ACOUSTIC EMISSION TECHNIQUE FOR CHARACTERIZING
DEFORMATION AND FATIGUE CRACK GROWTH IN
AUSTENITIC STAINLESS STEELS
Baldev Raj, C.K. Mukhopadhyay and T. Jayakumar
Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research
Kalpakkam-603102, India
ABSTRACT: Acoustic emission (AE) during tensile deformation and fatigue crack growth (FCG) of
austenitic stainless steels has been studied. In AISI type 316 stainless steel (SS), AE has been used to
detect micro plastic yielding occurring during macroscopic plastic deformation. In AISI type 304 SS,
relation of AE with stress intensity factor and plastic zone size has been studied. In AISI type 316 SS,
fatigue crack growth has been characterised using acoustic emission.
INTRODUCTION
Acoustic emission technique (AET) is extensively used for materials research, online process and structural integrity monitoring because of its capabilities for detection and
location of dynamic events [1]. An understanding of the acoustic emission (AE) signals
generated during deformation, fracture, crack growth etc. could help in better interpretation
of the AE data thus leading to comprehensive characterization of materials and processes.
This also leads to the development of reliable technology for monitoring structural integrity
of engineering components and plants.
Acoustic emission generated during deformation of flawed/notched specimens is
m
known to bear a correlation of the type N=AK where N is total AE count, K is stress
intensity factor and A and m are constants [2]. Since in materials with presence of flaws or
notches, the stresses developed at the notch tip are controlled by stress intensity factor, such
correlation between AE and stress intensity factor could be used to characteristic the stress
state at the notch tip. AET, which gives information on the dynamic changes can also be
applied for continuous monitoring of fatigue crack growth (FCG). During FCG, the major
sources of AE for a ductile material could be the cyclic plasticity occurring ahead of the
crack tip, whereas, for brittle materials, the crack extension at the crack tip could be the
major source. The successful demonstration of early detection of fatigue crack growth using
AET in laboratory specimens would imply that this technique could be advantageously
employed as an on-line monitoring technique for integrity monitoring of
components/structures made of stainless steel and experiencing fatigue damage.
In this paper, use of AET for characterizing different aspects of deformation and
crack growth have been discussed by citing specific examples. These include: modelling of
frequency of the AE generated during yielding in AISI type 316 stainless steel (SS), study
of the relation of AE with stress intensity factor and plastic zone size in AISI type 304 SS
and early detection of fatigue crack growth (FCG) in AISI type 316 SS.
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
1439
MODELLING OF FREQUENCY OF AE GENERATED DURING YIELDING IN
AISI TYPE 316 SS
AET has been used to detect the micro plastic yielding occurring during
macroscopic plastic deformation in AISI type 316 stainless steel. It is expected that
selection of different resonant frequency sensors would be essential for detecting the AE
signal with maximum sensitivity at different strain levels during tensile deformation. An
attempt has been made to develop a theoretical model to predict the approximate frequency
range of the AE signal generated from dislocation sources operating during pre-yield and
near-yield tensile deformation. The model is based on the assumption that a Frank-Reed
(FR) dislocation source at the centre of a grain starts generating new dislocations at a
critical shear stress and gets piled up against the grain boundary. The frequency of the AE
signal has been calculated from the event life time of the FR and grain-boundary (GB)
source operations. It is assumed that the centre of gravity in a dislocation pile up is located
at three quarters of the distance from the source to the head of the pile up. If it is considered
that the dislocations that have moved to the point of centre of gravity can take part in
generating a single AE pulse, then it is appropriate to assume that only about 3/4 of the total
dislocations, each moving an average distance of 7/8 of the radius of the grain (D/2), give
rise to a single AE pulse. The event lifetime (t) can then be estimated as [3]
t = ((3/4) x n x (7/8) x (D/2)) / v
(1)
where n is total number of dislocations piled up at the grain boundary, D is grain diameter
and v is free flight velocity of a dislocation. As suggested by Fleischmann et al. [4] and
Schaarwachter et al. [5], if we consider that the event lifetime (t) is equal to half the period
(l/2f) at a frequency f, a frequency of 133 kHz is predicted for the AE generation during
macroyielding for AISI type 316 stainless steel with a grain size of 45 |im. The model for
predicting the frequency of AE signal from FR source operation during pre-yield
deformation has been verified by the experiments on nuclear grade AISI type 316 SS [3].
Tensile specimens having gauge dimensions 32x6.5x3 mm prepared from the
annealed nuclear grade 316 SS were used. Tensile tests were carried out at a strain rate of
5.2xlO"4 s"1 at ambient temperature. AE signals generated were captured by a 170 kHz
resonant transducer. The expected frequency of the AE signal corresponding to the
generation of FR and GB sources during microyielding (shear stress of 47 MPa) was
theoretically estimated to be 133 kHz which is very close to the frequency (170 kHz) of the
resonant sensor used for the experiments. This model has been extended to predict the
frequency of the AE signal form the GB source operation near macroyielding region. The
validity of the model had also been verified by considering the AE results obtained from
aluminium, copper and AISI type 316 SS by different investigators [6-8]. Good agreement
observed between the theoretically estimated and experimentally observed frequencies of
AE at the microscopic yielding demonstrated the validity of the model.
CORRELATION OF AE COUNTS WITH STRESS INTENSITY FACTOR AND
PLASTIC ZONE SIZE
The correlation between total AE count and stress intensity factor has been studied
for notched specimens of the nuclear and commercial grade AISI type 304 stainless steels.
For this, single edge notched specimens of the two steels in the annealed condition were
used [9]. Notches with a/W ratio of 0.25 (a is notch length and W is specimen width) were
introduced in different specimens with varying thickness in the range 1.5 mm to 5 mm.
1440
Tensile tests were carried out at a nominal strain rate of 5x1 0"^ s"* at ambient temperature.
A part of the specimens was tested up to failure and another part was loaded to different
a/Gys values (a is the applied stress and oys is 0.2% offset yield strength in the unnotched
condition) between 0.2 and 0.6, unloaded and taken out for the estimation of plastic zone
size at the notch tip. AE signals generated during the tensile tests were recorded by a 175
kHz piezoelectric transducer. In order to correlate AE with the size of the plastic zone (rp) at
the notch tip, the values of rp for different unloaded notched specimens were estimated by
microhardness technique [10].
The results obtained have been analysed with the help of the relationship N=AKm.
The values of K were determined using two approaches, (i) sharp crack approach (K) and
(ii) maximum stress approach (Kp). The stress intensity factor values for these two
approaches were calculated as per the following formulations:
(i)
(ii)
Sharp crack approach [11]:
K=(P/WB)Va[1.99-0.41(a/W)+18.7(a/W)2-38.48(a/W)3+53.85(a/W)4]
(2)
Maximum stress approach [12]:
Kp- V(7C/2) lim
(3)
Vp}
(4)
<W = 2 (Va/p) a
where K and Kp are in MPaVm, P is load in kg, a is nominal stress in MPa, amax is
maximum stress developed at the notch tip in MPa, p is notch tip root radius in mm, and B,
W and a are in mm. The formulation given by Eq.l is for single edge notched tensile
specimens having fatigue precracks [11], whereas Eq.2 is based on the consideration of a
fixed notch root radius at the notch tip [12].
A typical plot for the variation of N with K is shown in Fig.l for the nuclear grade
steel (3 mm thickness). The correlation results are shown in Table la and Ib for the nuclear
and commercial grade steel respectively. It can be seen that the relationship N=AKm holds
good for both types of steels. The correlation results can be best described by considering
two domains of linear variation between N and K; one up to a K value corresponding to the
load of macroyielding and the other beyond macroyielding. The value of m is higher for the
pre-yield region compared to the post-yield region. The value of the exponent m in the
correlation N=AKm for both the steels lie below the theoretically predicted value of m=4
reported in the literature [1]. This is in agreement with the proposition of AE generation
form localised plastic deformation at the notch tip.
3.5
3.0-
4.0'
Nuclear grade steel
Thickness: 3 mm
3.5
^2.5-
Nuclear grade steel
Commercial grade steel
Z
. 3.0
c"
o 2.0-
§2-5
"<5
21.55?
-21.00.5'
0.5
•
•
*
-Specimen 1
-Specimen 2
-Specimen 3
£2.0
~ 1.5
Slope: 1.30
Regression coefficient: 0.93
1.0
1.0
1.5
2.0
2.5
log (plastic zone size, r )
log (stress intensity factor, K)
FIGURE 1. Typical log-log plot of total count (N)
vs. vs. stress intensity factor (K).
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FIGURE 2 Log-log plot of total count (N)
plastic zone size (rp).
TABLE la. Values of exponent (m) and corresponding values of stress intensity factor of nuclear grade 304
stainless steel as a function of specimen thickness (B).
Post-yield
Pre-yield
B
(mm)
1.5
2
3
4
5
Sharp Crack App.
m
K(MPaVm)
1.47
1.62
2.50
2.37
1.68
6.0-29.3
7.9-29.5
10.0-30.4
11.0-31.1
6.2-33.7
Maximum Stress App.
m
Kp (MPaVm)
1.47
1.62
2.53
2.37
1.70
5.9-28.8
7.4-27.1
9.4-27.9
10.2-28.8
5.8-31.7
Sharp Crack App.
m
K(MPaVm)
1.71
1.48
1.17
1.38
1.0
23.6-71.0
22.8-69.9
24.1-69.2
27.4-77.4
30.2-78.7
Maximum Stress App.
m
Kp (MPaVm)
1.79
1.48
1.17
1.38
1.0
22.9-69.9
22.4-69.6
22.9-69.2
25.7-77.0
29.4-74.0
TABLE Ib. Values of exponent (m) and corresponding values of stress intensity factor of commercial grade
304 stainless steel as a function of specimen thickness (B).
B
(mm)
1.5
2
3
4
5
Pre-yield
Sharp Crack App.
Maximum Stress App.
m
m
K(MPaVm)
Kp (MPaVm)
2.51
2.62
3.10
2.90
2.06
6.1-29.9
11.2-29.7
6.9-25.5
6.8-34.4
6.0-32.1
2.51
2.63
3.10
2.90
2.38
5.8-28.3
10.4-28.2
6.6-25.2
6.2-31.4
5.7-30.2
Post-yield
Sharp Crack App.
Maximum Stress App.
m
m
K(MPaVm)
Kp (MPaVm)
1.90
1.81
1.12
1.44
1.12
19.7-67.6
20.1-69.2
24.3-69.2
25.7-79.4
26.6-73.0
1.84
1.79
1.12
1.44
1.32
18.7-64.6
19.1-64.6
23.4-69.2
24.5-74.1
26.3-72.8
The magnitudes of m are higher for the commercial grade steel than that for the
nuclear grade one and dependent on thickness. The values of m obtained by both sharp
crack and maximum stress approaches are almost similar.
It was reported by Palmer and Heald [13] in an earlier investigation that the total AE
counts up to an imposed applied stress on a notched specimen are directly proportional to
the size of the plastic zone at the notch tip. The N-K relations described in the literature are
also given theoretical support with the assumption of some proportionality to exist between
N and rp under a given loading condition. An investigation has been made to probe the
relation of total AE counts with plastic zone size for notched specimens of nuclear and
commercial grade AISI type 304 stainless steels and this has shown that the relation
between N and rp can be expressed by the equation N=<xr^ [10]. The experimental
measurements of rp by microhardness technique have indicated the value of P to be 1.3
(Fig.2) and this is higher than the theoretically assumed value of p=l in the literature [13].
These investigations have indicated that the value of m in the equation N=AKm can be
obtained from the value of the exponent (p) in the relation N=a.r£ as m=2p and such values
of m can be correlated to the directly estimated values of the exponent in the N-K relation.
A synergistic analyses of relations between N and K and that between N and rp has been
made to provide the validity of the assumption mentioned in the literature [13] that N is
directly proportional to rp.
FATIGUE DAMAGE ASSESSMENT IN AISI TYPE 316 STAINLESS STEEL
The relation between AE and cyclic stress intensity factor during fatigue crack
growth of AISI type 316 stainless steel was investigated [14,15], These investigations were
carried out to explore the possibility of using AET for identifying the transition from stage
Ha and lib during fatigue crack growth in 316 stainless steel. For this part of the study,
1442
fatigue pre-cracked CT specimens of 25 and 12.5 mm thickness were used. AE generated
during fatigue crack growth tests was acquired using a 375 kHz resonant transducer. The
value of the cyclic stress intensity factor AK was calculated from the standard expression
for CT specimens (ASTM Standard 1986).
AE Behavior during Stage Ha and Stage lib
Investigations have been carried out to characterise AE generated during fatigue
crack growth studies of AISI type 316 stainless steel. These investigations showed that even
if the influence of microstructure on macro-crack growth rate is not significant during stage
II, its influence on acoustic mission should be very significant because of its microscopic
origin [14,15]. The results of such an investigation have been depicted in Fig.3 which
shows the plots of log(da/dn) and log (N) as a function of log (AK) for 25mm thick solution
annealed and thermally aged specimens of 316 stainless steel, where da/dn is crack growth
rate, N is cumulative ringdown counts and AK is cyclic stress intensity factor. Figure 4
shows similar plots for 12.5 mm thick parent metal and weld specimens of the same
material. In contrast to da/dn, the variation in log (N) with log (AK) shows a two slope
behavior for all the specimens which indicates a change in crack growth mechanism within
the linear Paris regime.
During stage II fatigue crack growth, extensive dislocation multiplication and
rearrangement of dislocations during cyclic plasticity within the cyclic plastic zone (CPZ) is
the major source of AE as compared to ductile crack extension, the expansion of the
monotonic plastic zone (MPZ) and crack closure. It is known that the size of the CPZ
increases with increase in AK [16]. It has been reported [16] that the CPZ is always
generated and developed only under plane strain condition. The actual measurement of CPZ
size using etchning technique in Fe-3Si steel [17] and that using microhardness technique in
medium strength steel [18] showed that the CPZ size always increases under plane strain
condition and decreases under plane stress condition. Therefore, the size of the CPZ is
expected to increase with increase inAK during stage Ha as the plane strain condition
dominates the major portion of the specimen thickness. The observed higher AE activity
during stage Ila is thus mainly attributed to the increasing CPZ volume and the increasing
extent of cyclic plasticity within the CPZ as the crack advances in several loading cycles.
Solution annealed
5
10
ho4
T6l Solution
eg f anneated(SA)
s
.
x
^
0
0
"Aj3 SA+Aged
p P3J for2hrs.
000
o °» » • •
*
QO°«
4! SA+Aged
[ for 8 hrs.
•"•
°^n
An
D
a
x
x
*
^*
*
2
X
5
*****
tfCP
:D
(..;.>
^1
3,
; io
da/dn, m/cycle
0
0
"a1**
10
20
30
40
50
60
70
AK MPa Vm
FIGURE 3. Variation in cumulative ringdown counts (N) and crack growth rate (da/dn) as a function of
cyclic stress intensity factor (AK) for 25 mm thick solution annealed (SA) and thermally aged (TA) specimens
of AISI type 316 stainless steel.
1443
105
Parent
Weld
i4
0°
0
lio
-10-
nj 103
-10-6
|io2
-ID'7
•£
C*
3
10
10
20
30
40 50 60
AK MPa /m
FIGURE 4. Variation in cumulative ringdown counts (N) and crack growth rate (da/dn) as a function of
cyclic stress intensity factor (AK) for 12.5 mm parent and weld specimens of AISI type 316 stainless steel
During stage lib, plane stress condition dominates [19]. As mentioned above, the
CPZ size is expected to decrease during this stage. The value of the CPZ size computed
from strain range distribution in an aluminum alloy by Davidson et al. [20] also showed that
the size of the CPZ decreases with increase in AK. This was attributed to the change in the
stress state from plane strain to plane stress. In addition, the plastic strain range in the CPZ
increases with increase in AK. It is also known that as the crack length increases, strain
intensification occurs in the plastic zone [21]. Therefore, the mean free path for the
dislocation movement is very much reduced at higher AK. Hence, there is a reduction in the
extent of cyclic plasticity. As a result, the AE activity decreases during stage lib. This study
has shown that acoustic emission technique can be successfully used to identify the
transition from stage Ila to stage lib during fatigue crack growth in materials.
Effect of Microstructure on AE during Stage Ila FCG
It is evident from Figs. 3 and 4 that the AE activity during stage Ila is strongly
influenced by microstructure variations. It has been observed that the presence of carbide
precipitates in thermally aged specimens do not significantly influence the macrocrack
growth behavior but the AE behavior is strongly influenced by the presence of carbides.
The AE activity (RDC) is reduced by 2 to 3 times in 2 hrs. aged specimens and 20 to 25
times for 8 hrs aged specimens as compared to that of solution annealed specimens. This is
attributed to the precipitation of carbides during thermal ageing which reduces both the
dislocation mean free path and dislocation source length. According to the model proposed
by Agarwal et al. [22], reduction in dilocation source length and mean free path reduce the
slip region diameter which in turn results in the reduced acoustic activity. It was also
observed that the slope of log (N) vs log (AK) plot during stage Ila decreases with increase
in ageing time. This has been attributed to the increase in the number density of carbide
particles at longer ageing times.
The FCG in weld metal generates higher AE ringdown counts as compared to that in
the parent metal, even though the crack growth behavior appears similar (Fig.4). The slope
of log (N) vs log (AK) plot during stage Ila showed the highest value for weld specimen. Xray diffraction based residual stress measurements showed the presence of tensile residual
stress of about 110 MPa in the weld region. The presence of tensile residual stresses in the
1444
weld structure could also aid to induce brittle microcracking and crack deflections which
result in crack path tortuosity in the weld specimens. The surface roughness induced crack
closure phenomenon resulting from crack path tortuosity would also contribute to AE
generated in the weld specimens, particularly during stage Ila. Microscopic investmgation,
indeed clearly showed zigzag crack path in the weld specimen as compared to the straight
path in the case of parent metal. Therefore, the enhanced AE activity in the weld specimen
has been attributed to the combined influence of cyclic plasticity, residual stress induced
microcracking and the roughness induced crack closure.
Empirical Relation between AE and Cyclic Stress Intensity Factor
As indicated earlier, during stage Ila of fatigue crack growth, the AE signal is
mainly generated by the cyclic plasticity within the CPZ. Hence, the total AE ringdown
counts generated at a given K should depend on the size of the CPZ, the average platic
strain within CPZ and the number of cycles before each crack extension. It was found that
the AE activity is strongly dependent on the combination of the volume of the CPZ (ocAK4),
average plastic strain range (°cAK2) and the number of cycles before each crack extension
(°cAK2/(da/dn)). Based on this, an empirical relationship between the cumulative RDC (N)
and AK was proposed as
N o c ( A K 4 * A K 2 ) * AK 2 /(da/dn)
(5)
Considering that da/dn ~ C AK3 for AISI type 316 SS,
NocAK 5
(6)
This empirical relation has been verified for solution annealed AISI type 316 stainless steel
(Fig.5) [14]. Since the transition from stage Ila to lib is found to occur at AK = 30 MPa
m17 , the data upto this value has been considered for the calculation of the slope in Fig.5.
There is a good agreement between the experimentally observed values (4.54 to 5.39) and
the predicted value of the exponent (5) in equation (6).
* SAB parent
o W34 weld
105
104
I
103
10
AK MPa Vm
FIGURE 5 Variation of cumulative ringdown counts (N) as a function of cyclic stress intensity factor (AK)
during stage Ila fatigue crack growth for 12.5 mm thick specimens.
1445
SUMMARY
Use of acoustic emission technique for understanding deformation and crack growth
has been discussed. A model has been given for estimating the frequency of AE generated
during microyielding in order to detect yielding in metals with high sensitivity using AET.
The correlation of acoustic emission count with stress intensity factor and plastic zone size
of notched specimens of 304 stainless steel has been studied. AET has also been used for
early detection of fatigue crack growth in 316 stainless steel. These studies have
demonstrated the capability and usefulness of AET as a materials science tool for
characterisation of dynamic events taking place on a microscopic scale in addition to its
applicability for monitoring structural integrity.
ACKNOWLEDGMENTS
Authors are thankful to our former colleague Dr. V. Moorthy, who is presently at
the University of Newcastle, U.K., for his contribution.
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1446