Crack Detection In Fastener Holes Using FG RFEC

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.

2501 N. Loop Drive, Ames, IA 50010, USA.

Tel. 515296 5328, Fax. 515 296 9910 email. suny@imtt-usa.com.



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 airgap 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



Drive Unit

FG RFEC Technique

Direct Coupling Path Pickup Unit

Indirect 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

USB

SSEC Board

& Software

Installed In a

Book-Size PC

Test Panel

Current Version

FG RFEC Probe

Customer preferred Computer

SSEC System

FG RFEC Probe

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)

Cross-Bolt Holes in Boeing 767 Landing Gear

• 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.




Probe

Carriage

Slip Ring

Rotation

Guide

Probe Coils

D = 5.84 mm

Probe Carriage

Modified

NDT 636

Reference

Standard

Modified NDT 636 Reference

Standard for Boeing 767




0.0 mm

6.3 mm

12.5 mm

18.8 mm

25.4 mm

0

°

A

EDM #2

90 ° EDM #1

C

1.25 mm



0.625 mm

B

2.5 mm



1.25 mm

180

° 270 ° EDM #3 360 °

A EDM #2 EDM #1 C B

Zoom-in version of above curve



Fastener Hole Crack Detection (1): Raster Scan

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

EDM

L13 L14 L15 L16

L23

EDM

L24 L25 L26



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

FG RFEC Probe

Fastener

Specimen

Probe is rotating around fastener center



Fastener Hole Crack Detection (2): Underlying Physics

Fastener- head Fastener-shank Fastener-head Fastener-shank

Crack

+ -

Differential Sensor

Inner Layer

Eddy Current

Streamlines

Excitation Coil

+

Differential Sensor

-

Excitation Coil

No- Crack Case: Zero signal measured by the sensor;

Crack Case : We see signal only when sensor passes a crack



Fastener Hole Crack Detection (2):

First Prototype of Rotational Probe

Rotation Guide &

Suction System

Probe Assembly

Probe head



Detecting Cracks in Raised-Head Fastener holes (1)

FG RFEC

Probe

A pocket closely matches fastener head

Additional

Guide for

Probe

Rotation

Specimen

Test

Specimen

A round probe head serves as a guide for probe rotation




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

Real, X

Imaginary, Y

Impedance Plane

Red – No EDM Notch

Magenta – 0.080” EDM Notch

Black – 0.050” EDM Notch

Blue – 0.110: EDM Notch




Results from Blind POD Tests at FAA AANC

OEM Target: Detection of 0.100” long cracks

1. 1 st layer crack detection in buttonhead (bucked) rivets -

90% POD number was 0.064” 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 0.047” 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.

RFEC Modified Probe

Crack Topside, Second Layer Exp.




RFEC M odifie d Probe - Cracks Topside Se cond Laye r Exp. (1/03)

0.5

0.4

0.3

0.2

0.1

0

0

1

0.9

0.8

0.7

0.6

0.02

0.04

0.06

Flaw Length (in.)

Crack Length (in)

0.08

0.1

0.12

Skin

Hidden

Crack

Subsurface

Crack



Detecting Crack in Flush-Head Fastener Holes (1)

Accurate Centering is Critical to Small Inner-

Layer Crack Detection

Signal Magnitude Minimum magnitude location

Fastener

Center

8 mils per step

No. 07 No. 08 No. 09 No. 10 No. 11 No. 12 No. 13 No. 14 No. 15

Signal magnitude evolution as probe rotation center passing over a cracked fastener from its cracked side



0

-0.5

0.5


Signal magnitude variations when rotation center moves around fastener center

Y [mm]

1

Min = 4.5 mv Y [mm]

1

Min = 16.6 mv

0.5

-1

A Crack-free hole

-1 -0.5

0 0.5

0

-0.5

-1

A 2.62mm 3:1 Triangle

-1 crack at 180

º

-0.5

0 0.5




1 st Prototype of Auto-Centering Centering Device

Cable & connectors

Rotational probe Assembly

Rotation,



, control motor

Y – control motor

X – control motor

From

Miniature

Vacuum

Specimen




Auto-Centering System

Auto-

Centering

Device

Vacuum-

Suction

System

Probe position & rotation control

Auto-Centering

Software in PC

Controller

SSEC

Rotational FG

RFEC Probe




Typical Example 1

Detecting 2nd Layer EDM Notches in Alodined Rivet Holes

Two layer of 0.063” 2024T3 Made by FAA CASR

Maximum among 5 times detections

Minimum among 5 times detections

1

3

1

3

1

3

1

3




Typical Example 2

Deep Crack Detection

7 mm

2.5 mm

7 mm

2.5 mm

45

°

No Crack 2 nd 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.

Crack Detection In Fastener Holes Using FG RFEC

Crack Detection In Aircraft Fastener Holes

Using FG RFEC Technique

And SSEC System

Results from Blind POD Tests at FAA AANC

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Crack Detection In Fastener Holes Using FG RFEC

Crack Detection In Aircraft Fastener Holes

Using FG RFEC Technique

And SSEC System

Results from Blind POD Tests at FAA AANC

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