red – No EDM Notch Black – 0.050” EDM Notch Magenta – 0.080” EDM Notch Blue – 0.110: EDM Notch Detecting Cracks in Raised-Head Fastener holes (2) Detecting 2nd Layer EDM Notches in Raised Head Fastener Holes on a 0.040”+0.040” 2024 T3 Specimen Using A FG RFEC Probe

17. Detecting Cracks in Raised-Head Fastener holes (3) Results from Blind POD Tests at FAA AANC OEM Target: Detection of 0.100” long cracks • 1st layer crack detection in buttonhead (bucked) rivets - 90% POD number was 0.064” with 0 false calls. • 1st layer crack detection detection in Cherrymax blind rivets - RFEC probe detected 100% of the flaws producing 0 false calls. • 2nd layer crack detection in buttonhead (placed) rivets - 90% POD number was 0.047” with 5 false calls. • 2nd layer crack detection in Cherrymax blind rivets - RFEC probe detected 100% of the flaws while producing 3 false calls.

18. RFEC Modified Probe Crack Topside, Second Layer Exp. Skin Hidden Crack Angled Subsurface Crack Probability of Detection Crack Length (in) Detecting Cracks in Raised-Head Fastener holes (4)

19. No. 07 No. 08 No. 09 No. 10 No. 11 No. 12 No. 13 No. 14 No. 15 Detecting Crack in Flush-Head Fastener Holes (1) Accurate Centering is Critical to Small Inner-Layer Crack Detection Minimum magnitude location Signal Magnitude Fastener Center 8 mils per step Signal magnitude evolution as probe rotation center passing over a cracked fastener from its cracked side

20. Detecting Crack in Flush-Head Fastener Holes (2) Signal magnitude variations when rotation center moves around fastener center Y [mm] Y [mm] Min = 4.5 mv Min = 16.6 mv A 2.62mm 3:1 Triangle crack at 180º A Crack-free hole X [mm] X [mm]

21. Detecting Crack in Flush-Head Fastener Holes (3) 1st Prototype of Auto-Centering Centering Device Cable & connectors Rotation, , control motor Rotational probe Assembly Y – control motor From Miniature Vacuum X – control motor Specimen

22. Detecting Crack in Flush-Head Fastener Holes (4) Auto-Centering System Auto-Centering Software in PC Controller SSEC Rotational FG RFEC Probe Probe position & rotation control Vacuum-Suction System Auto- Centering Device

23. 3 1 Maximum among 5 times detections 3 1 Minimum among 5 times detections 3 1 3 1 Detecting Crack in Flush-Head Fastener Holes (4) Typical Example 1 Detecting 2nd Layer EDM Notches in Alodined Rivet Holes Two layer of 0.063” 2024T3 Made by FAA CASR

24. 7 mm 2.5 mm 2.5 mm 7 mm 45° No Crack 2nd layer 2.5 mm 45°crack Detecting Crack in Flush-Head Fastener Holes (5) Typical Example 2 Deep Crack Detection

25. Summary Three applications of the FG RFEC and SSEC technique in aircraft Crack Detection have been introduced. Typical test examples provided have shown the effectiveness of the technique. The test results have shown good promise of this technique in aircraft NDI applications.