Thick & Multi-Layer Aircraft Structure NDI Using … at CO ASNT meeting...Innovative Materials Testing Technologies, Inc. 1 ASNT Colorado Chapter Meeting, April 8, 2008 Thick & Multi-Layer

Innovative Materials Testing Technologies, Inc.

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ASNT Colorado Chapter Meeting, April 8, 2008

Thick & Multi-Layer Aircraft Structure NDI Using Remote Field Eddy Current

(RFEC) Technique

Yushi Sun

Innovative Materials Testing Technologies, Inc.3141 W. Torreys Peak Drive

Superior, CO 80027Tel: 303 554 8000Fax: 303 554 8001

Email. [email protected]


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Contents• RFEC effect in tubing and flat geometries• Comparison of RFEC technique with conventional EC

technique• Example No. 1 - Corrosion detection in thick &

multilayer structures• Example No. 2 - Crack detection in thick & multilayer

metallic structures• Example No. 3 - Crack detection through thick

composite• Example No. 4 - Crack detection in bolt hole through

bushing• NDI automation


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RFEC Phenomenon in Metallic Tubes

Phenomenon:

Signals received by pick-up coils are sensitive to changes in wall thickness, conductivity, and permeability.

ΦRFExcitation coil Pick-up coil

Φ


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RFEC Phenomenon in Metallic Tubes

Phenomenon:

Signals received by pick-up coils are sensitive to changes in wall thickness, conductivity, and permeability.

Direct energy coupling path

Indirect energy coupling path

ΦRFExcitation

coilPick-up coil

Φ

Indirect energy coupling path

Underlying Physics:1. Direct energy coupling is restricted by EC in the wall.2. Pick-up coil signal ΦRF is dominated by the energy diffusing along the

indirect coupling path that traverses the wall twice.3. Changes in the phase of ΦRF are directly proportional to the thickness

of the wall.


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Magnetic Field Distribution around a Conducting Pipe


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POYNTING Vector Plot Showing Energy TransmissionAround Pipe Wall – No Defect Case

Finite Element Modeling Result (3)

Magnetic energy goes from pipe ID to OD, spreads along OD, then comes back to pipe ID in remote field region

Near field Remote field

Distance down pipe [pipe OD]


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POYNTING Vector Plot Showing Energy TransmissionAround Pipe Wall – Defect Case

Finite Element Modeling Result (4)

A defect provides a “short cut” to energy transmission path. Therefore, stronger and less phase lag magnetic field is seen at defect area in remote

field region

Near field Remote fieldDefect signal

seen here

Wall

Excitation Coil

Pipe inside

Pipe outside

Distance down pipe [pipe OD]


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IntroductionFG RFEC Probe

A Solution for Deep Penetration

The probe blocks the direct coupling path. The energy released from the drive unit is forced to go along the indirect coupling path.

Therefore, the entire signal received by the pickup unit has passed the wall twice and carries the whole information about the wall condition.

Indirect Coupling Path

Pickup UnitDrive Unit Direct Coupling Path

ΦRF


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FG RFEC Technique versus EC Techniques* ECT• Impedance Z is proportional to total flux, Φ.

In a reflection probe induced voltage V is proportional to Φ, too.• A flaw causes very limited change in Φ, since also in Z or V.• The change in Φ caused by a deeply hidden flaw may be less than

0.001% - 0.0001%.• Different approaches have been used to cancel/compensate the normal

signal and separate out the flaw signal. However, a perfect separation of the two signals is practically impossible.

------------------------------

* ECTs here represents all EC methods other than RFEC. It includes Pulsed EC, Self-Nulling Probe, SQUID, MOI, MWM and traditional EC.



ΦRF


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FG RFEC• V is proportional to a portion of the flux, ΦRF, that

has passed through the test object twice and represents the local condition of the object between the driver and receiver.

• The presence of a defect results in a large change inΦRF, and also in V.

• The change in phase of ΦRF has a linear relation with the wall thickness.

FG RFEC vs. EC



ΦRF


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EC• Signal level is high, but flaw-induced signal

variations are low. The ratio of flaw signal to normal-signal is low. This limits the gain value can be used in an instrument.

FG RFEC• Signal level is low, but flaw-signal/normal-

signal ratio is high. This allows higher gain for a given flaw signal.

RFEC vs. EC


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Super Sensitive Eddy Current Instrument SSEC II

Modification of conventional EC instrument; capable of working with FG RFEC probes as well as conventional EC probes

Higher sensitivity and larger gain to work with the extremely weak signal from an RFEC probe

Fully computerized system capable of on the spot automatic control, signal processing and pattern recognition

Light (2.4lbs), small and portable

Dimension:12”(L)x8.7”(W)x1.7”(H)


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SSEC II Instrument & System


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Example No. 1Corrosion detection in thick

& multilayer structures


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Sliding Probes Used for Corrosion Detection

RF4 V.3Footprint: 0.85” x 2.15”

Coil Center-to-Center Distance, CCD = 1.15”

RF2 V.3Footprint: 0.3” x 0.62”

Coil Center-to-Center Distance, CCD = 0.3”


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Example 1.1:5 Layer 2024 T3 Aluminum Specimen

0.1” + 0.1” + 0.19” +0.19” + 0.063”Total Thickness = 0.643”

Corrosion on Bottom of 5th LayerLocation = 0.603”

3” × 3” × 0.040”

Rf4 V3

Location = 0.603”

0.5” × 0.5” × 0.040”

0.100”

0.100”

0.063”

0.190”0.190”


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Example 1.1Corrosion Sample – the 5th Layer

0.063” thick aluminum, chemical thinning on the bottom side

0.5” × 0.5” × 0.040"

3.0” × 3.0” × 0.040"

Specimen provided by CNDE


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Test Result – Example 1.1.13” × 3” × 0.040” 5th Layer Bottom Side Corrosion, f=200Hz

Total Thickness = 0.643”, Location = 0.603”


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Test Result – Example 1.1.20.5” × 0.5” × 0.040” 5th Layer Bottom Side Corrosion, f=200Hz



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Example 1.23 Layer 2024 T3 Aluminum Specimen

0.26” + 0.05” + 0.063”Total Thickness = 0.373”

Corrosion on Bottom of 3rd LayerLocation = 0.367”

0.260”

0.063”

0.050”

0.5” × 0.5” × 0.006” corrosion

RF4 V3

0.367”


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0.5” × 0.5” × 0.006"





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Test Results - Example 1.20.5” × 0.5” × 0.006” 3rd Layer Bottom Side Corrosion, f=500Hz



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Corrosion detection through thick composite layer & metal + composite layers

Example 1.3

1st layer 0.350” thick graphite epoxy composite2nd Layer 0.190” thick 2024 T3 Aluminum

3rd layer 0.063” thick CNDE corrosion sample

0.350” composite

0.063”

0.190” aluminum

3.00” ×3.00” × 0.040” corrosion

RF4 V3


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0.5” × 0.5” × 0.040"

3.0” × 3.0” × 0.040"



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Example 1.3Typical Test Result


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Example No. 2Crack detection in thick &

multilayer metallic structures


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Sliding Probe Example 2.1Detecting 2nd Layer Notches in 0.25” + 0.25” Thick

B-52 Wing Spar Structure

181716

Y

Xx5x1

14 Scan Area

15 19

Edge Cutx0

29 mmX – Scan direction

19.7 mm

Excitation coil

Pickup coilFastener

15 – 2nd layer notches (0.100” & 0.245”)16-17 – 1st layer notch18-19 – 2nd layer notch


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Detecting Crack in 0.25” + 0.25” Thick B-52 Wing Spar Structure

15: 2nd layer vertical notches (0.100” & 0.245”)

16-17: 1st layer

Other holes

f = 100 Hz, EFD = 19.7 mm


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Rotational Scan Using A Rotary Probe Minimizing Noise from Fastener

Ball-bearing rotation guide

Slip ring Connector

Probe carriage

Composite layer

Titanium layer under inspection

Slip ringProbe carriage

Probe head with centering pin


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Rotational Scan – Minimizing noise from fastener

-+ -+

Diff Sensors

Fastener- head Excitation Coil

Diff Sensors

Fastener- head Excitation Coil

Probe 2 – No signal unless there is a crack

FastenerSensor

Drive coil

FastenerSensor

Drive coil

Probe 1 – Constant signal unless there is a crack


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Success Story 1 Detecting Cracks in Raised-Head Fastener holes

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


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0.100” rivet head overhang

Rotorcraft - Detection of Second Layer Cracks Topside

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.91

0 0.02 0.04 0.06 0.08 0.1 0.12

Flaw Length (in.)

Prob

abili

ty o

f Det

ectio

n

Phasec/Walking Probe Insp. #1 Phasec/Walking Probe Insp. # 2Phasec/Walking Probe Insp. # 3 Foerster-Rivet Check w/Custom ProbeRemote Field Eddy Current Concave Probe Insp. #1Concave Probe Insp. #2

Comparison of POD with other NDI TechniquesCarried out by FAA AANC, Sandia Labs


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Success Story 2 Detecting Lower Layer Cracks in Boeing 723 Lap-Joint


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New Challenge in Deep Crack DetectionWith Edge Effect and Steel Fastener

1. Signal magnitude is no longer an indication of existence of a crack

2. Other parameters, such as phase angle and/or shape of impedance plane must be used for crack identification

3. Concept of pattern recognition is needed


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Example 2.2 Detecting 0.200” 2nd Layer EDM Notch

in A Specimen Simulating0.500” + 0.140” C-130 Wing Structure

Specimen provided by Canadian Air Force

Notch

Steel fasteners

0.50

0” th

ick

1stla

yer

2nd

laye

r

Front View Bottom View Side View


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Rotational Scan: Result of Example 2.2 Crack free

Observation:In deep crack detection signal magnitude is no longer the indication of existence of a crack, other parameters, such as phase angle and/or shape of impedance plane must be used for crack identification. Concept of pattern recognition is needed.


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Step motor inside

Motor controller inside

Probe carriage

Probe head

Connectors

Probe carriage

Rotation guide with suction base

Ball bearing rotation guide with suction base

Probe head

Sealing rubber tube

Vacuum area

Automated Rotary ScannerEnsuring Constant Speed for Online Signal

Processing & Crack Identification


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Application Example 2.3Inspection of C130 Lower Wing Structure

Problem description

1st layer0.250” 7075-T7351

2nd layer0.140” thick & 1.0” wide 7075 T7351Steel fasteners

6


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1.0”

1.0”

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12

.600 .500 .400 .300 .200 .100

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12

1st layer – 0.250” 7075-T73512nd layer – 0.140” thick & 1.0” wide 7075 T73512nd Layer - Rectangular notch, Steel fasteners


Test Sample


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#4 0.6” EDM

P=262

#5 0.5” EDM

P=327

#6 0.4” EDM

P=235

#7 0.3” EDM

P=200

#8 0.2” EDM

P=101

#9 0.1” EDM

P=21

#10 no EDM

P=15

#11 no EDM

P=1

#12 no EDM

P=17

#2 no EDM

P=3

#3 no EDM

P=3


Test Results – Impedance Plane (1)


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Shape Factor P=101Shape Factor P=1.0

0.2” EDMNo EDM


Test Results – Impedance Plane (2)


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0

50

100

150

200

250

300

350

1 2 3 4 5 6 7 8 9 10 11

#9 0.1”

#8 0.2”

#7 0.3”

#6 0.4”

#5 0.5”

#4 0.6”

#2 0.0”

#3 0.0”

#10 0.0”

#11 0.0”

#12 0.0”

P

Shape Factor P

7


Test Results – Shape Factor


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Example 2.4 – Simulating Boeing 707 Wing Structure

Detection 2nd layer vertical crackswith Steel fasteners, crack very close to 2nd layer edge

A AB BC CD D0B C D

Upper Row

1st layer - 0.250” Aluminum

2nd layer - 0.313” Aluminum

0.75

0”

2nd layer edge

Steel Fasteners


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Example 2.4Detection 2nd layer vertically aligned cracksclose to 2nd layer edge & with steel fasteners

Challenges:1. In this particular case crack signal DECREASES with increase of

notch size as shown in next page.2. The underline physics of this phenomenon so far remains

unknown. 3. A notch can not be identified by any single parameter of detected

signal. A Shape Factor P, which is a combination of a few features of the shape of the impedance plane, is used for identification of a notch.


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Application Example 2.4Inspection of Boeing 707 Lower Wing Structure

Typical Test ResultsImpedance Plans and ellipse curve fitting

AB

P=0.66

BC

P=0.92

CD

P=0.57

EF

P=0.71

FG

P=0.58

H0

P=0.91

A

P=0.66

B

P=0.74

C

P=2.51

F

P=7.21

G

P=4.58

H

P=100

D0

P=1.0

D

P=3.04

E

P=6.11


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0.2” EDM notch

Shape Factor P=6.11

without notch

Shape Factor P=0.57

Example 2.4 - Signals from Fastener Holes w/ and w/o a NotchSignal decreases with increase of notch size

and varies in its size and shape


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Shape Factor P

0123456789

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Notched fastenersGood fasteners

C 0.15”

D 0.17”

E 0.20”

F 0.22”

G 0.25”

H 0.30”

P

B 0.13”

A 0.10”

AB

0.00”

BC

0.00”

CD

0.00”

D0 0.00”

EF 0.00”

FG 0.00”

H0 0.00”

9

Application Example 2.4Inspection of Boeing 707 Lower Wing Structure

Typical Test Results


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Challenges in Crack Detection in Titanium Layer through A Thick Composite Layer

1. Removal of composite for crack detection is practically impossible in new aircraft structures

2. Conventional ECT has limited capability in detecting a small crack after penetrating thru thick composite layer

3. Unknown capability of ultrasonic technique in this application

4. Our choice – Flat Geometry Remote Field Eddy Current (FG_RFEC) technique driven by Super Sensitive Eddy Current (SSEC) instrument.

Example 3Crack detection thru thick composite


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Example 3Crack detection thru thick composite

1. Raster scan using a sliding probeAl layer crack thru 1.5” thick polycarbonate layer

2. Rotational scan using a rotary probe or scanner• Detecting Ti layer fatigue crack thru 0.5” thick

graphite epoxy layer• Detecting Ti layer corner EDM notch thru 0.5”

thick graphite epoxy layer


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Example 3.1: Detection of aluminum layer crack through 1.5” of polycarbonate using sliding

probe RF4 V3A*

Three 7”×13” polycarbonate pieces with thicknesses:567-007 – 0.567”;483-007 – 0.483”442-007 – 0.442”

A 9.0”×1.25”×0.20” aluminum strip attached below each of themTitanium fasteners

Two layersOne layer Three layers

* Specimens provided by NAVAIR.


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T = 0.567”

One-Layer Test Results – Specimen 567-007 – 0.567” Thick


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Summary of all result s at 2.0 kHz

Part III – Crack detection through thick composite layer


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MaximumY = 0.1215v

f = 200Hz

Tree-Layers Test Results – All 3 Specimens on Top each otherTotal Thickness ≈ 1.500”, f = 0.2kHz



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No Notch600-500-400-S

Ti Thickness .60”Straight Groove

No Notch750-500-400-C

Ti Thickness .75”Curve Groove

0.400” Top Corner EDM Notch

750-500-400-CTTi Thickness .75”

Curve Groove

0.250” Top Corner Notch

600-500-400-STTi Thickness .60”

Curve Groove

0.250” Bottom Corner EDM Notch

750-500-400-CBTi Thickness .75”

Curve Groove

CS-500-5000.500” Thick

Composite Layer

CS-350-5000.350” Thick

Composite Layer

Real Large & through Crack

Specimen

Photos of Standards Provided by AFRL

Example 3.2Ti layer Corner EDM Notch detection Thru 0.35” –

0.50” Graphite Epoxy Layer


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A 0.35” thick composite on top of 0.75” thick Ti layer with a curved groove

Fitting ellipse angle=35.25°Fitting ellipse angle=-0.27°

No EDM notch 0.40” upper corner EDM notch

Example 3.2.1Detecting 0.40” upper corner notch

Thru 0.350” thick composite layer


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Example 3.2.2Detecting 0.250” upper corner notch

Thru 0.500” thick composite layerA 0.50” thick composite on top of 0.75” thick Ti layer

with a curved groove

Fitting ellipse angle=1.13° Fitting ellipse angle=30.96°

No EDM notch 0.40” upper corner EDM notch5


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Example 4Crack detection in bolt hole

through bushing


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Through Bushing Inspection System1. Fatigue cracking at fastener holes is a common problem in

aircraft. 2. A repair bushing to return the hole to its nominal diameter

after crack removed. 3. subsequent reinspection of the repaired hole often requires

removal of the bushing 4. This approach results in significant downtime and labor costs

and potentially damaging to the integrity of the aircraft structure.

5. IMTT’s Bushing Inspection System provides a unique and innovative approach to detecting these under bushing cracks.

6. No bushing removal required, high sensitivity, superior efficiency & reliability


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Thru Bushing Inspection Scanners

1. Scanners RF BSH 0.5 (for bushing ID 0.35” – 0.60”)

• Manual scanner• Automated scanner

2. Scanners RF BSH 1.5 (for bushing ID 0.50” – 2.00”)

• Manual scanner• Automated scanner


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Introducing Scanners RF BSH 0.5(for bushing ID 0.35” – 0.60”)

Automated scanner

The probe diameter is fixed. Probes can be built for each bushing diameter desired.

SBIR Phase II project 2005-2007 sponsored by AFRL

ball-bearing guide

Drive coil

Magnets holderPickup coil


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Hardware

1. FG_RFEC probe

2. An optical switch detecting probe circumferential location

3. A linear encoder & index switch detecting probe axial location

4. Built-in motor controller

5. Hands-free inspection using magnets

6. Light weight ~ 1.5 lb

Motors for rotation and Z-translation

Motor controller

Optic switch

Shaft driving probe

rotationStandard

Magnetic foot


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Hands-Free in A Vertical Inverted Position

Scanner

Real Aircraft WAF SampleReal Aircraft WAF Sample


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Movie Showing How the Scanner works

To Start the movie double left click on the picture

Part 2.1 RF BSH 0.5 Automated Scanner Hardware


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1. Computerized Instrument – SSEC II S

2. Automated Scanning and Image Display

3. Advanced On the Spot Signal Processing and Crack Identification/Quantification

4. Simple and User-Friendly User Mode

5. Sophisticate functions for Scanner Parameter Settings and Optimization in Advanced User Mode

6. Authorized Switching between Two Modes

Software


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Example Showing the Work of the Algorithm

Original Image After Signal processing


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• Two φ0.532” holes with Inconel bushings installed (yellow arrows/red outlined holes).• Hole #3, large crack, 0.075 x 0.090• Hole #4, small crack, 0.060 x 0.020

• Cracks at wing skin layer (red layer)

3 4

3 4

Aircraft Wing Attach Fitting section with bushed holes containing cracks0.090”at interface x 0.075”into bore 0.020”at interface x 0.060”into bore

A Typical Test ResultsReal Aircraft Sample


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Aircraft Wing Attach Fitting Section with Bushed Holes Containing A Cracks In Hole #4

Simplified Structure of Real Aircraft Sample

Fastener Hole0.000”

0.270”0.37”

0.610”

0.91”

1.66”

0.060”×0.020” crack

0.51”

Bushing

Steel

SteelAluminum

Aluminum

Aluminum

Aluminum


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Real aircraft hole #3A 0.075” x 0.090” crack

Real aircraft hole #4A 0.060” x 0.020” crack

No crack standardNo crack

1,088mV 460mV

Part 2.3 Test Results (1) Real Aircraft Sample


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NDI automationAuto-control & real time

signal processing and crack identification


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Automated Bushing Scanning System

1. Automatic probe position, in Z-direction, positionidentification and recording using a linear encoder and a built-in index;

2. Automatic identification of circumferential position of probe using an optic switch and display image;

3. Automatic scan one or two given ranges, or segments, in a bushing hole;

4. Automatic signal processing to remove noise;

5. Automatic crack identification and quantification;


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On going new development

On Going New Development1. Automatic Identification of Existence of Bushing – Success2. If yes, Automatic Measurement of Bushing Thickness – Success3. Following Automatic Instrument and Scanner Parameter settings

– On Going 4. Automatic Vertical Scan to Identify Scan Area – Success5. Automatically Move Probe to the Scan Area – On Going6. Etc.

Documents

Thick & Multi-Layer Aircraft Structure NDI Using … at CO ASNT meeting...Innovative Materials Testing Technologies, Inc. 1 ASNT Colorado Chapter Meeting, April 8, 2008 Thick & Multi-Layer