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Signal Invariance to Probe Characteristics

William C. Quinn* and Shaun Freed† Wyle Laboratories, Dayton, Ohio, 45440

The reproducibility of eddy current coils and probes is a serious concern in the field of eddy current non-destructive inspections. The Advanced NDI Group at Wyle Laboratories, Inc develops fully automated eddy current inspections for use on the United States Air Force Retirement for Cause (RFC) engine rotor inspection program. Probability of Detection (POD) is established from system response versus flaw size ( a vs. a ) data per methodology outlined in MIL-HDBK-1823 using two or a small number of probes. For the POD analysis to be meaningful on fielded inspections, it is necessary to consistently produce a response similar to that from which the POD is based. This is ensured through both calibration and acceptance testing. As part of an ongoing effort to better characterize the actual response of eddy current probes, data was taken using several differential eddy current coils in the “Split-D” configuration. Contrary to expectations, it was found that the pulse width from the differential coil did not depend on the width of the coil. The pulse width is dependent on the rate of travel of the probe, and is expected to relate to the physical construction of the coil, but only a slight correlation to the coil width was observed. Additionally, it has been found that the coil could be rotated within the coil plane +/- 15 degrees without significantly affecting the response on an EDM calibration notch or on reliability specimen cracks in transverse orientation. It was further observed that the signal from transverse cracks remains remarkably similar when the probe is rotated through the entire 90 degree range from ideal to non-ideal orientation.

I. Introduction The Advanced NDI Group at Wyle Laboratories (www.advancedndi.com) is an industry leader in the field of non-destructive eddy current inspections, providing automated inspections used on the United States Air Force Retirement for Cause (RFC) engine rotor inspection program. The Eddy Current Inspection Stations (ECIS) supplied by Wyle Laboratories provide millions of dollars in savings for the Air Force, because once critical areas on the engine rotors have been inspected for cracks they can be re-certified for extended lifetimes. Critical areas include many different features including bolt holes, scallops, knife edges, slots, and surfaces. The flaw detection requirements typically range in depth from 10 mils to 30 mils (20 mils to 60 mils in length).Detection limits are determined by taking data from cracked reliability specimens supplied by the Air Force and determining the 90% confidence boundary as described by Al Berens.1 For further details about the ECIS platform see Craig Benson.2

This study was performed as part of an ongoing effort to better characterize the signals from differential eddy current probes used in these inspections. Due to the success of the RFC program, new probes are continuously being manufactured. This has highlighted the necessity of better understanding the probe response to ensure consistent, accurate inspections. The following discusses two key results from this work, the first being a surprisingly slight relationship between the diameter of the eddy current coil and the observed pulse width, and the second a remarkably large tolerance found for the coil orientation.

II. Signal Invariance With Respect To Coil Diameter This first part of the study was performed to determine if the coil diameter could be determined from its response to a standard calibration notch. The eddy current probes used were differential coils in the “Split-D” configuration

* Senior Physicist, Advanced NDI Group, 2700 Indian Ripple Rd. Dayton, OH 45440. † Scientist/ Physics, Advanced NDI Group, 2700 Indian Ripple Rd. Dayton, OH 45440.

42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit9 - 12 July 2006, Sacramento, California

AIAA 2006-4583

Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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with two receive coils encircled by a drive coil as illustrated in Fig. 1.The receive coil signals for this style of probe are subtracted so that the output signal will be the difference between the two coils. When a flaw is encountered first by one coil and then the next this differential coil arrangement will produce a signal with a large peak-to-peak amplitude. The working theory for these probes is illustrated in Fig. 2.The difference signal should begin to rise once the effective footprint of the leading coil encounters a crack. After reaching it maximum value, the signal would be reduced as the first coil passes the crack and the trailing coil begins to see the crack .The signal would cross zero when the crack was equally detected by both coils, and then rise in the opposite polarity as the trailing coil moved across it, until the coil passes and the signal dies off with a limited amount of ringing anticipated. The effective footprint or range of sensitivity, of a differential eddy current probe has been assumed to be 01.33 times the nominal diameter. It is improper to refer to a single pulse as having a frequency, however the duration of the pulse can be thought of as roughly analogous to the period of a periodic signal. Using this analogy, the quantity “flaw frequency” has been defined as the inverse of the nominal pulse width. While not a frequency, this parameter has proven convenient for characterizing signals and for filter design. The typical implementation has been to calculate an expected flaw frequency and design the signal processing based on this value. The expected flaw frequency is calculated from the inverse of the amount of time it takes for the eddy current coil to pass over the flaw using the linear speed of the probe, the effective coil size, and the expected flaw width (often negligible).It was anticipated that flaw frequency could be used to either determine the effective size of an eddy current coil, or at least contribute to this determination.

A. Experiment Several probes were tested on an EDM cut calibration notch. The probes had nominal diameters of 0.030, 0.045 or 0.060 inches, which are commonly referred to as D20, D30 and D40 coils respectively. Two sets of data were taken about a year apart with the first (set A) consisting of three D20, two D30, and seven D60 coils, and the second (set B) eight D20, two D30, and three D60 coils. Each coil was operated at either two or six megahertz, depending on the standard operating frequency of the probe. The scan-rate was 300 inches per second, the sample rate 10,000 samples per second, the signal path included a band pass filter (post demodulation) set for the range of 20 to 1000Hz, and the signals were recorded at the maximum peak-to-peak responses. The calibration material was PWA 1074 mod IN100. The EDM calibration notch was optically measured to have a width of 0.004 inches and length of about 0.016 inches. For this feature size, the flaw frequency was predicted to double when the coil size doubles. The effect of the coil size was expected to be obvious in the recorded data. The waveforms were collected by indexing across the notch in steps not more than a quarter of the effective coil diameter to find the maximum probe response. Each waveform was then analyzed using MATLAB to determine the pulse width. The pulse width was measured from where the signal fell to within two standard deviation of the noise

Drive Coil

Receive Coils

Drive Coil

Receive Coils

Drive Coil

Receive Coils

Drive Coil

Receive Coils

Figure 1. Split-D Differential Eddy Current

Probe. Encircling drive coil around two balanced receive coils. Coils used in this paper ranged from 30 to 60 mils wide by design.

1

3

2

4

1

3

2

4

Figure 2. Anticipated pulse generation mechanism.

The characteristic differential eddy current signal is often said to derive directly from the time it takes the two receive coils to pass the feature.

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from zero on both the leading and trailing edge of the pulse. This proved to be a very consistent method for determining the period, consistently choosing points within about 0.004 seconds of the apparent zero crossing point, and provided better correlation to the coil size than other methods attempted.

B. Results

There was no more than slight correlation observed between the coil size and the width of the pulse recorded. This was in direct conflict with the working theory on how the signals relate back to the coil. The specific coils used in the experiment and the corresponding pulse widths are tabulated in the appendix to allow the reader to consider subsets of the collections, if desired. Figure 3 shows the measured flaw frequency verses the nominal coil diameter for both data sets. This data had correlation coefficients of 0.565 for set A and 0.819 for set B. Figure 4 shows the flaw frequency verses the coil size optically measured at the outside edge of the drive coil, perpendicular to the split between the receive coils. Note that measurements were not available for all coils. This data had correlation coefficients of 0.783 for set A and 0.718 for B. The difference in correlation values is partly due to the smaller data set; however the salient point is that physically measuring the coils did not improve the result. The predicted curve was calculated as the time for the effective coil width (01.33 * nominal coil size) to pass over the notch (0.004 in) at the inspection scan rate (300 in/ min).Note that except for the one outlying point shown in fig. 3 there is only a slight trend in the measured data instead of the doubling in pulse width predicted. The data also shows large amounts of probe to probe variation that nearly obscures what little trend there may be. It should be pointed out that the predicted trend line cannot be made to match the recorded data by simply increasing the effective coil width multiplier from 01.33, because this will cause the expected slope to actually be steeper, the opposite of what the data would suggest.

0

50

100

150

200

250

300

0.02 0.03 0.04 0.05 0.06 0.07

coil diameter, inches

10-4

sec

onds set A

set B

predicted

Figure 3. Pulse width versus nominal coil size.

Instead of the simple trend predicted, the slight trend is almost lost in the scatter. Note the outlier at 0.06”.

0

50

100

150

200

250

300

0.02 0.03 0.04 0.05 0.06 0.07

coil diameter (in)

sam

ples

(0.0

001s

)

set A

set B

predicted

Figure 4. Pulse width versus measured coil size.

Measuring the coil width did not improve the correlation, note that measurements were not available for all coils.

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C. Conclusions

The result described here should be viewed narrowly. The only conclusion asserted is that the physical width of the coil does not relate as simply to the flaw frequency as was previously understood. This result does not indicate that the signals from different coil sizes are equivalent because previous work has shown that the POD (probability of detection) curves are different for different coil sizes. It does, however, indicate that the signal generation mechanism is different from what had been assumed, and that models based on the earlier assumption should be reconsidered. It is anticipated that the flaw frequency does depend in part on the physical construction of the coil, however further studies will be required to determine the key parameters.

III. Signal Invariance With Respect To Coil Orientation The frequent manufacturing of differential eddy current probes by multiple vendors has driven the need for accurate manufacturing tolerances to be determined. This study was conducted to better understand the tolerance requirement for the rotation of a coil within the plane it lies in (Fig. 5). Differential probes are expected to produce a measurable response to any surface interruptions that are not simultaneously observed by both receive coils. When using a split-D style probe, the optimal configuration for detecting transverse flaws should then be with the coil oriented so that the split between the D coils is perpendicular to the direction of probe travel on the specimen as shown in Fig. 2.Transverse flaws, as opposed to longitudinal, are oriented perpendicular to the probe travel.

A. Experiment

The first issue examined was how rotating the coil in the inspection plane affected the calibration of the probe. The maximum response on an EDM cut calibration notch was recorded for ten different coils of various sizes at angles ranging from zero to ninety degrees away from ideal. The instrument setup was as described above, that is to say: Each coil was operated at either two or six megahertz, depending on the standard operating frequency of the probe. The scan-rate was 300 inches per second, the sample rate 10,000 samples per second, the signal path included a band pass filter (post demodulation) set for the range of 20 to 1000Hz, and the signals were recorded at the maximum peak-to-peak responses. The calibration material was PWA 1074 mod IN100. Following this, the effect rotating the coil has on flaw detection was investigated using titanium (Ti 6-4) POD specimens provided by the United States Air Force. POD collections were made with a single eddy current probe rotated through a variety of angles, ranging from 0 to 90 degrees from ideal. The data from each collection was plotted on a log-log plot against the flaw depth. For each collection the slope, intercept, and correlation factor σ were determined and used to calculate the threshold for an 18 mil flaw as described in Al Berens.1 It has been suggested that the calibration phase could affect this result, so a word of explanation about the signal path and the significance of phase calibration is included in Appendix 2.

angleangleangle

Figure 5. Rotation within the plane of the coil.

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B. Results

The effect on calibration was minimal. Even when the coil was rotated 15 degrees from the optimal orientation, the signal was still on average 96% ± 2% of the optimal. At a 45 degree rotation the signal retained 80% ± 7% its original strength. The effect on the POD collections was more difficult to interpret, but also surprisingly small. As shown in Fig. 6, the system response versus flaw size (â vs. a) 1 curve looks remarkably similar, even when the coil is rotated 90 degrees to the worst-case orientation. On closer inspection, the 90 degree data shows slightly less discrimination between flaw sizes, and had a higher noise level. The slope for all three shown in Table 1 was nominally two, but the intercepts vary from 02.66 counts to 03.317 counts. The intercept corresponds to the exponential term when plotting the log of exponential functions, so this obviously accounted for most of the variability in the resulting POD detection thresholds. The detection thresholds calculated for finding an 18 mil flaw with a 90% probability and 50% confidence bound are also shown in Table 1. The lower threshold found when the coil was turned 90 degrees shows the expected lower sensitivity in this orientation. It is not known if the trend in threshold values from 0 to 45 degrees is significant, nor why the probe appeared to be most sensitive when rotated 35 degrees.

C. Conclusions

These results suggest that the coil orientation may be allowed to drift over a large range before inspections are seriously impacted. While there is no reason for probe manufacturers to need this large of a window, the implication is that a coil may be rotated within its plane through a range of about ±15 degrees without significantly altering its detection abilities, either from reduced signal response or due to calibration error.

6

6.5

7

7.5

8

8.5

9

0 1 2 3 4

depth (ln mils)

ln c

ount

s 0 DEGREES

15 DEGREES

90 DEGREES

Figure 6. Ti POD Angled Data Collection. Data taken on POD specimen set with eddy current probe at the normal operating position, rotated 15 degrees, and rotated 90 degrees.

correlation a90/50

threshold Probe Angle Slope Intercept factor σ

for 18 mil flaw

degrees log(counts)/

log(mils) log(counts) counts

0 2.002 2.66 0.078 3810 5 2.026 2.57 0.075 3760 15 2.076 2.447 0.074 3836 25 2.022 2.633 0.077 3944 35 2.02 2.635 0.066 4044 45 1.857 3.057 0.075 3813 90 1.731 3.317 0.101 3280

Table 1. Angled Collection Fit Data. Data taken on POD specimen set with eddy current probe rotated at the angle specified from optimal.

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Appendix A 1. Probes Used For Flaw Frequency Experiment

A 2. Phase Calibration

In order to preserve phase information, eddy current signals are typically demodulated into two components, one in phase with the drive signal and one ninety degrees out of phase with the drive signal. When using absolute coils, a phase shift in the received data relates to the depth of penetration due to the diffusion time of eddy currents. This phase shift can be related to material properties, see for example Libby3 or similar. The phase information from a differential coil is less straight-forward because the phase information is from the difference between two phase shifted signals in independent receive coils. The reader is encouraged to convince themselves that the phase shift due to depth of penetration should be subtracted out when using a differential coil, observing that this phase lag is from the diffusion time rather than the time it takes for the electromagnetic signal to reach the sensor. For data reporting purposes, these two phase components can then be recombined the usual way:

output signal = ( A2 + B2 )1/2 * sin ( θ )

where: A is the in phase magnitude, B is the out of phase magnitude, and θ is defined by:

( θ ) = arctan ( A/ B ) + Φ Φ is chosen experimentally.

collection A collection B

drive nominal measured half max to

min pulse half max to

min pulse

frequency size width peak

separation width width peak

separation width MHz inches inches samples samples samples samples samples samples

2 0.03 0.034 48 57 166 77 52 174 2 0.03 38 48 147 * * * 2 0.03 0.034 * * * 40 47 157 2 0.03 * * * 40 48 163 2 0.06 52 67 192 * * * 2 0.06 110 65 193 * * * 2 0.06 47 60 188 224 60 190 2 0.06 0.058 116 69 189 52 68 193 2 0.06 48 89 282 144 63 194 6 0.03 0.032 41 54 156 54 49 164 6 0.03 0.033 * * * 61 47 160 6 0.03 0.031 * * * 71 50 161 6 0.03 0.032 * * * 126 42 143 6 0.03 0.033 * * * 113 50 134 6 0.045 0.041 * * * 104 54 164 6 0.045 47 70 196 * * * 6 0.045 0.046 51 70 201 124 52 167 6 0.06 61 59 184 * * * 6 0.06 80 66 185 * * *

Table A1. Coil Diameter vs. Pulse Width Data. This table shows the specific coil size and frequency combinations used in part II.

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Typically, for an absolute coil, Φ is chosen so that the lift-off signal (the signal when the coil is removed directly away from the conducting surface) is minimized. For all of the data in this experiment, which was taken with differential eddy current probes, the phase was set so that the maximum output response is achieved when scanning over a long (> 0.40 inches) notch nominally of similar width to the calibration notch. For measurements taken on the calibration block itself, this is expected to be similar to the absolute coil response. For measurements taken on the POD specimens, some phase effects will be present in the data, although no attempt is made here to describe them. For more information on the implications of phase shifts in eddy current signals see, for example Libby.3

IV. Acknowledgement The authors wish to acknowledge the assistance of Wyle Laboratories Advanced NDI Group and The United

States Air Force Engine Depot at Tinker Air Force Base for the use of inspection stations, probes and reference standards, as well as that of Katherine Mashburn for timely editing.

References 1Berens, Alan P., “NDE Reliability Data Analysis,” Metals Handbook, Vol. 17, 9th ed. 2Benson, Craig W., “Eddy Current Testing of Jet Engines,” Materials Evaluation, Vol., No., May 2004, pp. 516-519. 3Libby, Hugo L., Introduction to Electromagnetic Nondestructive Test Methods, 1971, pp. 39-56.