- Email: [email protected]
Characterisation and Optimization of in-process Eddy Current Sensor Arrays Using Computed Tomography
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 66 (2017) 243 – 248
1st Cirp Conference on Composite Materials Parts Manufacturing, cirp-ccmpm2017
Characterisation and optimization of in-process eddy current sensor arrays using computed tomography M.Sc. Dietrich Berger a,*, B.Sc. Tobias Will a, B.Sc. Hans-Christoph Töpper a, Prof. Dr.-Ing. Gisela Lanzaa, M.Sc. Dirk Koster b, Prof. Dr.-Ing. Hans-Georg Herrmannb,c a
wbk Institute of Production Science – KIT Karlsruhe Institute of Technology b IZFP Fraunhofer Institute for Nondestructive Testing c LLB Chair for Lightweight Systems – Saarland University
* Corresponding author. Tel.: +49-721-608-44016; fax: +49-721-608-45005. E-mail address: [email protected]
Abstract Due to increasing demands towards serial production of carbon fibre reinforced plastics (CFRP) the method development for process integrated quality control has become inevitable. The preforming step of Resin Transfer M oulding (RTM ) offers the possibility for process integration of eddy current sensor arrays, enabling 100% inspection of produced components. However, the application of this concept needs to take into account that optimal designs of applied sensors may vary depending on tool shape and inspected material. Therefore it is necessary to design and create a set of CFRP material standards with varying characteristics considering geometry and material. The created material standards are inspected via industrial computer tomography by using methods of image processing in order to acquire quantitative information about the structure of inspected material. Afterwards eddy current measurement is conducted with varying sensor properties in order to determine the optimal structure of an eddy current array that can be integrated in preforming tools. The presented paper includes detailed description of the approach focusing on the definition of reference geometries, measurement via computer tomography and eddy current testing. © by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2017 2017Published The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite M aterials Parts M anufacturing. Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing
Keywords: Eddy current, Material standard, CFRP, In-process quality control
1. Motivati on and objective The development of automated production technologies of carbon fibre reinforced plastics (CFRP) has great impact on the dissemination of produced CFRP parts and therefore the positive ecologic impact due to reduced pollutant emission [1]. Furthermore, innovative methods and systems for quality control need to be established in order to guarantee the reliability of co mponents in high volume production. To evaluate the functionality of these systems, calibrated reference parts can be used for the determination of measurement deviation and other specific values that can be considered as performance indicators of the examined quality control system. Resin Transfer Moulding (RTM ) offers the possibility to produce CFRP parts by automated process steps that can be separated into text ile cutting, draping, infiltration, curing and
fin ishing [2]. The draping process however, which is referred to as preforming, is crucial for the final quality of the produced parts [1, 3]. Fibre misalign ments, pleats or other defects can occur due to material or process characteristics and need to be avoided in high volume production [4]. Methods of non-destructive testing, such as eddy current testing, can be applied in this context to detect flaws in semifin ished textiles as could already be shown in various works [5, 6, 7]. The presented approaches are based on scanning methods using eddy current probes and kinematic systems. The common deficit of the presented approaches however is the necessity for additional stations on the shop floor and elongated process times due to time consuming scanning methods. One possibility to avoid these disadvantages but nevertheless benefit fro m the potential of eddy current testing is the static integration of eddy current arrays in the tools of
2212-8271 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing doi:10.1016/j.procir.2017.03.363
244
Dietrich Berger et al. / Procedia CIRP 66 (2017) 243 – 248
the preforming process [4]. These tools however may have complex three -dimensional shapes and there is no scientific knowledge about the effects and interactions between complex geo metries and according sensor designs in the testing of CFRP. Therefore, it is necessary to create a method for the determination of optimal eddy current array specifications for their process integrated application in preforming tools. The method must include the identificat ion and classification of possible tool geometries and the deduction of reference geometries. These reference parts are draped with CFRP text ile and analysed with industrial co mputer tomography, which is considered a reference measurement system. Methods of digital image processing can be used for the characterisation of the reference part. Subsequently, the effects of different eddy current array settings, such as coil tilt, distance or lift-off, can be examined by analysing eddy current signal characteristics as a function of both material and sensor shape. 2. Theoretical principles and state of the art Carbon fibre rein forced plastics (CFRP) have anisotropic electric properties that can be used for eddy current testing [5]. Eddy current testing is based on the generation of timedependent primary magnetic fields that induce eddy currents in CFRP [4]. Eddy currents cause secondary magnetic fields that oppose the change of their primary field. The intensity of this effect varies with the anisotropy of tested material due to the directionality of eddy currents and is used in the testing of CFRP. Various sensor types were already examined in the application on CFRP but however have advantages and drawbacks as a matter o f their functional principles. A qualitative comparison between applied probe types is shown in Tab. 1. [8, 9, 10, 11, 12]
diameters, inductor lengths, numbers of windings and therefore their accessible inductances. The inductance however influences the measurable voltage gain ୧୬ୢ according to (1) and (2) [14]. Ȱ ൌή ୧୬ୢ ൌ െ ή
(1) Ȱ
(2)
It is necessary to evaluate the characteristics of measured signals in order to make a conclusion towards identified fibre orientations. Semi-fin ished carbon fibre text iles can occur as fabrics with varying layer nu mbers and therefore can have intricate electrical properties . This is why the phenomenological description of eddy currents and the characteristics of measureable eddy current signals are challenging when applied to CFRP. Nu merical modelling and simu lation routines help in the determination of mechanical and electrical propert ies of complex shaped CFRP parts , however require a lot of expertise and effo rt in order to obtain realistic results. The systematic variation of probe parameters offers the possibility to determine the effects and interactions when examin ing signal characteristics [15]. Figure 1 shows the parameters of a half transmitting probe that can be adjusted in order to influence the signal characteristics in the determination of fibre orientations .
T able 1: Probe types for CFRP testing SQUID Magnetic sensitivity
GMR/ AMR
Hallprobe
Saturable Core
Inductor Coil
++
++
+
+
++
Number of components
-
--
+
+
++
Complexity
--
--
-
+
++
Size
+
++
-
-
+
Mechanical Robustness
--
--
+
+
++
O ve rall
--
--
+
+
++
Figure 1: Adjustable parameters for half transmitting probes
Inductor coils show a good applicability for CFRP testing according to the evaluation criteria of Tab. 1 and were used to determine fibre orientation, texture characteristics and waviness of CFRP parts in science and industrial applications [13]. The reliable characterisation of layer structures is important in means of quality control for CFRP production, because of the coherence between mentioned criteria and the carrying capacity of CFRP parts. The inductor types used in this context and their d imensions however strongly vary between different applicat ions in coil arrangements,
Extending the amount of receiving coils allo ws the application of eddy current sensor arrays that can be applied without time consuming mechanical movement using temporally changing switch positions to test areal CFRP samples. A basic technological approach was presented in [16] but is limited to flat CFRP parts and requires mechanical movement. The approach presented in this paper aims for the systematic determination of optimal designs for half transmitting probes in order to assemble co mp lex shaped eddy current arrays. These arrays are integrated in forming tools of the preforming process step of RTM in order to control layer structures of manufactured CFRP parts. According to the state of the art in science and industry the design of eddy current probes is strongly based on individual experience and therefore lacks for a method to systematically determine the transmission behaviour between inductors. The results can be
Dietrich Berger et al. / Procedia CIRP 66 (2017) 243 – 248
used to estimate optimal spatial resolutions and according geometric parameters when testing CFRP.
process. The geometry is then extracted fro m the CAD file of the tool geometry.
3. Scientific approach
3.2. Material standards
In order to integrate eddy current array probes in preforming tools, potential target geo metries need to be identified and classified to derive an appropriate empiric method in order to determine possible sensor designs. Afterwards, material standards need to be designed and measured in a reference system such as industrial co mputed tomography. A measurement rig is used thereafter under systematic variat ion of critical probe parameters. Signal characteristics are defined and used as target values for the sensor optimization afterwards.
Based on the geometries that are identified in the previous step, the reference part is manufactured and draped. Reference parts are typically used in machining applicat ions to make sure that machined parts reach desired qualities. These reference parts have similar geo metrical and physical properties compared to the part that is manufactured in the final application. In the context of non-destructive testing, reference parts need to be manufactured and measured with a reference measurement system in order to evaluate the reliability of newly developed systems. In the presented approach a system is required that is capable o f characterising the entire layer structure of draped semi-finished textiles without affecting it in any way. This information is used to compare it to data that is acquired with eddy current data afterwards. Industrial co mputer tomography has proven itself to be a reliable system to fu lfil these requirements. The reference parts presented in this publication consist of a carrier material and the draped carbon fibres. The carrier material is chosen to be Ureol as it has a low density, which is advantageous for the analysis in a CT. Furthermo re Ureo l can be mach ined quite well in order to gain similar geo metrical accuracies as the preformed part itself. [19] has already presented an approach where preformed reference parts are used for the estimat ion of measurement uncertainty of optical measurement systems in the quality control of semi-fin ished CFRP parts. One conclusion that was made is the necessity for additional geometrical co mponents that can be used as reference points for the defin ition of au xiliary planes in CT measurement. The definit ion of these planes is necessary in order to separate fibre layers in definite project ion planes that are used to create images containing informat ion about the inspected part. Fig. 4 illustrates the described relations.
3.1. Regions of interest Preforming can be implemented in various ways with the same intention to drape semi-finished textiles with a near net shape. The methods were described and characterized in [16] and can be seen in Fig. 2.
Figure 2: Preforming types [16] It is evident that the common characteristic in this process is the application of a lo wer tool, which resembles the geometry of the final part. Therefore specific co mponent areas that are quality critical in the final use can be inspected by eddy current testing during preforming itself. These so called regions of interest can appear as even, two dimensional (2D), single curved (2,5D) or co mp lex shaped (3D) geo metries and can be seen in Fig. 3.
Figure 3: Lower tool geometry segments [17] Restrictions that result fro m this segmentation are the available space for the insertion of force inductive elements, possible tilt angles and used carbon fibre materials that need to be respected in order to characterise potential sensor designs. By using static eddy current arrays in specific regions of interest quality critical material defects that were already discussed in [4, 18] can be detected and avoided in the final part. The outcome in the definition o f a reg ion of interest is the geometrical shape and size of the part that shall be tested in
Figure 4: Coordinate system alignment The planes that can be deduced fro m the aligned coordinate systems are used as projection planes in the acquisition of CT data with defined o rientations. Industrially used ruby balls have proven themselves to provide a high contrast compared to Ureol wh ich appears transparent in CT measurement. A 2D reference carrier with a convex angle of 45° and ruby references can be seen in Fig. 5.
245
246
Dietrich Berger et al. / Procedia CIRP 66 (2017) 243 – 248
(7)
ܶ ݕݕൌ ܹ ݕܫ כ ݕܫ כ
These matrices are used in the calculat ion of angles ߠ that represent the preferential orientation of detected edges in the analysed image. [21] ߠൌ Figure 5: Carrier material of an exemplary 2,5D reference material without carbon fibres 3.3. Reference measurement After the carrier material is draped, it is analysed via industrial computer tomography. The main advantage when using this system is the separability of single carbon fibre layers that can be evaluated individually. This offers the possibility to characterise the entire layer structure by using methods of digital image p rocessing. In the first step, manufactured reference materials are measured and an aligned coordinate system is created. It is used to establish planes that are shifted in order to identify single carbon fibre layers, which are separated from each other according to Fig. 6. Based on the coordinate system A* other coordinate systems can be established enabling the segmentation and thus characterisation of complex shaped geometries.
ͳ ʹ
ߨ ʹܶݕݔ ൰ כ൬ ʹ ܶ ݔݔെ ܶݕݕ
(8)
All calculated angles are grouped in order to set up orientation histograms. The analysis of h istograms is based on the identificat ion of local maximu ms h inting at specific fibre orientations. Figure 7 shows the result when combin ing the histograms of layers 1, 2 and 3 according to Figure 6.
Figure 7: Orientation histogram
Figure 6: Layer separation in computer tomography After separating the layers from each other, they are evaluated individually concerning their fibre orientation. After separating layer wise images within the reg ion of interest , gradient filters such as Sobel’s filter, are used to create derivatives . ͳ Ͳ െͳ (3) ݔܫൌ ʹ Ͳ െʹ ൩ ܣ כ ͳ Ͳ െͳ ͳ ݕܫൌ Ͳ െͳ
ʹ Ͳ െʹ
ͳ Ͳ ൩ܣ כ െͳ
(4)
It can be seen that there are t wo significant maximu ms in the illustrated angle spectrum. These maximu ms correlate to the nominal fibre orientations of 0°-90°-0° with a slight deviation. The amount of fibre layers can be determined by comparing the peak values of the maximu ms. It can be seen that the peak value of the first maximu m is approximately twice as h igh as the second maximu m at 89.7°. Th is circu mstance is caused by the twice amount of 0° no minally oriented fibres co mpared to 90°. Furthermore , this method offers the possibility to individually inspect single fib re layers and to extract statistical information about fibre orientation and distributions. The information about the inspected specimen is concentrated in Table 2. T able 2: Fibre orientation characteristics Layer No
The advantage when using Sobel’s filter is its simple implementation using 3x3 matrices without additional parameters. Other grad ient filters that can be applied are Prewitt, Laplacian-of-Gaussian filter or 2D Gaussian derivative filter. These filters were d iscussed in [ 20]. A p ixel wise mult iplication of the determined derivatives and a subsequent mu ltiplication with smoothing filter W deliver smooth derivation images of the originally acquired layers.
Nominal fibre orientation
Mean orientation
Standard deviation
1
0°
0.9°
3.44°
2
90°
89.7°
2.53°
3
0°
0.9°
6.20°
In the context of sensor development, this data can be used in order to interpret sensor characteristics by correlating orientation information according to Tab. 2 with signals that are presented in the following subsection.
ܶ ݔݔൌ ܹ ݔܫ כ ݔܫ כ
(5)
3.4. Eddy current analysis
ܶ ݕݔൌ ܹ ݕܫ כ ݔܫ כ
(6)
In order to expand the state of the art by developing a complex shaped eddy current sensor array it is necessary to
Dietrich Berger et al. / Procedia CIRP 66 (2017) 243 – 248
identify criteria that can be applied for the evaluation of different sensor array designs : x Small coil sizes/varying coil shapes for high spatial resolution x Large coil distances for high angular resolution x High inductivity for accordingly high electro magnetic sensitivity of receiving coils x Removal o f conducting material next to the coil arrangement/electromagnetic shielding When transferring these objectives to a generic eddy current array model it is evident that the described adjustable parameters as seen in Fig. 1 are co mp lemented by further, array specific design properties such as the resulting array size and the angular resolution. Fig. 8 shows the generic model of a 2D eddy current array.
evaluate the signal behaviour experimentally in order to determine the optimal co mpro mise between resulting array size and angular resolutions. In this context, eddy current measurements were conducted with co il specifications as described in Tab le 3. The co il pairs were rotated in 0.1° steps above a dry carbon fibre specimen consisting of a three layer unidirectional clutch fabric with fibre orientations in the order 0°-90°-0°, which corresponds to the nominal orientations in Table 2. The step size was chosen to be very small in order to increase the amount of measurement data and reduce the error potential due to interpolation fau lts. Different frequencies were used in order to make sure that the entire carbon fibre structure is penetrated with eddy currents: 500 kHz, 1.5 M Hz, 5 MHz and 10 M Hz. Fig. 10 shows the results from the eddy current test: (a) shows the results gained with 7 mm d istance between emitting and receiving co il, whereas the results in (b) were acquired with a coil d istance of 15.8 mm. The measurement signal represents the change of receiving coil impedance, real and imaginary parts. (a)
Figure 8: Generic 2D eddy current array model According to the image above, the presented approach aims for the expansion of half transmitting probe designs (see Fig. 1) by arranging multip le sensing coils around one single emitting coil with varying coil shapes . The advantage of varying coil shapes and distances is evident when analysing the resulting array sizes and estimated angular resolutions when arranging receiving coils as close as technically possible next to each other. Figure 9 shows the basic arrangement of two coils that was varied in this investigation.
(b)
Figure 9: Co ils applied in eddy current inspection under varied distances Table 3 shows the effects of different coil distances due to geometric boundaries. T able 3: Effects of sensor design on sensor size and angular resolution Emitting coil
Receiving coil
Cylindrical
Rectangular 1 mm x 10 mm
4 mm
Distance
Resulting array size
Estimated angular resolution
7 mm
44 mm
14°
15.8 mm
61.6 mm
3°
It can be seen that great advantages can be achieved in angular resolutions by increasing the coil distances. However, the restrictions resulting fro m space availability and decreasing electromagnetic coupling cannot be neglected when increasing the coil distances. Therefore it is necessary to
Figure 10: Comparison of measurement data The measurement was conducted at Fraunhofer Institute for Non-Destructive Testing in Saarbrücken. It can be seen that both sensor designs allow a reliab le determination of fibre orientations as it was already shown in other applications: x The lobes that are pointing in 0°/ 180° and 90°/270° directions are a significant indicator for fibre orientations
247
248
Dietrich Berger et al. / Procedia CIRP 66 (2017) 243 – 248
of the tested specimen. In spite of the increased distance between the applied coils, the clearness of the anisotropic signals does not seem to be compromised x The shape of the lobes pointing in 0°/180° direct ion suggest that there are more 0°/ 180° o riented fib res than 90°/270° oriented fibres The differences that can be seen between Fig. 9 (a) and (b) can be traced back to the varied coil distances according to Table 3: x The signal amplitude at individual frequencies intensifies with increasing coil distances x The lobe shapes as well as their size ratios do not change with increasing coil d istance. This means that the shape and lobe size ratios can be used for a reliable determination of fibre orientations in specific layers When transferring the scanning method to a static eddy current array as shown in Fig. 8 it is ev ident that a high angular resolution is required because regions between the visible lobes show low signal amp litudes in the inspected layer structure. After the acquisition of both CT and eddy current data, correlation analysis and other mathemat ical methods can be applied in order to estimate the effects of different sensor shapes on signal behaviour. Th is information will be applied in the design and development of a process integrated eddy current array. 4. Conclusion and outlook The presented paper shows an approach how to design and optimise a static eddy current array for the process integrated quality control during preforming in Resin Transfer Moulding. By identifying a reg ion of interest that shall be object to 100% testing of manufactured parts, reference materials are derived and draped in order to create a high similarity to the produced parts under large volume conditions. Industrial co mputer to mography is applied for the characterisation of reference parts in order to acquire quantitative informat ion about their overall structure with out affecting it. Here addit ional geometrical artefacts are necessary for the applicability of methods of image processing. Afterwards the reference part is measured via eddy current testing and the signal characteristics are analysed in order to derive an eddy current array design with maximu m spatial resolution. The co mparison between CT and eddy current data is necessary in order to evaluate the reliability of eddy current testing. Furthermore the described method of referencing eddy current measurements via co mputer tomography offers the possibility to link eddy current signals to different material structures. By successively increasing coil distances and other geometric properties such as coil t ilt and lift-off, geo metric boundaries can be determined on how two or mo re co ils in an array can be arranged relatively to each other. Further works will aim for the elaboration of an experimental design according to DoE in order to mathematically describe the characteristics of eddy current signals when applying the presented method on a set of various materials, geometries and further sensor shapes.
References [1] Thielmann et al.; Leichtbau BW GmbH; Wertschöpfungspotentiale im Leichtbau und deren Bedeutung für Baden; Württemberg; 2014; http://www.leichtbaubw.de/fileadmin/user_upload/Downloads/RZ_LeichtbauBW_Studie_WS K_Web.pdf; geladen 03.10.16 [2] Hopmann, C. et al, Einführung in die Kunststoffverarbeitung, Hanser, 10/2015 [3] Brabandt, D; Lanza, G; Data Processing for an Inline Measurement of Preforms in the CFRP-Production; 9th CIRP Conference on Intelligent Computation in Manufacturing Engineering – CIRP ICME ’14; Procedia CIRP 33 (2015) 269-274, 2014 [4] Berger, D et al.; Conception of an eddy current in-process quality control for the production of carbon fibre reinforced components in the RTM process chain, 10th CIRP Conference on Intelligent Computation in Manufacturing Engineering, CIRP ICME’16 (accepted for publication) [5] Mook, G. et al..; Electromagnetic imaging using probe arrays, In: 17th World Conference on Nondestructive T esting, October 25th -28th, Shanghai, China [6] Yin, W.; Withers, P. J.; Sharma, U.; Peyton, A.J.: Noncontract Characterization of Carbon-Fiber-Reinforced Plastics Using Multifrequency Eddy Current Sensors, In: IEEE Transactions on Instrumentation and Measurement, Vol. 58, pp. 738-742, Release Year: 2009 [7] Heuer, H. et. al.: Review on quality assurance along the CFRP value chain – Nondestructive testing of fabrics, preforms and CRFP hy HF radio wave techniques, In: Composites Part B 77, pp. 494-501, Release Year: 2015 [8] Yashan, A.; Über die Wirbelstromprüfung und magentische Streuflussprüfung mittels GMR-Sensoren, Dissertation, Universität des Saarlandes, 2008 [9] Hesse, S.: Sensoren für Prozess- und Fabrikautomation, 6., korregierte und verbesserte Auflage, Springer Vieweg Fachmedien Wiesbaden, 2014 [10] Andreeva, E.; Fertigung und Erprobung eines Mikro-Wirbelstromsensors zur Abstandsmessung, Dissertation, 2005 [11] Zielke, R.: Verbesserung der lateralen Auflösung der Wirbelstromprüfung durch Entwicklung magnetooptischer Sensorsysteme, Dissertation, 2004 [12] Sensortechnik, Handbuch für Praxis und Wissenschaft, Hans-Rolf Tränkler, Leonhard M. Reindl Hrsg., 2. Auflage, Springer-Verlag Berlin Heidelberg 2014 [13] Stegemann, D, Der Einsatz von Wirbelströmen für die Zerstörungsfreie Werkstoffprüfung, Published by DGZfP, 2010 [14] Li, X.; Eddy current Techniques for non-destrucitve Testing of Carbon FIbre Reinforced Plastic (CFRP), Dissertation, University of Manchest er, 2012 [15] Mallwitz, R.; Analyse von Wirbelstromsignalen mit problemangepassten Funktionen für die zerstörungsfreie Materialprüfung, Dissertation, Kassel Universität, 2000 [16] Wagner H. Featurebasierte Technologieplanung zum Preforming von textilen Halbzeugen. Dissertation, Karlsruher Institut für Technologie, 2016 [17] http://www.qpoint-composite.de/Produkte/FertigungstechnologiePreforming/445/, Loaded 01.03.2017, 13.30 [18] Berger, D,. et al. „Effects of defects in series production of hybrid CFRP lightweight components “, Measurement Journal, S. 389-394., 2016 [19] Evaluating the uncertainty of inline-measurements in the production of carbon fiber preforms by computed tomography Dipl.-Ing. Daniel Brabandt, Prof. Dr.-Ing. Gisela Lanza Paris CIRP ST C-P, 19/02/2015 [20] Mersmann, C.; Industrialisierende Machine-Vision-Integration im Faserverbundleichtbau, Dissertation, Apprimus, 2012 [21] Schmitt, R., et al.; A method for the automated positioning and alignment of fibre-reinforced plastic structures based on machine vision, CIRP Annals - Manufacturing T echnology, Vol. 57, Nr. 1, S. 501–504, 2008
ScienceDirect Procedia CIRP 66 (2017) 243 – 248
1st Cirp Conference on Composite Materials Parts Manufacturing, cirp-ccmpm2017
Characterisation and optimization of in-process eddy current sensor arrays using computed tomography M.Sc. Dietrich Berger a,*, B.Sc. Tobias Will a, B.Sc. Hans-Christoph Töpper a, Prof. Dr.-Ing. Gisela Lanzaa, M.Sc. Dirk Koster b, Prof. Dr.-Ing. Hans-Georg Herrmannb,c a
wbk Institute of Production Science – KIT Karlsruhe Institute of Technology b IZFP Fraunhofer Institute for Nondestructive Testing c LLB Chair for Lightweight Systems – Saarland University
* Corresponding author. Tel.: +49-721-608-44016; fax: +49-721-608-45005. E-mail address: [email protected]
Abstract Due to increasing demands towards serial production of carbon fibre reinforced plastics (CFRP) the method development for process integrated quality control has become inevitable. The preforming step of Resin Transfer M oulding (RTM ) offers the possibility for process integration of eddy current sensor arrays, enabling 100% inspection of produced components. However, the application of this concept needs to take into account that optimal designs of applied sensors may vary depending on tool shape and inspected material. Therefore it is necessary to design and create a set of CFRP material standards with varying characteristics considering geometry and material. The created material standards are inspected via industrial computer tomography by using methods of image processing in order to acquire quantitative information about the structure of inspected material. Afterwards eddy current measurement is conducted with varying sensor properties in order to determine the optimal structure of an eddy current array that can be integrated in preforming tools. The presented paper includes detailed description of the approach focusing on the definition of reference geometries, measurement via computer tomography and eddy current testing. © by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2017 2017Published The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite M aterials Parts M anufacturing. Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing
Keywords: Eddy current, Material standard, CFRP, In-process quality control
1. Motivati on and objective The development of automated production technologies of carbon fibre reinforced plastics (CFRP) has great impact on the dissemination of produced CFRP parts and therefore the positive ecologic impact due to reduced pollutant emission [1]. Furthermore, innovative methods and systems for quality control need to be established in order to guarantee the reliability of co mponents in high volume production. To evaluate the functionality of these systems, calibrated reference parts can be used for the determination of measurement deviation and other specific values that can be considered as performance indicators of the examined quality control system. Resin Transfer Moulding (RTM ) offers the possibility to produce CFRP parts by automated process steps that can be separated into text ile cutting, draping, infiltration, curing and
fin ishing [2]. The draping process however, which is referred to as preforming, is crucial for the final quality of the produced parts [1, 3]. Fibre misalign ments, pleats or other defects can occur due to material or process characteristics and need to be avoided in high volume production [4]. Methods of non-destructive testing, such as eddy current testing, can be applied in this context to detect flaws in semifin ished textiles as could already be shown in various works [5, 6, 7]. The presented approaches are based on scanning methods using eddy current probes and kinematic systems. The common deficit of the presented approaches however is the necessity for additional stations on the shop floor and elongated process times due to time consuming scanning methods. One possibility to avoid these disadvantages but nevertheless benefit fro m the potential of eddy current testing is the static integration of eddy current arrays in the tools of
2212-8271 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 1st Cirp Conference on Composite Materials Parts Manufacturing doi:10.1016/j.procir.2017.03.363
244
Dietrich Berger et al. / Procedia CIRP 66 (2017) 243 – 248
the preforming process [4]. These tools however may have complex three -dimensional shapes and there is no scientific knowledge about the effects and interactions between complex geo metries and according sensor designs in the testing of CFRP. Therefore, it is necessary to create a method for the determination of optimal eddy current array specifications for their process integrated application in preforming tools. The method must include the identificat ion and classification of possible tool geometries and the deduction of reference geometries. These reference parts are draped with CFRP text ile and analysed with industrial co mputer tomography, which is considered a reference measurement system. Methods of digital image processing can be used for the characterisation of the reference part. Subsequently, the effects of different eddy current array settings, such as coil tilt, distance or lift-off, can be examined by analysing eddy current signal characteristics as a function of both material and sensor shape. 2. Theoretical principles and state of the art Carbon fibre rein forced plastics (CFRP) have anisotropic electric properties that can be used for eddy current testing [5]. Eddy current testing is based on the generation of timedependent primary magnetic fields that induce eddy currents in CFRP [4]. Eddy currents cause secondary magnetic fields that oppose the change of their primary field. The intensity of this effect varies with the anisotropy of tested material due to the directionality of eddy currents and is used in the testing of CFRP. Various sensor types were already examined in the application on CFRP but however have advantages and drawbacks as a matter o f their functional principles. A qualitative comparison between applied probe types is shown in Tab. 1. [8, 9, 10, 11, 12]
diameters, inductor lengths, numbers of windings and therefore their accessible inductances. The inductance however influences the measurable voltage gain ୧୬ୢ according to (1) and (2) [14]. Ȱ ൌή ୧୬ୢ ൌ െ ή
(1) Ȱ
(2)
It is necessary to evaluate the characteristics of measured signals in order to make a conclusion towards identified fibre orientations. Semi-fin ished carbon fibre text iles can occur as fabrics with varying layer nu mbers and therefore can have intricate electrical properties . This is why the phenomenological description of eddy currents and the characteristics of measureable eddy current signals are challenging when applied to CFRP. Nu merical modelling and simu lation routines help in the determination of mechanical and electrical propert ies of complex shaped CFRP parts , however require a lot of expertise and effo rt in order to obtain realistic results. The systematic variation of probe parameters offers the possibility to determine the effects and interactions when examin ing signal characteristics [15]. Figure 1 shows the parameters of a half transmitting probe that can be adjusted in order to influence the signal characteristics in the determination of fibre orientations .
T able 1: Probe types for CFRP testing SQUID Magnetic sensitivity
GMR/ AMR
Hallprobe
Saturable Core
Inductor Coil
++
++
+
+
++
Number of components
-
--
+
+
++
Complexity
--
--
-
+
++
Size
+
++
-
-
+
Mechanical Robustness
--
--
+
+
++
O ve rall
--
--
+
+
++
Figure 1: Adjustable parameters for half transmitting probes
Inductor coils show a good applicability for CFRP testing according to the evaluation criteria of Tab. 1 and were used to determine fibre orientation, texture characteristics and waviness of CFRP parts in science and industrial applications [13]. The reliable characterisation of layer structures is important in means of quality control for CFRP production, because of the coherence between mentioned criteria and the carrying capacity of CFRP parts. The inductor types used in this context and their d imensions however strongly vary between different applicat ions in coil arrangements,
Extending the amount of receiving coils allo ws the application of eddy current sensor arrays that can be applied without time consuming mechanical movement using temporally changing switch positions to test areal CFRP samples. A basic technological approach was presented in [16] but is limited to flat CFRP parts and requires mechanical movement. The approach presented in this paper aims for the systematic determination of optimal designs for half transmitting probes in order to assemble co mp lex shaped eddy current arrays. These arrays are integrated in forming tools of the preforming process step of RTM in order to control layer structures of manufactured CFRP parts. According to the state of the art in science and industry the design of eddy current probes is strongly based on individual experience and therefore lacks for a method to systematically determine the transmission behaviour between inductors. The results can be
Dietrich Berger et al. / Procedia CIRP 66 (2017) 243 – 248
used to estimate optimal spatial resolutions and according geometric parameters when testing CFRP.
process. The geometry is then extracted fro m the CAD file of the tool geometry.
3. Scientific approach
3.2. Material standards
In order to integrate eddy current array probes in preforming tools, potential target geo metries need to be identified and classified to derive an appropriate empiric method in order to determine possible sensor designs. Afterwards, material standards need to be designed and measured in a reference system such as industrial co mputed tomography. A measurement rig is used thereafter under systematic variat ion of critical probe parameters. Signal characteristics are defined and used as target values for the sensor optimization afterwards.
Based on the geometries that are identified in the previous step, the reference part is manufactured and draped. Reference parts are typically used in machining applicat ions to make sure that machined parts reach desired qualities. These reference parts have similar geo metrical and physical properties compared to the part that is manufactured in the final application. In the context of non-destructive testing, reference parts need to be manufactured and measured with a reference measurement system in order to evaluate the reliability of newly developed systems. In the presented approach a system is required that is capable o f characterising the entire layer structure of draped semi-finished textiles without affecting it in any way. This information is used to compare it to data that is acquired with eddy current data afterwards. Industrial co mputer tomography has proven itself to be a reliable system to fu lfil these requirements. The reference parts presented in this publication consist of a carrier material and the draped carbon fibres. The carrier material is chosen to be Ureol as it has a low density, which is advantageous for the analysis in a CT. Furthermo re Ureo l can be mach ined quite well in order to gain similar geo metrical accuracies as the preformed part itself. [19] has already presented an approach where preformed reference parts are used for the estimat ion of measurement uncertainty of optical measurement systems in the quality control of semi-fin ished CFRP parts. One conclusion that was made is the necessity for additional geometrical co mponents that can be used as reference points for the defin ition of au xiliary planes in CT measurement. The definit ion of these planes is necessary in order to separate fibre layers in definite project ion planes that are used to create images containing informat ion about the inspected part. Fig. 4 illustrates the described relations.
3.1. Regions of interest Preforming can be implemented in various ways with the same intention to drape semi-finished textiles with a near net shape. The methods were described and characterized in [16] and can be seen in Fig. 2.
Figure 2: Preforming types [16] It is evident that the common characteristic in this process is the application of a lo wer tool, which resembles the geometry of the final part. Therefore specific co mponent areas that are quality critical in the final use can be inspected by eddy current testing during preforming itself. These so called regions of interest can appear as even, two dimensional (2D), single curved (2,5D) or co mp lex shaped (3D) geo metries and can be seen in Fig. 3.
Figure 3: Lower tool geometry segments [17] Restrictions that result fro m this segmentation are the available space for the insertion of force inductive elements, possible tilt angles and used carbon fibre materials that need to be respected in order to characterise potential sensor designs. By using static eddy current arrays in specific regions of interest quality critical material defects that were already discussed in [4, 18] can be detected and avoided in the final part. The outcome in the definition o f a reg ion of interest is the geometrical shape and size of the part that shall be tested in
Figure 4: Coordinate system alignment The planes that can be deduced fro m the aligned coordinate systems are used as projection planes in the acquisition of CT data with defined o rientations. Industrially used ruby balls have proven themselves to provide a high contrast compared to Ureol wh ich appears transparent in CT measurement. A 2D reference carrier with a convex angle of 45° and ruby references can be seen in Fig. 5.
245
246
Dietrich Berger et al. / Procedia CIRP 66 (2017) 243 – 248
(7)
ܶ ݕݕൌ ܹ ݕܫ כ ݕܫ כ
These matrices are used in the calculat ion of angles ߠ that represent the preferential orientation of detected edges in the analysed image. [21] ߠൌ Figure 5: Carrier material of an exemplary 2,5D reference material without carbon fibres 3.3. Reference measurement After the carrier material is draped, it is analysed via industrial computer tomography. The main advantage when using this system is the separability of single carbon fibre layers that can be evaluated individually. This offers the possibility to characterise the entire layer structure by using methods of digital image p rocessing. In the first step, manufactured reference materials are measured and an aligned coordinate system is created. It is used to establish planes that are shifted in order to identify single carbon fibre layers, which are separated from each other according to Fig. 6. Based on the coordinate system A* other coordinate systems can be established enabling the segmentation and thus characterisation of complex shaped geometries.
ͳ ʹ
ߨ ʹܶݕݔ ൰ כ൬ ʹ ܶ ݔݔെ ܶݕݕ
(8)
All calculated angles are grouped in order to set up orientation histograms. The analysis of h istograms is based on the identificat ion of local maximu ms h inting at specific fibre orientations. Figure 7 shows the result when combin ing the histograms of layers 1, 2 and 3 according to Figure 6.
Figure 7: Orientation histogram
Figure 6: Layer separation in computer tomography After separating the layers from each other, they are evaluated individually concerning their fibre orientation. After separating layer wise images within the reg ion of interest , gradient filters such as Sobel’s filter, are used to create derivatives . ͳ Ͳ െͳ (3) ݔܫൌ ʹ Ͳ െʹ ൩ ܣ כ ͳ Ͳ െͳ ͳ ݕܫൌ Ͳ െͳ
ʹ Ͳ െʹ
ͳ Ͳ ൩ܣ כ െͳ
(4)
It can be seen that there are t wo significant maximu ms in the illustrated angle spectrum. These maximu ms correlate to the nominal fibre orientations of 0°-90°-0° with a slight deviation. The amount of fibre layers can be determined by comparing the peak values of the maximu ms. It can be seen that the peak value of the first maximu m is approximately twice as h igh as the second maximu m at 89.7°. Th is circu mstance is caused by the twice amount of 0° no minally oriented fibres co mpared to 90°. Furthermore , this method offers the possibility to individually inspect single fib re layers and to extract statistical information about fibre orientation and distributions. The information about the inspected specimen is concentrated in Table 2. T able 2: Fibre orientation characteristics Layer No
The advantage when using Sobel’s filter is its simple implementation using 3x3 matrices without additional parameters. Other grad ient filters that can be applied are Prewitt, Laplacian-of-Gaussian filter or 2D Gaussian derivative filter. These filters were d iscussed in [ 20]. A p ixel wise mult iplication of the determined derivatives and a subsequent mu ltiplication with smoothing filter W deliver smooth derivation images of the originally acquired layers.
Nominal fibre orientation
Mean orientation
Standard deviation
1
0°
0.9°
3.44°
2
90°
89.7°
2.53°
3
0°
0.9°
6.20°
In the context of sensor development, this data can be used in order to interpret sensor characteristics by correlating orientation information according to Tab. 2 with signals that are presented in the following subsection.
ܶ ݔݔൌ ܹ ݔܫ כ ݔܫ כ
(5)
3.4. Eddy current analysis
ܶ ݕݔൌ ܹ ݕܫ כ ݔܫ כ
(6)
In order to expand the state of the art by developing a complex shaped eddy current sensor array it is necessary to
Dietrich Berger et al. / Procedia CIRP 66 (2017) 243 – 248
identify criteria that can be applied for the evaluation of different sensor array designs : x Small coil sizes/varying coil shapes for high spatial resolution x Large coil distances for high angular resolution x High inductivity for accordingly high electro magnetic sensitivity of receiving coils x Removal o f conducting material next to the coil arrangement/electromagnetic shielding When transferring these objectives to a generic eddy current array model it is evident that the described adjustable parameters as seen in Fig. 1 are co mp lemented by further, array specific design properties such as the resulting array size and the angular resolution. Fig. 8 shows the generic model of a 2D eddy current array.
evaluate the signal behaviour experimentally in order to determine the optimal co mpro mise between resulting array size and angular resolutions. In this context, eddy current measurements were conducted with co il specifications as described in Tab le 3. The co il pairs were rotated in 0.1° steps above a dry carbon fibre specimen consisting of a three layer unidirectional clutch fabric with fibre orientations in the order 0°-90°-0°, which corresponds to the nominal orientations in Table 2. The step size was chosen to be very small in order to increase the amount of measurement data and reduce the error potential due to interpolation fau lts. Different frequencies were used in order to make sure that the entire carbon fibre structure is penetrated with eddy currents: 500 kHz, 1.5 M Hz, 5 MHz and 10 M Hz. Fig. 10 shows the results from the eddy current test: (a) shows the results gained with 7 mm d istance between emitting and receiving co il, whereas the results in (b) were acquired with a coil d istance of 15.8 mm. The measurement signal represents the change of receiving coil impedance, real and imaginary parts. (a)
Figure 8: Generic 2D eddy current array model According to the image above, the presented approach aims for the expansion of half transmitting probe designs (see Fig. 1) by arranging multip le sensing coils around one single emitting coil with varying coil shapes . The advantage of varying coil shapes and distances is evident when analysing the resulting array sizes and estimated angular resolutions when arranging receiving coils as close as technically possible next to each other. Figure 9 shows the basic arrangement of two coils that was varied in this investigation.
(b)
Figure 9: Co ils applied in eddy current inspection under varied distances Table 3 shows the effects of different coil distances due to geometric boundaries. T able 3: Effects of sensor design on sensor size and angular resolution Emitting coil
Receiving coil
Cylindrical
Rectangular 1 mm x 10 mm
4 mm
Distance
Resulting array size
Estimated angular resolution
7 mm
44 mm
14°
15.8 mm
61.6 mm
3°
It can be seen that great advantages can be achieved in angular resolutions by increasing the coil distances. However, the restrictions resulting fro m space availability and decreasing electromagnetic coupling cannot be neglected when increasing the coil distances. Therefore it is necessary to
Figure 10: Comparison of measurement data The measurement was conducted at Fraunhofer Institute for Non-Destructive Testing in Saarbrücken. It can be seen that both sensor designs allow a reliab le determination of fibre orientations as it was already shown in other applications: x The lobes that are pointing in 0°/ 180° and 90°/270° directions are a significant indicator for fibre orientations
247
248
Dietrich Berger et al. / Procedia CIRP 66 (2017) 243 – 248
of the tested specimen. In spite of the increased distance between the applied coils, the clearness of the anisotropic signals does not seem to be compromised x The shape of the lobes pointing in 0°/180° direct ion suggest that there are more 0°/ 180° o riented fib res than 90°/270° oriented fibres The differences that can be seen between Fig. 9 (a) and (b) can be traced back to the varied coil distances according to Table 3: x The signal amplitude at individual frequencies intensifies with increasing coil distances x The lobe shapes as well as their size ratios do not change with increasing coil d istance. This means that the shape and lobe size ratios can be used for a reliable determination of fibre orientations in specific layers When transferring the scanning method to a static eddy current array as shown in Fig. 8 it is ev ident that a high angular resolution is required because regions between the visible lobes show low signal amp litudes in the inspected layer structure. After the acquisition of both CT and eddy current data, correlation analysis and other mathemat ical methods can be applied in order to estimate the effects of different sensor shapes on signal behaviour. Th is information will be applied in the design and development of a process integrated eddy current array. 4. Conclusion and outlook The presented paper shows an approach how to design and optimise a static eddy current array for the process integrated quality control during preforming in Resin Transfer Moulding. By identifying a reg ion of interest that shall be object to 100% testing of manufactured parts, reference materials are derived and draped in order to create a high similarity to the produced parts under large volume conditions. Industrial co mputer to mography is applied for the characterisation of reference parts in order to acquire quantitative informat ion about their overall structure with out affecting it. Here addit ional geometrical artefacts are necessary for the applicability of methods of image processing. Afterwards the reference part is measured via eddy current testing and the signal characteristics are analysed in order to derive an eddy current array design with maximu m spatial resolution. The co mparison between CT and eddy current data is necessary in order to evaluate the reliability of eddy current testing. Furthermore the described method of referencing eddy current measurements via co mputer tomography offers the possibility to link eddy current signals to different material structures. By successively increasing coil distances and other geometric properties such as coil t ilt and lift-off, geo metric boundaries can be determined on how two or mo re co ils in an array can be arranged relatively to each other. Further works will aim for the elaboration of an experimental design according to DoE in order to mathematically describe the characteristics of eddy current signals when applying the presented method on a set of various materials, geometries and further sensor shapes.
References [1] Thielmann et al.; Leichtbau BW GmbH; Wertschöpfungspotentiale im Leichtbau und deren Bedeutung für Baden; Württemberg; 2014; http://www.leichtbaubw.de/fileadmin/user_upload/Downloads/RZ_LeichtbauBW_Studie_WS K_Web.pdf; geladen 03.10.16 [2] Hopmann, C. et al, Einführung in die Kunststoffverarbeitung, Hanser, 10/2015 [3] Brabandt, D; Lanza, G; Data Processing for an Inline Measurement of Preforms in the CFRP-Production; 9th CIRP Conference on Intelligent Computation in Manufacturing Engineering – CIRP ICME ’14; Procedia CIRP 33 (2015) 269-274, 2014 [4] Berger, D et al.; Conception of an eddy current in-process quality control for the production of carbon fibre reinforced components in the RTM process chain, 10th CIRP Conference on Intelligent Computation in Manufacturing Engineering, CIRP ICME’16 (accepted for publication) [5] Mook, G. et al..; Electromagnetic imaging using probe arrays, In: 17th World Conference on Nondestructive T esting, October 25th -28th, Shanghai, China [6] Yin, W.; Withers, P. J.; Sharma, U.; Peyton, A.J.: Noncontract Characterization of Carbon-Fiber-Reinforced Plastics Using Multifrequency Eddy Current Sensors, In: IEEE Transactions on Instrumentation and Measurement, Vol. 58, pp. 738-742, Release Year: 2009 [7] Heuer, H. et. al.: Review on quality assurance along the CFRP value chain – Nondestructive testing of fabrics, preforms and CRFP hy HF radio wave techniques, In: Composites Part B 77, pp. 494-501, Release Year: 2015 [8] Yashan, A.; Über die Wirbelstromprüfung und magentische Streuflussprüfung mittels GMR-Sensoren, Dissertation, Universität des Saarlandes, 2008 [9] Hesse, S.: Sensoren für Prozess- und Fabrikautomation, 6., korregierte und verbesserte Auflage, Springer Vieweg Fachmedien Wiesbaden, 2014 [10] Andreeva, E.; Fertigung und Erprobung eines Mikro-Wirbelstromsensors zur Abstandsmessung, Dissertation, 2005 [11] Zielke, R.: Verbesserung der lateralen Auflösung der Wirbelstromprüfung durch Entwicklung magnetooptischer Sensorsysteme, Dissertation, 2004 [12] Sensortechnik, Handbuch für Praxis und Wissenschaft, Hans-Rolf Tränkler, Leonhard M. Reindl Hrsg., 2. Auflage, Springer-Verlag Berlin Heidelberg 2014 [13] Stegemann, D, Der Einsatz von Wirbelströmen für die Zerstörungsfreie Werkstoffprüfung, Published by DGZfP, 2010 [14] Li, X.; Eddy current Techniques for non-destrucitve Testing of Carbon FIbre Reinforced Plastic (CFRP), Dissertation, University of Manchest er, 2012 [15] Mallwitz, R.; Analyse von Wirbelstromsignalen mit problemangepassten Funktionen für die zerstörungsfreie Materialprüfung, Dissertation, Kassel Universität, 2000 [16] Wagner H. Featurebasierte Technologieplanung zum Preforming von textilen Halbzeugen. Dissertation, Karlsruher Institut für Technologie, 2016 [17] http://www.qpoint-composite.de/Produkte/FertigungstechnologiePreforming/445/, Loaded 01.03.2017, 13.30 [18] Berger, D,. et al. „Effects of defects in series production of hybrid CFRP lightweight components “, Measurement Journal, S. 389-394., 2016 [19] Evaluating the uncertainty of inline-measurements in the production of carbon fiber preforms by computed tomography Dipl.-Ing. Daniel Brabandt, Prof. Dr.-Ing. Gisela Lanza Paris CIRP ST C-P, 19/02/2015 [20] Mersmann, C.; Industrialisierende Machine-Vision-Integration im Faserverbundleichtbau, Dissertation, Apprimus, 2012 [21] Schmitt, R., et al.; A method for the automated positioning and alignment of fibre-reinforced plastic structures based on machine vision, CIRP Annals - Manufacturing T echnology, Vol. 57, Nr. 1, S. 501–504, 2008