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The effect of pressurization on the acoustic emission characteristics of a low carbon steel
The effect of pressurization on the acoustic emission characteristics of a low carbon steel L. Bolin and C-G. Gustafson The effect of a pre-pressurization of 1100 MN/m 2 on the acoustic emission (AE) characteristics of a commercial low carbon steel has been investigated. The results show that the pressurization decreases the AE intensity during the entire tensile test. A tendency for the upper and lower yield stress to be decreased is also noticed. This is probably caused by the formation of pressure-induced mobile dislocations and active dislocation sources at second phase particles. This work shows an effect very similar to the Kaiser effect, which is that a material in which plastic deformation or fracture has occurred during loading gives no emission before the previous load level has been reached. In the present work, the load is in the form of a hydrostatic pressure and this gives no plastic deformation. There are several theories on AE source mechanisms and it seems plausible that different mechanisms are operative in different cases. In some mechanisms dislocations are involved but not in others. In fiber-reinforced plastics for example debonding or fibre fracture may produce AE. In polycrystalline metals dislocation movements normally take place during deformation. This applies even at strains where the material behaves elastically on a macroscopic scale. As AE can be detected during deformation it is reasonable to assume that there exists some interrelation between dislocation movements and AE. Materials that exhibit discontinuous yielding very often have the type of AE behaviour shown in Fig. 1. Several semi-quantitative models have been discussed in the literature to explain this behaviour in physical terms. The most widely accepted theory suggests that the bursts of AE are caused by a sudden release of a pile-up of dislocations, which results in the generation of a great number of dislocation loops giving the AE bursts.
Pressurization of iron would therefore be expected to reduce the intensity of AE produced during a subsequent tensile test as compared with the AE from an unpressurized specimen. The amount, and possibly also the intensity of the dislocation bursts would be reduced by pressurization. The extent to which this behaviour will be present for different materials would be expected to depend on the material composition, heat treatment details and the level of hydrostatic pressure.
Experimental technique The material used in this study was a plain cold-roiled steel containing in the as received condition 0.08%C, 0.04% Si, 0.33% Mn, 0.009% P and 0.014% S. The tensile specimens were manufactured with a gauge length of 25 mm and with a cross-section of 8mm × 3 mm. No heat treatment was given. I000
15
750~
-g
AE
Also, when impurity-locked dislocations are torn away from their impurity atmospheres high AE will occur ~ and participate in the multiplication of dislocation loops. Another possible source of emission during this early stage of deformation is cracking of non-metallic inclusions such as various carbides. It has been shown that this cracking can occur at very small st rains. A hydrostatic pressure of a sufficiently high level changes the subsequent yield behaviour of metals containing second phase particles acting as elastic discontinuities. 2 Mobile dislocations and dislocation sources are thereby introduced at the second phase particles. In iron this results in reduced upper a u v s (upper yield stress) and lower aLYS (lower yield stress) and a decrease in Luder's strain. 3-6 At a sufficiently high level of pressurization it is possible to eliminate the initial yield discontinuity. 2'3
20
B
I0
500~
5
250
O, 0
2.5
a
± 7.5
5 Elongation(mm)
20
O00
75o'~ "i~ z
v o~ o 500 E
AE
-~
I0
Load
J bJ
250 <
5
o
i 0
D
i 2.5
.]
.on l
nh
5 Elongation ( mm )
_
71.5
The authors are at Link~ping Institute of Technology, Department of Mechanical Engineering, Division of Solid Mechanics, S-581 83
Fig. 1
LiB kiSping, Sweden.
for t w o unpressurised, cold-rolled steel specimens
NDT I NTE RNATIONAL. OCTOBER 1 9 7 8
0 I0
--
o
IO
Load and acoustic emission rate as a function of elongation
0308-9126/78/050227-02 $02.00 © 1978 I PC BusinessPress
227
20
IO00
Compare this with Fig. 2 where almost all signals in this region are lower than the threshold level of the counter.
15
750 "~
This behaviour of decreased AE intensity and total amount of emission caused by pressurization are more evident than the change in OuYS and crLvs.
Z
500
§ io .J
o hi
250 <
2.5
a
5 Elongation (mm)
0 IO
7.5
20t
I000
7 5 0 7 "1-
15
AE
z g
500 E=
IO
Load
.J
250 <
J ..,,h_
0
b
0
2.5
5
- -
71.5
0
I0
Elongotion(mm)
Fig. 2 Load and acoustic emission rate as a f u n c t i o n of elongation for t w o cold-rolled steel specimens pre-pressurised at 1100 M N / m 2
The specimens were placed in the bore of a high-pressure container and subjected to a pressure of 1100 MN/m 2 using castor oil as the pressure fluid. The maximum pressure was maintained for 30s before decompression. After the pressurization the specimens were kept at -50°C for 4-6h. Before testing the specimens were brought to room temperature. The tensile testing was carried out using a 50 kN servo-hydraulic testing machine from MTS Systems Corp. In all tests a crosshead velocity of 0.05 mm/s was used. The acoustic emission equipment used was a Dunegan/ Endevco, 3000-series, with a type S-140 transducer covering a frequency range of 100--300 kHz. The AE count rate was measured and registered together with the load-elongation curve on a two-channel x-y recorder. The total gain of the amplifier was 97 dB in the tests.
Discussion
The plain low carbon steel investigated responded to the pressurization with a decrease in Ours and oLVS. This is in accordance with the results reported in References 2-6; however, this effect is relatively small. A probable explanation for this fact is that the material was tested without prior annealing. In Reference 6 a low carbon steel with a similar composition was annealed to achieve a high initial Ouvs and a high initial difference in Ouvs and tTLyS. A subsequent pressurization then caused a considerable decrease in o u v s . A higher level of pressurization would probably remove the initial yield discontinuity but the strength of the pressure container limited the pressure to 1100 MN/m 2. The decrease in AE intensity is more pronounced though there are some individual variations. In some cases high AE bursts occur just prior to yielding. This behaviour could be explained by the assumption that there were not sufficient numbers of mobile dislocations and dislocation sources present to prevent all the locked-in dislocations from breaking away from their impurity atmospheres and taking part in the deformation process. A background emission from cracking at non-metallic inclusions and released dislocation concentrations may also be present. The results also show that AE behaviour is sensitive to small physical and mechanical changes and that the AE technique is well suited to measuring and observing these changes. Conclusions
Specimens exposed to a hydrostatic pressure of 1100 MN/m 2 respond with considerably lower AE intensity than unpressurized specimens. This behaviour is caused by pressure induced mobile dislocations and active dislocation sources at second phase particles.
Acknowledgement Results
Specimens tested in the unpressurized condition show high intensity of AE at the first stage of straining with a maximum just before the upper yield stress is reached. Typical examples are given in Fig. la and lb. A substantial AE also occurs during the strain hardening stage. This bechaviour is typical for the AE from most mild steels. The tests with pressurized specimens show a decrease in Ouvs and OLYS and in the Lilders strain value. This is illustrated in Figs 2a and 2b. It is also evident from Figs 1 and 2 that the AE intensity and total amount of emission are decreased by a pressure treatment of 1100 MN/m 2. It is also true throughout the strain hardening stage where the unpressurized material is noisy as evident from Fig. 1.
228
The pressurization was carried out using the high-pressureequipment at the department of Production Engineering, Chalmers University of Technology, Gothenburg, Sweden. References
1 2 3
4 5 6
Ono, K. Proceedings o f the Second Acoustic Emission Symposium (Tokyo, 1974) Radcliffe,S.V. in 'The mechanical behaviour o f materials under pressure' edited by H.L1.D. Pugh (Elsevier, 1970) Bullen, F.P.,Henderson, M.M. andWain, H.L. PhilMag9 (1964) p 285 Radcliffe,S.V. "Irreversible effects o f high pressure and temperature on materials" (ASTM, New York, 1964) Yajima,M. and Ishii, M. Acta Metall 15 (1967) p 651 Capp, D.J., McCormie, P.G. and Muir, H. Acta Metall 21 (1973) p 43
NDT INTERNATIONAL.
OCTOBER
1978
Pressurization of iron would therefore be expected to reduce the intensity of AE produced during a subsequent tensile test as compared with the AE from an unpressurized specimen. The amount, and possibly also the intensity of the dislocation bursts would be reduced by pressurization. The extent to which this behaviour will be present for different materials would be expected to depend on the material composition, heat treatment details and the level of hydrostatic pressure.
Experimental technique The material used in this study was a plain cold-roiled steel containing in the as received condition 0.08%C, 0.04% Si, 0.33% Mn, 0.009% P and 0.014% S. The tensile specimens were manufactured with a gauge length of 25 mm and with a cross-section of 8mm × 3 mm. No heat treatment was given. I000
15
750~
-g
AE
Also, when impurity-locked dislocations are torn away from their impurity atmospheres high AE will occur ~ and participate in the multiplication of dislocation loops. Another possible source of emission during this early stage of deformation is cracking of non-metallic inclusions such as various carbides. It has been shown that this cracking can occur at very small st rains. A hydrostatic pressure of a sufficiently high level changes the subsequent yield behaviour of metals containing second phase particles acting as elastic discontinuities. 2 Mobile dislocations and dislocation sources are thereby introduced at the second phase particles. In iron this results in reduced upper a u v s (upper yield stress) and lower aLYS (lower yield stress) and a decrease in Luder's strain. 3-6 At a sufficiently high level of pressurization it is possible to eliminate the initial yield discontinuity. 2'3
20
B
I0
500~
5
250
O, 0
2.5
a
± 7.5
5 Elongation(mm)
20
O00
75o'~ "i~ z
v o~ o 500 E
AE
-~
I0
Load
J bJ
250 <
5
o
i 0
D
i 2.5
.]
.on l
nh
5 Elongation ( mm )
_
71.5
The authors are at Link~ping Institute of Technology, Department of Mechanical Engineering, Division of Solid Mechanics, S-581 83
Fig. 1
LiB kiSping, Sweden.
for t w o unpressurised, cold-rolled steel specimens
NDT I NTE RNATIONAL. OCTOBER 1 9 7 8
0 I0
--
o
IO
Load and acoustic emission rate as a function of elongation
0308-9126/78/050227-02 $02.00 © 1978 I PC BusinessPress
227
20
IO00
Compare this with Fig. 2 where almost all signals in this region are lower than the threshold level of the counter.
15
750 "~
This behaviour of decreased AE intensity and total amount of emission caused by pressurization are more evident than the change in OuYS and crLvs.
Z
500
§ io .J
o hi
250 <
2.5
a
5 Elongation (mm)
0 IO
7.5
20t
I000
7 5 0 7 "1-
15
AE
z g
500 E=
IO
Load
.J
250 <
J ..,,h_
0
b
0
2.5
5
- -
71.5
0
I0
Elongotion(mm)
Fig. 2 Load and acoustic emission rate as a f u n c t i o n of elongation for t w o cold-rolled steel specimens pre-pressurised at 1100 M N / m 2
The specimens were placed in the bore of a high-pressure container and subjected to a pressure of 1100 MN/m 2 using castor oil as the pressure fluid. The maximum pressure was maintained for 30s before decompression. After the pressurization the specimens were kept at -50°C for 4-6h. Before testing the specimens were brought to room temperature. The tensile testing was carried out using a 50 kN servo-hydraulic testing machine from MTS Systems Corp. In all tests a crosshead velocity of 0.05 mm/s was used. The acoustic emission equipment used was a Dunegan/ Endevco, 3000-series, with a type S-140 transducer covering a frequency range of 100--300 kHz. The AE count rate was measured and registered together with the load-elongation curve on a two-channel x-y recorder. The total gain of the amplifier was 97 dB in the tests.
Discussion
The plain low carbon steel investigated responded to the pressurization with a decrease in Ours and oLVS. This is in accordance with the results reported in References 2-6; however, this effect is relatively small. A probable explanation for this fact is that the material was tested without prior annealing. In Reference 6 a low carbon steel with a similar composition was annealed to achieve a high initial Ouvs and a high initial difference in Ouvs and tTLyS. A subsequent pressurization then caused a considerable decrease in o u v s . A higher level of pressurization would probably remove the initial yield discontinuity but the strength of the pressure container limited the pressure to 1100 MN/m 2. The decrease in AE intensity is more pronounced though there are some individual variations. In some cases high AE bursts occur just prior to yielding. This behaviour could be explained by the assumption that there were not sufficient numbers of mobile dislocations and dislocation sources present to prevent all the locked-in dislocations from breaking away from their impurity atmospheres and taking part in the deformation process. A background emission from cracking at non-metallic inclusions and released dislocation concentrations may also be present. The results also show that AE behaviour is sensitive to small physical and mechanical changes and that the AE technique is well suited to measuring and observing these changes. Conclusions
Specimens exposed to a hydrostatic pressure of 1100 MN/m 2 respond with considerably lower AE intensity than unpressurized specimens. This behaviour is caused by pressure induced mobile dislocations and active dislocation sources at second phase particles.
Acknowledgement Results
Specimens tested in the unpressurized condition show high intensity of AE at the first stage of straining with a maximum just before the upper yield stress is reached. Typical examples are given in Fig. la and lb. A substantial AE also occurs during the strain hardening stage. This bechaviour is typical for the AE from most mild steels. The tests with pressurized specimens show a decrease in Ouvs and OLYS and in the Lilders strain value. This is illustrated in Figs 2a and 2b. It is also evident from Figs 1 and 2 that the AE intensity and total amount of emission are decreased by a pressure treatment of 1100 MN/m 2. It is also true throughout the strain hardening stage where the unpressurized material is noisy as evident from Fig. 1.
228
The pressurization was carried out using the high-pressureequipment at the department of Production Engineering, Chalmers University of Technology, Gothenburg, Sweden. References
1 2 3
4 5 6
Ono, K. Proceedings o f the Second Acoustic Emission Symposium (Tokyo, 1974) Radcliffe,S.V. in 'The mechanical behaviour o f materials under pressure' edited by H.L1.D. Pugh (Elsevier, 1970) Bullen, F.P.,Henderson, M.M. andWain, H.L. PhilMag9 (1964) p 285 Radcliffe,S.V. "Irreversible effects o f high pressure and temperature on materials" (ASTM, New York, 1964) Yajima,M. and Ishii, M. Acta Metall 15 (1967) p 651 Capp, D.J., McCormie, P.G. and Muir, H. Acta Metall 21 (1973) p 43
NDT INTERNATIONAL.
OCTOBER
1978