ALEXANDER ALEXIEV Institute of Mechanics-BAS, Sofia, Bulgaria. ANTON RAZYGRAEV CNIITMASH, Moscow, Russia

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1 11 th National Congress on Theoretical and Applied Mechanics, 2-5 Sept. 2009, Borovets, Bulgaria ULTRASONIC TESTING OF THICKNESS ON THE BASE OF ANGULAR PROBES FOR SHEAR WAVES. COMPARISON BETWEEN RESULTS FROM THEORETICAL MODELS AND EXPERIMENTAL INVESTIGATIONS ALEXANDER ALEXIEV Institute of Mechanics-BAS, Sofia, Bulgaria ANTON RAZYGRAEV CNIITMASH, Moscow, Russia ABSTRACT. In the work are investigated prototypes of probes for ultrasonic testing of thickness, constructed on the base of developed theoretical models of acoustical tract of probes. For the prototypes of probes was defined ability resolving and sensitivity of the method for testing of thickness. Those angular probes are designed based for control of elements of nuclear power stations and pipelines. The results from theoretical models are compared with experimental investigations. For theoretical models and the probes are made conclusions about their abilities. KEY WORDS: Ultrasonic testing of thickness, modelling of acoustical tract, Special angular probes for shear waves 1. Introduction One of the main tasks of non-destructive testing in power engineering, oil and gas industry and pipeline installations is the control of erosion-corrosion wear of equipment. In common practice used to control ultrasonic thickness gauges, working with normal probes for longitudinal waves. Often, however, reflected by the bottom surface information signal is too weak against the signals obtained from the corrosive deposits and possible degradation of the bottom surface. On the other hand, wellknown fact is the use of transverse waves reaches a better resolution than the use of longitudinal. Given the above, the measurement of wall thickness of responsible use of equipment offered special angular of the transverse wave probes for working on the method of transmission. Specifically examined in nuclear power equipment with typical initial thickness of the walls H 0 =3, 6, 9, 14, 18, 24 and 30 mm and working

2 Alexander Alexiev, Anton Razigraev range of probes ΔH=H 0 0,7H 0. The scheme of the proposed method is shown in Fig. 1, where the emitter 1 and receiver 2 are located in general housing, and designed probes are unique for each measured thickness of the device. Defectoscope Ho Ho Fig.1. Scheme of the method. 2. Modelling research In the construction of such probes is applied numerical modelling of acoustic tract using pulsed flaw that generates electrical impulses to the bell shaped shell and number of pulses of oscillation in the level 6 db n = 3. The spectrum of the signal is passed information recorded in the form [1] Sa 2 (2.1) S(H, ω ) = S(0, ω ) R( (H))cos( (H)) (H, ) 2 λ( ω )r( θ(h )) θ θ Φ ω, where S(0, ω) is a spectrum of the transmitted signal, estimated as proposed in [2] approach, r-twice the distance from the fake source to the bottom surface, R - coefficient of reflection from an infinite plane, ω = 2πf - cyclic frequency, Sa area of the fake source, Φ - diagram of the orientation of the transmitter and receiver, H - thickness of the object. Diagram of the orientation of the angular probe with square piezoplate in the plane of incidence of the wave is presented by the expression [1] (2.2) ' 2 ' a sin(α ) sh (a σ p sin(α )) + Φ(H, ω ) = ' ' 2 sh(a σa sin(α ) (a σ p sin(α )) ' 2 + sin a k( ω )( sin(γ sin(θ( H ))) ' 2 + ak( ω )(sin(γ sin(θ(h)),

3 Ultrasonic Testing of Thickness on the base of Angular Probes for Shear Waves where a' = a/cos (α), a - length of piezoplate, γ - angle of entry into the material, α - angle of prism, σ р - coefficient of attenuation in the prism, k - wave number. Amplitude of the signal is passed information obtained by back Fourier - Transformation in the form + 1 (2.3) A(H,t) = S(H, ω )exp(iωt)dt 2π. Were investigated for modelling the influence of the frequency of generated impulses, the magnitude of piezoplate, angle of entry of probe, and the way in plexiglass prism [3]. Based on model studies have selected the optimal parameters of acoustic tract square piezoplate of probe with side length a = 4 mm, time in the light - 4 mm, resonance frequency f 0 = 5 MHz, the velocity of the wave in prism C lp = 2730 m / s, angle of entry γ = 40 o, velocity of longitudinal and transverse wave in the controlled object, respectively, C l = 5920 m / s and C t = 3255 m / s [4]. There was even distribution of the amplitude of the signal information passed across the range (Н Н 0 ) for all the cases. Differences in the amplitudes of the signals from the maximum thickness Н 0 to minimum 0.7Н 0 ranging from 5.6 db for H0 = 3 mm to 19.9 db for H0 = 30 mm. On Fig. 2 is presented a diagram of the orientation of the transmitter (receiver) as specified in the parameters. Fig. 2. Diagram of orientation of probes. The angle of the solution diagram of orientation of the probe at level 0.7 is θ 0.7 = ( + θ 0.7 = , - θ 0.7 = ), and at level 0.5 is θ 0.5 = ( + θ 0.5 = , - θ 0.5 = ).

4 Alexander Alexiev, Anton Razigraev 3. Experimental studies Based on the research results in the model are made prototypes of probes. Photograph of probes is shown in Fig. 3. Fig. 3. Prototypes of the probes. Some of characteristics of constructed probes are shown in Table 1. Table 1. Characteristics of probes Н 0, mm 0.7 Н 0, mm r, mm γ, grad θ 0,5, grad θ 0,7, grad ΔA, db

5 Ultrasonic Testing of Thickness on the base of Angular Probes for Shear Waves To assess the sensitivity of probes, in particular the probe designed to work with the thickness of H 0 = 18mm is constructed special block with spherical reflectors with different diameters φ and depth h, resembling shells corrosion. The results obtained for the amplitude of the reflected signals from the reflector relative to the amplitude of the bottom of the model are presented in Table 2. Table 2. Assessment of the sensitivity of probes φ/h, mm 2/2 3/2 4/2 5/2 6/2 7/2 8/2 9/2 A, db φ/h, mm 3/3 4/3 6/3 8/3 3/4 4/4 6/4 8/4 A, db Results showed that sensitivity reached can detect small defects on bottom surface of the controlled objects. 3. Comparison of model and experimental studies For evaluation of the probes for thicknesses in the range ΔH=H 0 0,7H 0 is made a control block to work with probe intended to gauge ΔH= mm. The data obtained for the amplitude of the reflected signals from different thicknesses, and the data obtained from modelling for the same case are presented in Fig A, db 10 5 MODEL EXPERIMENT H, mm Fig.4. Comparison of model and experimental studies There was good consistency with the theoretical model and experimental studies.

6 Alexander Alexiev, Anton Razigraev Conclusion This work has proposed an innovative approach for ultrasonic monitoring of corrosion - erosion wear of pipelines and equipment through the use of angular probes and transverse waves. Numerical modelling has been conducted on acoustic tract, which is achieved in an even distribution of the amplitudes of the received data signals across a range of controlled thicknesses. Reached acceptable minimum size for the practice of probes all set for wall thickness of pipes. They are made prototypes of probes, control samples and test methodology. Conducted experiments show the adequacy of the proposed model and good consistency with experimental data. Achieved resolution and sensitivity allow the detection of small defects on the bottom surface and the small changes in the thickness of the controlled devices. R E F E R E N C E S [1] MIHOVSKI, M., A Complex Use of Non-destructive Methods in the Study of the Structure and Physico-Mechanical Properties of Metallic Materials, D.Sc. Thesis, Institute of Mechanics, Bulg. Acad. Sci., 1991 (in Bulgarian). [2] ERMOLOV, I. N., Theory and Practice of the Ultrasonic Testing, M., Mashinostroenie, 1981 (in Russian). [3] RAZYGRAEV А., AL. ALEXIEV, Model studies in ultrasonic thickness control of pipelines, XVIII National Conference NDT`03, Sozopol, 2003, (in Bulgarian). [4] ALEXIEV AL., А. RAZYGRAEV, Ultrasonic Testing of Corrosion-Erosion Wear of Pipelines and Equipment, XX National Conference NDT`05, Sozopol, 2005, (in Bulgarian).