V. Uchanin, G. Lutsenko, А. Dzhaganian, V. Buga, V. Derecha

G. V. Karpenko Physical-Mechanical Institute of the National Academy of Sciences of Ukraine, Lvov

“Promprylad” LLC, Kiev,

“ANTONOV” Aeronautical Scientist/Technical Complex (Antonov ASTC), Kiev

Introduction

 Eddy current technique is very important for providing reliable operation of aeronautical engineering. At the initial stage the eddy current technique was widely applied for surface defects detection. However, in the last decades it has been used for subsurface defects detection more and more, including detection of defects of fatigue and corrosion origin in inner layers of multilayer aerostructures, flaws under the sealant layer, rivet head etc.

 Researches performed earlier in G. V. Karpenko Physical-Mechanical Institute of the National Academy of Sciences of Ukraine proved that it was possible to increase the testing depth owing to the optimization of exciting field frequency and to the usage of eddy current probes (ECP) with spaced coils. Basing on these researches and ECP, there have been developed low-frequency eddy current flaw detectors, such as DUET, POLIOT, POLIOT-ZS, VID and VDN 1.01. Here, the testing depth is increased on the grounds of application of rather low operating frequencies. Effective techniques of eddy current testing of aerostructures which can be easily adapted when using modern flaw detectors. The experience of eddy current flaw detectors application while inspecting the units of aeronautical engineering is summed up in the reference manual.

  Nowadays, special-purpose low-frequency eddy current flaw detectors aren’t practically manufactured. At the end of 80’s in the last century universal eddy current flaw detectors have been developed and produced. They enable to perform the testing in a wide range of frequencies, including low ones. Flaw detectors versatility is provided by connecting ECP of different types. The instruments assure the carrying out of automatic compensation of ECP unbalancing and ECP signal observation in a complex plane (Х/У mode) or registration of real (vertical) or imaginary (horizontal) constituents of ECP signal in the time scan mode (У/Т or Х/Т modes) on the display. Whereby, it is possible to tune out from interferences influence by turning the complex plane from 0 to 360°. To reduce the impact of interferences, some universal flaw detectors had filters of low and high frequencies with an option of cutoff frequency control. When applying both filters at a time and performing appropriate tuning, the mode of bandpass filter was assured. Because of this, flaw detectors enabled to carry out inspection not only in static, but also in dynamic mode. Elotest В2 flaw detector (Rohmann GmbH company, Germany) has been widely used in aviation. The instrument weight was rather big (11,5 kg) due to the usage of electron-beam tube for display. These instruments were purchased by aircraft enterprises in 80’s in the last century and are successfully applied till today (for instance, at Kiev aircraft repair and overhaul enterprise № 410 and in “Borispol” airport).

Main requirements to universal eddy current flaw detectors

 Over the last years universal eddy current flaw detectors have been seriously upgraded. The following features are characteristic of the majority of modern universal eddy current flaw detectors to this or that extent:

• application of digital techniques of signals processing on the basis of built-in processor or self-contained personal computer;
• usage of luminescent or liquid-crystal graphics (in some instruments - color) display;
• wide range of operating frequencies from units of Hz to several MHz;
• simultaneous application of up to 4 operating frequencies (separately or in combination) in certain instruments;
• usage of 2, 4 and more independent channels;
• possibility of inspection in static or dynamic testing modes;
• possibility of various ECP (parametric, transformer, absolute, differential etc.) connection;
• different modes of information presentation (complex plane, time scan etc.);
• automation of flaw detector setup and possibility of flaw detector setups storage when performing certain testing techniques to simplify and speed the setup;
• automatic going off of flaw detector alarm when the signal hodograph gets in the complex plane window which boundaries and configurations can be adjusted;
• possibility of received defectograms storage in files of standard formats (for example, ТІF or ВМР) and their transmission to PC or printer via different ports for the purpose of storage and registration of testing results;
• possibility of integration in automatic testing systems;
• portable version with self-contained supply.

 The task to create a domestic universal eddy current flaw detector which grade would meet modern requirements was set and solved in “Promprylad” LLC. Two variants of the instrument have been designed – ОKО-01 and VD 3-71. ОKО-01 instrument enables to solve a wide range of tasks and is convenient when creating automated systems of testing (including a complex one) owing to a big number of channels.

 The capacities of simpler flaw detector, such as VD 3-71, are sufficient for the majority of tasks of eddy current testing or aeronautical engineering (particularly in operation). Due to its small weight and dimensions as well as to built-in storage battery presence, the instrument is especially convenient for operation in field conditions and airfield conditions.

Characteristics of ОKО-01 eddy current flaw detector

 While developing a universal eddy current flaw detector which meets up-to-date requirements, the modular approach of flaw detector construction has been chosen. Such approach provides a possibility of flexible extension of flaw detector capacities by connecting additional units and modules. Central module is a special computer. Besides processor, membrane keyboard, TFT display, central module comprises a flash-card which functions as read-only memory, encoder controller and USB interface. Encoder controller makes it possible to connect two encoders what is essential for the plotting of testing surface scan. The central module is attached to eddy current one by means of a special transceiver. Eddy current module is based on the erasable programmable logic device (EPLD). It controls frequency synthesizers, amplifiers, compensation of ECP unbalancing voltage, ADC. Moreover, EPLD carries out the filtering of received signal, controls external switches and prepare the pattern of eddy current signal for display on the central module screen.

 From one up to four eddy current modules each of which has one physical eddy current channel can be connected to ОKО-01 flaw detector. Thus, one instrument can provide up to 4 independent eddy current ducts. Each of these ducts is able to function in multifrequency mode. The values of operating frequencies are adjusted in the range from 100 Hz to 2 МHz. ОKО-01 flaw detector can operate with various ECP (absolute, differential, parametric and transformer).

 There is a keyboard for the instrument operation on the front panel of flaw detector. The values of operating frequencies, sampling frequency, exciting voltage, scale, turn are displayed. Flaw detector assures the display of signal in a complex plane what makes it possible to single out defects among interferences by analyzing the shape of signal hodograph. Four signals are separately turned within the limits from -360° to 360° with a step of one degree on (X/Y)-area of display. Four working points of coordinate origin are independently located on the display. Image browsing allows to display up to 12 pages on the screen. Every page permits to display up to 4 eddy current channels. This makes it possible to configure up to 32 channels and easily perform 12 pages flipping for fast viewing of all 32 channels of eddy current data. Sampling frequency is adjusted by an operator and can reach up to 1000 samples per second at four frequencies. It is also possible to set up to 4 alarm frames (which can be combined by means of “And” or “Or” logical function) for each channel.

 Automatic measurement of signal amplitude and phase assures the evaluation of defect size during data analysis. The measured values of phase or voltage amplitude are necessary for the flaw size estimation in accordance with the selected calibration curve. Such curve provides the association of signal amplitude or phase parameters with defect parameters in millimeters or per cent from the wall thickness.

Characteristics of VD3-71 eddy current flaw detector

 VD3-71 flaw detector (Fig. 1) has one physical eddy current duct which provides the operation of up to 2 frequencies. The range of operating frequencies is selected within the limits from 500 Hz to 6 МHz what enables to connect low-frequency and high-frequency ECP and solve the tasks of detection of both surface and subsurface defects. Due to the adjustment of gain and ECP exciting voltage it becomes possible to work with absolute and differential, parametric and transformer ECP produced by different companies.

  The instrument display allows to present the values of operating frequencies, sampling frequency, exciting voltage, scale and phase (signal rotation angle on a complex plane). Image browsing makes it possible to display up to 3 pages on the screen. Every page permits to display 1 display area and 2 time scans. Each display area enables to display eddy current signal in the following views: vector display of signals or complex plane. Sampling frequency is adjusted by an operator and can reach up to 3000 samples per second. There is also the mode of automatic measurement of signal amplitude or phase for the defects of different depth with the storage of corresponding calibration curve which is used for subsequent evaluation of the flaw size during testing. Creation of up to 4 alarm “frames” is also provided in VD3-71 flaw detector. These frames and signal together form an event (for example, threshold level exceeding by a signal) which can be indicated by a sound signal, LEDs lighting on the instrument panel, alarm my means of program indicator or combination of above-listed responses. Flaw detector is capable of creating the mixes of two channels. For mixing an operator can select one of 5 algorithms: addition, subtraction, addition with horizontal inversion and addition with vertical inversion, multiplication.

Fig. 1. Appearance of VD3-71 eddy current flaw detector.

 VD3-71 flaw detector assures filtering of ECP signal in real time with the help of the following filters: low-frequency filter, high-frequency filter, passband filter, differential filter and averaging filter. Availability of filters enables to carry out the mode of dynamic testing which is prospective for the detection of defects on the side surface of holes. It is also possible to store up to 100 setups of flaw detector and 10 testing results what increases the operation efficiency.

Fig. 2. Working filed of program for the analysis of eddy current testing results on PC.

 Testing data obtained by this flaw detector can be transmitted to PC for long-term storage, processing, visualization, creation of databases by the inspected objects (Fig. 2).

Examples of flaw detector application for aeronautical engineering testing

 As it has been mentioned above, eddy current technique has no alternative when solving many air tasks. The technique is especially effective when detecting defects in multilayer aerostructures, defects under the layer of sealant or paint, flaws in holes, defects in the rivet area, including the area under its head and so on.

 Detection of hidden flaws in multilayer aircraft units (or while testing from unaffected side) is based on the application of low-frequency ECP. To solve these problems, there have been designed a set of low-frequency ECP which showed great testing depth in conjunction with good sensitivity and resolution. What is important is that these ECP are tuned out well from the influence of changes of gap between ECP and testing surface. A set of special calibration blocks made from D16Т aluminum alloy have been developed and produced for metrological assurance during flaw detection of subsurface flaws. The block consists of two fayed parts. An extensive defect 2 mm in depth is simulated by a vertical joint of block parts. The indicated defect depth is 1 mm, 2 mm, 3 mm and 4 mm. It is possible to use the block for selecting an optimum operating frequency when detecting defects at a certain depth and for setting up flaw detector with the purpose of sampling flaws detection at a specified depth.

 Let’s consider several examples of prospective usage of universal flaw detectors for aircraft units inspection.

Rivet joint area testing

 An important advantage of eddy current testing techniques is the possibility to detect fatigue and corrosion flaws in internal layers of multilayered constructions, including its execution without fasteners dismantling and constructions disassembling. It allows to use efficiently eddy current technique of rivets area testing not only during repair of aeronautical engineering, when it is possible to dismantle fasteners, but also directly during tests and operation. Typical example of the simplest air two-layer construction where the stringer is jointed with the outer skin of an aircraft by a regular line of countersunk rivets is given in Fig. 3.

Fig. 3. Typical unit, such as “stringer-skin” with a rivet line.

 The testing area is marked with a dash-and-dot line in Fig. 3. The position of possible crack is shown on the leader. The cracks which do not go beyond the rivet head are considered dangerous in many cases. In general, a number of jointed layers may be larger and defects can be located in inner layers. The structural elements are jointed to the outer skin of an aircraft by countersunk rivets in the majority of cases. Internal joint is often performed with the help of rivets with heads of various shapes. Bolt joints are sometimes used, too.

 According to the type of testing area scanning, all techniques of eddy current testing of rivet joints can be conditionally divided in three main groups:

1. Static mode – carried out by placing ECP on the rivet or next to it;
2. Sliding mode – performed by progressive ECP movement along the rivets line or near it;
3. Rotary mode – ECP is put on the rivet and rotated around it manually or by means of a driver.

 The modes of each group are used for the detection of flaws both in outer skin and in inner layers of multilayered constructions. In this regard, the techniques of each group have its merits and drawbacks. That is why, the possibility and efficiency of their application is determined by the features of aircraft constructions and required level of sensitivity and depth of testing.

  The highest sensitivity is provided by rotary mode according to which ECP is placed in alignment with the rivet, unbalancing compensation is performed and the testing signal changes are observed when rotating ECP. The balance is kept when rotating ECP over the flaw-free rivet. If there is a defect, the balance is disturbed. For technique realization it is sufficient to provide rotation in both sides at an angle from 45 to 90 degrees. It is also important to assure the rivet and ECP centering.

 For technique carrying out in case of countersunk rivets it is possible to use ECP of Leotest series with working area diameter from 7 to 15 mm depending on the rivet diameter. For ECP centering in this case dielectric guides with the hole of appropriate diameter are used. To detect defects under the rivets overhanging the testing surface and with round and cylindrical heads (head diameter - 10 mm, hole diameter - 6 mm), a special ECP of Leotest MDF 2201/10 R with central hole of corresponding diameter has been designed for its placement on the rivet. It should be noted that in this case it is not necessary to apply dielectric guides for ECP centering during rotation, as ECP is centered by the rivet itself. Special ECP of Leotest MDF 2201/10 R has been tested at the operating frequency of 2 kHz with signal registration in a complex plane. An experiment was carried out on the block from D16 aluminum alloy with 6 mm diameter hole from which artificial defects, such as cracks with opening of 0,1 mm, length from 1 to 6 mm (through 1,0 mm) was cut by electrospark technique. The block with a defect was covered with a flaw-free 2 mm thick plate with the hole of 6 mm in diameter and jointed by the rivet. Fig. 4-а and 4-b presents the hodographs of ECP signals received when turning ECP over the block with a crack of 1 mm (Fig. 4-а) and 2 mm (Fig. 4-b) in length. Having put ECP on the block, the unbalancing was compensated and complex plane was turned in such way so that the signal from defect was directed vertically from bottom to top. The sensitivity was the same during registration of signals from 1 mm long crack and signals from interferences. The sensitivity for 2 mm long crack was 12 dB less. Hodograph analysis shows that the crack length influences strongly the signal amplitude. In particular, the signal from 2 mm long crack is over 3 times more than the signal from 1 mm long crack. The signal from 1 mm long crack exceeds the level of signal from main interferences for more than 6 dB what proves high selectivity of testing with the help of presented ECP.

Fig. 4. Rotary ECP signals from defects, such as a crack of 1 mm (а) and 2 mm (b) in length under the rivet head and 2 mm thick skin.

 Comparative study showed that the presented ECP has better sensitivity and selectivity while detecting flaws in the second layer from the surface than standard ECP intended for solving the same task.

Eddy current testing of air wheels

 Nowadays there have been used a bulky and outdated instrument Elotest В2 to which MDF 0701 probes were connected for manual and automated testing. Eddy current flaw detectors, such as ОKО-01 and VD3-71 complete with MDF 0701 probes, can solve this task even better. Upon that, ОKО-01 instrument is more effective for the development of automated testing system, and VD3-71 instrument – for manual testing in airfield conditions. Fig. 5 presents the signals from defects with depth of 0,3 mm and 0,5 mm on ОKО-01 flaw detector screen. On the left there are signals on the time scan, on the right – signals in a complex plane. It is seen on defectogram that the signals from defect stand out well against a background of interferences (dependence of signal from smaller flaw and signal from gap change of about 10 dB).

Fig. 5. Signal from defects, such as a crack, with depth of 0,3 mm (1) and 0,5 mm (2) in the air wheel hub.

Detection of defects under the layer of sealant.

 The research performed by us showed that it is possible to provide a rather high sensitivity to fatigue cracks on the basis of low-frequency multidifferential ECP usage during testing through the layer of sealant (without its removal). The presence of sealant layer which has dielectric properties in the range of eddy current operating frequencies influences the same way as the gap between the probe and testing surface does. Distinctive feature of multidifferential ECP of Leotest МDF series is almost complete suppression of noises connected with the changes during inspection of the gap between the probe and testing surface. Dependences of ECP signal amplitude from defect when changing the gaps (or, in our case, the sealant layer thickness) are given in Fig. 6. For ECP of Leotest МDF 0901 type the dependence is got for two operating frequencies - 1 kHz and 10 kHz. The amplitudes of signals from through surface defect in 5 mm thick plate for different gaps are reduced to the amplitude of signal from the same defect during testing without a gap (layer of protective dielectric cover). For ECP of Leotest МDF 0701 type the dependence is got only for operating frequency of 10 kHz, as the frequency of 1 kHz lies beyond the operating range of this ECP. The mentioned dependences prove that for MDF 0901 ECP the signals from defect at the operating frequencies of 1 kHz and 10 kHz attenuate with the gap increase almost in the same way, i.e. the reduction of signal from defect with the gap increase depends only a little on its operating frequency. It is worth mentioning that this is applied to the case when the operating frequency is selected within an optimal range of operating frequencies of the present ECP. The comparison of dependence for MDF 0901 and MDF 0701 ECP shows that the attenuation of signal from defect with the gap increase depends more on its size. At the same time, it should be noted that attenuation difference is not significant and in many cases smaller dimensions of MDF 0701 ECP can give it an advantage during testing of complex aircraft units with projections, holes, fillet transitions etc. In general, it should be observed that for multidifferential ECP of Leotest МDF type the signals from coarse defects can be surely registered (with signal/noise ratio of more than 6 dB) at gaps of up to 15,0 mm for MDF 0701 ECP and at gaps of up to 10,0 мм for MDF 0701 ECP.

Fig. 6. Dependence ECP signals amplitude from the couplant layer thickness.

 When detecting cracks under the layer of couplant, the critical areas are singled out; its list is determined by the design manager. First of all, these are the reinforcement sheet in places of skins and stringers joining, areas of stringer coarse thinnings in the zone of their end, bolt joints areas etc. In consequence of these areas analysis, there have been already developed the technique of detection of fatigue cracks in critical areas without removing the sealant which proved its efficiency in the aircraft repair enterprise conditions. Here is an example of critical area.

Fig. 7. Scheme of eddy current testing of stringers joint: 1 – stringer, 2 – stringers joint, 3 – reinforcing rib, 4 – skin.

  Fig. 7 presents the scheme of testing of joint 1 with reinforcing rib 3. The layout of rivets and bolt joints are marked with crosses in this figure. The areas subject to inspection are marked with arrows, brackets with arrows and hatched sections with arrows. To make it simple, the sealant layer is not shown in the figure. The testing sections are divided into characteristics areas which are called “separate testing areas” (SRA). Flaw detector setup and operation procedure in each of these areas are different. The following characteristic sections are inspected on such unit: upper flanges of stringers 1 at rib ends (SRA are marked by the letters Г, Ж and Е), skin 4 in the area of stringers 1 joint 2 (hatched SRA – K) and reinforcing rib 3 in the area of stringer joint (SRA is marked by the letters И and Д).

Conclusion

1. The presented universal eddy current flaw detectors, such as ОKО-01 and VD3-71, which allow to solve a wide range of NDT tasks on the basis of possibility to operate in a wide range of operating frequencies. It is possible to present signals in a complex plane or signals components with time scan in flaw detectors. The filters of different kind can be applied as well what permits to improve signal/noise ratio while detecting small and deeply seated flaws (in nonmagnetic alloys). Flaw detectors enable to connect different ECP. The instruments provide the storage of setups and testing results. Due to small weight and dimensions and to the presence of built-in storage battery, such instrument as VD3-71 is convenient for the usage in field and airfield conditions.
2. Flaw detectors, such as ОKО-01 and VD 3-71, have sufficient sensitivity when connecting low-frequency eddy current probes. This allows to detect not only surface, but also subsurface flaws, including in multilayer aerostructures.
3. A special calibration block with internal flaws of depth from 1 to 4 mm have been designed for metrological assurance of eddy current techniques of hidden subsurface defects detection.
4. On the basis of VD 3-71 and ОKО-01 instruments with the application of special multidifferential ECP of Leotest series there can be developed the techniques of field inspection of aerostructures solid units, including:
• detection of local corrosion damages (including in “stringer-skin” area) while testing from the unaffected side;
• detection of fatigue cracks under the skin with thickness of up to 10-12 mm;
• detection of cracks in critical areas under the sealant layer with thickness of up to 15 mm, including in the caisson tanks;
• detection of cracks on the side hole surface;
• detection of cracks in aircraft wheel hubs;
• detection of cracks which doesn’t go beyond the rivet head (including in the second and third layers);
• detection of cracks under the layer of chromium with thickness of up to 200 μm in shock absorbers gears.