Transient (Pulsed) Eddy Current Inspection of CF188 Inner ... · • Simulated Wing Spar – 7075-T6 aluminum spar – Nylon 6 polymer “skin” – Ferrous Hi-Lok fasteners –

Transient (Pulsed) Eddy Current Inspection of

CF188 Inner Wing Spar Through Carbon/Epoxy

Skin with Ferrous Fasteners Present

Capt. Peter F. Horan

T.W. Krause, P.R. Underhill, V. Babbar

Royal Military College of Canada

Kingston, ON

[email protected]

Outline

• Introduction

• Experimental Setup

– Principal Components Analysis

• Results

• Conclusion

Introduction• RCAF CF188 Fleet has a need to detect cracks in Inner Wing spar over

carbon-epoxy skin without ferrous fastener or skin removal

• Transient Eddy Current (TEC) inspection seen as possible solution

Experimental Setup

• Central Driver Differential Pickup probe design

Source: P. Whalen, TEC Inspection in the presence of

ferrous fasteners in multi-layered aluminum structures,

RMC Thesis, 2010

Experimental Setup

• Simulated Wing Spar

– 7075-T6 aluminum spar

– Nylon 6 polymer “skin”

– Ferrous Hi-Lok fasteners

– Notches used to simulate cracks

• Only full-through notches between

fasteners examined, i.e.

12 June 2012

Fastener with

no notch

Fastener

with notch

Through notch

in aluminum

Experimental Technique

• Measurement technique used meant to simulate on-aircraft inspection method for

hundreds of fasteners

– Removing and replacing probe on spar, aligning by eye

– Aligning 180-degree coils in-line with fasteners (determined to be optimum detection

position)

Experimental Methodology

• Signal response difference due to technique:

TEC responses of 21 measurements of fastener #16 (blank) (right) with controlled measurement technique, and (right) with realistic technique

#16 (Blank)

Experimental Methodology

• Analysis of raw TEC, time-domain responses incapable of

accurate crack differentiation:

TEC responses of 21 measurements of fastener #16 (blank) (right) and fastener #17 (left) (270°) using modified measurement technique

#16 (Blank) #17 (Notched)

Principal Components Analysis (PCA)

• Principal Components Analysis (PCA) used on numerous measurements of like

fasteners

• Statistical technique used to reduce the # of relevant dimensions of large data

arrays to a few vectors that best describes the signals in a least squares sense

• PCA can then reduce each signal measurement to 3 or 4 values (scores) that

describe almost all of the variance between them

• PCA (Z) scores – uncorrelated combinations of original data and eigenvectors,

expressing each signal as a single dimension with maximum amount of variation in

the data accounted for

z� =�X��u��

�

Principal Components Analysis

� = � � � ����+���� �⋯

Results• Eigenvectors

Eigenvector ui

u1

u2

u3

u4

Eigenvalue λiλ1 = 14334.02 λ2 = 102.83 λ3 = 66.02 λ4 = 6.340

Results• PCA Scores – 1st and 2nd components (Z1 and Z2)

Scores of first two principal components for TEC responses of numbered fasteners.Left: fasteners with notches at bore. Right: fasteners without notches at bore

Z1 vs. Z2 – fasteners w/notched holes Z1 vs. Z2 – fasteners w/out notched holes

Results• PCA Scores – 2nd and 3rd components (Z2 and Z3)

Scores of the second and third principal components for TEC responses of numbered, cracked ferrous fastenersLeft: fasteners with cracks at bore. Right: fasteners without cracks at bore

No Crack

Z2 vs. Z3 – fasteners w/notched

holes

Z2 vs. Z3 – fasteners w/out

notched holes

Results

• Eigenvectors

– 3rd eigenvector is what is describing the presence of a defect

Results

• PCA score outliers indicate measurement anomalies

Results• Anomalies – likely due to probe not being centered over fastener, and/or

pickup coils not aligned properly with crack/spar direction

#15

Conclusions

• Reliable method using PCA to detect notches beneath a

non-conducting layer without ferrous fastener removal

has been developed

• A realistic measurement technique, and statistical analysis

method allow for signal variation caused by user errors to

be identified and differentiated from variation caused by

the presence of cracks

• Ongoing development of probe design to detect

• Low-cost system, potentially fast and efficient with real-

time analysis capability

Questions?