Eddy current technique is widely used in the practice of non-destructive testing of critical constructions and parts in various branches. At the initial stage 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. Taking into consideration an exigency of modern eddy current flaw detector which would allow to carry out quality testing and possess a wide range of capabilities, being quite easy in operation, “Prompribor” LLC specialists developed VD3-71 universal eddy current flaw detector (Fig.1).

Fig. 1. VD3-71 manual flaw detector

  VD3-71 flaw detector is intended for manual testing by eddy current technique for the presence of surface and subsurface defects, the evaluation of detected defects depth and the storage of testing results for further analysis. Flaw detector detects such defects as: surface cracks, subsurface cracks, material properties change, fatigue cracks, potholes, corrosion defects, flaws related to the surface thickness change. When inspecting, there is no need to clean the testing surface. Inspection can be performed with a gap between the testing object and ECP (eddy current probe). Friendly interface of flaw detector allows to perform the instrument setup maximally easy and quickly. VD3-71 flaw detector has one physical eddy current duct which provides operation of up to 2 frequencies. Values of operating frequencies are adjusted in the range from 500 Hz to 6 МHz. VD3-71 flaw detector enables to connect eddy current probes of various configurations: absolute and differential (both parametric and transformer), and also to realize bridge circuits of ECPs connection (Fig. 2).

Fig. 2. EC probes for VD3-71 flaw detector

This instrument feature considerably extends testing capacities, increases the range of testing objects, what makes it possible to solve the following tasks:

• Inspection of separate sections of bearing structures;
• Welded joint testing using single-element probes, inspection of beams, pipelines, various-diameter pipes, bars, stripes, sheathed cable;
• Inspection of different metal sections (from ferromagnetic and colored materials), (Fig. 3, а; b) with dielectric insulation or without it, parts of cars, bodies, bearing structures, geared engine pinion, connecting rods of wing spar, generator actuators hubs, gears, coupling boxes, compressor cases, lead screws, diverse cylinders, hydraulic pump gears, thrust rings, wheel axles, springs, screws, power turbine blades;
• Complete testing of rail wheels and other parts of rolling stock (Fig. 4).

Moreover, there in no alternative to eddy current technique when solving many aircraft tasks. The technique is particularly effective and has no alternative during detection of defects in multilayer aerostructures, defects under the layer of sealant or paint, flaws in holes, defects in rivet area, including under its head etc. Specific character of testing objects and techniques in the present case created incentives for the development of specific probes. The need of good ECP sensitivity reception at low operating frequencies (multilayer construction inspection), high resolution (for the detection of local defects, such as pores and short cracks) resulted in the appearance of set of new multidifferential ECP, such as Leotest МDF for the testing at different operating frequencies.

a)

b)

Fig. 2. EC probes for VD3-71 flaw detector
a)trolley bolster;
b)engine cylinder sleeve

These ECP have the following advantages, among with are:

• high sensitivity to small short and extensive defects;
• high sensitivity to local flaws, such as voids;
• small influence of a gap (due to multidifferential structure);
• good suppression of primary electromagnetic field;
• high resolution at wide testing area;
• great length of testing at low operating frequencies;
• possibility of defects parameters determination;
• small overall dimensions (in particular, the best ratio of testing depth and ECP diameter).

  The keyboard for the instrument operation is situated on the front panel of flaw detector.

  Values of operating frequencies, sampling rate, excitation voltage, scale, phase are displayed on the screen. Scrolling of images enables to display up to 3 pages on the screen at a time. Every page allows to show 1 display area and 2 time scans. (Fig. 6).

a)

b)

c)

Fig. 4. Testing of rail rolling stock parts
a)carriage axle;
b)carriage wheel;
c)fragment of side frame.

  Each display area enables to show eddy current signal in the following views: vector image: with that, the straight line (vector) is displayed in the area, one its end is the center, the other one indicates the current signal value; complex plane: allows to single out flaws among interferences by analyzing the shapes of hodograph of signal from defect; Sampling frequency is controlled by an operator and can reach 3000 samplings per second. Automatic measurement of signal amplitude and phase provides a possibility to estimate the defect size during data analysis. Measured values of phase or voltage amplitude are essential to evaluate the defect size in accordance with the selected calibration curve. Such curve provides the association of parameters of signal amplitude or phase with defect parameters in mm or in % from the wall thickness (Fig.5).

Fig. 5. Calibration curve plotting

  It is worth mentioning that a rather powerful mechanism of Automatic Flaw Alarm (ALARM) is realized in VD3-71. Creation of up to 4 “frames” (threshold levels) of alarm for each display area (Fig. 6) is also provided. These frames and the signal together form an event (for instance, signal exceeding the threshold level). An operator is able to adjust the response to the event. The response to the event may be the following:

• Sound signal;
• LED backup on the instrument panel;
• Alarm by means of software indicators;
• Or the combination of above-listed responses.

Fig. 6. Signals display

  Flaw detector allows to create the mix of two signals (for mixing an operator can choose one of 5 algorithms: addition, subtraction, addition with horizontal inversion and addition with vertical inversion, multiplication); VD3-71 is capable of filtering the signal in real time. There are 5 filter types: Lowpass filter, Highpass filter, Bandpass filter, Differential filter, Averaging filter. An operator can store and load flaw detector settings what makes it possible to adapt the instrument to the necessary task almost immediately.

Fig. 7. Software for connection with personal computer

  It is also possible to store and load testing results what allows an operator to solve several flaw detection tasks simultaneously saving the data integrity. Flaw detector is able to store up to 20 setups and testing results what substantially increases the operation efficiency. Setups and testing results are stored on Flash card, such as CompactFlash, with memory capacity of 128 Mb. Flaw detector is equipped with USB port for connection to PC for data transmission. Testing data received by this flaw detector can be transmitted to PC for long-term storage, processing, viewing, creation of databases on the inspected objects, solving of tasks of their technical state monitoring. There is also a possibility to analyze and create documentary reports on the testing results by means of specially developed software (included in the delivery set) (Fig.7).

Conclusion

  

1. The article presents VD3-71 portable eddy current flaw detector which enables to solve a wide range of NDT tasks basing on the capability to work in a wide range of operating frequencies. Flaw detector also provides a possibility to display signals in complex plane or signal components with time scan. Flaw detector easily adapts to eddy current probes of various types. The whole variety of performed functions and application sphere are carried out by means of software. VD3-71 flaw detector has adequate sensitivity what permits to detect not only surface cracks, but also subsurface defects, including flaws in multilayer constructions. The instrument assures the storage of testing results and setups.
2. VD3-71 flaw detector properties allow to recommend it for solving other urgent tasks of nondestructive testing, namely:
   • welded joint inspection;
   • quality testing when producing pipes, bars, wires, rods etc.;
   • inspection of power equipment elements, including the elements of heat exchanger with internal ID\OD probes;
   • testing of parts of transport vehicles during their manufacture and operation,
   • inspection of vessels in chemical, oil-and-gas and other branches.