Defektoskopia ultradźwiękowa bezkontaktowa Bolesław AUGUSTYNIAK

Defektoskopia ultradźwiękowa bezkontaktowa

Bolesław AUGUSTYNIAK

Metody bezkontaktowe badania

Wzbudzanie drgań – klasyczny nadajnik poprzez powietrzewiązka laserowasiła Lorentza (EMAT)

Detekcja drgań – klasyczny odbiornik poprzez powietrze wiązka laserowa (interferometr)elektromagnetyczna (EMAT)

Badanie odkształceń – za pomocą wiązki laserowej shearography

UT wzbudzanie i detekcja drgań ze sprzężeniem przez powietrze

This technique uses a high sound pressure to compensate losses and low frequencies (50kHz to some 100 kHz).

http://www.ndt.net/article/cdcm2006/papers/schnars.pdf

A double shell structure with foam core inside is investigated by impact tests. The stringer debonding is clearly visible in the C-scan.

Laserowe wzbudzanie i detekcja drgań

Sketch of the principle of laser-ultrasonics or of a “laser-ultrasonic transducer”.

http://www.ndt.net/article/wcndt2004/pdf/non-contact_ultrasonics/722_monchalin.pdf

Laserowe wzbudzanie i detekcja drgań 2

http://www.ndt.net/article/ecndt2006/doc/P223.pdf

Laserowe wzbudzanie i detekcja drgań 3

http://www.ndt.net/article/cdcm2006/papers/schnars.pdf

Laserowe wzbudzanie drgań

There are essentially two kinds of mechanisms for generating ultrasound: - the first one is perfectly nondestructive and is based on a

thermoelastic mechanism - the second one is invasive and is based on the ablation of

the sample or on the vaporization of some surface layer.


Laserowe wzbudzanie drgań efekt termoelastyczny

The principle of thermoelastic generation is the following: laser light is absorbed to some depth inside the material releasing heat locally; the heated region then expands producing a strain and a corresponding stress that is the source of waves propagating in the material or at its surface. When light penetration is small and the excitation spot much less than an acoustic wavelength, a complex emission pattern is observed in the far field with inclined lobes (from 30 to 60°) for the longitudinal and shear waves. There is in particular no longitudinal emission along the normal to the surface.


efekt termoelastyczny cdSurface waves and plate waves can also be generated efficiently and in a very versatile manner. When the laser beam is focused to a small circular spot, a surface wave with a cylindrical symmetry is emitted from this spot. Good directivity is obtained by focusing the beam with a cylindrical lens to get a line source. More complicated patterns can even be used, such as an array of lines, giving narrower band emission but having the advantage to distribute the laser energy over a broader area so surface damage can be avoided. A converging circular Rayleigh surface wave giving very strong displacement at the center of convergence can be readily obtained by using, in addition to the conventional spherical lens, an axicon (conical lens). Enhancement techniques have been developed by sweeping the line or line array source with a proper velocity.

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efekt plastyczny

If one increases the energy density, particularly for small light penetration (metals), one reaches the threshold where the surface starts to melt and then to get vaporized. At this point, matter is ejected from the surface and through various physical processes this vapor and the surrounding air is ionized, thus producing a plasma plume that expands away from the laser spot on the surface. Generation of ultrasound originates from the initial recoil produced by the material ablation and from the plasma pressure. A similar vaporization effect occurs also when the material is covered by a thin absorbing layer, which is blown off, leaving the substrate underneath substantially unaffected if the energy density is below some threshold. In the strong plasma regime on the other hand, a crater mark is left on the surface. .

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Laserowa detekcja drgań

To detect ultrasound, the surface is illuminated by a laser beam, continuous or of pulse duration sufficiently long to capture all the ultrasonic signal of interest. Scattered or reflected light is then collected by an optical receiver, which is in the case of most industrial applications an interferometer


Principle of optical detection of ultrasound with an interferometer (here sketched as a confocal Fabry-Perot): the interfrometer converts the phase or

frequency modulation produced by the surface motion in an intensity modulation.

Intereferometr Febry- Perot

A practical interferometer, which is by now widely used, is the confocal Fabry-Perot. This is a simple system made of two concave identical mirrors separated by a distance equal to their radius of curvature. In excellent approximation each ray retraces its path after multiple reflections in the resonator, which gives to the system a high collection efficiency (etendue or throughput). It can be used in transmission or in reflection. In transmission the responsivity peaks at a frequency equal to about a resonance width. The reflection scheme has on the other hand nearly flat frequency response above this frequency, except for periodic drops at integer of the free spectral range. It should be noted that the responsivity is practically zero at very low frequencies, which means that this system is intrinsically insensitive to vibrations, a key advantage for use in industrial environments. The main weakness of Fabry-Perot demodulators is their lack of sensitivity at low ultrasonic frequencies (below 2 MHz), which is circumvented by devices based on two-wave mixing in photorefractive materials.


Laserowa detekcja drgań – układ dwuwiązkowy

A signal beam which acquires phase shift and speckle after reflection on a surface in ultrasonic motion, is mixed in a photorefractive crystal with a pump plane wave to produce a speckle adapted reference wave that propagates in the same direction as the transmitted signal wave and interferes with it.


Laserowa detekcja drgań – układ dwuwiązkowy cd

One important advantage of the photorefractive demodulator with respect to the confocal Fabry-Perot is its better sensitivity at low ultrasonic frequencies (below 1 MHz), thus allowing probing more easily materials with strong ultrasonic attenuation. The system has also the advantage to be easily combined with a differential or balanced scheme (two detectors giving responses to phase modulation of opposite sign), so the noise coming from the laser intensity fluctuations can be eliminated to a large extent.


Laserowa detekcja drgań - cechy lasera One key element of the detection scheme is the detection laser. It should be high power, since sensitivity increases with power, and should not contribute to any noise in addition to the fundamental photon or shot noise. High power is particularly needed when the surface is absorbing and detection is at a large distance giving a small collection solid angle. The pulse duration should be sufficiently long to capture all the signal of interest, which means for many applications a duration between 10 and 100µs. Nd-YAG technology at 1.06 µm, which is known to provide high amplification gain, is particularly suited for realizing such a laser by amplifying a small and very stable Nd-YAG laser oscillator. A suitable oscillator is commercially available with power from 100 mW to about 2 W and is based on a small monolithic cavity pumped by a laser diode. Depending upon the repetition rate, the amplifier could be flashlamp pumped (up to 100 Hz) or diode pumped (above 100 Hz). Peak powers of 1 kW and more are typically obtained with pulse duration of the order of 50 µs. More recently, since such a master oscillator-pulsed amplifier system is complex and costly, IMI/NRC has been working on the development of a pulsed oscillator without seeding giving directly about 100 W


Laserowa detekcja drgań – obróbka obrazu Regarding imaging, a numerical imaging approach such as the one based on the Synthetic Aperture Focusing Technique (SAFT) can be combined with laser-ultrasonics to obtain high quality images.

Processing can be done in the time domain but since this is can be very computation intensive and fairly long, methods that operate in Fourier space and make use of Fast Fourier Transform algorithms have been developed. These methods, significantly different from the conventional time SAFT, although sometimes called F-SAFT, are based on a plane wave decomposition of the acoustic field for each frequency combined with a back-propagation algorithm.

This processing technique has been applied in particular to the imaging of stress corrosion cracks in steel.


Przykład obrazu – pęknięcia korozyjne

Comparison of crack opening images obtained by liquid penetrants (left) and laser-ultrasonic F-SAFT processing using shear waves and an annular aperture (right). The horizontal lines observed at right are artifacts originating

from the small size of the test sample.


Przykład zastosowań – 1


Effect of optical scaning occurs as if a conventional piezoelectric transducer was moved over the surface

Przykład zastosowań – lotnictwo


Laser-ultrasonic C-scan image of part of the horizontal stabilizer of a CF-18 airplane in undismantled and ready for take-off conditions. One will notice that, unlike conventional water jet ultrasonics, laser-ultrasonic allows scanning to the very edge of the part.

Przykład zastosowań – lotnictwo 2

http://www.ndt.net/article/wcndt2004/pdf/noncontact_ultrasonics/722_monchalin.pdfhttp://www.ndt.net/article/cdcm2006/papers/schnars.pdf

The inspected surface is at the corner..

Przykład zastosowań – beton


Specimen with artificial voids: Thin Styrofoam plates (150 x 150 x 10 mm) were embedded at different depths in concrete

Przykład zastosowań – beton cd


Comparison of waveforms between with and without defects

Velocity dispersion curve without defect Velocity dispersion curve with defect (defect-A)

Metoda mieszana : laser wzbudza – detekcja poprzez

powietrze

http://www.ndt.net/article/wcndt2004/pdf/laser_ultrasonics/276_guillorit.pdf

The Nd:YAG oscillator is coupled to a 1 mm diameter multimode optical fiber (ended with an SMA connector), and a variable optical attenuator is used to adjust the maximum power level that can be delivered onto the sample without damage. In order to concentrate as much as possible the ultrasound energy in the direction of the air coupled transducer, a focusing probe was developed to focus the laser beam into a line. In this way sufficient energy can be deposited without raising the power density so high as to cause ablation, and the wave propagation direction is restricted to be perpendicular to the line. The focusing arrangement is made of a plano-convex lens to collimate the laser beam emerging from the fiber, and a cylindrical lens to focus the beam into the line (about 10 mm x 0.5 mm) at the top surface of the sample. The optical assembly is placed into a stackable lens tube. Rayleigh waves propagating in the direction perpendicular to the laser line are detected with the air-coupled transducer placed at few millimeters in front of the optic probe. .

Metoda mieszana :

cd


This scanner moves the probe holder above the sample and keeps a constant gap between the transducer/optic probe block and the sample. The ultrasonic signal is amplified with a 25 dB LECOEUR Electronique preamplifier, and filtered with a WIDERANGE Decade Filter module. A digital AGILENT 54622A oscilloscope displays the ultrasonic waveforms, and the data are transferred and stored into a PC using an AGILENT E8408A VXI mainframe. A software was developed on HP VEE to control the generation laser, the XY scanner and the VXI board. Finally, the signal processing and the plot of the C-scan are performed with a

dedicated PV WAVE software.

Metoda mieszana :

cd 2


(a): Portable head C-scan of a 4.6 mm thick CFRP composite sample with Teflon

foils, and (b): detail drawing of the sample

Metoda mieszana : cd 3


C-scan of a composite honeycomb sandwich sample with structural inserts.

(a): Portable head equipment, (b): Through transmission air-coupled equipment

Shearography laserowe badanie

odkształceń powierzchni

http://www.ndt.net/article/wcndt2004/pdf/optical_techniques/534_yang.pdf

. Typical shearographic setup in which the modified Michelson interferometer is used as a shearing device. The modified Michelson interferometer brings the light waves from two points: P1 and P2, on the object surface into one point: P, on the image plane by tilting the mirror 1 a very small angle. The intensity of the interferogram is then registered by a CCD camera and saved in the computer

Shearography laserowe badanie – zasada działania


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Digital shearography is a laser measuring technique based on digital data processing, phase-shifting techniques and interferometry. According to the common sense of interferometry, two beams with an identical wavelength are required for a purpose of interference. Usually, an identical wavelength for the two beams can be obtained from one laser by using a beam splitter, such as the object beam and the reference beam used in the setups of holography and electronic speckle pattern interferometry (ESPI)7. A distinguishing feature of shearography is the use of a self-reference interference system. Instead of using a reference beam, shearography utilizes a shearing device to bring the light waves from two points on the object surface into one point on the image plane, which results in an interference phenomenon, i.e. so-called speckle interferogram, without

using an additional reference beam.

Shearography – a holografia


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Holography detects a flaw by looking for displacement anomalies induced by the defect which look like a circular fringe patterns, whereas, shearography detects a flaw by looking for strain anomalies induced by the defect which look like a butterfly pattern in fringe pattern. Compared with holography which measures full-field displacement, shearography directly measures strain information and thus it is more direct and simple for detection of strain concentrations which are usually created at the positions of defects that decrease the strength of the structure.

Shearography – przykłady


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Left: holographic NDT, in which circular anomalous fringe patterns appear at the debond positions, right: Shearographic NDT, in which butterfly patterns appear at the debond positions.


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NDT of a GFRP rotor vane about 5 meters in length by internal pressure (about .p = 0.01 MPa); (a) view of the rotor vane, (b) showing the passage of lamination, (c) showing a delamination, and (d) displaying a micro crack.

Shearography – przykład 2

EMAT electromagnetic acoustic transducer

http://www.ndt.net/article/wcndt2004/pdf/petrochemical_industry/67_murayama.pdf

The EMAT consists of a magnet that produces a bias magnetic field and a sensor coil that produces a dynamic magnetic field. The driving force uses a high frequency vibration of magnetostriction generated in the direction of the compound’s magnetic field by combining the dynamic magnetic field generated by a high frequency electric current in the sensor coil and the static field by the electromagnet.

EMAT sondy

http://www.ndt.net/article/wcndt2004/pdf/non-contact_ultrasonics/691_alers.pdf

Construction details of an EMAT. (a) Simplified structure to show the essential parts – a wire to induce eddy currents in a conducting surface and a static magnetic field. (b) A meander line coil under a large permanent magnet pole. (c) A periodic array of magnets that apply alternating magnetic fields to an eddy current sheet flowing in a race-track pattern.

EMAT głowice dla fal poprzecznych

Głowice meandrowe bez magnesu

http://www.ndt.net/article/wcndt2004/pdf/noncontact_ultrasonics/599_salzburger.pdf

The RF-coil is placed very close 0.5 mm) to the materials surface underneath a periodic arrangement of permanent magnets of alternating polarity

Głowice z magnesami

EMAT zastosowania -

spoiny


The probe (T/R-type with a focus in a distance of 65 mm) is placed on one of the surfaces of the thinner sheet and radiates the ultrasonic pulse into the weld. Back reflected signals (echoes) from weld defects such as pores, holes, incomplete through-welding… are digitized; the maxima of the echo-amplitudes are displayed versus the probe position as Amplitude-Locus Curves (Scan). Fig. 7a shows such a scan with the indication of an internal lack of fusion between the weld and the base metal of the thicker blank.

EMAT automatyczne badanie spoiny 2


The combined evaluation of the pulse-echo- and pulse transmission inspection using the dispersive SH-mode propagating in circumferential direction of the pipe wall. The inspection frequency is 0.8 MHz. The echo- and roundtrip signals are gated out electronically. Their maximum is displayed on-line as scan (see fig. 11). Fig. 10 shows in the foreground the tube; the weld is in the 12 o’clock position. At the backside the probe holder is located; the probe is (hidden by the tube) fixed in the central part of the holder. The tube is moving along the probe in a fixed distance of 0.2 mm (assured by the holder rollers) with a speed of up to 1m/s

EMAT automatyczne badanie spoiny 3


The US-scans of a 16 m long weld. The curves represent the echo maxima of a well scraped weld (up) and of a very badly scraped weld (down). The well scraped weld shows a very low amplitude level along the whole 16 m; whereas the scan of the badly scraped weld has a much higher amplitude level. The oscillations in the first 7 meters show a periodic change of the quality of the scraping. In the second half of the scan the decreasing level indicates a slow increasing of the scraping quality.

EMAT badanie grubości ścianek


In case of external defects the wall (upper) thickness can be calculated using the time-offlight of the ultrasonic backwall echo signal. The shape of the artificial defects and the measured wall thickness fit very well. As expected, the EC technique does not show any indication. Figure (bottom) shows the results when the defects are located at the internal side. In the defect areas the EMAT signal breaks down, and the ultrasonic thickness measurement is no longer possible. However, the EC technique now detects the inside defects.

EMAT badanie rur

http://www.ndt.net/article/wcndt2004/pdf/non-contact_ultrasonics/569_alters.pdf

Example of an E-scan of a 14” diameter x 0.25” wall pipe containing simulated corrosion pits in the form of 1” diameter. hemispherical depressions with depths of 0.050” (20%), 0.102” (40%), 0.145” (60%) and 0.194” (80 %).

EMAT badanie rur cd

http://www.ndt.net/article/wcndt2004/pdf/non-contact_ultrasonics/569_alters.pdf

Three E-scan presentations of a field of general corrosion and pits near a circumferential weld in a 30” diameter pipe.

Podsumowanie

Zalety technik bezkontaktowych

- ‘szybkie’ badanie dużych powierzchni i w wysokich temperaturach

- automatyzacja badań

Wady

- ograniczona ‘rozdzielczość’

- znaczące koszty aparatury

- konieczne specjalne szkolenie personelu

Documents

Defektoskopia ultradźwiękowa bezkontaktowa Bolesław AUGUSTYNIAK