A Study on Boeing 747 Thick Structure NDI Using a Combination of GRFEC and LFEC Techniques towards a Goal of Improved Aircraft Safety Level

Transcription

1 A Study on Boeing 747 Thick Structure NDI Using a Combination of GRFEC and LFEC Techniques towards a Goal of Improved Aircraft Safety Level Yushi Sun 1, Jianhua Pan2 2, John Bohler3 3, David Piotrowski 3, Qingdong Yun4 4, Xueliang Wu 4, Haiou Zhu 1, Changhong Sun 1, Weiqiang Wan 1, Hai Lin 1 Background and Motivation Development of General Remote Field Eddy Current (GRFEC) technique has reached outstanding progress in a number of applications related to military airplane crack and corrosion detection. Currently we are looking for opportunity of deploying the GRFEC technique in commercial airplane applications. The newest development includes the newest version of the Super Sensitive Eddy Current Instrument, SSEC III, which can drive most of the conventional eddy current (EC) Nondestructive Testing (NDT) probes and provide inspection performances equal or better than those which being used in Boeing commercial airplane maintenance. We believe that it is very possible that a large number of Boeing NDT procedures can be implemented using a combination consisting of SSEC III driving EC and GRFEC probes (Combination) and the Combination may provide higher level of safety in airworthiness for commercial airplanes in deployed around the world. It is the purpose of this work to verify experimentally the possible improvement in Nondestructive Inspection (NDI) quality using the Combination. This is a big effort. The work stated in this paper is just a feasibility study. More work will be done jointly with The Boeing Company, FAA and AANC, Delta Airlines, AMECO- Beijing, Air China, NDT Solutions and RevoNDT Inc. Acknowledgements The Author of this presentation would like to extend his greatest appreciation to following individuals and organizations: Great thanks to Dave Galella, Paul Swindell, Mike Bode, FAA, for the support and encouragement to the activity towards potential higher level of safety in airworthiness and allowing using all the NDT specimens in AANC for the validation. Great thanks to Jeff Kollgaard and Jeff Thompson for encouraging promise in trying our equipment in future Boeing NDT study and re-evaluation of this technique for Boeing s in-service applications. 1 Innovative Materials Testing Technologies, Inc. 2 Ameco Beijing 3 Delta Airlines 4 Air China 1

2 Great thanks to Delta Air Lines, AMECO-Beijing and NDT Solutions who provided a number of Boeing 747 NDT reference standards to support this work. Great thanks to Air China who provided the opportunity to use its Boeing 747 Airplanes for the On-Aircraft Test. Great thanks to NDE Services, RevoNDT, VM Products and EC NDT who provided their equipment for the testing and directly performing test for this work. Scope of Work The work is a systematical study of the techniques included in Combination containing a number of topics including: 1. Verify possibility of using SSEC instruments to drive different EC probes: a. High Frequency Eddy Current (HFEC) pencil probes, in absolute and differential modes, for surface crack detection, b. Mid and Low Frequency Eddy Current (LFEC) ring probes, spot probes and sliding probes, in reflection and differential modes, for subsurface crack detection, c. LFEC probes, in reflection mode, for thick aircraft structure NDI. 2. Verify feasibility of possible improvement in Aircraft Safety Level by driving different GRFEC probes using SSEC instruments for thick aircraft structure NDI: a. GRFEC sliding probe, b. GRFEC manual and motorized rotary probe, and c. GRFEC ring probe. However, the paper will be focused on Topics 1c and 2c for Boeing747 thick structure NDI, because ring probes are considered as the most suitable approach for commercial airplane thick-wall structure NDI. Example Selected for this Presentation Boeing747 NDT Manual Part 6, Eddy Current Mid-spar Lower Chord The photos of the three 2-layer reference standards are shown in Figure 1. A about long and wide through saw-cut is made on the 2 nd layer as defined in the NDT Procedure shown in Figure 2. 2

3 4062C 4062A 4062B Figure 1 Photos of 3 reference standards 2 Jeweler s saw-cut (0.76mm) max width Figure 2 References standards defined in the NDT Procedure A probe is placed on top of the gap for the saw-cut detection. The probe locations are defined in the NDT Procedure as shown in Figure 3. 3

4 Instruments Used Figure 3 Probe positions defined in the NDT procedure The newest versions of SSEC instrument are used in this test. They are SSEC III PC version, which is suitable for working in an indoor environment, and SSEC III Portable version, which is good for working for on-airplane or field environment. Their pictures are shown in Figure 4. SSEC III PC Version SSEC III Portable Version Figure 4 SSEC III Instrument used in the tests SSEC III is an eddy current instrument of general purpose. It is capable of driving GRFEC probes as well as other conventional EC probes. 4

5 Probes Used Two types of probes are used in the test: LFEC probes and GRFEC probes. A large number of different LFEC probes have been tested in the test, but to simplify the paper only a probe made by EC-NDT Inc. is shown here as an example. Figure 5 A LFEC ring probe made by EC-NDT to simulate an NEC probe referred in the NDT procedure probe NEC RR019-1 The GRFEC ring probe used is RF Ring 125/50 as shown in Figure 6. Side view Top view Figure 7 GRFEC Ring Probe RF Ring 125/50 OD = 1.250, ID =

6 Complete system using SSEC PC Complete system using SSEC portable Test #1 Figure 8 Photos of 2 complete test systems Testing 4062 Standards using a LFEC probe and SSEC III per Boeing NDT Procedure Instructions SSEC instrument parameter settings used for Test 1 are shown in Figure 10 blow. Figure 10 SSEC parameter settings for Test 1 Signal rotation angle to have off-center signal horizontal Drive current Determined by software Pre-Gain Software determined Post Gain Common to 3 tests X & Y Grids Can be varied to change signal magnitude shown on screen To start the test first the probe is placed on Position 1, as shown in Figure 9, null the system and start data acquisition for a couple of seconds. The cursor remains at the origin in the impedance plane window. Then stop acquisition, move the probe to position 2 then assune data acquisition for another couple of seconds. The cursor moves to a new location, the signal imaginary of the new location indicates the signal of the saw-cut. 6

7 Position 1 Position 2 Probe Standard 2 nd layer saw-cut location Figure 11 Testing Standards using LFEC Probe and SSEC per Boeing NDT Procedure The signal impedance planes from the three tests are cut out from the system screen and pasted on Figure 12. The signal values copied directly from the system screen are pasted on the three impedance plane as well. 4062A 1 st Layer thickness: 0.50 Signal Change: 0.350V 4062B 1 st Layer thickness: 0.60 Signal Change: 0.274V 4062C 1 st Layer thickness: 0.35 Signal Change: 0.914V Test #2 - Figure 12 Signal impedance planes of Test 1 results copied from system screen Test 4062 Standards using a GRFEC Ring Probe and SSEC III per Boeing NDT Procedure Instructions Similarly to Test 1, but using a GRFEC ring probe to replace the LFEC probe as shown in Figure 13. Positio n 1 Positio n 2 2 nd layer saw-cut location Figure 13 Test #2 - Testing Standards using GRFEC probe and SSEC per Boeing NDT Procedure 7

8 The key parameters used in the tests are: Ref = 100 ma, Pre Gain = 8 db, Post Gain = 32 db, Rot = 50. The signal impedance planes of the test results are shown in Figure A 1 st layer: 0.50 Signal Change: 0.502V 4062B 1 st layer: 0.60 Signal Change: 0.332v 4062C 1 st layer: 0.35 Signal Change: 1.182v Figure 14 Signal impedance planes of Test #2 results copied from system screen The signal-to-1 st layer thickness relations of the two tests are shown in Table 1 below: Table 1 Signal-to-1 st layer thickness relations from for Test #1 and Test #2 EC-NDT GRFEC Difference V 1.182V V.502V V.332V.058 The noises seen in the signal are not significant, especially in Test 2. This is evident for the A-scan signal imaginary strips shown in Figure 15. Test 1 using a LFEC Ring Probe Test 2 using a GRFEC Ring Probe Figure 15 No significant noise is seen in the A-scan strips of signal imaginary for both tests. Observation from Tests 1 & 2 8

9 It is evident from the two tests that: 1. Combination of SSEC-LFEC ring probe is capable of performing NDI for Boeing747 mid-spar lower chord. 2. Combination of SSEC-GRFEC ring probe is also capable of performing NDI for Boeing747 mid-spar lower chord Test #3 Test 4062 Standards using GRFEC Ring Probe and SSEC Exploring Capability for Higher Level of Safety in Airworthiness It is understood why in the Boeing NDT Procedure defines that the 2 nd layer crack/sawcut is detected on top of an air-gap, because when the procedure was wrote, there was not any NDT technique which was capable of penetrate the first Al layer at a fastener location and get the crack detected. It was the only possibility to detect a crack through an air-gap on top of a crack that was below a LFEC probe. Recent development of GRFEC technique has enabled crack detection through thick and multiple layer structure. It is the purpose of Test#3 to experimentally verify this capability. In the test the GRFEC ring probe is first placed on top of a no-crack fastener and centered with the fastener-head, null the system to have the cursor locate right at the origin of the impedance plane. Then, stop data acquisition and move the probe to a fastener under inspection and resume data acquisition. See figure 16. The signal impedance planes of the test results are shown in Figure 17. Position 1 2 nd layer saw-cut locations Position 2 Figure 16 Test #3 Testing Standards using GRFEC probe and SSEC from the top of a fastener The key parameters used in the tests are: Ref = 100 ma, Pre Gain = 26 db, Post Gain = 27 db, Rot = 60 for 4062B or Rot = 55 (for other two standards). The signal-to-1 st layer thickness relations of the three tests are shown in Figure 18. 9

10 0.717v v 0.372v v 1.517v v 0.001v 0.001v 0.015v 4062A 1 st layer: 0.50 Signal Change: 0.716V 4062B 1 st layer: 0.60 Signal Change: 0.371v 4062C 1 st layer: 0.35 Signal Change: 1.502v Figure 17 Signal impedance planes of Test #3 results copied from system screen Signal [V] GRFEC on Gap Top LFEC on Gap Top GRFEC on Fastener Top Thickness of layer above saw-cut Figure 18 Signal-to-1 st layer thickness relations from for all three tests Observation from Tests 3 Combination of SSEC-GRFEC ring probe is also capable of performing NDI for Boeing747 mid-spar lower chord when probe is placed right on top of fasteners. Potential Improvement in Safety Level in Airworthiness and NDI practice by deploying GRFEC & SSEC system How much is the potential improvement is depended on the two issues: 1. Could cracks be detected earlier, before it reaches 1.4 long, by SSEC-GRFEC system? 10

11 The answer depends on how much room is there in SNR of the test results for smaller crack detection? 2. To cover crack detection in fastener holes in the aircraft NDI practice, a large number of LFEC ring probes with different ID and OD must be built to fit the variation in fastener material, fastener diameter, fastener styles (e.g. flush-head or raised-head) and host material and thickness. Does a single GREFC ring probe cover a larger range of fastener sizes and materials? Discussion on Signal to Noise Ratio (SNR) Issue We know Standard Definition of SNR is Where μ is the signal mean or expected value SNR = μ / σ (1) σ is the standard deviation of the noise, or an estimate thereof. The actual S/N definition used by Boeing 5 : SNR = [RMS(X anomaly ) - RMS(X noise )]/ 3σ (2) Where 3σ is a noise level that is greater than the values of 99.7% population of the noise, as shown in noise distribution curve below. Figure 19 Noise distribution curve Signal variations in multiple time measurement for estimation of SNR 14 times measurement on top of 8 fasteners of each specimen including Holes 2, 3, 4, 5, 8, 9, See Figure 20. Holes 1, 6, 7, 12 are excluded because there is visible edge effect. 5 Provided by Paul Rutherford, Phantom Work of The Boeing Company, at Seattle. 11

12 Top View Bottom View Figure 20 Fastener-hole number used for SNR study The 14 times test results and the SNR estimations are shown in Figure 21. The SNR-to- 1 st -layer-thickness relation is shown in Figure 22. Room for better detect-ability From study we can see: 1. Using SSEC-GRFEC to detect saw-cut by placing probe right from top of a fastener ensures high-enough SNR for all the 3 standards. 2. There is no question SSEC-GRFEC can detect flaws smaller than those built in Standards 4062A and 4062C. This is evident by the large values in SNR. 3. SSEC-GRFEC can also detect saw-cuts smaller than 1.40 in Standard 4062B if consider the acceptable SNR is

13 Signal Imagina ry [mv] 14 times Tests on 4062A, the saw-cut below surface SNR =5.74 Saw-cut location Signal Imagina ry [mv] SNR =3.34 Fastener Number 14 times Tests on 4062B, the saw-cut below surface Saw-cut location Signal Imagina ry [mv] SNR =12.51 Fastener Number 14 times Tests on 4062C, the saw-cut below surface Saw-cut location Fastener Number Figure times test results and the SNR estimations for Test #3 combination SNR vs Distance from Surface to Saw- Cut SNR Distance from Surface to Saw-Cut [0.001 Figure 22 SNR to 1 st ] layer thickness relation for Test #3 combination 13

14 On-Aircraft Test on Boeing747 B-2445 in an Air China/AMECO-Beijing Hanger Test Date: April 4, 2011 Figure 24 On-Aircraft Test on Boeing747 B-2445 The system, SSEC instrument and probe, was first calibrated on 4062B reference standard. Then, about 30 fastener locations were checked using the system. The cursors returned to null point for each check meaning the system work fine and there was no crack found among the 30 fastener holes. Probe coverage issue Based on the test experiences we have had until today, the GRFEC ring probe can be used for: 1. Fastener materials: Al, Steel and possibly also for Ti. 2. OD of steel fastener heads can vary from OD of AL fastener heads can vary from Flush head and raised head fasteners Example 1 - NDI of C130 lower wing rainbow structure, OD steel flush-head fasteners and horizontally oriented notches Front Side Back Side 2 nd layer aluminum strip 1 st lay 0.25 steel fasteners long made in these holes Figure 23 Coverage issue Example 1 - NDI of C130 lower wing rainbow structure 14

15 Signal imagin ary [mv] NO NO NO NO SNR = 2.73 for = 4.50 fro Fastener Figure times test results and SNR estimation for C130 lower wing rainbow structure Example 2 - NDI of C structure, OD steel flush-head and raised-head fasteners Top View A1 A2 Bottom View B1 B long long Figure 24 C test specimen 15

16 0.464v C flush head fastener v v v C Raised head Fastener Figure 24 Signal impedance planes and A-scan strip charts of testing C specimen copied from system screen Example 3 NDI of a layers standard simulating Boeing737 lap-joint structure This is a thin structure. Thickness of this specimen is There two types of small fastener with and head diameters, respectively. Photo of the specimen is shown in Figure 25. Front View Back View U1 U2 U3 U4 U5 U6 U7 U7 U6 U5 U4 U3 U2 U L1 L2 L3 L4 L5 L6 L7 L7 L6 L5 L4 L3 L2 L1 Figure 25 Photo of the layers standard with numbered fasteners 16

17 NDT fastener NDT fastener Figure 26 Signal impedance planes and A-scan strip charts of testing the specimen copied from system screen Observations 1. SSEC-GRFEC ring probe combination placing probe on top of a fastener provides wider coverage range on fastener material, OD and flush/raised head. 2. More work is needed to determine the range and verify each of the applications considering the SNR issue. Summary and future work (1) 1. Both combination of SSEC - LFEC ring probe and combination of SSEC III- GRFEC ring probe are capable of performing NDI for Boeing747 mid-spar lower chord per current Boeing NDT procedure. 2. Combination of SSEC-GRFEC ring probe is also capable of performing NDI for Boeing747 mid-spar lower chord by placing probe right on top of fasteners. 3. Using SSEC-GRFEC ring probe combination to detect saw-cut by placing probe right from top of a fastener ensures high-enough SNR for all the 3 standards and can detect flaws smaller than those built in 4062 Standards. This may provide higher safety level in airworthiness. 4. SSEC-GRFEC ring probe combination provides wider coverage range on fastener material, OD and flush/raised head. 17

18 5. More work is needed to determine the capability in detecting smaller flaws. 6. More work is needed to determine the range and verify each of the applications considering the SNR issue. Summary and future work (2) - What does GRFEC bring me? 1. Provides much better sensitivity and reliability on deeply hidden crack detections. 2. Provides direct external inspection (on top of fastener) even though for thick structure, resulting in much less aircraft disassemblies and tremendous time and cost savings. 3. Since SSEC instrument is backward compatible to existing EC probes, therefore it preserve the previous investments of existing EC probes. It will also compliment and enhance the performance these EC probes. 18