Flaw cross-section reconstruction using the correlation synthesizing delayed amplitude technique

Flaw cross-section reconstruction using the correlation synthesizing delayed amplitude technique Guo Zhongxiong, Tao Liang, Gao Shaolun, Li Yanzhang* and Yu Shisheng In order to give the flaw reconstruction technique a wider application in ultrasonic nondestructive testing, a new technique of cross-sectional imaging using the correlation synthesizing delayed amplitude technique (CSDAT) with signals from a transducer linear array has been developed. This technique is suitable for wider use because of its simple installation, convenient operation and fast algorithm. In this paper, the imaging system, the principle of correlation synthesizing delayed amplitude, some experimental results and test results for a large forging in the Harbin Turbine Works are introduced.

Keywords: ultrasonic cross-section imaging, transducer linear array, correlation synthesis, delayed amplitude

Imaging techniques have become increasingly important in ultrasonic NDT. Some techniques such as B-scan and C-scan are limited in application by their requirements for focused transducers and scanning systems. Both SAFTtl,21 (synthetic aperture focusing technique ), which identifies the defects using the sums of the corresponding amplitudes of the signals received by transducers at different positions, and ALOK c3~ (amplitude and transit time dynamic curves), which reconstructs images of defects by processing many data sets of probe position, amplitude and transit time, need a great deal of data from transducers at many different positions to reconstruct a complete and high signal-to-noise-ratio (SNR) image. Therefore, expensive subsidiary devices such as precise scanning systems are needed, and furthermore processing so many data will inevitably lower the imaging speed. The CSDAT described in this paper is an approach to NDT suitable for large forgings. To be applicable for field testing, the technique is combined with traditional A-scan testing, which can still be applied as a major method for practical NDT of large forgings in which there are few defects approaching or exceeding the standard limit. In eases where it cannot be decided whether the workpiece is usable or not using only the height of a reflected wave from the flaw, the technique can be used to reconstruct the flaw cross-section to provide evidence for fracture mechanics. Harbin Institute of Technology, Harbin, Heilongjiang, Post Code 150006, Box 333, People's Republic of China. * Harbin Turbine Works, 1 Daqing Road, Dongli District, Harbin, Heilongjiang, Post Code 150040, People's Republic of China. Paper received 17 June 1991

As shown in Figure 1, the imaging system consists only of five transducers, a five-channel amplifier, an ultrasonic pulser and a personal computer. The CSDAT is based on the SAFT principle with the purpose of increasing the SNR of the image derived from fewer data. A satisfactory image can be reconstructed by processing signals received by a five-transducer linear array at a definite position, so the technique also had the advantage of being suitable for testing complicated-surface workpieces.

Principle of CSDAT Delayed amplitude First, a point A(x, z) us assumed in the imaging region. As shown in Figure 2, ro is the distance between A and transmitting transducer T;.ri is the distance between A and receiving transducer Ri; Wi is the distance between T and R~. If a pulse 6(t) is emitted by T at time to, the time of flight (TOF) of an echo reflected from A to R~is :

zi(X,Z) = [r0(x, z) + ri(x, z)]/v

(1)

where v is the longitudinal wave velocity. That is, if A is a defect point, the signal E~(t) received by R~ has a pole at t, t = to + z~(x, z). On the other hand, the delayed time z~(x, z) can be derived from the pole of the received signals. In the case of Figure 2:

Ti(x,z) = {(x 2 + z2) 1/2 + [(x + Wi) 2 + z21112}lv (2) Equation (2) demonstrates that the locus in the imaging region corresponding to the pole of the time domain received signals is an ellipse with foci T and R~.

0963-8695/91/040203-04 © 1991 Butterworth-Heinemann Ltd NDT& E International Volume 24 Number 4 August 1991

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amplitudes of several channels, the synthetic values will correlate more with these ellipses. Corresponding amplitudes of signals received by transducers in a linear array are correlation synthesized as follows:

im

× Ej[h(x,z)]

x

E~[z~(x,z)]

(3)

where n is the number of transducers in the array and m is a parameter controlling the correlation synthetic aperture which determines the definition. As to a defect point, the product of the corresponding amplitudes of signals received by the three transducers i,j, k will become very large or very small, and therefore, after synthesizing these products, a comparatively high SNR image can be reconstructed using fewer data. In our work, one transceiving transducer and four receiving transducers are used as a linear transducer array (n = 5), and the parameter m is 2. Finally, the flaw image can be reconstructed by comparing synthetic values of each point with a threshold Eo : (4)

f ( x , z ) = ( s g n [ E ( x , z ) - Eo] + 1}/2

Fig. 1 shaft

(a) Imaging system; (b) five-transducer array on turbine Disc drive

Printer

Transducer array Ri

I

Wi

T

I

I

X

Ii

Microcomputer Monitor

T A/D converter A (X,Z}

Fig. 2

Correlation synthesis

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Time controller

1 Five-channel amplifier

Calculation of duration

There are many poles in the signals of each channel when the defect is large, such as a crack. As far as one channel corresponding to one transducer position is concerned, all the ellipses are parallel, and there are ellipses each corresponding to a different channel intersecting at a defect point. The sum of the amplitudes of each channel's signals corresponding to one point increases with the number of ellipses passing the point. If summing amplitudes is replaced by summing the products of

{ L

Ultrasonic pulser

R5 (R3)

Workpiece

Fig. 3

Block diagram of the CSDAT imaging system

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Array

where

f(x,z) = {12

if point (x, z) is inside the defect if point (x, z) is outside the defect

(5)

It has been verified by the experiments referred to below that the CSDAT is suitable for both holes and cracks.

Flaw reconstruction procedure [4] Data acquisition Figure 3 is a block diagram illustrating the CSDAT imaging system. First, an imaging window is assumed at an area including the flaw detected by A-scan. The five-transducer array is arranged on the surface of the workpiece as shown in Figure 3: four receiving transducers R 1, R2, R4, R 5 are arranged symmetrically about the transceiving transducer T(R3), with equal spacing between them.

0

a

87.5

175

262.5

350

(mrn)

The microcomputer excites the ultrasonic pulser at a fixed frequency by means of an interface circuit to send an electrical pulse to T. Then T transforms the electrical pulse into a divergent ultrasonic pulse to irradiate the imaging area inside the workpiece. The transducer array receives pulses scattered by defects and transforms them back into electrical signals. After being amplified and quantized, the data segment corresponding to the imaging window will be sampled by computer at a fixed interval. Fig. 5

Array

0

(a) Specimen; (b) image

Data processing

S0

a

100 (mm)

150

200

Pre-processing of the original data is first carried out to remove negative values and normalize the signals in each channel by their maximum values. The computer calculates the durations zi(x, z) of pulses scattered by point (x, z ) to each transducer according to the geometry, and then obtains the delayed amplitudes Ei[zi(x,z)] from signals from each channel, and the synthetic value E(x, z) of this point can be derived from Equation (3). The synthetic value is calculated point by point in the imaging window. Finally, reconstruction of the flaw can be obtained by eliminating noise with a threshold E o as shown in Equation (4).

Imaging examples of CSDAT Broad-band normal ultrasonic probes were adopted as the transducers, with a 5 MHz centre frequency. The ultrasonic pulser was a CTS-23 flaw detector having been simply refitted. The microcomputer was an IBM PC/AT, operating at 10 MHz. The sampling frequency of the A/D converter was 30 MHz. The specimen with artificial flaws was a 34CrNiMoV alloy steel forging, and the cracks inside the specimen were machined by line-cutter.

Fig. 4

(a) Specimen; (b) image

NDT& E International August 1991

Figure 4 shows an experiment with a cylindrical surface specimen with a 20 mm long crack. The parameters displayed in the photographs are: W is the distance between two neighbouring transducers; D is the depth of the specimen; L is the length of the flaw; TIME is the imaging speed. In the case of a cylindrical surface specimen W is the arc length and D is the diameter.

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popular use. This new technique can reconstruct a flaw cross-section in a comparatively complicated condition with simple installation, convenient operation and fast calculation. The results of imaging holes, inclusions and cracks are satisfactory. The C S D A T imaging system has been applied to N D T of shafts in the Harbin Turbine Works.

Fig. 6 Cross-sectional image of turbine impeller

The technique can overcome certain practical problems, but the imaging precision is not very high because of the reduction of a large number of data. For instance, because the reflection of an ultrasonic pulse from the middle section of a crack is more directional than that from the tip of a crack, the reconstructed crack is sometimes discontinuous, though the length and orientation of the crack can be depicted accurately. It is supposed that the technique could be improved by increasing the amount of data used.

Acknowledgements

Figure 5 shows a specimen with three 5.5 m m long cracks and its resulting image. The horizontal distance between the tips of crack A and crack B is 2.5 mm, and the vertical distance between crack B and crack C is 3 mm. The image shows that all can be easily discriminated.

The research was supported by the Harbin Turbine Works. Some equipment was supplied by the Shantou Institute of Ultrasonic Instruments.

Figure 6 shows a result of practical testing of a turbine impeller. The flaw size determined by the amplitude of A-scan signals is equivalent to a 3 . 6 m m diameter plane-bottom hole.

References

The results indicate that under the above conditions, both the axial and lateral resolutions are 2 mm, and the location error is less than 2 m m ; the crack length error is less than 10%.

Conclusion This research based on existing theories has been developed to increase the technique's suitability for

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l

Langenberg, K.J., Berger, M., Kreutter, Th., Mayer, K. and Schimitz,

V. 'Synthetic aperture focusing technique signal processing' N D T hltern 19 3 (June 1986) pp 177-189 Doctor, S.R., Hall, T.E. and Reid, L.D. "SAFT - the evolution of a signal processing technology for ultrasonic testing' N D T hltern 19 3 (June 1986) pp 163-167 Grohs, B., Barbian, D.A., Kappes, W., Paul, H., Licht, R. and Hoh,

F.W. 'Characterization of flaw location, shape, and dimensions with the ALOK system' Mater Eral40 1 (January 1982) pp 84 89 Guo, Z.X., Chea, Z.X., Cl~ag, T.A., Yu, S.S., Li, Y.Z., Li, J.C. and Metooka Seiieh 'Cross-section image formation of metallic materials by syntheses of delayed ultrasonic waves' Appl Acoust 9 4 (July 1990) pp 10-12

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