Innovative Materials Testing Technologies, Inc. Crack Detection In Aircraft Fastener Holes Using FG RFEC Technique And SSEC System Y. Sun, T. Ouyang, J

Innovative Materials Testing Technologies, Inc.

Crack Detection In Aircraft Fastener HolesUsing 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. [email protected].


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


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


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.


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


FG RFEC Technique

Indirect Coupling Path

Pickup UnitDrive 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.


SSEC System

Monitor

FG RFEC Probe

Test Panel

SSEC Board & Software

Installed In a Book-Size PC

SSEC SystemFG RFEC Probe

Customer preferred Computer

USB

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.


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



Modified NDT 636 Reference Standard

Probe Carriage

Probe CarriageProbe CoilsD = 5.84 mm

Modified NDT 636 Reference Standard for Boeing 767

Slip Ring

Rotation Guide


EDM #3EDM #1EDM #2

A BC

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 #2 BA CEDM #1


1.25 mm 0.625 mm

2.5 mm 1.25 mm


Fastener Hole Crack Detection (1): Raster Scan

L16L15L14L13

L26L25L24L23

EDM

EDM

Inner-layer crack signals are submerged by the fastener signals/noises


Fastener Hole Crack Detection (2): Rotational FG RFEC Probe

Specimen

FG RFEC Probe

Fastener

Probe is rotating around fastener center


Fastener Hole Crack Detection (2): Underlying Physics

Differential Sensor

+ -

Fastener- head Fastener-shank

Differential Sensor

Fastener-head Fastener-shank

Excitation CoilExcitation 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


Probe Assembly

Probe head

Rotation Guide & Suction System

Fastener Hole Crack Detection (2): First Prototype of Rotational Probe


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


Detecting 2nd Layer EDM Notches in Raised Head Fastener Holeson a 0.040”+0.040” 2024 T3 Specimen Using A FG RFEC Probe


Real, X Imaginary, Y Impedance Plane

Red – No EDM Notch Black – 0.050” EDM NotchMagenta – 0.080” EDM Notch Blue – 0.110: EDM Notch



1. 1st layer crack detection in buttonhead (bucked) rivets - 90% POD number was 0.064” with 0 false calls.

2. 1st layer crack detection detection in Cherrymax blind rivets - RFEC probe detected 100% of the flaws producing 0 false calls.

3. 2nd layer crack detection in buttonhead (placed) rivets - 90% POD number was 0.047” with 5 false calls.

4. 2nd 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 0.100” long cracks


RFEC Modified ProbeCrack Topside, Second Layer Exp.


Pro

bab

ility

of

Det

ecti

on

Crack Length (in)

Angled Subsurface

Crack

Skin

HiddenCrack


Detecting Crack in Flush-Head Fastener Holes (1)

Accurate Centering is Critical to Small Inner-Layer Crack Detection

No. 07 No. 08 No. 09 No. 10 No. 11 No. 12 No. 13 No. 14 No. 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


Signal magnitude variations when rotation center moves around fastener center

Detecting Crack in Flush-Head Fastener Holes (2)

-1 -0.5 0 0.5 1

-1

-0.5

0

0.5

1

-1 -0.5 0 0.5 1

-1

-0.5

0

0.5

1Y [mm] Y [mm]

A Crack-free holeA 2.62mm 3:1 Triangle

crack at 180ºX [mm] X [mm]

Min = 16.6 mvMin = 4.5 mv


Rotation, , control motor

Y – control motor

X – control motor

Rotational probe Assembly

Cable & connectors

From Miniature Vacuum

Specimen

1st Prototype of Auto-Centering Centering DeviceDetecting Crack in Flush-Head Fastener Holes (3)


ControllerAuto-

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


Detecting Crack in Flush-Head Fastener Holes (4)Typical Example 1

Detecting 2nd Layer EDM Notches in Alodined Rivet HolesTwo layer of 0.063” 2024T3 Made by FAA CASR

Minimum among 5 times detections

13

13

13

13

Maximum among 5 times detections


Detecting Crack in Flush-Head Fastener Holes (5)Typical Example 2

Deep Crack Detection

7 mm

7 mm

2.5 mm2.5 mm

45°

No Crack 2nd layer 2.5 mm 45°crack


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.

Documents

Innovative Materials Testing Technologies, Inc. Crack Detection In Aircraft Fastener Holes Using FG RFEC Technique And SSEC System Y. Sun, T. Ouyang, J