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Eddy Current Array Inspection of Riveted Joints
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Transportation Research Procedia 43 (2019) 48–56
8th International Conference on Air Transport – INAIR 2019 8th International Conference on Air Transport – INAIR 2019 GLOBAL TRENDS IN AVIATION GLOBAL TRENDS IN AVIATION
Eddy Current Array Inspection of Riveted Joints Eddy Current Array Inspection of Riveted Joints Michal Janovec a,a,*, Martin Bugajaa, Milan Smetanabb Michal Janovec *, Martin Bugaj , Milan Smetana
Department of Air Transport, Faculty of Operation and Economics of Transport and Communications , University od Žilina, Univerzitná 1, 010 26 Žilina, Slovakiaand Communications , University od Žilina, Univerzitná 1, Department of Air Transport, Faculty of Operation and Economics of Transport b Department of Electromagnetic and Biomedical Engineering, Faculty ofSlovakia Electrical Engineering and Information Technology, University od 010 26 Žilina, b Žilina, Univerzitná 1, 010 26 Žilina, Slovakia Department of Electromagnetic and Biomedical Engineering, Faculty of Electrical Engineering and Information Technology, University od Žilina, Univerzitná 1, 010 26 Žilina, Slovakia a a
Abstract Abstract A Non-destructive Testing (NDT) plays an important role in the safety of aircraft components and structures. As this term suggests, A Non-destructive plays an important roleapproaches in the safetytoofexplore aircraftan components and structures. this term suggests, NDT refers to test Testing methods(NDT) that use special devices and object without damaging As or affecting its future NDT refers NDT to testismethods that to use special and approaches to explore an object without damagingoror affectingbut its future usefulness. mainly used detect anddevices characterize hidden defects or damage to critical components structures, it also has applications dimensional and thehidden evaluation oforsome material properties. NDTorisstructures, a very broad and usefulness. NDT isinmainly used to measurements detect and characterize defects damage to critical components but it also has applications area in dimensional measurements and the evaluation of of some material properties. NDTpublic is a security. very broad and interdisciplinary that finds application in many sectors where failure critical parts could jeopardize NDT is interdisciplinary thattool finds application in many sectors where failure of critical parts that could public security. is used as a quality area control during production to ensure materials and finished products dojeopardize not contain internal errors,NDT or that the abnormalities detected areduring belowproduction the size considered potentiallyand dangerous. NDT is also used theinternal operation of objects used as a quality control tool to ensure materials finished products that do notduring contain errors, or that the abnormalities detected or areother below the size consideredand potentially NDT is also used during the of objects to detect cracks, corrosion forms of degradation damage dangerous. that can cause components or systems to operation fail or interfere with to detect cracks, corrosion or other of degradation and of damage that can cause components or systems to fail or interfere with their proper functioning. This paperforms deals with the inspection the riveted joints of the produced riveted specimen, which simulates damage around the riveted joints. Surface and the subsurface damage well as corrosion in the region of riveted joints are simulated. their proper functioning. This paper deals with inspection of theas riveted joints of the produced riveted specimen, which simulates damage around riveted joints. Surface andMX subsurface damage as well corrosion in Eddy the region of array riveted jointsSAB-067-005are simulated. Experiments willthe utilize Olympus OmniScan device with presence ofas ECA module. current probes Experiments will utilize Olympus OmniScan MX device witheddy presence of ECA module. Eddy array probes SAB-067-005032 and SBB-051-150-032 are used, respectively. Harmonic current excitation is used forcurrent this purpose. 032 and SBB-051-150-032 are used, respectively. Harmonic eddy current excitation is used for this purpose. © 2019 The Author(s). Published by Elsevier B.V. © 2019 The Authors. Published by Elsevier B.V. © 2019 The under Author(s). Publishedof bythe Elsevier B.V. Peer-review under responsibility responsibility scientific committee Peer-review of the scientific committee of of the the 8th 8th International International Conference Conference on onAir Air Transport Transport –– INAIR INAIR 2019, 2019, GLOBAL TRENDS TRENDS IN AVIATION AVIATION Peer-review under responsibility of the scientific committee of the 8th International Conference on Air Transport – INAIR 2019, GLOBAL IN GLOBAL TRENDS IN AVIATION Keywords: cracks; defectoscopy; eddy current methods; rivetted joint; subsurface inspection Keywords: cracks; defectoscopy; eddy current methods; rivetted joint; subsurface inspection
* Corresponding author. Tel.: +421/41/513 35 60 E-mail address:author. [email protected] * Corresponding Tel.: +421/41/513 35 60 E-mail address: [email protected] 2352-1465 © 2019 The Author(s). Published by Elsevier B.V. Peer-review responsibility the scientific committee 2352-1465 © under 2019 The Author(s).of Published by Elsevier B.V.of the 8th International Conference on Air Transport – INAIR 2019,
GLOBAL TRENDS IN AVIATION Peer-review under responsibility of the scientific committee of the 8th International Conference on Air Transport – INAIR 2019, GLOBAL TRENDS IN AVIATION 2352-1465 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 8th International Conference on Air Transport – INAIR 2019, GLOBAL TRENDS IN AVIATION 10.1016/j.trpro.2019.12.018
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1. Introduction The eddy current method is a very commonly used technique for non-destructive testing and material evaluation (Bugaj, 2005). The method is very suitable for detecting cracks that result from fatigue or cracking as a result of stress corrosion. Eddy current control can be performed with minimal part preparation and a high degree of sensitivity (Bugaj, 2012). In the eddy current method, a tested object is subjected to an alternating magnetic field, which is generated by an excitation coil powered by alternating current. The tested object of certain dimensions must be electrically conductive, thus having magnetic permeability. In the tested object, eddy currents are indicated, which act by their magnetic effect on the magnetic field, the original ones, thus exciting. The two magnetic fields (primarily from the excitation coil and the reaction from the eddy currents) are vectored in the resulting field, which is dependent on the electrical conductivity and magnetic permeability of the tested object. The principle of evaluation of materials by eddy currents is the structural state of the material changes above mentioned magnetic and electrical parameters. In the event of an error in the material, a portion of the eddy current paths is interrupted when the reverse effect is changed to the excitation field (Bugaj & Rostáš, 2016). The eddy current method is used to detect surface and subsurface defects, corrosion in aircraft structures, openings for fixing various components and cracks on threads (Bugaj, 2018). The eddy current inspection is mainly carried out on the landing gear of the aircraft in the area of the wheel hubs. The eddy current method is also used to check aircraft engine components. The method is only suitable for conductive materials (iron, non-ferrous and austenitic materials) and requires calibration standards and a trained operator. The method is fast and portable, but special probes are needed to test different types of materials (Janovec, 2017). The aim of this work is to apply powerful ECA method to inspect concrete metallic material structures and to evaluate its detection abilities under specific conditions. This means that this paper deals with non-destructive evaluation of the riveted joints by using eddy current array method. Selected experimental results are presented and discussed at this paper. 2. Eddy Current Array Technique The ECA eddy current technology (Eddy Current Array) was used to check the produced riveted joints. The ECA is a method that allows electronic control of coils of eddy currents placed side by side in one probe assembly. Collecting data from individual coils is accomplished by multiplexing the coils in a special pattern to prevent mutual inductance between the individual coils. The ECA passing through the coil generates a magnetic field. When the coil is positioned over the conductive portion, opposite alternating currents (eddy currents) are formed. In the event of a fault, the eddy current path is interrupted and the eddy currents change is recorded by the measurement coil. In the case of ECA probes, each individual coil in the probe generates a signal with respect to the phase and amplitude of the structure below it. These data refer to the encoded position and time and are graphically represented as a C-scan image (top view). C-scan view allows fast orientation on the tested area and good interpretation of the resulting data. In the eddy current coil yarn applications that pass through the damaged riveted joint, they produce a unique signal response. For coils that are affected by a crack that starts from the thread hole, the C-scan display shows the amplitude change. For coils with no change, the color display on the C-scan display remains constant 2.1. Multiplexing of the individual coils Multiplexing is the process by which multiple analog message signals are combined into one digital signal on a shared medium, Fig.1. When eddy current array data is multiplexed, the individual eddy current coils are excited at different times, allowing the system to excite all of the coils in the probe without ever exciting any two adjacent coils at the same time. An undesirable effect known as mutual inductance (magnetic coupling between coils in close proximity) is minimized with the use of an internal multiplexing system to carefully program the exact time that each coil is excited to transmit its eddy current signal (Smetana et al., 2016). The signals are then reassembled before being displayed as an image. In addition to the enhanced imaging capabilities of multiplexed data, multiplexing allows any individual coil (data) channel to be analyzed after inspection (Smetana et al., 2018).
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With the benefits of single-pass coverage, and enhanced imaging capabilities, ECA technology provides a remarkably powerful tool and significant time savings during inspections. ECA technology includes the following advantages: larger area can be scanned in a single-probe pass, while maintaining a high resolution, less need for complex robotics to move the probe; a simple manual scan is often enough; C-scan imaging improves flaw detection and sizing. Complex shapes can be inspected using probes customized to the profile of the part being inspected (Shull, 2002).
Fig. 1. ECA method: the principle of multiplexing between individual elements and 2D raster scanning principle. (Source: Webpage Olympus, 2018).
3. Riveting joints ECA inspection: experimental set-up For the control measurements, several types of riveted joints were created to simulate the defined damage. The riveted joints were made of aluminum non-anodized sheets 200 x 1000 x 1mm. Aluminum solid rivets STN 02 2311 head 4x6 mm were used as fasteners (Figure 2).
Fig. 2. Solid rivet with countersunk head. (Source: https://www.metalcom.cz/sk/nity-klasicke-so-zapustnou-hlavou-plochou-din-661). Table 1 Specification of the rivet used to form the riveted sample. Source: Author. Rivet
Solid rivet with countersunh head
Shank diameter [mm] 4 (+0,1, mm)
Head diameter [mm] 7
Height head [mm]
Material
Shank head [mm]
1
AlMgCu 2,5
6,0 mm (+/0,2 mm)
The riveted joints for inspection measurements were formed by riveting three aluminum sheets, each having a thickness of 1 mm. The resulting thickness of the riveted sample was thus 3 mm. The rivet holes were formed by a column drill machine. A 4 mm diameter drill was used to make the holes. In the sheet metal, where the rivet heads were inserted into the holes, it was necessary to create a chamfer of the hole for the correct insertion of the thread head into the sheet. The riveted joints were created manually, using a pneumatic riveting hammer, in the four double-row regions indicated in Figure 3, under items 1,2,3,4.
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Fig. 3. The riveted sample created in the program Catia V5. (Source: Author).
Artificial damages were created on the riveted sample to simulate the defects of riveted joints. These were formed on the riveted sheets by grinding the notches in the region of the rivet holes. The length of the simulated faults was 2,4,6 and 8 mm and the width was 1 mm. The depth of the cut notches ranged from 0,5 to 1 mm. The depths of the notches in the individual sheets cannot be precisely determined because the notches were made manually by an electric hand micro drill (Figure 4 a). The faults were oriented along the rivet holes or inclined at 45 °.
a)
b)
Fig. 4. a) Notches formed in the area of the holes for rivets 2,4,6 and 8 mm long, b) detail of the formed rivet joint. (Source: Author).
In the area of riveted joints, the defects caused by surface corrosion of aluminum sheet were also simulated. Aluminum powder was glued to the sheet metal, simulating the various corrosion shapes between the riveted sheets. A very fine aluminum powder CAS: 7429-90-5 was used to simulate corrosion. The aluminum powder was adhered to the surface of the sheet by a flexible, solvent-free, FLEXTEC polymer-based gel adhesive.
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Fig. 5. Aluminum powder glued to the surface of aluminum sheet. (Source: Author).
The Olympus OmniScan MX was used to measure riveted joints. This device is a modular and portable test unit. A swirl current module was used in the instrument. The device can be used for manual as well as automated inspection. The number of channels of the device is 32 with internal multiplexing, or 64 channels with external multiplication. The OmniScan connector has a probe ID function that allows for the physical detection and detection of the probe attached. This function is to set the resolution of the C-scan ECA probes and read the correct probe parameters. Appropriate ECA probes SBB-051-150-032 and SAB-067-005-032, which are suitable for such riveted joint inspection, have been used with the test unit Olympus OmniScan MX. The probes specifications are shown in Table 2.
Fig. 6. Olympus OmniScan MX Defectoscope (left) and ECA Probes (right). (Source: Author). Table 2. Measuring probe specifications. (Source: Olympus, 2010) Probe type Probe coverage Probe resolution Frequency range Number of coils
SBB-051-150-032 51 mm 1,6 mm 50 kHz – 500 kHz 32 two-row absolute bridge mode
Operating mode
SAB-067-005-032 67 mm 2,1 mm 1 kHz – 25 kHz 32 receiving in two rows with an orientation of approximately 30 ° from 1 mm up to 3.5 mm
surface inspection
The penetration of eddy currents in aluminum materials
Detection capability
-
2 mm cracks in the first layer of bonded material a surface well with a diameter of 0,5 mm
-
3,8 mm long cracks in the second layer, the first layer can be up to 2 mm thick 5% corrosion at a depth of 2 mm with a diameter of 6 mm 10% corrosion at a depth of 3 mm with a diameter of 6 mm
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4. Methodic of inspection riveted joints Before performing individual measurements, the probes must first be calibrated on a known material sample with artificial defects. The Sample with Artificial Failure includes calibration failures whose dimensions (length, depth) are known. Probe calibration should be performed on as many test specimens as possible. They should be of the same material (or material of the same conductivity) and thickness as the parts to be tested, and if a multilayered part is to be tested, each layer should be of the same material (or the same conductivity) and thickness as the test parts. In addition, they should have real or artificial errors that simulate the desired type, position, and orientation as errors that have to be detected. Prior to measurement, probe reference signals should be identified so that they are compared to the actual measured signals where cracks or corrosion may occur. The calibration process must be performed before each measurement. The ECA method is called comparative method. This means that two signals must be compared: no damage (no cracks or errors), an example of a no-fault signal is shown in Figure 7 and a crack presence signal. The differential of these two signals gives a different response, which is important for signal analysis and evaluation. Based on the comparison of these signals, the conclusion of any measurement can be stated and confirmed. All calibration measurements were performed on calibration samples. Calibration samples means a block sample of the same material as the actual structure with the presence of a crack of defined dimensions. The resulting signals are displayed after excitation and are basic signals to evaluate other unknown signals obtained by measuring the actual structure.
Fig. 7. Recording without material defects (left) and recording with known failure dimensions of 11mm and 0,2mm wide. (Source: Author).
5. Evaluation of measurements of simulated faults and corrosion As a first measurement, a hole measurement was made for a rivet that was not yet filled with rivet. This initial measurement had to be done to improve calibration, defectoscope settings and measuring probes. The measurement also provided more detailed information on the structured riveted sample, which is then checked for other simulated failures. The result measurement of the hole for rivet is shown in Figure 8. The C-scan displays the measurement results in a two-dimensional view as a planar view of the structure being inspected. This view is similar to that of an X-ray image. The color spectrum displayed by C-scan represents the amplitude curve. The individual colors represent the depth of the failure signals when the probe passes over the test surface. This type of display is only possible with the ECA probe. Material surface scan is here faster than the conventional ECT method. At the same time, it is possible to display the waveform of the Gaussian plane (impedance raw) for a given segment of the selected linear path. This display shows the progress of the impedance recorded during measurement. Depending on the shape of the curve and in which quadrant the signal is located, it is possible to distinguish between the edge of the object, the defect, and so
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on. The last display is the amplitude coding of the signal in the time domain. The presence of inhomogeneity is displayed as a signal deflection. The determination of the failure dimensions can be estimated from the C-scan display, in which the cursors can be used to subtract the size of the fault. The resulting dimension of the diameter of the hole for rivet read from the Cscan cursors is: 30,45 mm – 26,25 mm = 4,2 mm. The hole diameter value for the rivet subtracted from the cursors for the C-scan display corresponds to the actual hole for rivet of 4 mm. Another parameter that can be detected from the defectoscope screen is the fault depth, in this case the hole length for the rivet. The fault depth is determined from the waveform signal impedance signal (graph strip raw). The measurement was carried out on a 1 mm thick aluminum sheet. The maximum impedance value was approximately 9V. In the following measurements, it will be known the impedance value used to estimate the depth of the simulated faults that is the impedance comparison value.
Fig. 8. Determining the hole size for the rivet. (Source: Author).
5.1. Defects in the third layered sample In this section of the article, the indication of failure in the third layered sample is evaluated. A 4 mm failure was scanned. The SBB-051-150-032 probe was used to scan the fault. Optimal failure display was obtained with a 45 kHz excitation frequency setting, a gain of 85 dB, no filter or multiplexer was used. The scan result is shown in Figure 9. The failure rate recorded by the defectoscope was 4,8 mm. The impedance signal varied from - 10 V to + 10 V. The probe signal saturation can be seen from the waveform, which is reflected in the maximum permissible waveform amplitude (red) and at the same time the probe signal limitations. It follows that the eddy currents flow around the inhomogeneity is maximized at these circumstances.
Rivet
Fig. 9. Result inspection (left) and inspection method for riveted joint (right). (Source: Author).
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5.2. Simulated corrosion in a third layered riveted sample Aluminum powder was used to simulate corrosion between riveted sheets. The following inspection was aimed at revealing such simulated corrosion. The areas of rivets were inspected and the inspection was aimed at detecting the presence of aluminum powder in the second layered sample. The approximate shape of the glued aluminum powder around the rivets is shown in Figure 10 (right) in red color.
Fig. 10. Aluminum powder glued around the holes for rivets (left) and inspection method (right). (Source: Author).
The control result is shown in Figure 11. A probe SAB-067-005-032 was used for scanning, which is more suitable for detecting corrosion. The best results were obtained by setting the excitation frequency of 10 kHz, gaining 75 dB, without using a filter. While scanning, simulated corrosion with aluminum powder was revealed with very good resolution. The maximum impedance signal value is approximately +9 V and the estimated fault depth based on this impedance value is 1 mm.
Fig. 11. Result of simulated corrosion inspection in second layered riveted sample. (Source: Author).
However, aluminum powder was only trapped in the area of the rivets. In the space between the rivets, the aluminum powder was not captured by the probe (the area indicated by the red arrows). Such result can be made due to the fact that the sheets between the rivets do not lie together and there is an air gap between them. There is no change in eddy currents in this area, and aluminum powder is not revealed. It can be seen from the measurement that the use of a given probe to detect simulated corrosion is very effective. Due to the dimensions of the individual coils forming the probe, it was possible to create a surface map with a relatively good resolution. Simulation of subsurface corrosion is a specific problem and special probes and measurement standards are required for its detection.
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6. Conclusion In today's industry, destructive and non-destructive control techniques have an irreplaceable place to ensure product quality. While destructive techniques are only used for some product samples, since these products are destroyed after test. However, in modern industrial processes, non-destructive techniques are much more advantageous since all testing is performed without permanent changes to the test object. The foundations of this non-destructive technique have been laid down decades ago, the research and development of new probes, techniques and instrumentation is constantly being carried out by manufacturers and research groups around the world. New probes are being developed to meet the increasingly high quality standards required in almost every industry and the use of new superconducting quantum interference devices is being explored. Continuous probe development is also aimed at optimizing crack detection and operating variables such as frequency and signal ratio to disturb eddy currents. For a large number of simulated failures on the riveted sample in the article, the results of all measurements are not evaluated in detail. However, all simulated failures made on the riveted sample were checked. Meter setting parameters for other failures in the first, second, and third layered samples were the same as those for the measurements described in the article. In the measurements, all the simulated failures of the riveted sample were reliably detected. Minor measurement inaccuracies were recorded in the third sheet of the riveted sample. In this case, the probe has reached the boundary of the eddy current penetration depth. In conclusion, non-destructive eddy current testing provides industry with reliable information on the occurrence of various undesirable material and component anomalies. With the development of new materials used also in the aeronautical technology, there is a continuous development of new techniques for reliable control of these materials. In recent years, excellent improvements have been made to this non-destructive technique and it can be said that improvements will continue. References Bugaj, M. 2005. Aircraft maintenance - new trends in general aviation. Promet - Traffic - Traffico, 17(4), pages 231-234. Bugaj, M. 2012. Basic step of applying reliability centered maintenance in general aviation. Transport Problems, Volume 7, p. 77-86. Bugaj, M., et al., 2018. Analysis and implementation of airworthiness directives. Transport Means, Proceedings of the International Conference, Volume 2018, p. 1174-1178. Bugaj, M., Rostáš, J. 2016. Diagnostika lietadlovej techniky. In: Zvyšovanie bezpečnosti a kvality v civilnom letectve 2016: medzinárodná vedecká konferencia organizovaná v rámci riešenia projektu Základný výskum tarifnej politiky na špecifickom trhu letiskových služieb VEGA 1/0838/13: Zuberec, 27.-29. január 2016. Žilina: Žilinská univerzita. 2016. ISBN 978-80-554-1143-9. s.64-66. Douglas, A. 2007. Health monitoring of structural materials and components, ISBN 978-0-470-03313-5. Holoda, Š. et al. 2017. Modification in Structural Design of L-13 “Blanik” Aircraft's Wing to Obtain Airworthiness. TRANSCOM 2017: International scientific conference on sustainable, modern and safe transport, vol. 192, pages 330-335. Janoušek, L., Smetana, M. and Alman, B. 2011. Impact of partially conductive cracks on perturbation field in eddy current non-destructive inspection, proceedings of the ISEM 2011 - 15th International Symposium on Applied Electromagnetics and Mechanics, Italy. Janovec, M., Bugaj, M. 2017. Nedeštruktívne metódy kontroly konštrukcie lietadiel. In Aero-Journal: international scientific journal of air transport OmniScan MXE. (2013). Eddy Current Inspection Data Acquisition and Analysis Software. User‘s Manual, Olympus NDT Shull, P. 2002. Nondestructive evaluation: Theory, Techniques and Applications, Marcel Dekker, Inc., ISBN: 0-8247-8872-9. Škeřík, M., Mařánek, P. 2014. Skripta NDT metody. Praha: Advanced Technology Group. Smetana, M., et al. 2016. Identification of biomaterial fatigue cracks by ECT method, In: Electromagnetic nondestructive evaluation (XIX). Amsterdam: IOS Press BV. ISBN 978-1-61499-638-5, Studies in applied electromagnetics and mechanics, Vol. 41, ISSN 1383-7281, p. 223229. Smetana, M., et al. 2018. Identification of biomaterial fatigue cracks by ECT method, In: Electromagnetic.S Webpage http://www.olympus-ims.com, accessed on November 2018.
ScienceDirect ScienceDirect
Transportation Research Procedia 00 (2019) 000–000 Available online at www.sciencedirect.com Transportation Research Procedia 00 (2019) 000–000
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Transportation Research Procedia 43 (2019) 48–56
8th International Conference on Air Transport – INAIR 2019 8th International Conference on Air Transport – INAIR 2019 GLOBAL TRENDS IN AVIATION GLOBAL TRENDS IN AVIATION
Eddy Current Array Inspection of Riveted Joints Eddy Current Array Inspection of Riveted Joints Michal Janovec a,a,*, Martin Bugajaa, Milan Smetanabb Michal Janovec *, Martin Bugaj , Milan Smetana
Department of Air Transport, Faculty of Operation and Economics of Transport and Communications , University od Žilina, Univerzitná 1, 010 26 Žilina, Slovakiaand Communications , University od Žilina, Univerzitná 1, Department of Air Transport, Faculty of Operation and Economics of Transport b Department of Electromagnetic and Biomedical Engineering, Faculty ofSlovakia Electrical Engineering and Information Technology, University od 010 26 Žilina, b Žilina, Univerzitná 1, 010 26 Žilina, Slovakia Department of Electromagnetic and Biomedical Engineering, Faculty of Electrical Engineering and Information Technology, University od Žilina, Univerzitná 1, 010 26 Žilina, Slovakia a a
Abstract Abstract A Non-destructive Testing (NDT) plays an important role in the safety of aircraft components and structures. As this term suggests, A Non-destructive plays an important roleapproaches in the safetytoofexplore aircraftan components and structures. this term suggests, NDT refers to test Testing methods(NDT) that use special devices and object without damaging As or affecting its future NDT refers NDT to testismethods that to use special and approaches to explore an object without damagingoror affectingbut its future usefulness. mainly used detect anddevices characterize hidden defects or damage to critical components structures, it also has applications dimensional and thehidden evaluation oforsome material properties. NDTorisstructures, a very broad and usefulness. NDT isinmainly used to measurements detect and characterize defects damage to critical components but it also has applications area in dimensional measurements and the evaluation of of some material properties. NDTpublic is a security. very broad and interdisciplinary that finds application in many sectors where failure critical parts could jeopardize NDT is interdisciplinary thattool finds application in many sectors where failure of critical parts that could public security. is used as a quality area control during production to ensure materials and finished products dojeopardize not contain internal errors,NDT or that the abnormalities detected areduring belowproduction the size considered potentiallyand dangerous. NDT is also used theinternal operation of objects used as a quality control tool to ensure materials finished products that do notduring contain errors, or that the abnormalities detected or areother below the size consideredand potentially NDT is also used during the of objects to detect cracks, corrosion forms of degradation damage dangerous. that can cause components or systems to operation fail or interfere with to detect cracks, corrosion or other of degradation and of damage that can cause components or systems to fail or interfere with their proper functioning. This paperforms deals with the inspection the riveted joints of the produced riveted specimen, which simulates damage around the riveted joints. Surface and the subsurface damage well as corrosion in the region of riveted joints are simulated. their proper functioning. This paper deals with inspection of theas riveted joints of the produced riveted specimen, which simulates damage around riveted joints. Surface andMX subsurface damage as well corrosion in Eddy the region of array riveted jointsSAB-067-005are simulated. Experiments willthe utilize Olympus OmniScan device with presence ofas ECA module. current probes Experiments will utilize Olympus OmniScan MX device witheddy presence of ECA module. Eddy array probes SAB-067-005032 and SBB-051-150-032 are used, respectively. Harmonic current excitation is used forcurrent this purpose. 032 and SBB-051-150-032 are used, respectively. Harmonic eddy current excitation is used for this purpose. © 2019 The Author(s). Published by Elsevier B.V. © 2019 The Authors. Published by Elsevier B.V. © 2019 The under Author(s). Publishedof bythe Elsevier B.V. Peer-review under responsibility responsibility scientific committee Peer-review of the scientific committee of of the the 8th 8th International International Conference Conference on onAir Air Transport Transport –– INAIR INAIR 2019, 2019, GLOBAL TRENDS TRENDS IN AVIATION AVIATION Peer-review under responsibility of the scientific committee of the 8th International Conference on Air Transport – INAIR 2019, GLOBAL IN GLOBAL TRENDS IN AVIATION Keywords: cracks; defectoscopy; eddy current methods; rivetted joint; subsurface inspection Keywords: cracks; defectoscopy; eddy current methods; rivetted joint; subsurface inspection
* Corresponding author. Tel.: +421/41/513 35 60 E-mail address:author. [email protected] * Corresponding Tel.: +421/41/513 35 60 E-mail address: [email protected] 2352-1465 © 2019 The Author(s). Published by Elsevier B.V. Peer-review responsibility the scientific committee 2352-1465 © under 2019 The Author(s).of Published by Elsevier B.V.of the 8th International Conference on Air Transport – INAIR 2019,
GLOBAL TRENDS IN AVIATION Peer-review under responsibility of the scientific committee of the 8th International Conference on Air Transport – INAIR 2019, GLOBAL TRENDS IN AVIATION 2352-1465 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 8th International Conference on Air Transport – INAIR 2019, GLOBAL TRENDS IN AVIATION 10.1016/j.trpro.2019.12.018
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1. Introduction The eddy current method is a very commonly used technique for non-destructive testing and material evaluation (Bugaj, 2005). The method is very suitable for detecting cracks that result from fatigue or cracking as a result of stress corrosion. Eddy current control can be performed with minimal part preparation and a high degree of sensitivity (Bugaj, 2012). In the eddy current method, a tested object is subjected to an alternating magnetic field, which is generated by an excitation coil powered by alternating current. The tested object of certain dimensions must be electrically conductive, thus having magnetic permeability. In the tested object, eddy currents are indicated, which act by their magnetic effect on the magnetic field, the original ones, thus exciting. The two magnetic fields (primarily from the excitation coil and the reaction from the eddy currents) are vectored in the resulting field, which is dependent on the electrical conductivity and magnetic permeability of the tested object. The principle of evaluation of materials by eddy currents is the structural state of the material changes above mentioned magnetic and electrical parameters. In the event of an error in the material, a portion of the eddy current paths is interrupted when the reverse effect is changed to the excitation field (Bugaj & Rostáš, 2016). The eddy current method is used to detect surface and subsurface defects, corrosion in aircraft structures, openings for fixing various components and cracks on threads (Bugaj, 2018). The eddy current inspection is mainly carried out on the landing gear of the aircraft in the area of the wheel hubs. The eddy current method is also used to check aircraft engine components. The method is only suitable for conductive materials (iron, non-ferrous and austenitic materials) and requires calibration standards and a trained operator. The method is fast and portable, but special probes are needed to test different types of materials (Janovec, 2017). The aim of this work is to apply powerful ECA method to inspect concrete metallic material structures and to evaluate its detection abilities under specific conditions. This means that this paper deals with non-destructive evaluation of the riveted joints by using eddy current array method. Selected experimental results are presented and discussed at this paper. 2. Eddy Current Array Technique The ECA eddy current technology (Eddy Current Array) was used to check the produced riveted joints. The ECA is a method that allows electronic control of coils of eddy currents placed side by side in one probe assembly. Collecting data from individual coils is accomplished by multiplexing the coils in a special pattern to prevent mutual inductance between the individual coils. The ECA passing through the coil generates a magnetic field. When the coil is positioned over the conductive portion, opposite alternating currents (eddy currents) are formed. In the event of a fault, the eddy current path is interrupted and the eddy currents change is recorded by the measurement coil. In the case of ECA probes, each individual coil in the probe generates a signal with respect to the phase and amplitude of the structure below it. These data refer to the encoded position and time and are graphically represented as a C-scan image (top view). C-scan view allows fast orientation on the tested area and good interpretation of the resulting data. In the eddy current coil yarn applications that pass through the damaged riveted joint, they produce a unique signal response. For coils that are affected by a crack that starts from the thread hole, the C-scan display shows the amplitude change. For coils with no change, the color display on the C-scan display remains constant 2.1. Multiplexing of the individual coils Multiplexing is the process by which multiple analog message signals are combined into one digital signal on a shared medium, Fig.1. When eddy current array data is multiplexed, the individual eddy current coils are excited at different times, allowing the system to excite all of the coils in the probe without ever exciting any two adjacent coils at the same time. An undesirable effect known as mutual inductance (magnetic coupling between coils in close proximity) is minimized with the use of an internal multiplexing system to carefully program the exact time that each coil is excited to transmit its eddy current signal (Smetana et al., 2016). The signals are then reassembled before being displayed as an image. In addition to the enhanced imaging capabilities of multiplexed data, multiplexing allows any individual coil (data) channel to be analyzed after inspection (Smetana et al., 2018).
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With the benefits of single-pass coverage, and enhanced imaging capabilities, ECA technology provides a remarkably powerful tool and significant time savings during inspections. ECA technology includes the following advantages: larger area can be scanned in a single-probe pass, while maintaining a high resolution, less need for complex robotics to move the probe; a simple manual scan is often enough; C-scan imaging improves flaw detection and sizing. Complex shapes can be inspected using probes customized to the profile of the part being inspected (Shull, 2002).
Fig. 1. ECA method: the principle of multiplexing between individual elements and 2D raster scanning principle. (Source: Webpage Olympus, 2018).
3. Riveting joints ECA inspection: experimental set-up For the control measurements, several types of riveted joints were created to simulate the defined damage. The riveted joints were made of aluminum non-anodized sheets 200 x 1000 x 1mm. Aluminum solid rivets STN 02 2311 head 4x6 mm were used as fasteners (Figure 2).
Fig. 2. Solid rivet with countersunk head. (Source: https://www.metalcom.cz/sk/nity-klasicke-so-zapustnou-hlavou-plochou-din-661). Table 1 Specification of the rivet used to form the riveted sample. Source: Author. Rivet
Solid rivet with countersunh head
Shank diameter [mm] 4 (+0,1, mm)
Head diameter [mm] 7
Height head [mm]
Material
Shank head [mm]
1
AlMgCu 2,5
6,0 mm (+/0,2 mm)
The riveted joints for inspection measurements were formed by riveting three aluminum sheets, each having a thickness of 1 mm. The resulting thickness of the riveted sample was thus 3 mm. The rivet holes were formed by a column drill machine. A 4 mm diameter drill was used to make the holes. In the sheet metal, where the rivet heads were inserted into the holes, it was necessary to create a chamfer of the hole for the correct insertion of the thread head into the sheet. The riveted joints were created manually, using a pneumatic riveting hammer, in the four double-row regions indicated in Figure 3, under items 1,2,3,4.
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Fig. 3. The riveted sample created in the program Catia V5. (Source: Author).
Artificial damages were created on the riveted sample to simulate the defects of riveted joints. These were formed on the riveted sheets by grinding the notches in the region of the rivet holes. The length of the simulated faults was 2,4,6 and 8 mm and the width was 1 mm. The depth of the cut notches ranged from 0,5 to 1 mm. The depths of the notches in the individual sheets cannot be precisely determined because the notches were made manually by an electric hand micro drill (Figure 4 a). The faults were oriented along the rivet holes or inclined at 45 °.
a)
b)
Fig. 4. a) Notches formed in the area of the holes for rivets 2,4,6 and 8 mm long, b) detail of the formed rivet joint. (Source: Author).
In the area of riveted joints, the defects caused by surface corrosion of aluminum sheet were also simulated. Aluminum powder was glued to the sheet metal, simulating the various corrosion shapes between the riveted sheets. A very fine aluminum powder CAS: 7429-90-5 was used to simulate corrosion. The aluminum powder was adhered to the surface of the sheet by a flexible, solvent-free, FLEXTEC polymer-based gel adhesive.
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Fig. 5. Aluminum powder glued to the surface of aluminum sheet. (Source: Author).
The Olympus OmniScan MX was used to measure riveted joints. This device is a modular and portable test unit. A swirl current module was used in the instrument. The device can be used for manual as well as automated inspection. The number of channels of the device is 32 with internal multiplexing, or 64 channels with external multiplication. The OmniScan connector has a probe ID function that allows for the physical detection and detection of the probe attached. This function is to set the resolution of the C-scan ECA probes and read the correct probe parameters. Appropriate ECA probes SBB-051-150-032 and SAB-067-005-032, which are suitable for such riveted joint inspection, have been used with the test unit Olympus OmniScan MX. The probes specifications are shown in Table 2.
Fig. 6. Olympus OmniScan MX Defectoscope (left) and ECA Probes (right). (Source: Author). Table 2. Measuring probe specifications. (Source: Olympus, 2010) Probe type Probe coverage Probe resolution Frequency range Number of coils
SBB-051-150-032 51 mm 1,6 mm 50 kHz – 500 kHz 32 two-row absolute bridge mode
Operating mode
SAB-067-005-032 67 mm 2,1 mm 1 kHz – 25 kHz 32 receiving in two rows with an orientation of approximately 30 ° from 1 mm up to 3.5 mm
surface inspection
The penetration of eddy currents in aluminum materials
Detection capability
-
2 mm cracks in the first layer of bonded material a surface well with a diameter of 0,5 mm
-
3,8 mm long cracks in the second layer, the first layer can be up to 2 mm thick 5% corrosion at a depth of 2 mm with a diameter of 6 mm 10% corrosion at a depth of 3 mm with a diameter of 6 mm
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4. Methodic of inspection riveted joints Before performing individual measurements, the probes must first be calibrated on a known material sample with artificial defects. The Sample with Artificial Failure includes calibration failures whose dimensions (length, depth) are known. Probe calibration should be performed on as many test specimens as possible. They should be of the same material (or material of the same conductivity) and thickness as the parts to be tested, and if a multilayered part is to be tested, each layer should be of the same material (or the same conductivity) and thickness as the test parts. In addition, they should have real or artificial errors that simulate the desired type, position, and orientation as errors that have to be detected. Prior to measurement, probe reference signals should be identified so that they are compared to the actual measured signals where cracks or corrosion may occur. The calibration process must be performed before each measurement. The ECA method is called comparative method. This means that two signals must be compared: no damage (no cracks or errors), an example of a no-fault signal is shown in Figure 7 and a crack presence signal. The differential of these two signals gives a different response, which is important for signal analysis and evaluation. Based on the comparison of these signals, the conclusion of any measurement can be stated and confirmed. All calibration measurements were performed on calibration samples. Calibration samples means a block sample of the same material as the actual structure with the presence of a crack of defined dimensions. The resulting signals are displayed after excitation and are basic signals to evaluate other unknown signals obtained by measuring the actual structure.
Fig. 7. Recording without material defects (left) and recording with known failure dimensions of 11mm and 0,2mm wide. (Source: Author).
5. Evaluation of measurements of simulated faults and corrosion As a first measurement, a hole measurement was made for a rivet that was not yet filled with rivet. This initial measurement had to be done to improve calibration, defectoscope settings and measuring probes. The measurement also provided more detailed information on the structured riveted sample, which is then checked for other simulated failures. The result measurement of the hole for rivet is shown in Figure 8. The C-scan displays the measurement results in a two-dimensional view as a planar view of the structure being inspected. This view is similar to that of an X-ray image. The color spectrum displayed by C-scan represents the amplitude curve. The individual colors represent the depth of the failure signals when the probe passes over the test surface. This type of display is only possible with the ECA probe. Material surface scan is here faster than the conventional ECT method. At the same time, it is possible to display the waveform of the Gaussian plane (impedance raw) for a given segment of the selected linear path. This display shows the progress of the impedance recorded during measurement. Depending on the shape of the curve and in which quadrant the signal is located, it is possible to distinguish between the edge of the object, the defect, and so
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on. The last display is the amplitude coding of the signal in the time domain. The presence of inhomogeneity is displayed as a signal deflection. The determination of the failure dimensions can be estimated from the C-scan display, in which the cursors can be used to subtract the size of the fault. The resulting dimension of the diameter of the hole for rivet read from the Cscan cursors is: 30,45 mm – 26,25 mm = 4,2 mm. The hole diameter value for the rivet subtracted from the cursors for the C-scan display corresponds to the actual hole for rivet of 4 mm. Another parameter that can be detected from the defectoscope screen is the fault depth, in this case the hole length for the rivet. The fault depth is determined from the waveform signal impedance signal (graph strip raw). The measurement was carried out on a 1 mm thick aluminum sheet. The maximum impedance value was approximately 9V. In the following measurements, it will be known the impedance value used to estimate the depth of the simulated faults that is the impedance comparison value.
Fig. 8. Determining the hole size for the rivet. (Source: Author).
5.1. Defects in the third layered sample In this section of the article, the indication of failure in the third layered sample is evaluated. A 4 mm failure was scanned. The SBB-051-150-032 probe was used to scan the fault. Optimal failure display was obtained with a 45 kHz excitation frequency setting, a gain of 85 dB, no filter or multiplexer was used. The scan result is shown in Figure 9. The failure rate recorded by the defectoscope was 4,8 mm. The impedance signal varied from - 10 V to + 10 V. The probe signal saturation can be seen from the waveform, which is reflected in the maximum permissible waveform amplitude (red) and at the same time the probe signal limitations. It follows that the eddy currents flow around the inhomogeneity is maximized at these circumstances.
Rivet
Fig. 9. Result inspection (left) and inspection method for riveted joint (right). (Source: Author).
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5.2. Simulated corrosion in a third layered riveted sample Aluminum powder was used to simulate corrosion between riveted sheets. The following inspection was aimed at revealing such simulated corrosion. The areas of rivets were inspected and the inspection was aimed at detecting the presence of aluminum powder in the second layered sample. The approximate shape of the glued aluminum powder around the rivets is shown in Figure 10 (right) in red color.
Fig. 10. Aluminum powder glued around the holes for rivets (left) and inspection method (right). (Source: Author).
The control result is shown in Figure 11. A probe SAB-067-005-032 was used for scanning, which is more suitable for detecting corrosion. The best results were obtained by setting the excitation frequency of 10 kHz, gaining 75 dB, without using a filter. While scanning, simulated corrosion with aluminum powder was revealed with very good resolution. The maximum impedance signal value is approximately +9 V and the estimated fault depth based on this impedance value is 1 mm.
Fig. 11. Result of simulated corrosion inspection in second layered riveted sample. (Source: Author).
However, aluminum powder was only trapped in the area of the rivets. In the space between the rivets, the aluminum powder was not captured by the probe (the area indicated by the red arrows). Such result can be made due to the fact that the sheets between the rivets do not lie together and there is an air gap between them. There is no change in eddy currents in this area, and aluminum powder is not revealed. It can be seen from the measurement that the use of a given probe to detect simulated corrosion is very effective. Due to the dimensions of the individual coils forming the probe, it was possible to create a surface map with a relatively good resolution. Simulation of subsurface corrosion is a specific problem and special probes and measurement standards are required for its detection.
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6. Conclusion In today's industry, destructive and non-destructive control techniques have an irreplaceable place to ensure product quality. While destructive techniques are only used for some product samples, since these products are destroyed after test. However, in modern industrial processes, non-destructive techniques are much more advantageous since all testing is performed without permanent changes to the test object. The foundations of this non-destructive technique have been laid down decades ago, the research and development of new probes, techniques and instrumentation is constantly being carried out by manufacturers and research groups around the world. New probes are being developed to meet the increasingly high quality standards required in almost every industry and the use of new superconducting quantum interference devices is being explored. Continuous probe development is also aimed at optimizing crack detection and operating variables such as frequency and signal ratio to disturb eddy currents. For a large number of simulated failures on the riveted sample in the article, the results of all measurements are not evaluated in detail. However, all simulated failures made on the riveted sample were checked. Meter setting parameters for other failures in the first, second, and third layered samples were the same as those for the measurements described in the article. In the measurements, all the simulated failures of the riveted sample were reliably detected. Minor measurement inaccuracies were recorded in the third sheet of the riveted sample. In this case, the probe has reached the boundary of the eddy current penetration depth. In conclusion, non-destructive eddy current testing provides industry with reliable information on the occurrence of various undesirable material and component anomalies. With the development of new materials used also in the aeronautical technology, there is a continuous development of new techniques for reliable control of these materials. In recent years, excellent improvements have been made to this non-destructive technique and it can be said that improvements will continue. References Bugaj, M. 2005. Aircraft maintenance - new trends in general aviation. Promet - Traffic - Traffico, 17(4), pages 231-234. Bugaj, M. 2012. Basic step of applying reliability centered maintenance in general aviation. Transport Problems, Volume 7, p. 77-86. Bugaj, M., et al., 2018. Analysis and implementation of airworthiness directives. Transport Means, Proceedings of the International Conference, Volume 2018, p. 1174-1178. Bugaj, M., Rostáš, J. 2016. Diagnostika lietadlovej techniky. In: Zvyšovanie bezpečnosti a kvality v civilnom letectve 2016: medzinárodná vedecká konferencia organizovaná v rámci riešenia projektu Základný výskum tarifnej politiky na špecifickom trhu letiskových služieb VEGA 1/0838/13: Zuberec, 27.-29. január 2016. Žilina: Žilinská univerzita. 2016. ISBN 978-80-554-1143-9. s.64-66. Douglas, A. 2007. Health monitoring of structural materials and components, ISBN 978-0-470-03313-5. Holoda, Š. et al. 2017. Modification in Structural Design of L-13 “Blanik” Aircraft's Wing to Obtain Airworthiness. TRANSCOM 2017: International scientific conference on sustainable, modern and safe transport, vol. 192, pages 330-335. Janoušek, L., Smetana, M. and Alman, B. 2011. Impact of partially conductive cracks on perturbation field in eddy current non-destructive inspection, proceedings of the ISEM 2011 - 15th International Symposium on Applied Electromagnetics and Mechanics, Italy. Janovec, M., Bugaj, M. 2017. Nedeštruktívne metódy kontroly konštrukcie lietadiel. In Aero-Journal: international scientific journal of air transport OmniScan MXE. (2013). Eddy Current Inspection Data Acquisition and Analysis Software. User‘s Manual, Olympus NDT Shull, P. 2002. Nondestructive evaluation: Theory, Techniques and Applications, Marcel Dekker, Inc., ISBN: 0-8247-8872-9. Škeřík, M., Mařánek, P. 2014. Skripta NDT metody. Praha: Advanced Technology Group. Smetana, M., et al. 2016. Identification of biomaterial fatigue cracks by ECT method, In: Electromagnetic nondestructive evaluation (XIX). Amsterdam: IOS Press BV. ISBN 978-1-61499-638-5, Studies in applied electromagnetics and mechanics, Vol. 41, ISSN 1383-7281, p. 223229. Smetana, M., et al. 2018. Identification of biomaterial fatigue cracks by ECT method, In: Electromagnetic.S Webpage http://www.olympus-ims.com, accessed on November 2018.