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Seismic and strain studies on large laboratory rock samples being stressed to failure
Tectonophyncs,
144 (1987) 55-68
Elsevier Science Publishers
55
B.V., Amsterdam
- Printed
in The Netherlands
Laboratory Modelling
Seismic and strain studies on large laboratory being stressed to failure H. SPETZLERI,
C. SO~D~RGELD
of Geologxal Sciences and Cooperative Insfirute
’ Department
rock samples
I**, G. SOBOLEV 2 and B. SALOV ’
forResearch
in Environmenral
Sciences,
ofColorado/NOAA,
University
Boulder, CO 80309 (U.S. A.) ’ Institute of the Ph,vsics
oftheEarth,
(Received
Academy
January
of Sciences of the USSR,
1.5,1986;
IO Grunzinskaya,
revised version accepted
Moscow (USSR)
July 28,1986)
Abstract Spetzler,
H., Sondergeld,
samples
C., Sobolev,
being stressed
to failure.
G. and Salov,
B., 1987. Seismic
In: R.L. Wesson (Editor),
and
Mechanics
strain
studies
of Earthquake
on large
Faulting.
laboratory
rock
Teer~noph~~ie~, 144:
55-68. Large granite and ultrasonic
(700 x 700 X 700 mm) and basalt velocity
measurements
ments
were correlated
patterns
of emission
initially
active then quiet and active again
rate toward
in space activity
failure. At constant
in terms of frequency
content
zone were very reproducible The Kaiser detected
effect
time. Several
applied
before
zones of acoustic greatly
failure,
and amplitude
one-half
and relatively
more power in dilatancy
was recorded in the sample.
sources stress:
increasing
The acoustic
active then quiet, (b)
continuousty
and at accelerating
emanating
signals was quite different from within
from transducer
i.e. very few acoustic
a particular
to transducer. emissions
stress 0. as long as o was less than about
the individual Strain
measurements
acoustic
emission
and ultrasonic
on much smaller
Stram
All measure-
were identified.
but quite different
of the stress of failure.
to failure.
(a) initially
zones. The signals
attained
loaded
deformation.
emission
at lower frequencies. Similar
emission
during
of the acoustic
transducer,
below previously
were cyclicly
recorded
with increasing
and (c) activity
for the different
on a particular
stress at failure. As failure was approached
large variations
emissions
stress. the seismic character
as recorded was cycled
and acoustic
these zones varied
seems to hold up to about
when the sample
the eventual
and
within
(580 X 580 x 915 mm) samples
were made
events became velocity
samples
were
one-half
of
more energetic
measurements are consistent
indicate with these
results.
Introduction
(Spetzler
With few exceptions (e.g. Dieterich, 1981; Okubo and Dieterich. 1984) failure experiments on laboratory samples have been restricted to samples having linear dimensions of less than 200
1982; Brodsky and Spetzler, 1984) and their possible interactions was severely restricted by sample size. For this reason several tests on large samples were performed at the 50,000 ton press at the High-Pressure Institute near Moscow, USSR. In addition to several large concrete samples, two
mm. Locations of zones preceeding and indicating failure have been identified by various experimental techniques. The identification of multiple zones
* Present
address:
Amoco
Production
Co., Research
and Estey,
trical conductivity, self potential. electromagnetic emissions, strain, acoustic velocity and emissions
Center.
0 1987 Elsevier Science Publishers
1974; Sondergeld
igneous rocks were broken: a granite specimen 700 X 700 X 700 mm and a basalt specimen 580 X 580 X 915 mm. The complete results of all measurements on the granite specimen involving elec-
P.O. Box 591, Tulsa, OK 74102 (U.S.A.)
00~-1951/u7/$03.50
and Martin,
B.V.
56
are reported
in the Soviet literature
1982).
Here
results
obtained
velocity
and emission
periments
we will
concentrate
from
the
strain
et al.,
0.5%) and contained
on
major
biotite
acoustic
rather
uniform
biotite
concentration
the
and
measurements
on the granite
(Sobolev
from the ex-
and basalt
samples.
and
thick crossed
procedures and results
The upper l/3
are different
of the 50,000 ton press is shown
in Fig. 1. It can accept
a sample
with a vertical
dimension
up to 3 m. The samples
were deformed
uniaxially,
but were constrained
tom by steel beams stress concentrations contact
on top and bot-
Both the granite
40% albite,
Its grain
2-3 mm. A vein of
approximately
the sample note
10 to 15 mm
(see Figs. 9 and 12). In
that
the experimental
for the basalt and granite
and
acoustic
20%
size was
emission
details
samples.
measurements
in
basalt
to minimize failure at the where the samples were in
with the pistons.
Velocity
of pyrite.
being between
the following, Experimental
40% quartz,
a trace
and the
Nine
compressional
transducers,
with
a free
resonance frequency of 150 kHz, were mounted on each of the four free faces of the sample. Their
basalt samples were loaded cyclicly, the loading history of the latter being shown in Fig. 2. The
locations and the 96 paths over which the velocities were measured are indicated in Fig. 3a. For data analysis the block was divided into 27 ele-
basalt
ments
sample had a porosity
of approximately
6%
and contained 60% plagioclase, 20% olivine, 10% pyroxene and a total of 10% of augite, diopside, magnetite, apatite mm). The granite
(see Fig. 3b) and the average
termined
for each block. The velocity
all traces
velocity
de-
averages
for
and for all cycles are shown
in Fig. 2.
etc. It was fine grained (< 1.5 sample had a low porosity (-c TABLE
1
Summary
of acoustic
emission
activity
Cycle No.
Basalt 2
24
-
11
+4%
0.31
3
42
25
113
-5%
0.54
4
63
40
17
-13%
0.81
5
78
55
(-55%)
1.00
lhr
Granite 3
23
17
+4R
0.13
4
32.5
24
66
- 20%
0.18
5
45
26
17
-4%
0.25
6
75
43
13
~ 20%
0.42
7
86
60
14
- 36%
0.48
8
139
55
8000
D,,,~~= maximum
(-50%)
stress reached
v* = stress at onset of acoustic
0.77
during
the n th cycle (MPa).
emissions
during
cycle number
n +1. Ar = Number
of hours following
cycle n before the start of the
next cycle. (( OA - %I, )/‘%,a~ ) x 100 is a measure of the memory The two values in parentheses correspond to data during occurred Fig. 1. The top portion High-Pressure shown.
Institute
of the 50,000 tom research in Troitzk
near
Moscow
press at the USSR
is
additional
loading
180 MPa was reached reloading
cycles.
8000 hrs (11 months) occurred
before
For
granite
loss. obtained
the reloading
later, at which time a stress of failure.
In the case of basalt,
1 hr after cycle 5 and the maximum
that could be reached
was 75 MPa.
the
stress
57
2468 250881
I Fig. 2. Five loading average
cycles are shown
of the velocities
velocities
in basalt
that
for the basalt
were measured
is contrasted
specimen.
in granite
9,233
v&9
644
4’8
T V---7
2
4
6
paths
8
0
310661
Note the time elapsed
over 96 different
with the increase
x3
2 4 11 12 08 B1 270661 26 I I
2
4
hours dates
0109.61
between
the loading
(see Fig. 3a) are also shown.
cycles (see Table 1). The
The general
decrease
for the
(see Fig. 7).
6,677 5.6.7
Yo
5 . 2 .
915
1
2 .
!
6 .
6
.
l
9 .
.
T
8.-
.
I
1”’
ta
I
5,4,3
I
6.1,2
2 ,I 96
/
L
(a)
(b)
Fig. 3. a. The sample sample.
There
calculation.
coordinates,
are 14 paths
the sample
transducers
locations
in each of three horizontal
was partitioned
into 27 elements
and the 96 paths planes
for the velocity
and nine paths
for which the velocities
measurements
in each of six vertical were determined.
are shown planes.
for the basalt
b. For purposes
of
SVi % +20
+15
Element
Number 0 4 n 5
+10 1 ;
.6 13 ;
14
*
'5
c
22
*
23
0
24
l-----EH-1-l 4
2 Fig. 4. The percentage given: ((I’, ~ F)/i;) were calculated
difference
between
the average
velocity
X 100. v, is the velocity in block number
similarly
to the method
used by Nishizawa
ucoustic
emission
measurements
t
I_
(see Fig. 2) and those in the interior
I, r is the algebraic
average
of velocities
elements
hrs of layer B (Fig. 3b) are
in all 27 elements.
The velocities
et al. (1985).
The changes from these average velocities that occurred during the last two cycles are shown in Fig. 4 for the nine elements of the interior slice B of Fig. 3b. Acoustic emission activity was recorded for the basalt sample and is shown in tabular form in Table 1. Velocit?, und grunite
8
6
in
Before and after cycling at zero axial stress, detailed P-velocity measurements were made over
290 horizontal (perpendicular to the loading axis) paths. The measurement procedure involved the bonding of a transducer pair, one on each opposing face exactly opposite each other. The transducer locations were marked on the free surfaces as illustrated in Fig. 5a. The z-axis was the vertical axis and thus the axis where the load was applied. Each set of 145 measurements in one direction yielded data for a velocity map. Such velocity maps were produced in pairs, one for the y direction and one for the x direction. Two such map pairs were produced for the granite specimen, one before stress was applied and one at the end of the
59
700mm
etc
i
(b) Fig. 5. a. For the granite locations
where
constructed
sample
the transducers
for the velocity
refers to velocities
between
(700
x
700
for these
measurements
x
700 mm) 145 velocity
velocity
measurements
in the x and ,V directions
4.8 and 4.9 mm ps-‘).
No signals
measurements
were made
were bonded both before
were observable
are shown.
in the .x and the )’ directions. b. Iso-velocity
and after loading
after loading
contour
of the gramte
in the zones Indicated
sample.
maps
The were
(e.g. 4.8’
by shading.
7th loading cycle. The four data sets are shown as velocity contour maps in Fig. 5b. Velocity histograms showing the change in velocity distributions for both directions The continuous
stress for 14 different paths are These velocity data were used in emissions. The loading history cycles of the granite sample and
Y,-Y,
-20
after--I
-before
ing acoustic gions within
-‘“3.
s-1
4.4
Fig. 6. Histograms Fig. 5 are shown larger ing.
for the velocity
measurements
for both
the x and
range and multimodal
distribution
after loading
are given in Fig. 6. change in velocities versus axial
when they are compared
described
y direction,
Note
in the
of the measurements with those before
load-
shown in Fig. 7. locating acoustic for the last five the accompany-
emission activity for five distinct rethe specimen are given in Fig. 8. The
locations of the transducers that were used to locate the acoustic emissions are shown in Fig. 9. A total of about 3500 events were assigned to these regions. The cumulative total of acoustic emissions for each of the last five cycles is plotted as a function of axial stress in Fig. 10. Examples of amplitude versus time traces for all eight receiv-
60
5.4
5.2 7 m 5.1 E Y S 5.0
4.9
4.8
4.7
/
“/ /
11
-
12
-0
I
5-7'
I-2'
13 14
-A -+
5-6' 5-3'
4.8
I
0
Fig. 7. In contrast the granite sample
to the 96 different
sample during
contrasts
20
sharply
measured
are indicated
transducer
locations.
loading.
40
60 80 stress MPa
paths over which velocities
The velocities
with the velocity by the transducer
during
decrease numbers,
100
were measured
in the basalt sample.
the final cycle {No. 8) are shown.
observed
in our basalt
sample
e.g.. 5-7’ refers to transducer
120
The general
(Fig. 2). The paths
5 sending
only 14 paths were measured velocity
increase
over which the velocities
and transducer
7’ receiving.
in
in the granite
were
See Fig. 9 for
61
T 5
0
I ,
0
50
,,
, I.
,
5
IO
I
,.. , ,
0
5
, , ,,, ,
0
hrs
5
,,,,,,
a
b
I
Fig. 8. a. The stress history increases regions
-m IO
within the sample. to the various
.
I-
-
for the last five loading Region
,d
rL
dl
5
events
to allow the many measurements
or on 2 and 8 and similarly assigned
-
I
cycles is shown for the granite
to be made at constant
I is characterized
for the other regions.
sample.
stress. b. The acoustic
by events with first and second Note the different
behavior
arrivals
of the various
The stress was held constant emission
activity
is shown
on either transducers regions.
after stepwise for five distinct
1 and 8 respectively
A total of about
3500 events were
regions.
c
0
001
05
041
lo
0 I’
006
*b face
z
O6’
of the transducers stress cycling
used for locating
2
IT
YL
measurements
X0
O*’ 03’
080
Fig. 9. The positions
02
07’
5b
during
20
face
07
face
0
XL
0
40
acoustic
as open circle pairs. A biotite
emissions
vein crossing
are shown
as filled circles,
the upper
part of the sample
those used for velocity is also included.
62
dominantly
equal dimensional
to collapse cracks
and
beginning
thus the decrease
values of nearly
elements
of the largely
The extremes
occur
interior
slip on the bottom
5 and 23, both
From the relatively
high
4, 5 and 6 we suspect
that
between
By analogy piston
the top piston
we conclude
that the
must have been substan-
tially greater than at the top. This interpretation further during
stress.MPa
Fig. 10. The extent
to which the Kaiser
effect is applicable
these
becomes
when
experiments
number
of acoustic
emission
apparent
events is plotted
stress for the last five cycles. For details
ing
transducers
are
shown
the
in
cumulative
as a function
of
see text and Fig. 14.
in Fig.
11 for
four
distinct acoustic emission events and at two stress levels during cycle 8. Each set of eight traces was recorded simultaneously on synchronized pretriggered digital oscilloscopes and then stored on floppy disks. A discriminator circuit (Sondergeld, 1980) was used to differentiate between noise and acoustic emissions and to subsequently trigger the oscilloscopes. The entire discriminator and recording system was described earlier by Sondergeld and Estey (1981). Struin measurements
on the granite sample
Sets of 84 and 104 strain gauges were glued to the X0 and Y, faces respectively (See Figs. 5b and 9). Those on face Y, are depicted in Fig. 12. The strain invariants were calculated for sets of three strain gauges and isostrain lines are drawn in Fig. 13 for three stress levels during
cycle 8.
Discussion
It is surprising that in spite of the large porosity of the basalt (6%) the sound velocity decreases perpendicular to the loading direction from the very start of load application. The total decrease is over 20% (see Fig. 2). We conclude that the pre-
enhanced unloading
in
slice B (Fig. 3b).
for elements
little or no slip occurred and the sample.
At the
a spread
20% (Fig. 4) in the nine
being next to the pistons. in elements
of axial
in velocity.
of the 4th cycle we observe
velocity
velocities
cavities are difficult
but lead to the early formation
through
the
observation
is that
at the end of the 4th cycle, those
elements in intimate contact with the top piston (4, 5, 6) show a relative increase in velocity (probably because of the radial restraint due to the lower compliance of the piston) while those that are less constrained radially show a further decrease in velocity. We conclude that during unloading considerably more axial cracks closed in elements 4, 5 and 6 than did in the other elements. Note that there is a decrease in the average velocity (Fig. 2). The final range of velocity within block B approaches 35%. The acoustic
velocities
in the granite
values sample
behave quite differently. Instead of a 20% decrease in the measured velocities there is a slight increase until very close to the end of cycle 8. The high rate of velocity increase with stress that is shown in Fig. 7 occurs up to the maximum stress level reached in cycle 7. Note that the maximum stress reached in cycle 7 was only 0.77 of the failure stress (Table 1). When the maximum values in Fig. 7 are compared with the velocity distributions before loading shown in Fig. 6, the increase in the measured velocities from the unloaded to the fully loaded case is found to be between 5 and 10%. The increase in velocity due to the closing of preexisting cracks seems to dominate over the effect of the newly created cracks which would be expected to result in a decrease in velocity. The effect of the newly created cracks is perhaps best seen in Figs. 5b and 6 where velocity measurements are compared that were made before cycle 1 and after unloading from cycle 8. The histograms in Fig. 6 clearly show that the
63
average velocities directions
at zero load in both the x and _y
decreased
smooth
distributions
changed
to broad
loading.
We interpret
form distribution greater
isovelocity
6%.
obtained
Relatively
before
loading
distributions
after
this to be due to a non-uni-
of cracks within of the
the sample.
velocity
maps before
and after loading
changes
12% in the x and
the minimum
changes
in velocity
4’ directions about
The
distributions
can also be seen when comparing
5b. The maximum and
about
multimodal
complexity
after loading
by
firms an earlier observation (-
a small
50 mm)
by one of us (C.S.) on
sample
which had lost all memory
of previous
it was stored in the laboratory (Yoshikawa general
and
validity
Mogi
in Fig.
were 13.5% respectively,
4%.
levels by stress cycling ing changes
also
question
in the acoustic
emission
after the completion in the present 180 MPa.
to find previous
their samples
months
at
stress after
for several months.
(1981)
sample failed
granite
the
of the Kaiser effect in rocks. They
have devised a new method
the
of Westerly
stress
and observrate). Eleven
of cycle 8. the granite
study Judging
was reloaded from
the
and
velocity
The acoustic emission activity (Fig. 8b) shows drastically different behavior in different regions
decrease
within
have expected such high strength. Is the memory loss that is evident in acoustic emissions some
the rock. showing
increasing
activity
in one
region while others show a decrease in activity. Through the end of cycle 6 regions 1 and 5 show similar behavior. They are indistinguishable from the other regions for cycle 4 and 5, but show markedly increased activity during cycle 6. Region 5 becomes quiet again until near the end of cycle
and the acoustic
the end of cycle 8 (Figs.
emission
increase
during
7 and 8) we would
not
healing process that can also increase the ultimate strength? The data for basalt suggest a partial loss of memory commencing at an axial stress of ap-
in-
proximately 60% u,,,~~. The datum at umax/ufallure = 100 is not unexpected since in a virtually failed
crease in activity with increasing stress. What may be interpreted as quiescent periods for region 1
sample acoustic emissions must start again at low levels of reapplied stress; i.e. there must be an
during
upper limit to the validity of the Kaiser effect. In contrast to the granite, no time dependent effect is observed that could be attributed to the long time interval between cycles 3 and 4. A thought experiment should illustrate the point of an upper limit for the Kaiser effect. Assume
8, while
region
1 shows
cycle 8, upon
a nearly
monotonic
closer examinations,
can be
correlated with periods of either constant or decreasing stress. Region 2 shows little activity until cycle 8 and then its maximum activity occurs long before unloading. In region 3 the maximum activity is recorded during cycle 5. Region 4 has the lowest average activity well past the middle of cycle 8 after which it becomes the second most active region. The cumulative 10 hints
in Fig. Kaiser
acoustic emission activity shown at the limited validity of the
effect (Kaiser,
1953) i.e. acoustic
emission
activity in cyclically loaded samples is absent until the maximum previously applied stress is exceeded. In Table 1 we present the available data from our two experiments for the resumption of acoustic emissions during cyclic loading of our granite and basalt samples. Graphically these data are presented in Fig. 14. The data show that the memory loss is nearly complete for granite when the axial stress has reached approximately 30% of the failure strength. The apparent memory loss for the granite sample at 20% of u,,, may be due to the long time interval between cycles 4 and 5. This con-
that in a servo controlled
press a rock had been
deformed at a constant strain terminated in the post-failure
rate. The cycle was region, i.e., a stress
maximum had been reached. On reloading, acoustic emissions surely will occur long before the previously
achieved
maximum
stress.
Viewing
a
rock as a heterogeneous material implies that when loading it to near the maximum stress which it can support, some regions of the rock are already in the post failure region. We expect these regions, on reloading, to support some of the load and become acoustically noisy. Under otherwise equal conditions, the larger a specimen, the more likely it is to be heterogeneous and the lower we expect the stress to be at which memory loss occurs. The character of acoustic emission events changed from region to region (compare Fig. 11 a, b, c with lid) but remained quite recognizable
d E
s
5 :
I
< ‘
=====-:
c
z c
: --+---m
1
co
events between
on the granite
b and c, an event from region
l-8-7
is shown.
,
sample
in terms of arrival times and amplitudes. The axial stress was 130 MPa.
in arrival
longer
and periods. but the considerably
times, amplitudes
locations
130 MP.
periods
from
to
as in Fig.
is shown. of this event. d. For comparison
c. An event from the same region
as in Fig. lla
see Fig. 9. a. Traces b. An event from the same region
during cycle 8 (see Fig. 8). For transducer
6 etc.) are shown. The axial stress was 119 MPa.
this event and that in Fig. lla,
on transducer
and b is shown. The axial stress was again 130 MPa. Note the similarities
the events shown in Fig. lla,
lla
The axial stress was 130 MPa. Note the similarity
7, second
shown in this figure were obtained
at transducer
emission
(first arrival
Fig. 11. All traces for acoustic
an event in region 7-6-l
c
130MP.
66
within
one region
(compare
Fig. lla,
While
the average
frequency
content
emissions
decreased
was a mixture events
of high frequency
throughout
onstrated Fig. lib event,
at high axial the loading
by comparing and llc;
b and
c).
of acoustic
stresses,
there
and low frequency cycles. This is dem-
the events
depicted
in
(a)
the latter being a low frequency
the former
a high
have the same location
frequency
event.
Both
(note the arrival sequence),
the same sense of first motions
with the possible
exception
of those on transducer
8 and both events
occurred
at the same axial stress. The two events
‘_____I
/
i____i___J
1
‘1
depicted in Fig. lla and b were recorded at different stresses, 119 and 130 MPa respectively, but appear very similar, with respect to frequency content, motions,
location,
amplitudes,
and
sense
of first
(b)
Conclusion It has been repeatedly shown that large samples fail at lower stresses than small samples. This is thought to be due to large samples containing a larger number of potentially weak regions where a failure zone could nucleate. In the present experiments we have identified various regions within
Fig. 13. Contours are plotted
of isostrain
for results obtained
in Fig. 12. The strain
invariants
( c,,~~,~~,~,~, + c,,er,,cil,)
from the straingauge
all for cycle 8, the final loading
cycle, and correspond
levels of 85 MPa, 123 MPa and 134 MPa respectively. of strain dashed
\I/ \I/ 1 &i x
\l/\I/
-m-----m
r”
r
--t
l-
s
,
Fig. 12. The location
was approx.
sample
50 mm.
w
are shown.
occurred
in those
areas
The length
to stress Failure
circumscribed
by
lines.
our samples as potential nucleation The samples were clearly much
of the 104 strain gauges that were bonded
to the J = 0 face of the granite of the gauges
I
700 mm
-I
gauges
set shown
units are 10K5. The maps a, b and c are
sites for failure. larger than the
failure zones. The changes in the average properties of the samples, such as strains and acoustic velocities, were smaller than those observed in smaller laboratory samples. In typically uniform and small samples the failure zone becomes a substantial fraction of the sample volume and
67
(4) Based
upon
our
emission
activity
there is no reason
activity
should
be
volume.
Therefore,
ion of precise exponential
uniform
insufficient
failure
increase
ticular
region
They
indicate
Fig. 14. The extent to which the Kaiser effect holds is shown in this graph.
may vary from region
difference start
between
in a particular
was reached pared memory
cycle, and
in the previous
the maximum
the ultimate
n,&, the stress at which
cycle. The horizontal
stress reached
stress at failure
acoustic
em, the maximum in a particular
6,. Calculated
emissions stress
axis comcycle with
curves for complete
loss at a, .= 30%. 40% and 60% of et are plotted.
data and equation
that
For
see Table 1.
in velocity
become
as large as 50% (Spetz-
ler and Martin. 1974; Granryd et al., 1983). Dilatant volumetric strains of several percent may be observed (Kurita et al., 1983). Acoustic emission activity in small samples, exposed to monotonically increasing stress, are less likely to show quiet periods than specifically identifiable regions within
emis-
quiescence
and
very
signals similar and
failure
one region
from a par-
but
in
terms
of
first motions.
mode
for many
the failure
mode
to region.
(6) As failure of the sample is approached the acoustic emission signals become generally larger and their spectra frequency. The
contain larger amplitudes at low continued occurrence of high
frequency events indicates for the frequency content found
changes
both
amplitude,
sources
axis gives a scale for the percentage
the acoustic
emission
the same
within
that
a sample
data for the predict-
time;
appear
content,
to suspect
have been observed.
(5) Many acoustic frequency
of acoustic
through
monitoring
sion rate provides
The vertical
observation
predominantly
that the primary reason of the signal must be
in the source rather
the path effect. (7) Strain patterns on established at about 60% of only slightly as the stress is (8) The regional activity the velocity maps and the identifying gions.
the locations
than in
the surface that are ultimate stress change further increased. of acoustic emission, strain maps correlate,
of incipient
failure
re-
a large sample. We thus conclude that within a single large rock sample: (1) Several incipient failure zones can develop concurrently and semi-independently.
Acknowledgments The authors
wish to acknowledge
the support
(2) Changes in acoustic velocities within these zones may be as large as those observed in small samples; i.e. about 50% decrease in compressional
provided through the United States Geophysical Survey and the Soviet Academy of Sciences in the realm of the US-Soviet bilateral agreement in the field of Environmental Protection. To name all of
wave velocity. The average velocity change on a path through the entire sample is, however, considerably smaller. Some elastic waves might not even sample the deformed zone.
those on the technical and on the administrative sides that helped in making this undertaking possible would make this the longest part of the
(3) The Kaiser effect, i.e. the reappearance of acoustic emissions at previously attained maximum stress levels, has only limited applicability in uniaxial tests. It has an upper stress limit beyond which there is no memory. This upper limit may vary with rock type. The stress at which memory is lost may be greatly reduced if the rock is unstressed for an extended period of time.
paper. We are deeply appreciative, however, for their enthusiasm and perseverance even when the odds of success seemed remote. References Brodsky,
N.S. and Spetzler,
an incipient
H.A., 1984. Direct
fault zone using the scanning
scope. Eos, Trans.
Am. Geophys.
Union,
observation electron
of
micro-
65 (45): 1081.
68
Dieterich.
J.H.. 1981. Potential
large scale tests. Geophys. Granryd. dence
for geophysical
L.. Getting,
I.C. and Spetzler,
of acoustic
velocity
tally deformed
westerly
experiments
in
and
H., 1983. Path depen-
attenuation
granite.
Geophys.
in experimenRes. Lett.. 10(l):
71-74. Kaiser,
J., 1953. Erkentnisse
Werkstoffen.
Arch.
K.. Swanson.
Surface
deformation
Nishizawa,
0.. Onai.
distribution
und Folgerungen
bei Zugbeanspruchung Eisenhiittenwes., P.L.. Getting,
granite
Terent’ev,
during
H., 1983.
creep. Geo-
K. and Kusunose,
K., 1985. Hypocenter
of AE events during Andesite.
two
Pure Appl.
Geo-
J.H., 1984. Effects of physical
fault
122: 36-52.
faults. J. Geophys. G.A..
instabilities
produced
on simulated
Res.. 89(B7): 581775827.
Semerchan,
A.A.,
Salov,
B.G., Spetzler,
Izvestiya,
1980. An effective emission
A.V.,
studies.
Los’. I.R..
I.M., 1982. Precursors
of a large rock sample. C.H.,
of the
18: 572-580.
noise discriminator
for
Rev. Sci. Instrum..
51:
C.H.
and
Estey,
L.H.,
1981.
Acoustic
study of microfracturing
during
erly granite.
Res., 86(B4): 2915-2924.
Sondergeld,
J. Geophys.
C.H.
and Estey,
Appl. Geophys.. Spetzler,
and velocity
1982. Source
uniaxial
cycling
of west-
mechanisms of rock. Pure
120: 151-166.
during
R.J.. III. 1974. Correlation dilatancy.
Nature.
stress from cored
tory
the
in
case
74: 323-339.
of
of strain
252:30-31.
S. and Mogi, K., 1981. A new method
tion of the crustal study
emission
the cyclic loading
L.H.,
during
H. and Martin,
Tectonophysics. H.A.,
Kol’tsov.
A.V.. Stakhovskii,
134221344. Sondergeld,
Yoshikawa,
on frictional
V.N.,
Ponomarev,
V.A. and Turetskii,
destruction Sondergeld,
Badanov.
R.M.,
and microfracturing
and focal mechanism
P.G. and Dieterich,
properties Sobolev,
24: 43-45.
75-78.
stress stage creep in Yugawara phys..
aus der Messung von metallischen
I.C. and Spetzler,
of westerly
phys. Res. Lett.. 10(l):
Okubo.
C.H.,
use in acoustic
von Gerluschen Kurita,
Sondergeld.
V.F.. Nasimov.
Res. Lett.. 8(7): 653-656.
for estima-
rock samples:
uniaxial
labora-
compression.
144 (1987) 55-68
Elsevier Science Publishers
55
B.V., Amsterdam
- Printed
in The Netherlands
Laboratory Modelling
Seismic and strain studies on large laboratory being stressed to failure H. SPETZLERI,
C. SO~D~RGELD
of Geologxal Sciences and Cooperative Insfirute
’ Department
rock samples
I**, G. SOBOLEV 2 and B. SALOV ’
forResearch
in Environmenral
Sciences,
ofColorado/NOAA,
University
Boulder, CO 80309 (U.S. A.) ’ Institute of the Ph,vsics
oftheEarth,
(Received
Academy
January
of Sciences of the USSR,
1.5,1986;
IO Grunzinskaya,
revised version accepted
Moscow (USSR)
July 28,1986)
Abstract Spetzler,
H., Sondergeld,
samples
C., Sobolev,
being stressed
to failure.
G. and Salov,
B., 1987. Seismic
In: R.L. Wesson (Editor),
and
Mechanics
strain
studies
of Earthquake
on large
Faulting.
laboratory
rock
Teer~noph~~ie~, 144:
55-68. Large granite and ultrasonic
(700 x 700 X 700 mm) and basalt velocity
measurements
ments
were correlated
patterns
of emission
initially
active then quiet and active again
rate toward
in space activity
failure. At constant
in terms of frequency
content
zone were very reproducible The Kaiser detected
effect
time. Several
applied
before
zones of acoustic greatly
failure,
and amplitude
one-half
and relatively
more power in dilatancy
was recorded in the sample.
sources stress:
increasing
The acoustic
active then quiet, (b)
continuousty
and at accelerating
emanating
signals was quite different from within
from transducer
i.e. very few acoustic
a particular
to transducer. emissions
stress 0. as long as o was less than about
the individual Strain
measurements
acoustic
emission
and ultrasonic
on much smaller
Stram
All measure-
were identified.
but quite different
of the stress of failure.
to failure.
(a) initially
zones. The signals
attained
loaded
deformation.
emission
at lower frequencies. Similar
emission
during
of the acoustic
transducer,
below previously
were cyclicly
recorded
with increasing
and (c) activity
for the different
on a particular
stress at failure. As failure was approached
large variations
emissions
stress. the seismic character
as recorded was cycled
and acoustic
these zones varied
seems to hold up to about
when the sample
the eventual
and
within
(580 X 580 x 915 mm) samples
were made
events became velocity
samples
were
one-half
of
more energetic
measurements are consistent
indicate with these
results.
Introduction
(Spetzler
With few exceptions (e.g. Dieterich, 1981; Okubo and Dieterich. 1984) failure experiments on laboratory samples have been restricted to samples having linear dimensions of less than 200
1982; Brodsky and Spetzler, 1984) and their possible interactions was severely restricted by sample size. For this reason several tests on large samples were performed at the 50,000 ton press at the High-Pressure Institute near Moscow, USSR. In addition to several large concrete samples, two
mm. Locations of zones preceeding and indicating failure have been identified by various experimental techniques. The identification of multiple zones
* Present
address:
Amoco
Production
Co., Research
and Estey,
trical conductivity, self potential. electromagnetic emissions, strain, acoustic velocity and emissions
Center.
0 1987 Elsevier Science Publishers
1974; Sondergeld
igneous rocks were broken: a granite specimen 700 X 700 X 700 mm and a basalt specimen 580 X 580 X 915 mm. The complete results of all measurements on the granite specimen involving elec-
P.O. Box 591, Tulsa, OK 74102 (U.S.A.)
00~-1951/u7/$03.50
and Martin,
B.V.
56
are reported
in the Soviet literature
1982).
Here
results
obtained
velocity
and emission
periments
we will
concentrate
from
the
strain
et al.,
0.5%) and contained
on
major
biotite
acoustic
rather
uniform
biotite
concentration
the
and
measurements
on the granite
(Sobolev
from the ex-
and basalt
samples.
and
thick crossed
procedures and results
The upper l/3
are different
of the 50,000 ton press is shown
in Fig. 1. It can accept
a sample
with a vertical
dimension
up to 3 m. The samples
were deformed
uniaxially,
but were constrained
tom by steel beams stress concentrations contact
on top and bot-
Both the granite
40% albite,
Its grain
2-3 mm. A vein of
approximately
the sample note
10 to 15 mm
(see Figs. 9 and 12). In
that
the experimental
for the basalt and granite
and
acoustic
20%
size was
emission
details
samples.
measurements
in
basalt
to minimize failure at the where the samples were in
with the pistons.
Velocity
of pyrite.
being between
the following, Experimental
40% quartz,
a trace
and the
Nine
compressional
transducers,
with
a free
resonance frequency of 150 kHz, were mounted on each of the four free faces of the sample. Their
basalt samples were loaded cyclicly, the loading history of the latter being shown in Fig. 2. The
locations and the 96 paths over which the velocities were measured are indicated in Fig. 3a. For data analysis the block was divided into 27 ele-
basalt
ments
sample had a porosity
of approximately
6%
and contained 60% plagioclase, 20% olivine, 10% pyroxene and a total of 10% of augite, diopside, magnetite, apatite mm). The granite
(see Fig. 3b) and the average
termined
for each block. The velocity
all traces
velocity
de-
averages
for
and for all cycles are shown
in Fig. 2.
etc. It was fine grained (< 1.5 sample had a low porosity (-c TABLE
1
Summary
of acoustic
emission
activity
Cycle No.
Basalt 2
24
-
11
+4%
0.31
3
42
25
113
-5%
0.54
4
63
40
17
-13%
0.81
5
78
55
(-55%)
1.00
lhr
Granite 3
23
17
+4R
0.13
4
32.5
24
66
- 20%
0.18
5
45
26
17
-4%
0.25
6
75
43
13
~ 20%
0.42
7
86
60
14
- 36%
0.48
8
139
55
8000
D,,,~~= maximum
(-50%)
stress reached
v* = stress at onset of acoustic
0.77
during
the n th cycle (MPa).
emissions
during
cycle number
n +1. Ar = Number
of hours following
cycle n before the start of the
next cycle. (( OA - %I, )/‘%,a~ ) x 100 is a measure of the memory The two values in parentheses correspond to data during occurred Fig. 1. The top portion High-Pressure shown.
Institute
of the 50,000 tom research in Troitzk
near
Moscow
press at the USSR
is
additional
loading
180 MPa was reached reloading
cycles.
8000 hrs (11 months) occurred
before
For
granite
loss. obtained
the reloading
later, at which time a stress of failure.
In the case of basalt,
1 hr after cycle 5 and the maximum
that could be reached
was 75 MPa.
the
stress
57
2468 250881
I Fig. 2. Five loading average
cycles are shown
of the velocities
velocities
in basalt
that
for the basalt
were measured
is contrasted
specimen.
in granite
9,233
v&9
644
4’8
T V---7
2
4
6
paths
8
0
310661
Note the time elapsed
over 96 different
with the increase
x3
2 4 11 12 08 B1 270661 26 I I
2
4
hours dates
0109.61
between
the loading
(see Fig. 3a) are also shown.
cycles (see Table 1). The
The general
decrease
for the
(see Fig. 7).
6,677 5.6.7
Yo
5 . 2 .
915
1
2 .
!
6 .
6
.
l
9 .
.
T
8.-
.
I
1”’
ta
I
5,4,3
I
6.1,2
2 ,I 96
/
L
(a)
(b)
Fig. 3. a. The sample sample.
There
calculation.
coordinates,
are 14 paths
the sample
transducers
locations
in each of three horizontal
was partitioned
into 27 elements
and the 96 paths planes
for the velocity
and nine paths
for which the velocities
measurements
in each of six vertical were determined.
are shown planes.
for the basalt
b. For purposes
of
SVi % +20
+15
Element
Number 0 4 n 5
+10 1 ;
.6 13 ;
14
*
'5
c
22
*
23
0
24
l-----EH-1-l 4
2 Fig. 4. The percentage given: ((I’, ~ F)/i;) were calculated
difference
between
the average
velocity
X 100. v, is the velocity in block number
similarly
to the method
used by Nishizawa
ucoustic
emission
measurements
t
I_
(see Fig. 2) and those in the interior
I, r is the algebraic
average
of velocities
elements
hrs of layer B (Fig. 3b) are
in all 27 elements.
The velocities
et al. (1985).
The changes from these average velocities that occurred during the last two cycles are shown in Fig. 4 for the nine elements of the interior slice B of Fig. 3b. Acoustic emission activity was recorded for the basalt sample and is shown in tabular form in Table 1. Velocit?, und grunite
8
6
in
Before and after cycling at zero axial stress, detailed P-velocity measurements were made over
290 horizontal (perpendicular to the loading axis) paths. The measurement procedure involved the bonding of a transducer pair, one on each opposing face exactly opposite each other. The transducer locations were marked on the free surfaces as illustrated in Fig. 5a. The z-axis was the vertical axis and thus the axis where the load was applied. Each set of 145 measurements in one direction yielded data for a velocity map. Such velocity maps were produced in pairs, one for the y direction and one for the x direction. Two such map pairs were produced for the granite specimen, one before stress was applied and one at the end of the
59
700mm
etc
i
(b) Fig. 5. a. For the granite locations
where
constructed
sample
the transducers
for the velocity
refers to velocities
between
(700
x
700
for these
measurements
x
700 mm) 145 velocity
velocity
measurements
in the x and ,V directions
4.8 and 4.9 mm ps-‘).
No signals
measurements
were made
were bonded both before
were observable
are shown.
in the .x and the )’ directions. b. Iso-velocity
and after loading
after loading
contour
of the gramte
in the zones Indicated
sample.
maps
The were
(e.g. 4.8’
by shading.
7th loading cycle. The four data sets are shown as velocity contour maps in Fig. 5b. Velocity histograms showing the change in velocity distributions for both directions The continuous
stress for 14 different paths are These velocity data were used in emissions. The loading history cycles of the granite sample and
Y,-Y,
-20
after--I
-before
ing acoustic gions within
-‘“3.
s-1
4.4
Fig. 6. Histograms Fig. 5 are shown larger ing.
for the velocity
measurements
for both
the x and
range and multimodal
distribution
after loading
are given in Fig. 6. change in velocities versus axial
when they are compared
described
y direction,
Note
in the
of the measurements with those before
load-
shown in Fig. 7. locating acoustic for the last five the accompany-
emission activity for five distinct rethe specimen are given in Fig. 8. The
locations of the transducers that were used to locate the acoustic emissions are shown in Fig. 9. A total of about 3500 events were assigned to these regions. The cumulative total of acoustic emissions for each of the last five cycles is plotted as a function of axial stress in Fig. 10. Examples of amplitude versus time traces for all eight receiv-
60
5.4
5.2 7 m 5.1 E Y S 5.0
4.9
4.8
4.7
/
“/ /
11
-
12
-0
I
5-7'
I-2'
13 14
-A -+
5-6' 5-3'
4.8
I
0
Fig. 7. In contrast the granite sample
to the 96 different
sample during
contrasts
20
sharply
measured
are indicated
transducer
locations.
loading.
40
60 80 stress MPa
paths over which velocities
The velocities
with the velocity by the transducer
during
decrease numbers,
100
were measured
in the basalt sample.
the final cycle {No. 8) are shown.
observed
in our basalt
sample
e.g.. 5-7’ refers to transducer
120
The general
(Fig. 2). The paths
5 sending
only 14 paths were measured velocity
increase
over which the velocities
and transducer
7’ receiving.
in
in the granite
were
See Fig. 9 for
61
T 5
0
I ,
0
50
,,
, I.
,
5
IO
I
,.. , ,
0
5
, , ,,, ,
0
hrs
5
,,,,,,
a
b
I
Fig. 8. a. The stress history increases regions
-m IO
within the sample. to the various
.
I-
-
for the last five loading Region
,d
rL
dl
5
events
to allow the many measurements
or on 2 and 8 and similarly assigned
-
I
cycles is shown for the granite
to be made at constant
I is characterized
for the other regions.
sample.
stress. b. The acoustic
by events with first and second Note the different
behavior
arrivals
of the various
The stress was held constant emission
activity
is shown
on either transducers regions.
after stepwise for five distinct
1 and 8 respectively
A total of about
3500 events were
regions.
c
0
001
05
041
lo
0 I’
006
*b face
z
O6’
of the transducers stress cycling
used for locating
2
IT
YL
measurements
X0
O*’ 03’
080
Fig. 9. The positions
02
07’
5b
during
20
face
07
face
0
XL
0
40
acoustic
as open circle pairs. A biotite
emissions
vein crossing
are shown
as filled circles,
the upper
part of the sample
those used for velocity is also included.
62
dominantly
equal dimensional
to collapse cracks
and
beginning
thus the decrease
values of nearly
elements
of the largely
The extremes
occur
interior
slip on the bottom
5 and 23, both
From the relatively
high
4, 5 and 6 we suspect
that
between
By analogy piston
the top piston
we conclude
that the
must have been substan-
tially greater than at the top. This interpretation further during
stress.MPa
Fig. 10. The extent
to which the Kaiser
effect is applicable
these
becomes
when
experiments
number
of acoustic
emission
apparent
events is plotted
stress for the last five cycles. For details
ing
transducers
are
shown
the
in
cumulative
as a function
of
see text and Fig. 14.
in Fig.
11 for
four
distinct acoustic emission events and at two stress levels during cycle 8. Each set of eight traces was recorded simultaneously on synchronized pretriggered digital oscilloscopes and then stored on floppy disks. A discriminator circuit (Sondergeld, 1980) was used to differentiate between noise and acoustic emissions and to subsequently trigger the oscilloscopes. The entire discriminator and recording system was described earlier by Sondergeld and Estey (1981). Struin measurements
on the granite sample
Sets of 84 and 104 strain gauges were glued to the X0 and Y, faces respectively (See Figs. 5b and 9). Those on face Y, are depicted in Fig. 12. The strain invariants were calculated for sets of three strain gauges and isostrain lines are drawn in Fig. 13 for three stress levels during
cycle 8.
Discussion
It is surprising that in spite of the large porosity of the basalt (6%) the sound velocity decreases perpendicular to the loading direction from the very start of load application. The total decrease is over 20% (see Fig. 2). We conclude that the pre-
enhanced unloading
in
slice B (Fig. 3b).
for elements
little or no slip occurred and the sample.
At the
a spread
20% (Fig. 4) in the nine
being next to the pistons. in elements
of axial
in velocity.
of the 4th cycle we observe
velocity
velocities
cavities are difficult
but lead to the early formation
through
the
observation
is that
at the end of the 4th cycle, those
elements in intimate contact with the top piston (4, 5, 6) show a relative increase in velocity (probably because of the radial restraint due to the lower compliance of the piston) while those that are less constrained radially show a further decrease in velocity. We conclude that during unloading considerably more axial cracks closed in elements 4, 5 and 6 than did in the other elements. Note that there is a decrease in the average velocity (Fig. 2). The final range of velocity within block B approaches 35%. The acoustic
velocities
in the granite
values sample
behave quite differently. Instead of a 20% decrease in the measured velocities there is a slight increase until very close to the end of cycle 8. The high rate of velocity increase with stress that is shown in Fig. 7 occurs up to the maximum stress level reached in cycle 7. Note that the maximum stress reached in cycle 7 was only 0.77 of the failure stress (Table 1). When the maximum values in Fig. 7 are compared with the velocity distributions before loading shown in Fig. 6, the increase in the measured velocities from the unloaded to the fully loaded case is found to be between 5 and 10%. The increase in velocity due to the closing of preexisting cracks seems to dominate over the effect of the newly created cracks which would be expected to result in a decrease in velocity. The effect of the newly created cracks is perhaps best seen in Figs. 5b and 6 where velocity measurements are compared that were made before cycle 1 and after unloading from cycle 8. The histograms in Fig. 6 clearly show that the
63
average velocities directions
at zero load in both the x and _y
decreased
smooth
distributions
changed
to broad
loading.
We interpret
form distribution greater
isovelocity
6%.
obtained
Relatively
before
loading
distributions
after
this to be due to a non-uni-
of cracks within of the
the sample.
velocity
maps before
and after loading
changes
12% in the x and
the minimum
changes
in velocity
4’ directions about
The
distributions
can also be seen when comparing
5b. The maximum and
about
multimodal
complexity
after loading
by
firms an earlier observation (-
a small
50 mm)
by one of us (C.S.) on
sample
which had lost all memory
of previous
it was stored in the laboratory (Yoshikawa general
and
validity
Mogi
in Fig.
were 13.5% respectively,
4%.
levels by stress cycling ing changes
also
question
in the acoustic
emission
after the completion in the present 180 MPa.
to find previous
their samples
months
at
stress after
for several months.
(1981)
sample failed
granite
the
of the Kaiser effect in rocks. They
have devised a new method
the
of Westerly
stress
and observrate). Eleven
of cycle 8. the granite
study Judging
was reloaded from
the
and
velocity
The acoustic emission activity (Fig. 8b) shows drastically different behavior in different regions
decrease
within
have expected such high strength. Is the memory loss that is evident in acoustic emissions some
the rock. showing
increasing
activity
in one
region while others show a decrease in activity. Through the end of cycle 6 regions 1 and 5 show similar behavior. They are indistinguishable from the other regions for cycle 4 and 5, but show markedly increased activity during cycle 6. Region 5 becomes quiet again until near the end of cycle
and the acoustic
the end of cycle 8 (Figs.
emission
increase
during
7 and 8) we would
not
healing process that can also increase the ultimate strength? The data for basalt suggest a partial loss of memory commencing at an axial stress of ap-
in-
proximately 60% u,,,~~. The datum at umax/ufallure = 100 is not unexpected since in a virtually failed
crease in activity with increasing stress. What may be interpreted as quiescent periods for region 1
sample acoustic emissions must start again at low levels of reapplied stress; i.e. there must be an
during
upper limit to the validity of the Kaiser effect. In contrast to the granite, no time dependent effect is observed that could be attributed to the long time interval between cycles 3 and 4. A thought experiment should illustrate the point of an upper limit for the Kaiser effect. Assume
8, while
region
1 shows
cycle 8, upon
a nearly
monotonic
closer examinations,
can be
correlated with periods of either constant or decreasing stress. Region 2 shows little activity until cycle 8 and then its maximum activity occurs long before unloading. In region 3 the maximum activity is recorded during cycle 5. Region 4 has the lowest average activity well past the middle of cycle 8 after which it becomes the second most active region. The cumulative 10 hints
in Fig. Kaiser
acoustic emission activity shown at the limited validity of the
effect (Kaiser,
1953) i.e. acoustic
emission
activity in cyclically loaded samples is absent until the maximum previously applied stress is exceeded. In Table 1 we present the available data from our two experiments for the resumption of acoustic emissions during cyclic loading of our granite and basalt samples. Graphically these data are presented in Fig. 14. The data show that the memory loss is nearly complete for granite when the axial stress has reached approximately 30% of the failure strength. The apparent memory loss for the granite sample at 20% of u,,, may be due to the long time interval between cycles 4 and 5. This con-
that in a servo controlled
press a rock had been
deformed at a constant strain terminated in the post-failure
rate. The cycle was region, i.e., a stress
maximum had been reached. On reloading, acoustic emissions surely will occur long before the previously
achieved
maximum
stress.
Viewing
a
rock as a heterogeneous material implies that when loading it to near the maximum stress which it can support, some regions of the rock are already in the post failure region. We expect these regions, on reloading, to support some of the load and become acoustically noisy. Under otherwise equal conditions, the larger a specimen, the more likely it is to be heterogeneous and the lower we expect the stress to be at which memory loss occurs. The character of acoustic emission events changed from region to region (compare Fig. 11 a, b, c with lid) but remained quite recognizable
d E
s
5 :
I
< ‘
=====-:
c
z c
: --+---m
1
co
events between
on the granite
b and c, an event from region
l-8-7
is shown.
,
sample
in terms of arrival times and amplitudes. The axial stress was 130 MPa.
in arrival
longer
and periods. but the considerably
times, amplitudes
locations
130 MP.
periods
from
to
as in Fig.
is shown. of this event. d. For comparison
c. An event from the same region
as in Fig. lla
see Fig. 9. a. Traces b. An event from the same region
during cycle 8 (see Fig. 8). For transducer
6 etc.) are shown. The axial stress was 119 MPa.
this event and that in Fig. lla,
on transducer
and b is shown. The axial stress was again 130 MPa. Note the similarities
the events shown in Fig. lla,
lla
The axial stress was 130 MPa. Note the similarity
7, second
shown in this figure were obtained
at transducer
emission
(first arrival
Fig. 11. All traces for acoustic
an event in region 7-6-l
c
130MP.
66
within
one region
(compare
Fig. lla,
While
the average
frequency
content
emissions
decreased
was a mixture events
of high frequency
throughout
onstrated Fig. lib event,
at high axial the loading
by comparing and llc;
b and
c).
of acoustic
stresses,
there
and low frequency cycles. This is dem-
the events
depicted
in
(a)
the latter being a low frequency
the former
a high
have the same location
frequency
event.
Both
(note the arrival sequence),
the same sense of first motions
with the possible
exception
of those on transducer
8 and both events
occurred
at the same axial stress. The two events
‘_____I
/
i____i___J
1
‘1
depicted in Fig. lla and b were recorded at different stresses, 119 and 130 MPa respectively, but appear very similar, with respect to frequency content, motions,
location,
amplitudes,
and
sense
of first
(b)
Conclusion It has been repeatedly shown that large samples fail at lower stresses than small samples. This is thought to be due to large samples containing a larger number of potentially weak regions where a failure zone could nucleate. In the present experiments we have identified various regions within
Fig. 13. Contours are plotted
of isostrain
for results obtained
in Fig. 12. The strain
invariants
( c,,~~,~~,~,~, + c,,er,,cil,)
from the straingauge
all for cycle 8, the final loading
cycle, and correspond
levels of 85 MPa, 123 MPa and 134 MPa respectively. of strain dashed
\I/ \I/ 1 &i x
\l/\I/
-m-----m
r”
r
--t
l-
s
,
Fig. 12. The location
was approx.
sample
50 mm.
w
are shown.
occurred
in those
areas
The length
to stress Failure
circumscribed
by
lines.
our samples as potential nucleation The samples were clearly much
of the 104 strain gauges that were bonded
to the J = 0 face of the granite of the gauges
I
700 mm
-I
gauges
set shown
units are 10K5. The maps a, b and c are
sites for failure. larger than the
failure zones. The changes in the average properties of the samples, such as strains and acoustic velocities, were smaller than those observed in smaller laboratory samples. In typically uniform and small samples the failure zone becomes a substantial fraction of the sample volume and
67
(4) Based
upon
our
emission
activity
there is no reason
activity
should
be
volume.
Therefore,
ion of precise exponential
uniform
insufficient
failure
increase
ticular
region
They
indicate
Fig. 14. The extent to which the Kaiser effect holds is shown in this graph.
may vary from region
difference start
between
in a particular
was reached pared memory
cycle, and
in the previous
the maximum
the ultimate
n,&, the stress at which
cycle. The horizontal
stress reached
stress at failure
acoustic
em, the maximum in a particular
6,. Calculated
emissions stress
axis comcycle with
curves for complete
loss at a, .= 30%. 40% and 60% of et are plotted.
data and equation
that
For
see Table 1.
in velocity
become
as large as 50% (Spetz-
ler and Martin. 1974; Granryd et al., 1983). Dilatant volumetric strains of several percent may be observed (Kurita et al., 1983). Acoustic emission activity in small samples, exposed to monotonically increasing stress, are less likely to show quiet periods than specifically identifiable regions within
emis-
quiescence
and
very
signals similar and
failure
one region
from a par-
but
in
terms
of
first motions.
mode
for many
the failure
mode
to region.
(6) As failure of the sample is approached the acoustic emission signals become generally larger and their spectra frequency. The
contain larger amplitudes at low continued occurrence of high
frequency events indicates for the frequency content found
changes
both
amplitude,
sources
axis gives a scale for the percentage
the acoustic
emission
the same
within
that
a sample
data for the predict-
time;
appear
content,
to suspect
have been observed.
(5) Many acoustic frequency
of acoustic
through
monitoring
sion rate provides
The vertical
observation
predominantly
that the primary reason of the signal must be
in the source rather
the path effect. (7) Strain patterns on established at about 60% of only slightly as the stress is (8) The regional activity the velocity maps and the identifying gions.
the locations
than in
the surface that are ultimate stress change further increased. of acoustic emission, strain maps correlate,
of incipient
failure
re-
a large sample. We thus conclude that within a single large rock sample: (1) Several incipient failure zones can develop concurrently and semi-independently.
Acknowledgments The authors
wish to acknowledge
the support
(2) Changes in acoustic velocities within these zones may be as large as those observed in small samples; i.e. about 50% decrease in compressional
provided through the United States Geophysical Survey and the Soviet Academy of Sciences in the realm of the US-Soviet bilateral agreement in the field of Environmental Protection. To name all of
wave velocity. The average velocity change on a path through the entire sample is, however, considerably smaller. Some elastic waves might not even sample the deformed zone.
those on the technical and on the administrative sides that helped in making this undertaking possible would make this the longest part of the
(3) The Kaiser effect, i.e. the reappearance of acoustic emissions at previously attained maximum stress levels, has only limited applicability in uniaxial tests. It has an upper stress limit beyond which there is no memory. This upper limit may vary with rock type. The stress at which memory is lost may be greatly reduced if the rock is unstressed for an extended period of time.
paper. We are deeply appreciative, however, for their enthusiasm and perseverance even when the odds of success seemed remote. References Brodsky,
N.S. and Spetzler,
an incipient
H.A., 1984. Direct
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and
H., 1983. Path depen-
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erly granite.
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