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Comparison of Two Ultrasonic Methods for Materials Characterisation P. Petculescu, C. Oprea, Ovidius University, 124 Mamaia Ave. 8700 CONSTANTA ROMANIA Contact

Abstract

We present in this paper the USSI (Ultrasonic System for Structure Investigation) program in order to determinate the acoustic parameters in the austenitic welded joint. We have determinated the velocity and the ultrasonic attenuation using the pulse - echo method in direct contact and immersion techniques for different frequencies. We analyzed two samples of the steel 1646 and 1680. The measurements are made in different points of the same samples and the acoustic parameters values determined by USSI program are compares with those obtained with conventional A - scan method.
Keywords: material characterization, acoustic parameters, USSI program, A - scan, austenitic samples.

1.Introduction

The use of automatic interpretation techniques serves first of all the increase in the velocity of the data interpretation, however, very important goals are also the increase in the reability and reproductibility of the examination. Also, in some situation, computer system can help to interpret complex data because they are capable of analyzing more data simultaneously than it can be done by an operator. Ultrasonic spectroscopy is a method for testing materials with relatively low acoustic attenuation by exciting the sample under test into mechanical resonance. By progressively sweeping the frequency of a driving transducer across a range and recording the amplitude and time of flight of a receiving transducer one can build up a spectrum of the sample under examination. A spectrum therefore contains information about the sample that is of interest for the purpose of inspecting it, the challenges of the technique are to sample the spectrum accurately and to interpret it correctly.

2. Experimental setup

2.1 Examined samples
The experiment has been made on two welded samples of austenitic steel types 1646 and 1680.
Their chemical composition is given in Table 1, where one can notice that the two samples show close chemical composition.

Type	C	Si	Mn	Cr	Ni	Mo	S	P	V
1646	0.02	0.53	1.61	20.9	10.9	0.08	0.014	0.022	0.095
1680	0.03	0.49	0.72	20.38	10.56	0.58	0.01	0.012	_
Table 1: Chemical composition for austenitic steel types 1646 and 1680

Due to the small dimensions of the samples, the acoustic parameters measurements have been made only on a few points on both perpendicular directions with the requirement that a measurement point to be on the welded joint. The points where the measurements have been made are showed for sample 1680 in Figure 1 and for sample 1646 in Figure 2.For illustration, in the case of sample 1646, points1 through 4 are on the upper surface of the sample (a^'b^'c^'d^') while points 5 through 9 are located on the lateral surface (a b b^'a^' ).For sample 1646 the measurement point located on the weld is 7, while for 1680 there are two such points, 3 and 6. The differences between the number of measurement points are due to the different shapes of the two samples.At each of the measurement points the ultrasonic attenuation and velocity were measured by the pulse-echo technique in both direct contact (A-scan ) and immersion (USSI ) at frequencies of 4 MHz and 10 MHz .We note that for the immersion method ,when the USSI program is used ,due to restrictions imposed by the experimental setup, measurements are performed for only part of these points.

Fig 1: Sample 1680

Fig 2: Sample 1646

2.3. The system experimental description
The system experimental has two parts: on automatic mechanical system for immersion scanning and a data analysis software (USSI) for the determination of the acoustic parameters. The automatically mechanical system allows the displacement over three degrees of freedom with a maximum displacement of 100 mm in each direction according to the immersion technique. The transducer displacement is achieved using several step - by - step motors (1.8 deg/step) while the automated position control is accomplished through the acquisition software USCAN 95 [1]
The obtained data in each measurement point by the ultrasonic instrument SONIC - 136 ULTRA are conveyed through the serial interface RS - 232 to the PC and then are saved under the form of data files. For each measured point the following informations are saved, radio frequency signal, amplitude and time of flight. The main parts of this system are: PC computer 233 MHz, National Instruments A/D hoard, SONIC - 136 ULTRA instrument, stepper motor controller. (Figure 3)

Fig 3: USSI experimental system

2.4. The USSI program description
The USSI(Ultrasonic System for Investigation Structures) program has been concept and carried out for the ultrasonic examination of materials and structures characteristics. An analysis of the frequency spectrum involves differentiation of two successive echoes, E₁ and E₂ obtained from the time domain generating from the same emission impulse.Then,based on Fast Fourier Transforms ,the power spectra of the echoes, S₁ and S₂ , are determined. The ratio of the two power spectra in the region of the peak allows the calculation of the ultrasonic attenuation. In the frequency domain the amplitudes ratio for a series of frequency components forms the basis for the deduction of the functional relation between the attenuation coefficient and the frequency. After obtaining ranges S₁ and S₂ corresponding to the selected echoes the results are graphically displayed indicating the peak frequency and the amplitude corresponding to the peak frequency. An option of USSI program permits the selection of a frequency range in order to obtain the frequency dependence of the attenuation. The corrections calculation concerning the attenuation depending on the choose configuration is shown in [2]. Visually, in a window of USSI program appears the graphic of the experimental results and the theoretical curve fitting in the frequency domain as

a(f) = af + bf 4

(1)

which represents the expression of the attenuation coefficient a(f) in Rayleigh scattering domain.This relation is valid in the approximation of small grain sizes compared to the wavelenght of the signal, which is the case for our samples.
The inputs of the USIS program are:

• the sample thickness;
• the sample density;
• the propagation velocity (preliminary,obtained independently by the A-scan method and used only for the determination of the real time );
• the gate width;
• the range.
The output of the USSI program consists in the following information:
• determinate the attenuation at peak frequency;
• computes the propagation velocity;
• computes the time delay corresponding to the shifts echoes E₁,E₂, E₃ to overlap by applying a coincidence criteria;
• performs the Fourier transforms of the E₁, E₂, and E₃ echoes;
• computes the spectrum amplitude ratios

  a(f) = Ai / Ai+1

  over a specified frequency range;
• computes the diffraction correction required by the chosen configuration;
• the attenuation values in the considered frequency range;
• the calculated polynomial coefficients of the proposed theoretical dependence a(f) (obtained by the filtering of the experimental results);
• display the results obtained;

3. Results - Discussions

3.1. Sample 1646
The experimental results obtained on sample 1646 concerning the acoustic parameters (ultrasonic attenuation and the velocity ) are displayed in 3D in Figure 4 for both A-scan (direct contact) (Fig. 4a) and USSI program (immersion method) (Fig. 4b). The ultrasonic attenuation determined by the A-scan method has a variation with position ,with a peak at the joint (point7) for both frequencies but more striking at 10 MHz. Note ,that when using the direct contact the propagation velocities depend on the scan plan (a^'a b b^') or (a^'d^'c^'b^') for 4MHz and are independent for 10MHz. In the immersion method ,using the USSI program (Fig. 4b), the ultrasonic attenuation is no longer qualitatively similar for the two frequencies; there is a peak in the attenuation at point 2 for 4MHz and 10 Mhz. In this case, velocities are a little different for the two used frequences but there is a good corelation between the same measurement points.Note that through the immersion method the values of velocities are bigger and more precise .

Fig 4a:

Fig 4b:

Fig 4: Ultrasonic attenuation and velocity for sample 1646 measured at various points at the frequencies of 4 MHz and 10 MHz and determined with both the A-scan (Fig. 4a) and USSI (Fig. 4b)

3.2. Sample1680
For this sample ,the ultrasonic attenuation and velocity are displayed in 3D in Fig. 5 for both the A-scan (Fig. 5a) and the USSI methods (Fig. 5b). In this case, both methods give qualitatively similar attenuation and velocity profiles at both frequencies.The ultrasonic attenuation determined with the A-scan method has a local maximum at point 3 ,where the weld is located and minimum at point 4.The global maximum is located at point 6 ,where the weld is viewed from the lateral surface.When using A-scan method the values of velocities are different after the two scan plane at 4MHz but are the same for 10 MHz.In the case of USSI program, the attenuation and velocity have similar overall behaviour at the two working frequencies.The acoustic parameters have a maximum at the joint (point 3 ) and a sharp decrease at point 4 ,corresponding to the thickness of the sample.The USSi program (immersion techique ) shows a little variation in velocity values being independent on the used frequency.

Fig 5a:

Fig 5b:

Fig 5: Ultrasonic attenuation and velocity for sample 1680 measured at various points at the frequencies of 4 MHz and 10 MHz and determined with both the A-scan (Fig. 5a) and USSI (Fig. 5b)


A complet vizualization of acoustic parameters in real time through the USSI program for sample 1680 at the frequency of 4 MHz ,in immersion at point 3(on the weld) is displayed on Figure 6. After representing successive echoes in RF, the frequency domain response is reached through FFT with a window of spectral analysis of attenuation, after which the experimental results are being displaying by comparison the theoretical curve of equation (1).

Fig 6: Front panel of the USSI program.

At the same time ,the visualization from Figure 6 presents the determined velocity values from the ultrasonic instrument SONIC 136- ULTRA and the USSI program (based on the attenuation values at the peak frequency and at the lowest frequency in the spectrum ). There are also displayed the corecctions in the attenuation above mentioned.Comparing the ultrasonic attenuation for the two samples we note that for the sample 1680 the values of the attenuation are consistently larger. Based on optical metallography studies we found that the average grain size for the two samples are different, being larger for 1680.Consequently, we propose that the larger attenuation is due to the larger average grain size ,which probably determines increased scattering losses.We also note that, comparing the two methods for the ultrasonic measurements, the A-scan and USSI (immersion technique), some of the variations of the ultrasonic parameters observed in the later case may be due to the fact the propagation of ultrasounds in water is nonlinear [3]. The errors introduced by the nonlinear propagation in water are seen in the values of the attenuation, velocity, and the reflection coefficient at the water-air interface.It was estimated that the errors in the ultrasonic attenuation could be up to 0.3dB/mm at low frequencies, while at small amplitudes and short distances of propagation the errors are negligible. The advantage of the USSI method is that it allows to take into account these errors, which are estimated and then used to corect the final result.

4.Conclusions

We presented in this paper the results of ultrasonic studies of austenitic welds and we introduced a recently developed analysis software, USSI program. From the experimental results obtained through the two methods A-scan and USSI, we come to the conclusions that:
• the USSI method using the program designed and created in our laboratory has major advantages to the A-scan method.In a single measurement, a wide range of information concerning the acoustic parameters in the examined region can be obtained; thus, acoustic parameters are determined and displayed, together with the time domain and the frequency domain responses;
• assuming that the attenuation dependence on frequency is given by the Rayleigh scattering ,the theoretical curve is plotted displaying a and b coefficients values by comparison with the experimental curve;
• the are also visualized the attenuation values for the peak frequency and for the lowest one and the concerned range of frequency. -Ultrasonic attenuation values are identical in the same scan plane and they are in the same side of the weld (see 1,2 and 3,4 or 5,6 to 8 points for 1646 sample and 1,2 points for 1680 sample );
• ultrasonic attenuation values in the weld points are more dependent on the scan plane and frequency;
• the values of propagation velocities depend on the scan plane;
• different values of velocity and attenuation after the scan plane for the two samples show that the sample structure isn't homogene being possible to appreciate grains size and orientation (1680sample present a more dense structure and the big grain size).
• metallographical analyse confirms experimental results about acoustic parameters for the two studied samples.
The USSI program presents the advantage of a limitation of the researcher intervention and interpretation and of a bigger amount of information display by comparison with A-scan method. Due to the complete and immediate visualization of the successive echoes in the time domain we came to the conclusion that USSI program facilitate the display and determination on of the acoustic parameters in real time. The use of automatic interpretation techniques increases the velocity of the data interpretation, the reability and reproductibily of the measuraments. Also, computer system can help to interpret complex data because they are capable of analyzing more data simultaneously.

References

1. Petculescu P ,Ciocan R ,Bokas V, "Investigation of the austenitic structures by ultrasonic spectral analysis".Journal of Nondestructive Testing and Ultrasonics ,vol.3, nr.11, 1998.
2. Krautkramer J & H "Ultrasonic testing of materials " Springer Verlag, 1977.
3. Smith R.A., Lequiri B., Jones LD, Hodnett M.: "Nonlinear propagation in water and its effects on ultrasonic C - scanning" Insight vol. 40, nr. 1, 1998.

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