High-resolution eddy current sensor system

High-resolution eddy current sensor system

Martin H. Schulzea, Henning Heuera, Martin Küttnera a Fraunhofer Institute for Non-Destructive Testing, Maria-Reiche-Str. 2, D-01109 Dresden ABSTRACT Carbon fiber materials become more and more important for many applications. Unlike to metal, the technological parameters and certificated quality control mechanisms have not been developed yet. There is no efficient and reliable testing system for an inline inspection and a consecutively manual inspection of the Raw Carbon Fiber materials (RCF) and the post laminated Carbon Fiber Reinforced Plastics (CFRP). Based upon the multi-frequency Eddy Current device developed at Fraunhofer IZFP structural and hidden defects such as missing carbon fiber bundles, lanes, suspensions, fringes, missing sewing threads and angle errors can be detected. Due to the help of an optimized sensor array and an intelligent image pre-processing algorithm the complex impedance signal can be allocated to different carbon fiber layers. This technique enables the possibility to detect defects in the depth up to 5 layers including the option of free scale resolution and testing frequency. Appropriate parameter lists for an optimal error classification are available. The dimensions of the smallest detectable defects are in the range of a few millimeters. A prototype of a special single sensor and an eddy-current sensor array are developed and establish the way to transfer the prototype into an industrial application. Keywords: Eddy Current, Carbon Fiber, RCF, CFRP, Fraunhofer, IZFP

1. INTRODUCTION

-

Carbon fiber materials become more and more important in many applications. Unlike to metal, the technological parameters and certificated quality control mechanisms have not been developed yet. There is no efficient and reliable testing system for an inline inspection and a consecutively manual inspection of the Raw Carbon Fiber materials (RCF) and the post laminated Carbon Fiber Reinforced Plastics (CFRP) available at the moment. Hidden defects during the production may cause far-reaching consequences with high-risk about material fatigue and even higher costs due to scrap coming out after machining. Typical defects are missing bundles, lanes by overlaying or fringes. Actually for mono or biaxial materials (two layers), surface inspection is realized by manual inspection or by an Automated Optical Inspection (AOI). However, some products require more than two layered materials, e.g. three-directional or up to five-directional material will be used for future applications. In order to guarantee the highest possible level of quality and reliability (aircraft or space application) new methods for the inspection of hidden RCF or CFRP layers are requested.

Fig. 1 Typical defects in RCF and CFRP materials

Carbon fiber based materials show a lightly electrical conductivity, but sufficient to measure deviations in the material by using eddy current methods. Eddy current (EC) measuring is an approved method for characterization of surfaces (crack detection) or material differentiations, caused by conductivity or permeability variations. To acquire information for process control or quality assurance the method needs to be robust, non-destructive and economically reasonable. The data acquisition methods, analyzing algorithms and techniques for image pre-processing are described in the next chapters. Smart Sensors, Actuators, and MEMS IV, edited by Ulrich Schmid, Carles Cané, Herbert Shea Proc. of SPIE Vol. 7362, 73621A · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.821443

Proc. of SPIE Vol. 7362 73621A-1

Eddy currents are created through electromagnetic induction by an excitation coil. When alternating current is applied to the coil, a magnetic field develops in and around the coil. This magnetic field expands as the alternating current rises to maximum and collapses as the current is reduced to zero. If another electrical conductor is brought into close proximity to this changing magnetic field, current will be induced in the second conductor (e.g. RCF or CFRP). Eddy currents are induced electrical currents that flow in a circular path through the specimen which caused a new magnetic field reversely to the excitation field. The excited field can be measured with a second, receiving coil.

2. EXPERIMENT Due to the upcoming demand of testing methods for raw carbon fiber materials, an experimental setup for basic investigation and feasibility studies was developed. This experimental device allows the acquisition of complex eddy current signals in amplitude and phase shift as an A-, B- or C-Scan by moving the sensor over the sample. Therefore a high precision manipulator table with an external resolver was modified to scan non-destructively over the sensitive surface to realize the needed high resolution of such low conductivity samples. In scanning direction (X) the manipulator has a minimum step width of 78.125µm, in Y-direction analogy a minimum step width of 12.5µm. Therefore this manipulator can be used as a microscope for RCF or CFRP materials.

Fig. 2 Experimental setup at IZFP Dresden

Description

Value

Unit

scanning area

500 x 500

mm

maximum scan speed

60

mm / s

usable sensor types

single

sensor

16 channel

line array

minimal resolution

78.125 x 12.5

µm

communication

RS232

Table 1: Demonstrator hardware specifications

The sensors were developed and optimized for carbon materials. For testing the WS-98 eddy current inspection hardware from Fraunhofer IZFP Saarbruecken was used. The features are shown in Table 2. This triggered device was combined with a multiplexer and a controlling unit. The electronic is connected to a LabViewTM interface via Ethernet, digital I/O ports and RS232. This setup enables the possibility to scan and measure every kind of plate samples as described in the introduction using an internal developed high resolution sensor.


Feature

Description

Oscillator

f = 10Hz - 10MHz, 4 frequencies (sensors), multiplexed up to 2.5kHz sampling rate

AC Amplifier

10 to 50dB, optional pre-amp

A/D Converter

16 bit resolution, 6.4μs sample time

IIR Analysis Filter

high-pass 1.0Hz to 3.2kHz, low-pass 10Hz to 7.9kHz, attenuation: 80dB per decade

Input

four (4) real-time inputs, eight (8) control inputs, external trigger, external gate, external positioning (up/down, incremental), sync input

Output

real-time multiplexed controller, eight (8) programmable control outputs, sync output

others

FIR decade filter, DC gain, phase shift, offset compensation, 8 bit expansion port Table 2: WS-98 features

The applicator shown in Fig. 3 was constructed under the limitations of variability. This means that different sensors types, such as absolute, differential and compensated sensors at different measurement angles can be used for single sensor probing. The optimum single sensor prototype is a directional property probe with a very high spatial resolution because of its high focusing Point-Spread-Function (PSF). The used sensor is a half transmission type with a lateral pitch of 3.5mm.

Fig. 3 Elastic and rotatable single sensor holder

This sensor principle was utilized to develop a sensor line optimized for carbon materials testing by eddy current with 16 individual sensor pairs to speed up the measuring cycle by parallel measuring.

Fig. 4 Scalable demonstrator line array (left) and new 875µm 2D array (right)

Having a good ratio between hardware complexity, costs and scan speed a new 2D sensor-array with a higher lateral resolution of 875µm is under development (right part of Fig. 4).


3. RESULTS 3.1 Hidden defects in raw carbon fiber materials Defects in multidirectional RCF materials during web proceedings like missing fiber bundles, lanes, fringes or angle errors may cause far-reaching consequences with a high risk of material fatigue of the post processed CFRP. Unidirectional RCF layers have a high anisotropic character in the tension direction. Hence they are webbed into multidirectional layers to reduce or optimize this parameter for the specific application. The following images are acquired with a pitch of 1.5 x 1.5mm. They are representing the results we expect from the new row, which would be finer and with a higher information depth. Sample

Photograph ( front)

Photograph (rear)

Complex EC image front

Complex EC image rear

#1 J.

3 layer RCF with one small lane in the 1st, some missing fiber bundles in the 3rd and in the invisible 2nd layer

#2

1

3 layer RCF with a huge fringe in the 3rd layer and one missing bundle in the invisible 2nd layer

#3

3 layer RCF with 2 missing bundles in the 1st layer Table 3: RCF sample #1 - #3


Sample

Photograph ( front)

Photograph (rear)

Complex EC image front

#4

3 layer RCF with a lane in the 1st and a missing bundle in the 3rd layer

#5

3 layer RCF with 2 missing bundles and 2 lanes in the 1st layer, visible even in rear C-Scan

Complex C-Scan of front layer not available

#6

3 layer RCF with missing bundles in the 2nd invisible layer

#7

*

COMPLEX IMAGE

PHASE IMAGE

High resolution image of marked area in sample #6 (left: complex image; right: phase image) Table 4: RCF sample #4 - #7


Complex EC image rear

To show the functionality of high resolution imaging a so called microscopic eddy current image (sample #7) of sample #6 of the marked area was captured. The size of the square scan area is 50 x 50mm at a resolution of 75µm in both directions. An image with 640 x 640 pixels was the result. The main defect is shown very clearly. By taking the phase information of the complex values into a 2D matrix scaled from -180° to 180° images result like in sample #8 and #9. An optional windowing of these values by a histogram function can separate the errors more efficiently (phase image of sample #7). This result is the basic condition for an easy and even fast digital image preprocessing and an inline inspection process aimed with the scalable 875µm 2D-Array of Figure 4. 3.3 Hidden defects in CFRP Two main defects in the laminated CFRP can be found. First, the described web proceeded faults and second the invisible delaminating between the glue and the fiber layers, which can caused by a abnormal high in-plane or orthogonal pressure of the CFRP. Discontinuities can be located in different layers of RCF and also in the laminated material. Sample

Photograph

Complex C-Scan (EC)

Phase image (EC)

#8

A marcel in the 3rd layer was built in this 5-layer CFRP during production process. 5

-

Uncoated area _ -

#9

coated area

---a

Missing bundles and lanes Metal reference under sample

2 bundles were removed during production. Photo of CFRP picture of production process unavailable Table 5: Hidden defects in CFRP

The C-Scans and phase images represent integral information about the material characteristic. An adapted setup of measuring frequencies allows penetrating the sample down to various depths of the CFRP layers. To check the depth is high enough for inspecting the whole volume, a tinplate (metal reference) was attached under the sample. By increasing the measuring frequency, the conspicuity of this plate in the C-Scan is decreasing due to the skin effect. The second


specified patterns of defects are delaminating effects between the carbon fiber and the glue layer and cracks of the fiber plate trough different layers. In sample #10 too high in-plane compressive forces were inducted into the 12 layer CFRP with the angle settings of 90° / 0° / 120° / 30° / 150° / 60° / 60° / 150° / 30° / 120° / 0° / 90°. Sample

Photograph

Complex C-Scan (EC)

Phase image (EC)

#10

Delamination combined with some inner material cracks. Table 6: Delamination in CFRP

In the complex C-Scan image, the first six layers are visible. If there were defects like in sample #8 or #9 they would become visible. The sensor coil is not optimized for the required low frequencies, which are suitable to detect the deeper defects. That’s why other layers and the real volume extension (visible in the phase image) might not be complete. All C-Scans shown in this paper are results of phase shifts from the acquired complex values. The projected absolute vector length is put into a 2D matrix shown in sample #1 - #11. With this technique it is possible to separate and maximize defects or material parameters or eliminate parasitic effects like impedance changes of measurement cables. Sample

Real image

Imaginary image

#11

Real and imaginary images with parasitic effects, eliminated by phase shift Table 7: Image optimizing with phase shift


Complex image (EC) at 115°

4. CONCLUSIONS The shown acquisition types, some extracts of analyzing methods and optimizing techniques smooth the way for an industrial application. Changes in sensor geometry and frequency range can open new fields of non-destructive testing and C-Scan imaging of micro structural properties for even other worse conductive parts like Nickel-base Superalloys or titanium alloys.

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Gerhardt Mook, Fritz Michel, Jouri Simonin: Electromagnetic imaging using probe arrays; 17th World Conference on Nondestructive Testing, 25-28 Oct 2008, Shanghai, China W. Weber: Zerstörungsfreie Prüfung dickwandiger austenitischer Rohre und Rohrbögen mit fortschrittlicher Wirbelstromtechnik; Dissertation Universität Hannover, Hannover; 2002 A. Yashan, W. Bisle, T. Meier: Inspection of Hidden Defects in Metal-Metal Joints of Aircraft Structures using Eddy Current Technique with GMR Sensor Array. - In: 9th European Conference on NDT: ECNDT Berlin 2006 ; September 25 - 29, 2006 Berlin (2006), Paper Tu.4.4.4 W. Yin, P. J. Withers, U. Sharma, A. J. Peyton: Non-Contact Characterisation of Carbon-Fiber-Reinforced Plastics (CFRP) using multi-frequency Eddy Current Sensors Gerhardt Mook, Juergen Pohl, Fritz Michel: Non-destructive characterisation of smart CFRP structures A. Yashan: Über die Wirbelstromprüfung und magnetische Streuflussprüfung mittels GMR-Sensoren, Dissertation am IZFP Saarbrücken; 2008 X. E. Gros, K. Ogi, K. Takahashi: Eddy Current, Ultrasonic C-Scan and Acoustic Microscopy Testing of Delaminated Quasi-Isotropic CFRP Materials: A Case Study, Journal of Reinforced Plastics and Composites 1998


High-resolution eddy current sensor system

High-resolution eddy current sensor system

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