·Table of Contents
·Materials Characterization and testing
Comparison of Two Ultrasonic Methods for Materials CharacterisationP. Petculescu, C. Oprea, Ovidius University, 124 Mamaia Ave. 8700 CONSTANTA ROMANIA
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.
|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 
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 . 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:
|a(f) = Ai / Ai+1|
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 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)|
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 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)|
|Fig 6: Front panel of the USSI program.|
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