- Email: [email protected]
Application of a signal recovery technique to acoustic emission analysis
Application of a signal recoverytechnique to acoustic emission analysis E. ESMAIL
and I. GRABEC
The dependence of ringdown counting on signal peak height and trigger level, in acoustic emission during martensitic transformation, was determined experimentally using a signal recovery technique. The corresponding relation between the total ringdown counts and the trigger level, and consequently the system gain, was made. The results show a very sensitive dependence of the total ringdown counts on both trigger level and system gain. A method of obtaining the peak height distribution using both the above relations is described.
Introduction
Experimental
Due to the diffusionless, shearlike nature of martensitic transformations, acoustic emission (AE) occurs. The mechanism of this phenomenon is still relatively unknown, in contrast to that of AE during tensile tests, which is now well explained and related to the dislocation motion.‘32
The average shape of AE waves was obtained for signals with different peak heights using the arrangement shown in Fig. 1. The cydndrical steel specimen of 5 cm length and 0.5 cm diameter is welded to a stainless steel waveguide of the same diameter and 25 cm length. Its end is enlarged to 2 cm diameter to suit the 200 kHz transducer used. The transducer is connected to a pre-amplifier and amplifier which had an 80 dB gain. The output of the amplifier is led into two branches; the first to a correlator, and the second to a trigger circuit with an adjustable trigger level.
Since the work of Liptai et al,3 some experimental work was done to study martensitic transformations by AE techniques. All this work used ringdown counting,4 which provides a qualitative analysis of the transformation, although it was proved by Speich and Fisher,’ and by Speich and Schwoeble,6 that the total number of ringdown counts is proportional to the amount of martensite formed. This shows that ringdown counting can also be used for quantitative analysis. Amplitude analysis of AE bursts seems to be of great importance, since it is expected to be in direct relation to the local stress generated by the martensitic transformation. Pollock’ discussed the significance of amplitude analysis and described the theory and application of amplitude sorting. In this article, a signal recovery technique for obtaining the average shape of the AE waves, and hence the ringdown counts for different signal-peak heights and trigger levels, is described. The experim,,ntal relation between the ringdown counts per acoustic burst, its signal peak height, and the trigger level is obtained. The dependence of the total ringdown counting on the trigger level is also obtained from the experimental data. Finally, it is indicated how the signal peak height distribution may be obtained from these experimental relations. This is a new technique for amplitude analysis using the normal instruments for ringdown counting.
procedure
If the signal peak height exceeds the preset trigger level, an output pulse from the circuit synchronizes the correlator to examine a series of 100 samples of the waveform following the synchronizing pulse, and then averages the corresponding samples from each series. The coherent pattern is reinforced at each repetition and any noise present in the signal averages to zero. The interval between successive samples was taken as ins, giving a duration time of 100 ps, and an average of 1024 repetitions was taken, giving the average waveform which was then recorded on the XY recorder. Hence the number of counts exceeding different levels were counted directly.
-Transducer
The authors are at the Mechanical Engineering Department, University of Ljubljana, p.p. 394, Ljubljana, Yugoslavia. Paper received 18 May
1977.
ULTRASONICS.
Fig. 1
MARCH 1978
The apparatus
components
87
The specimen was heated to 1050°C in a tubular furnace for 10 minutes and then taken out, so that the cooling rate was 2°C s-r , when the specimen transformed martensitically between 425°C and 250°C. The waveform was obtained for signals with different peak heights following the above procedure several times, each time with a different trigger level. Also, the total ringdown counts were obtained for different trigger levels using the counter connected to the trigger circuit.
6-
I
1
O-03 -0.2
-0.1
I
I
0
0.1
I
0.2
I
0.3
I
0.4
0.5
( 7
0.6
Lo9 u,
Log total number of ringdown counts versus log trigger
Fig. 4 level
Results
0
IO
20
30
40
50
60
70
80
90
Time C@
Fig. 2
The average wave form of AE burrts
I’
I
A sample of the recorded waveforms of the AE burst of 3.6 V peak height is given in Fig. 2. The shape was found to be qualitatively the same for bursts with different peak heights. It is important to ensure that the envelope is not exponentially decaying. To obtain the dependence of the ringdown counts per pulse (Nn) on the signal peak height (Jr) and the trigger level (UJ*, pulses exceeding the levels 0.5, 1,l S, 2,2.5 and 3 V were counted for signals with peak heights 0.5,0.75,1,1.8,2, 2.5,3.15 and 3.6 X. The number of counts is plotted in Fig. 3 against the signal peak height for different trigger levels. The straight lines obtained were found to intersect at the same point (- 2, - 5.3), and each straight line can be represented by the equation:
1413-
12-
II-
NR
=
Ah+B
(1)
where A and B are constants for each trigger level, and they can be obtained from the experimental relations: A
8-
2”
I-
1 = 0.1611 + U,) ’
B = - (2 + 3.56 log U,)
(2)
6-
Substituting the values of A and B from (2) in (l), we get the general expression:
5-
4-
NR
3P-
II
OO
I
I
I
2 h CVI
I
I
3
4
Ringdown counts versus signal peak height for different Fig. 3 trigger levels
88
=
h
0.16 (1 + U,)
- (2 + 3.56 log U,)
To obtain the peak height distribution, the total number of ringdown counts (NT) exceeding the trigger levels 0.75, 1.5,2.5 and 3.6 was measured, and found to be 7.3 x 106, 1.42 x 10s, 1.23 x lo4 and 2.56 x lo’, respectively. Fig. 4 shows a plot of log NT against log Ut which is a straight line, from which the relation between the total number of ringdown counts and the trigger level is: lh
and Ut measured in volts
ULTRASONICS.
MARCH
1978
NT
=
1.58
x
lo6 ut-S*l
(4)
The total number of ringdown counts is given by:
NT
=
NR
f(h) dh
(5)
ut where f(h) is the peak height distribution function which is unknown. Functions N,(U,) and NR(Ut, h) were experimentally determined, but have no simple form. Therefore, the inverse of the transformation which gives f(h) cannot be found analytically and has to be obtained numerically.
130 M. This shows that the obtained waveform cannot be explained as a consequence of end reflections. The relation obtained between the ringdown counts and the signal peak height is practically linear, and not logarithmic, as in the case of an exponentially decaying envelope. The total ringdown counting was found to be proportional to Ut-‘.‘, which is also different from that obtained in tensile tests. Consequently, the dependence of NT on the system gain is NT (gain)“‘, which shows that the total ringdown counting, and also the rate of counts of AE during martensitic transformation, are very dependent on the system gain. So special attention must be paid to the precise determination of both the trigger level and the system gain in such experiments. References
Discussion
1 2
The signal recovery technique for obtaining the average waveform of AE bursts was described. It was seen that the envelope obtained is different from the usually assumed exponential decay. This could be attributed to beating of several possible vibrational modes of the whole system. The first reflected wave, corresponding to double the waveguide’s length and the speed of sound of 3000 m s-l, should appear after
ULTRASONICS.
MARCH 1978
3 4
Giiis, P.P. Acoustic Emission, ASTM STP 505 (1972) 20-29 Frederick, J.R and Felbeck, D.K. Acoustic Emission, ASTM STP 505, (1972) 129-139 Liptai, RG., Dunegan, H.L., and Tatro, CA., Inf J Nondest Test l(1969) 213-221 Brindley, B.J., Holt, J., and Palmer, I.G., Nondest Test (Dee
1973) 299-306 5 6 I
Speich, G.R 505, (1972) Speich, G.R Integrity by Pokck, bk
and Fisher, RM. Acoustic Emission, ASTM STP 140-151 and Schwoeble, A.J. Monitoring Structural Acoustic Emission, ASTM STP 571 (1975) 40-58 Nondest Test (Ckt 1973) 264-269
89
and I. GRABEC
The dependence of ringdown counting on signal peak height and trigger level, in acoustic emission during martensitic transformation, was determined experimentally using a signal recovery technique. The corresponding relation between the total ringdown counts and the trigger level, and consequently the system gain, was made. The results show a very sensitive dependence of the total ringdown counts on both trigger level and system gain. A method of obtaining the peak height distribution using both the above relations is described.
Introduction
Experimental
Due to the diffusionless, shearlike nature of martensitic transformations, acoustic emission (AE) occurs. The mechanism of this phenomenon is still relatively unknown, in contrast to that of AE during tensile tests, which is now well explained and related to the dislocation motion.‘32
The average shape of AE waves was obtained for signals with different peak heights using the arrangement shown in Fig. 1. The cydndrical steel specimen of 5 cm length and 0.5 cm diameter is welded to a stainless steel waveguide of the same diameter and 25 cm length. Its end is enlarged to 2 cm diameter to suit the 200 kHz transducer used. The transducer is connected to a pre-amplifier and amplifier which had an 80 dB gain. The output of the amplifier is led into two branches; the first to a correlator, and the second to a trigger circuit with an adjustable trigger level.
Since the work of Liptai et al,3 some experimental work was done to study martensitic transformations by AE techniques. All this work used ringdown counting,4 which provides a qualitative analysis of the transformation, although it was proved by Speich and Fisher,’ and by Speich and Schwoeble,6 that the total number of ringdown counts is proportional to the amount of martensite formed. This shows that ringdown counting can also be used for quantitative analysis. Amplitude analysis of AE bursts seems to be of great importance, since it is expected to be in direct relation to the local stress generated by the martensitic transformation. Pollock’ discussed the significance of amplitude analysis and described the theory and application of amplitude sorting. In this article, a signal recovery technique for obtaining the average shape of the AE waves, and hence the ringdown counts for different signal-peak heights and trigger levels, is described. The experim,,ntal relation between the ringdown counts per acoustic burst, its signal peak height, and the trigger level is obtained. The dependence of the total ringdown counting on the trigger level is also obtained from the experimental data. Finally, it is indicated how the signal peak height distribution may be obtained from these experimental relations. This is a new technique for amplitude analysis using the normal instruments for ringdown counting.
procedure
If the signal peak height exceeds the preset trigger level, an output pulse from the circuit synchronizes the correlator to examine a series of 100 samples of the waveform following the synchronizing pulse, and then averages the corresponding samples from each series. The coherent pattern is reinforced at each repetition and any noise present in the signal averages to zero. The interval between successive samples was taken as ins, giving a duration time of 100 ps, and an average of 1024 repetitions was taken, giving the average waveform which was then recorded on the XY recorder. Hence the number of counts exceeding different levels were counted directly.
-Transducer
The authors are at the Mechanical Engineering Department, University of Ljubljana, p.p. 394, Ljubljana, Yugoslavia. Paper received 18 May
1977.
ULTRASONICS.
Fig. 1
MARCH 1978
The apparatus
components
87
The specimen was heated to 1050°C in a tubular furnace for 10 minutes and then taken out, so that the cooling rate was 2°C s-r , when the specimen transformed martensitically between 425°C and 250°C. The waveform was obtained for signals with different peak heights following the above procedure several times, each time with a different trigger level. Also, the total ringdown counts were obtained for different trigger levels using the counter connected to the trigger circuit.
6-
I
1
O-03 -0.2
-0.1
I
I
0
0.1
I
0.2
I
0.3
I
0.4
0.5
( 7
0.6
Lo9 u,
Log total number of ringdown counts versus log trigger
Fig. 4 level
Results
0
IO
20
30
40
50
60
70
80
90
Time C@
Fig. 2
The average wave form of AE burrts
I’
I
A sample of the recorded waveforms of the AE burst of 3.6 V peak height is given in Fig. 2. The shape was found to be qualitatively the same for bursts with different peak heights. It is important to ensure that the envelope is not exponentially decaying. To obtain the dependence of the ringdown counts per pulse (Nn) on the signal peak height (Jr) and the trigger level (UJ*, pulses exceeding the levels 0.5, 1,l S, 2,2.5 and 3 V were counted for signals with peak heights 0.5,0.75,1,1.8,2, 2.5,3.15 and 3.6 X. The number of counts is plotted in Fig. 3 against the signal peak height for different trigger levels. The straight lines obtained were found to intersect at the same point (- 2, - 5.3), and each straight line can be represented by the equation:
1413-
12-
II-
NR
=
Ah+B
(1)
where A and B are constants for each trigger level, and they can be obtained from the experimental relations: A
8-
2”
I-
1 = 0.1611 + U,) ’
B = - (2 + 3.56 log U,)
(2)
6-
Substituting the values of A and B from (2) in (l), we get the general expression:
5-
4-
NR
3P-
II
OO
I
I
I
2 h CVI
I
I
3
4
Ringdown counts versus signal peak height for different Fig. 3 trigger levels
88
=
h
0.16 (1 + U,)
- (2 + 3.56 log U,)
To obtain the peak height distribution, the total number of ringdown counts (NT) exceeding the trigger levels 0.75, 1.5,2.5 and 3.6 was measured, and found to be 7.3 x 106, 1.42 x 10s, 1.23 x lo4 and 2.56 x lo’, respectively. Fig. 4 shows a plot of log NT against log Ut which is a straight line, from which the relation between the total number of ringdown counts and the trigger level is: lh
and Ut measured in volts
ULTRASONICS.
MARCH
1978
NT
=
1.58
x
lo6 ut-S*l
(4)
The total number of ringdown counts is given by:
NT
=
NR
f(h) dh
(5)
ut where f(h) is the peak height distribution function which is unknown. Functions N,(U,) and NR(Ut, h) were experimentally determined, but have no simple form. Therefore, the inverse of the transformation which gives f(h) cannot be found analytically and has to be obtained numerically.
130 M. This shows that the obtained waveform cannot be explained as a consequence of end reflections. The relation obtained between the ringdown counts and the signal peak height is practically linear, and not logarithmic, as in the case of an exponentially decaying envelope. The total ringdown counting was found to be proportional to Ut-‘.‘, which is also different from that obtained in tensile tests. Consequently, the dependence of NT on the system gain is NT (gain)“‘, which shows that the total ringdown counting, and also the rate of counts of AE during martensitic transformation, are very dependent on the system gain. So special attention must be paid to the precise determination of both the trigger level and the system gain in such experiments. References
Discussion
1 2
The signal recovery technique for obtaining the average waveform of AE bursts was described. It was seen that the envelope obtained is different from the usually assumed exponential decay. This could be attributed to beating of several possible vibrational modes of the whole system. The first reflected wave, corresponding to double the waveguide’s length and the speed of sound of 3000 m s-l, should appear after
ULTRASONICS.
MARCH 1978
3 4
Giiis, P.P. Acoustic Emission, ASTM STP 505 (1972) 20-29 Frederick, J.R and Felbeck, D.K. Acoustic Emission, ASTM STP 505, (1972) 129-139 Liptai, RG., Dunegan, H.L., and Tatro, CA., Inf J Nondest Test l(1969) 213-221 Brindley, B.J., Holt, J., and Palmer, I.G., Nondest Test (Dee
1973) 299-306 5 6 I
Speich, G.R 505, (1972) Speich, G.R Integrity by Pokck, bk
and Fisher, RM. Acoustic Emission, ASTM STP 140-151 and Schwoeble, A.J. Monitoring Structural Acoustic Emission, ASTM STP 571 (1975) 40-58 Nondest Test (Ckt 1973) 264-269
89