Innovative Materials Testing Technologies, Inc. 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 N. Loop Drive, Ames, IA 50010, USA. Tel , Fax

Innovative Materials Testing Technologies, Inc. Contents 1.Aging of aircraft & Aircraft NDI 2.Requirements to Aircraft NDI Techniques 3.Existing & Emerging Aircraft NDI Techniques 4.FG RFEC & SSEC Technique 5.Detecting Cross-Bolt Hole Crack Through Bushing 6.Fastener Hole Crack Detection: Raster Scan versus Rotational Scan 7.Detecting Cracks in Raised-Head Fastener holes 8.Detecting Cracks in Flush-Head Fastener 9.Summary

Innovative Materials Testing Technologies, Inc. Aging of Aircraft & Aircraft NDI 1.Impact from Aging of In-Service Airplanes 2.Cracking – A Critical Issue Affecting Aircraft Life 3.Current Status of Aircraft Crack Detection Period of Maintenance Disassembly and Removal of Component Cost Breakdown 4.Demand for Advanced NDI Techniques

Innovative Materials Testing Technologies, Inc. Requirements to Future Aircraft NDI Technique 1.Multi-layered structures: up to 5 layers. 2.Deep penetration: total thickness can be up to 5-10 cm. 3.Materials: Al, Ti, Composite, etc. and Combinations. 4.Sensitivity to small-sized inner layer cracks. 5.Fasteners of different materials. 6.Reliability of detection without human factors involved. 7.High-speed and large-area inspection. 8.Discriminate noises, such as edge effect, Tapered thickness, etc. 9.Low cost. 10.Portability and convenience in use.

Innovative Materials Testing Technologies, Inc. Existing & Emerging Aircraft NDI Techniques do not meet the above requirements: 1.UT and Guide Wave UT do not penetrate through air- gap between layers. 2.X-Ray – heavy equipment and high cost. 3.Most ECTs and alternatives, limited by Skin-Depth effect, have little potential in increase of penetration depth. 4.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

Innovative Materials Testing Technologies, Inc. FG RFEC Technique Indirect Coupling Path Pickup Unit Drive Unit Direct Coupling Path 1.Probe blocks the direct coupling path. 2.Energy released is forced to go along the indirect coupling path. 3.Signal received by the pickup unit has passed the wall twice and carries the entire information about the wall condition. 4.Signal can be extremely weak, but is very clean without noise coming from the driving unit.

Innovative Materials Testing Technologies, Inc. SSEC System Monitor FG RFEC Probe Test Panel SSEC Board & Software Installed In a Book-Size PC SSEC System FG RFEC Probe Customer preferred Computer USB Current VersionNext 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.

Innovative Materials Testing Technologies, Inc. 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

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

Innovative Materials Testing Technologies, Inc. EDM #3 EDM #1 EDM #2 A B C 90° 0° 180° 270° 360° 25.4 mm 18.8 mm 12.5 mm 6.3 mm 0.0 mm Zoom-in version of above curve EDM #2BACEDM #1 Detecting Cross-Bolt Hole Crack Through Busing (3) 1.25 mm mm 2.5 mm 1.25 mm

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

Innovative Materials Testing Technologies, Inc. Fastener Hole Crack Detection (2): Rotational FG RFEC Probe Specimen FG RFEC Probe Fastener Probe is rotating around fastener center

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

Innovative Materials Testing Technologies, Inc. Probe Assembly Probe head Rotation Guide & Suction System Fastener Hole Crack Detection (2): First Prototype of Rotational Probe

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

Innovative Materials Testing Technologies, Inc. Detecting 2 nd Layer EDM Notches in Raised Head Fastener Holes on a T3 Specimen Using A FG RFEC Probe Detecting Cracks in Raised-Head Fastener holes (2) Real, X Imaginary, Y Impedance Plane Red – No EDM NotchBlack – EDM Notch Magenta – EDM NotchBlue – 0.110: EDM Notch

Innovative Materials Testing Technologies, Inc. Detecting Cracks in Raised-Head Fastener holes (3) 1.1 st layer crack detection in buttonhead (bucked) rivets - 90% POD number was with 0 false calls. 2.1 st layer crack detection detection in Cherrymax blind rivets - RFEC probe detected 100% of the flaws producing 0 false calls. 3.2 nd layer crack detection in buttonhead (placed) rivets - 90% POD number was with 5 false calls. 4.2 nd layer crack detection in Cherrymax blind rivets - RFEC probe detected 100% of the flaws while producing 3 false calls. Results from Blind POD Tests at FAA AANC OEM Target: Detection of long cracks

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

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

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

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

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

Innovative Materials Testing Technologies, Inc. Detecting Crack in Flush-Head Fastener Holes (4) Typical Example 1 Detecting 2nd Layer EDM Notches in Alodined Rivet Holes Two layer of T3 Made by FAA CASR Minimum among 5 times detections Maximum among 5 times detections

Innovative Materials Testing Technologies, Inc. Detecting Crack in Flush-Head Fastener Holes (5) Typical Example 2 Deep Crack Detection 7 mm 2.5 mm 45° No Crack 2 nd layer 2.5 mm 45°crack

Innovative Materials Testing Technologies, Inc. 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.