Building Calibration Standard for Remote Field Eddy Current Technique Detecting Deeply Hidden Corrosion in Aircraft Structures

Transcription

1 Building Calibration Standard for Remote Field Eddy Current Technique Detecting Deeply Hidden Corrosion in Aircraft Structures Innovative Materials Testing Yushi Sun Innovative Materials Testing 2501 N. Loop Drive, Suite 1610, Ames IA, Tel Fax Zhongqing You & XXXX Magnetic Analysis Corporation 535 S. 4 th Avenue, Mt. Vernon, NY Tel

2 Outline Introduction Distinguishing Features of RFEC Technique Building Calibration Standards for RFEC Technique Exploiting Fundamental Features of RFEC Technique Location (Remaining Thickness) to Signal Phase Angle Relation Corrosion Depth to Signal Magnitude Relation Corrosion Depth to Signal Y Relation Estimation of Corrosion Shapes Correction Factor in Estimating Small-Sized Corrosion Summary and Future Work

3 Introduction 1. Early detection of hidden corrosion in aircraft becomes big public, manufacturer and government concern for economy and safety. 2. Currently deeply hidden corrosion remains undetectable unless aircraft components are disassembled that is costly and time consuming. 3. A number of new techniques are currently under development. The Remote Field Eddy Current Technique is among the tops.

4 RFEC Phenomenon in Metallic Tubes Indirect energy coupling path Innovative Materials Testing F Excitation coil Phenomenon: Direct energy coupling path Indirect energy coupling path F RF Pick-up coil Signals received by pick-up coils are sensitive to changes in wall thickness, conductivity, and permeability..

5 RFEC Phenomenon in Metallic Tubes Underlying Physics: 1. Direct energy coupling is restricted by EC in the wall. 2. Pick-up coil signal, F RF, is dominated by the energy diffusing along the indirect coupling path that traverses the wall twice. 3. Changes in the phase of F RF are directly proportional to the thickness of the wall. Innovative Materials Testing

6 RFEC Phenomenon in Metallic Tubes Innovative Materials Testing Typical RFEC signals: double-peak signal Signals Real Component, X Time Imaginary Component, Y

7 2. RFEC Phenomenon In Flat Geometries RFEC Probe Drive Coil Direct Coupling Path Pickup Coil Indirect Coupling Path 1. In the absence of a test part, only direct coupling from the drive coil is detected, i.e., No RFEC signal is present 2. The RFEC probe is designed to minimize signal from direct coupling and focuses on the indirect coupling path

8 Distinguishing Features of RFEC Technique & SSEC System Compared to Commercial ECT: Higher sensitivity: capable of detecting SCC. Deep penetration: capable of detecting deeply hidden corrosion & cracks. Simple to use: similar to a conventional ECT system. Lower power requirement Current drive power is ~ 0.4 [Ampere- Volt] Capable of accommodating alternative, non-coil, types of magnetic sensors Capable of driving multi-phase traveling/rotating magnetic wave probes.

9 Examples from Two RFEC Probes RF-4 mm V.3 Footprint: 0.85 x 2.15 RF-2 mm V.3 Footprint: 0.3 x 0.62

10 RF4mm V. 3 Three Typical Examples In Detecting Deeply Hidden Corrosion It detects (in multi-layer Aluminum Structures) : Innovative Materials Testing Φ 0.75 Spherical Metal Loss with Maximum Depth of that is below Surface. ~ 4.2% thinning chemical thinning below surface (CNDE Specimen). ~ 6.6% thinning chemical thinning below surface (CNDE Specimen). ~ 1.6% thinning.

11 Example 1: 3 Layer 7075 T6 Specimen, Total Thickness = Spherical-Shaped Corrosion on Bottom of 2 th Layer Location RFEC Probe Rf4mm Location #1 F 0.75 #2 F 0.75 Max. Depth = Max. Depth = Location = Location = 0.575

12 Example 1.1 F 0.75 Spherical-Shaped Corrosion Max. Depth = On 2 nd Layer Bottom Side, f=200hz Total Thickness = 0.750, Remaining Thickness = 0.600

13 Example 1.2 F 0.75 Spherical-Shaped Corrosion Max. Depth = On 2 nd Layer Bottom Side, f=200hz Total Thickness = 0.750, Location = 0.575

14 Example 2.1: 5 Layer 2024 T3 Specimen Total Thickness = Corrosion on Bottom of 5 th Layer Location = Innovative Materials Testing RFEC Probe Rf4mm Location = A Scan Direction deep corrosion

15 Corrosion Sample # " chemical thinning on the bottom side CNDE Specimen #15 (0.063 thick) " chemical thinning on the bottom side

16 EXAMPLE th Layer Bottom Side Corrosion, f=200hz Total Thickness = 0.643, Location = 0.603

17 EXAMPLE th Layer Bottom Side Corrosion, f=200hz Total Thickness = 0.643, Location = 0.603

18 Example Layer 2024 T3 Specimen Total Thickness = Corrosion on Bottom of 4 th Layer Location = Innovative Materials Testing RFEC Probe Rf4mm Remaining Depth = deep corrosion

19 EXAMPLE th Layer Bottom Side Corrosion, f=200hz Total Thickness = 0.643, Location = 0.413

20 EXAMPLE th Layer Bottom Side Corrosion, f=200hz Total Thickness = 0.643, Location = 0.413

21 Example 3 3 Layer 2024 T3 Specimen Total Thickness = Corrosion on Bottom of 3 rd Layer Location = Innovative Materials Testing Remaining Depth = RFEC Probe Rf4mm corrosion

22 Corrosion Sample #5 CNDE Specimen #5 (0.063 thick) " chemical thinning on the bottom side

23 EXAMPLE rd Layer Bottom Side Corrosion, f=500hz Total Thickness = 0.373, Location = 0.333

24 RF2mm V. 3 - Two Typical Examples In Detecting Deeply Hidden Corrosion It detects (in multi-layer Aluminum Structures): chemical thinning below surface (CNDE Specimen). ~1.1% thinning. (in multi-layer Titanium Structures) chemical thinning below surface (CNDE Specimen). ~4.0% thinning.

25 Example 4 4 Layer 2024 T3 Specimen Total Thickness = Corrosion on Bottom of 4 th Layer Location = Innovative Materials Testing RFEC Probe Rf4mm corrosion

26 Corrosion Sample #1 CNDE Specimen #5 (0.063 thick) " chemical thinning on the bottom side

27 Detecting a 0.5 x0.5 x0.002" thinning below surface 1.1% Thinning Corrosion Area Direction for Scanning & Probe Orientation

28 Rf2mm Probe Example 5 3 Layer 2024 T3 Specimen Total Thickness = Corrosion on Bottom of 3 rd Layer Location = Innovative Materials Testing deep corrosion

29 Corrosion Sample # " chemical thinning on the bottom side CNDE Specimen #5 (0.063 thick) " chemical thinning on the bottom side

30 EXAMPLE th Layer Bottom Side Corrosion, f=2.0 khz Total Thickness = 0.163, Location = 0.157

31 EXAMPLE th Layer Bottom Side Corrosion, f=2.0 khz Total Thickness = 0.163, Location = 0.157

32 Building Calibration Standards Basic Corrosion Parameters We Need to Find out: Innovative Materials Testing 1. Corrosion Depth, D. 2. Location, L. Or, which layer? Top or Bottom Surface? 3. Size and Approximate Shape. L D

33 Building Calibration Standards Two Major Challenges: Innovative Materials Testing 1. Location & Depth Estimation Knowing signal magnitude isn t enough to tell anything, because it is a function of both Depth and Location. Phase angle is what used in ECT to tell a defect location. However, the relation is not straightforward. How the phase angle of an RFEC signal behaves? 2. Size & Shape Estimation It is known, from RFECT for tube inspection, that RFEC probe gives a two-peaks signal when it passes a single defect and is difficult to be used for defect shape characterization. Does an RFEC Probe for Inspection Flat Geometry Objects (FG RFEC probe) tell defect shape information?

34 Exploiting Fundamental Features of RFEC Technique Several experimental studies have been carried. They show: 1. For Location & Depth Estimation Signal phase angle obtained from a FG RFEC probe is a monotonic function of defect location. 2. For Size & Shape Estimation A FG RFEC probe does give a single-peak signal from defect if the defect is located at certain depth from the inspection surface. Therefore, the shape & size can be estimated from signal image.

35 Location to Signal Phase Relation (1) Innovative Materials Testing Case L = , or Case L = Case L =

36 Remaining Thickness to Signal Phase Relation (2)

37 Corrosion Depth to Signal Imaginary, Y, Relation

38 Suggested Calibration Standard for Location & Depth

39 Corrosion Size & Shape Estimation Innovative Materials Testing A Single-Peak Signal from RF4mm V.3 Opt. 3 Probe F = 200Hz Varying # of layers & Location, D Material Break with air-gap length, G= 0.00

40 Magnitude Corrosion Size & Shape Estimation At L = 0.06, it is a double-peak signal Innovative Materials Testing Imaginary, Y Phase Angle Drive coil passes Pickup coil passes

41 Corrosion Size & Shape Estimation Innovative Materials Testing At L = 0.160, it becomes a single-peak signal Magnitude Imaginary, Y Phase Angle

42 Corrosion Size & Shape Estimation Innovative Materials Testing At L = 0.54, signal peak approaching the defect position Magnitude Imaginary, Y Phase Angle

43 Corrosion Size & Shape Estimation A Single-Peak Signal from A FG RFEC Probe To go deeper we use f = 100Hz & Air-gap Length = 0.15 Varying # of layers & Location, L Material Break with air-gap length, G= 0.15

44 Corrosion Size & Shape Estimation When f = 100 Hz signals when L 0.54 Innovative Materials Testing L = 0.54 L = 0.64 L = 0.73

45 Corrosion Size & Shape Estimation Imaginary Y Innovative Materials Testing For shallow corrosion shape distortion decreases with increase of frequency RF4mm 200 Hz L=0.105 D= *Ymax Contours RF4mm 2 khz L=0.105 D=0.02 Corrosion Edges

46 Corrosion Size & Shape Estimation Shape distortion can be minimized with small-sized probe Rf2mm 2 khz L=0.123 D=0.003 Rf2mm 2 khz L=0.060 D=0.003

47 Corrosion Size & Shape Estimation Innovative Materials Testing About Shape Factor 0.32 RF2mm Probe , L= , L= , L=0.192

48 Corrosion Size & Shape Estimation About Shape Factor RF4mm Probe Innovative Materials Testing , L= , L=0.413

49 Corrosion Size & Shape Estimation About Shape Factor RF4mm Probe Innovative Materials Testing , L= , L=0.603

50 Corrosion Size & Shape Estimation Enhanced Edge in RFEC Signal Due to Designed Probe Structure Innovative Materials Testing F , L = F , L = 0.123

51 Summary and Future Work 1. Corrosion defects can be calibrated based on the following features of RFEC probe signals: a. Monotonic relation of signal phase to defect location, L; b. At a given L imaginary,y value increases with increase of defect depth, D; c. An RFEC signal becomes of single-peak when L is greater a certain value.

52 Summary and Future Work 2. A possible calibration process has been suggested. It consists of three steps: a. Determine location L, or on which layer and top or bottom surface, the defect is located; b. Estimate corrosion depth, D, based on the determined L and signal Y. c. Estimate defect size and shape based on the image of signal Y of a defect.

53 Summary and Future Work 3. The accuracy of size & shape estimation depends on: a. Defect size/probe size ratio; b. Frequency and defect location. 4. The enhanced edge feature of current probes creates error in defect size & shape estimation. Therefore, new probes with less edge enhancement need to be developed. 5. Application of the suggested calibration process to aircraft applications.