a highly sensitive system for aircraft crack detection – the rotational remote field...

FAA Hughes Technical CenterInnovative Materials Testing Technologies, Inc.

A Highly Sensitive System For Aircraft Crack Detection

– The Rotational Remote Field Eddy Current Probe And Super Sensitive Eddy Current

System Yushi Sun

Innovative Materials Testing Technologies, Inc.2501 N. Loop Drive, Suite 1601, Ames, IA 50010

Tel. 515 296-5328; Fax. 515 296-9910, Email. [email protected]

Cu NguyenFAA William J. Hughes Technical Center, AAR-480

Airport & Aircraft Safety R&D Division, Atlantic City Intl Airport, NJ 08405Tel. 609 485-6649 Fax: (609) 485-4569 Email. [email protected]

[1] The work is currently supported by Federal Aviation Administration William J. Hughes Technical Center (FAA/HTC), its Airworthiness Assurance NDI Validation Center (AANC). It was partly supported by The Center for Advanced Technology Development of Iowa State University.


Challenge in Aircraft Inner Layer Crack Detection

1. Detect inner layer cracks from outside aircraft skin (to avoid expensive and time-consuming work of

removal of aircraft interior inspection).2. Deep penetration.3. Sensitivity to small-sized inner layer crack.4. Reliability of detection.5. High-speed and large-area inspection.6. Discriminate noises from different factors, such as edge

effect, etc.7. Low cost – affordable by commercial airlines.8. Convenience in use.

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FG RFEC1 & SSEC2 System

1. Deep penetration in multi-layered aircraft structures – up to 0.8”.

2. High sensitivity to hidden cracks/corrosion.

3. No limit to customized software for enhanced features

4. Simplicity in use

5. Low cost

6. Portability1 FG RFEC – Remote Field Eddy Current technique for inspection of conductive

objects of Flat Geometries.2 SSEC – Super-Sensitive Eddy Current system.

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Indirect energy coupling path

ΦRF

Drive coilPick-up coil

Φ

Indirect energy coupling path

Direct energy coupling path

Energy from indirect coupling path dominates in remote field region, therefore, pick-up coil signal represents the wall

conditions.

RFEC Technique – A Brief Introduction

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FG RFEC Probe

Drive Coil Pickup CoilDirect Coupling Path

Indirect Coupling Path

Specially designed FG RFEC probe blocks direct couple path and forces energy going along indirect

coupling path

FG RFEC Probe

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RF-4 mm V.3Footprint: 0.85” x 2.15”

Coil Center-to-Center DistanceCCD = 1.15”

RF-2 mm V.3Footprint: 0.3” x 0.62”

Coil Center-to-Center DistanceCCD = 0.3”

FG RFEC Probe (Continued)

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Monitor

FG RFEC Probe

Test Panel

SSEC Board & SoftwareInstalled In a Book-Size

PC

Current System Configuration: a PCB inserted in a book-size PC

SSEC System

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Notebook or PC

SSEC SystemFG RFEC Probe

New System Configuration: System connected to PC via USB

USB

SSEC System (Continued)

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Faster Hole Crack Detection

Mode 1

Raster Scan Using A Sliding FG RFEC Probe

A Traditional Way of Crack Detection


Bottom View

Top View

Inspected Area

Crown Left

0.4”0.3”

DC-10 Crown Panel & Inspected AreaFive layers with a total thickness varying from 0.4” to 0.3”

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L16L15L14L13

L26L25L24L23

Detected EDM #1 on the 5th layer EDM #1

A raster-scan over the standard top.A 5th layer EDM notch is detected, but challenge to signal Interpretation .

Raster-Scan Using An FG RFEC Probe

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Possible Ways for Future Improvement

1. Improve probe design and performances;

2. Reduce/remove fastener signals/(background noises);

3. Develop signal process methods that enhance crack signal or suppress fastener signals;

4. Develop pattern recognition algorithms;

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Faster Hole Crack Detection

Mode 2

Rotational Scan Using A Rotational FG RFEC Probe

A Way to Minimize Fastener Signal In Crack Detection

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Probe Assembly

Probe head

Rotation Guide & Suction System

First Prototype of Rotational Probe

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Noise/edge effect

EDM Notch

Noise/edge effect Noise/edge effect

Real Component, X Imaginary Component, Y Impendence Plane

EDM Notch EDM Notch

Initial Test ResultsSignal detected from a fifth layer fastener hole EDM Notch on

DC-10 Crown Standard


Requirement in Accurate Centering Probe Rotation & 1st Centering Device Prototype

Non-powered suction cups

Strain-releaser

Slip-ring

FG RFEC Rotational probe

Rotation guide/Theta-Y table

X-adjustment screw

Y/theta-adjustment screw

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Rotation, ϑ, control motor

Y – control motor

X – control motor

Rotational probe Assembly

Cable & connectors

From Miniature Vacuum

Specimen

1st Prototype of Motorized Centering Device

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Study on Probe Signal Behavior with Variation of Probe Rotational Center

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

FastenerCenter

Minimum magnitude

locationSignal Magnitude

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

8 mils per step


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Y = F(X)MagnitudeReal, X Imaginary, Y

Y = F(X)Real, X MagnitudeImaginary, Y

No. 10

No. 13


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Min = 0.0334 v

Min = 0.0165 v

Signal magnitude variations when rotation center moves around fastener center

A Crack-free fastener hole A Cracked (left) fastener hole


Challenges –

1. Signal/electric center may not be the geometric center of a fastener head;

2. For a cracked fastener hole the location with minimum signal magnitude is neither a geometrical center, nor an electric center.

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Developing Auto-Centering System

1. To automatically discriminate a cracked fastener hole from crack-free holes;

2. To find the signal/electric center for each fastener hole and provide a right crack signal at the location.


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Current Progresses Made

1. Capable of automatically discriminating cracked fastener holes from crack-free holes in tested panels; It takes 40 – 60 seconds.

2. Working on the issue of finding the signal/electric centers for cracked fastener holes and provide right crack signals at the location for a tested panels.


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Rotational probe carriage

ϑ rotational drive motor

Y-axis drive motor

X-axis drive motor

Test panel

Protection case

2nd Auto-Center Device Prototype


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Next Step & Future Work

1. Continue to study probe behavior around a fastener hole on different panels and fasteners;

2. Continue to develop auto-centering software and to find more efficient indicator(s) for crack discrimination and quantification;

3. Test real aircraft panels with different situations of fastener installation;

4. Consider possible solutions for moving a probe from one faster to another;

5. 3rd prototype design, fabrication, test and POD study.


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CONCLUSIONS1. An FG RFEC rotational probe, is currently under

development. It shows a great potential of rapid detection of deeply hidden cracks and corrosion with high sensitivity and reliability.

2. An accurate centering of probe rotation center is a key to meet the increasing demands in detecting deep and small cracks hidden in aircraft structures and components;

3. A 3D ϑ-X-Y scan mode shows when an FG RFEC probe is rotating around a fastener hole, the signal/electric center may not be the geometrical center of a fastener head.


CONCLUSIONS(continued)

4. When scanning over a cracked hole, the minimum magnitude location is not the signal/electric center where to collect crack signal.

5. An automatic centering device that is capable of discriminating crack signals, locating signal electric center, and providing representative crack signals in currently under development and is supported by FAA William J. Hughes Technical Center.

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a highly sensitive system for aircraft crack detection – the rotational remote field...

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