CHARACTERIZATION OF THE FRACTURE BEHAVIOUR OF
ELASTOMERS UNDER COMPLEX LOADING CONDITIONS
Z. Major1,2, K. Lederer1 and R.W. Lang1,2
(1) Polymer Competence Center Leoben GmbH
(2) Institute of Materials Science and Testing of Plastics
University of Leoben, Leoben, A
ABSTRACT
In engineering applications elastomers are frequently exposed to complex combinations of mechanical loads
(monotonic, static, intermittent and cyclic loads) superimposed by various environmental effects. A better
understanding of the material resistance against crack initiation and propagation becomes of increasing practical
importance, which can be illustrated by the example of the rapid gas decompression failure behavior of
pressurized seals for oilfield applications. Modern experimental methods and techniques provide novel
opportunities to better understand the deformation and failure behaviour of materials under complex loading
conditions.
Hence, experiments were performed on both component level under near service loading conditions and on
laboratory test specimen level using various specimen configurations for two model elastomers. The results are
summarized as the kinetics of the loading and the material response. Special emphasis was devoted to the
observation and the adequate characterization of the crack initiation and crack growth process by fracture
mechanics parameters.
.
Introduction
A phenomenon termed rapid gas decompression (RGD) damage occurs if elastomer seals exposed to high gas
pressure fail upon the sudden release of the gas pressure in a brittle manner. The rapid gas decompression
failure of elastomer seals is an important issue for the oil exploration industry and recently the objective of an
intensive theoretical and practical research [1, 2]
As to the characterization of the rapid gas decompression failure behaviour of pressurized elastomer seals, the
paper deals with (i) the instrumented autoclave tests of pressurized O-rings and (ii) the crack initiation and crack
growth at high deformation rates and under various constraint conditions. The main objectives of this paper are (i)
the characterization of the failure process in elastomers by novel non-contact experimental techniques and (ii) the
determination of relevant fracture parameters.
Experimental
Materials
Two elastomer types, a HNBR and an FKM type were selected for the investigations described and discussed
below. The materials were then produced by the company partner (Economos Austria GmbH, Judenburg, A) as
O-rings and test specimens.
Test methods and test set-ups
In the first part of the study pressurization/depressurization experiments were performed in an autoclave on
component level using O-rings made from two rubber types (HNBR and FKM) and also on laboratory test
specimen level using round bar (RBT) test specimens. The main focus was to develop instrumented experiments,
where the pressure and temperature change of the chamber and the expansion of the O-rings and test specimens
were continuously measured and recorded.
In the second part of the study fracture tests were run on a servohydraulic test system (MTS 831.59 Polymer Test
System). A single edge notched pure shear specimen configuration with a faint waist in the mid-section (SENFWPS specimen) with a nominal thicknesses of 2 mm and unnotched (RBT) and notched cylindrical specimen
configuration (CRB specimen) with 14 mm nominal diameter were used in this study. Both single parameter
critical tearing energy values and crack growth resistance curves of various elastomer types under monotonic test
conditions were determined over a wide loading rate range and the results are analysed based on the crack
growth kinetics (da/dt-T curves). The tearing energy values were calculated based on the nominal stress-strain
curves of FWPS specimens. For more information on the data reduction please refers to [5].
In addition, to gain more insight into the real material behavior under complex loading conditions full field strain
and temperature measurements were performed both in the near crack tip and in the far field in the specimen
ligament and subsequently analyzed. Novel test systems like a full-field strain analysis system (Aramis, GOM,
Braunschweig, Germany; WestCam, Mills/Hall, Austria) and an IR-thermography testing system (CEDIP Infrared
Systems, France) were used. The Aramis system allows for 2 and 3-dimensional full-field strain analysis whereas
the thermo camera can measure temperature distributions across the specimen. Both optical non-contact
measuring systems are capable of monitoring tests up to high testing speeds. The complete test system is shown
in Figure 1 for RBT test set-up.
4
1
5
3
2
1.. Servohydraulic testing machine, MTS 831.59 Polymer Test System
2.. Aramis full-field strain analysis system
3.. IR-Thermograph camera
4.. Exemplary picture of a FWPS-specimen strain distribution
5.. Exemplary picture of a FWPS-specimen temperature distribution
Figure 1. Test set-up for the round bar test (RBT) and CRB specimens.
Furthermore, the influence of strain rate (loading rate of 100 mm/s and 1000 mm/s) and grade of pre-compression
(0%, 10%, 20%, and 30%) on the fracture behaviour was examined for both rubbers, FKM and HNBR.
Results and Discussion
The change of the test parameters and the volumetric deformation behaviour of a RBT test specimen are shown
in Figure 2 during an instrumented RGD test.
600
p
250
500
200
400
300
150
T
100
200
∆V,
∆h
50
volume expansion
100
%
pressure p, bar
temperature T, °C
300
0
0
0
20000
40000
time
t,
60000
80000
s
(a)
100
80
70
250
60
200
∆V
50
40
150
30
100
20
300
500
p
explosion
250
∆V
300
FKM, 100°C, 260bar, CO2
150
decompression
T
0
50
100
150
time
t,
(b)
200
min
250
200
100
100
50
0
-10
300
400
200
%
%
10
50
600
∆V,
T
∆V,
pressure p, bar
temperature T, °C
compression
350
volume expansion
p
90
volume expansion
300
FKM, 100°C, 260bar, CO2
pressure p, bar
temperature T, °C
350
0
0
1359,0
1359,5
1360,0
time
t,
1360,5
1361,0
min
(c)
Figure 2. The change of the test parameters and the size of the round bar test specimen during an instrumented
RGD test; (a) the complete test sequence, (b) pressurization phase and (c) decompression phase.
The various stages of deformation during the pressurization/depressurization phase are also seen in Figure 3. In
the pressurization phase a moderate increase of material volume was observed up to the saturation (compare
Figures 3a and 3b). After the pressure release a sudden increase of the volume up to very high volumetric strain
was observed in a short time (40 % to 500 % in 30 sec, compare Figures 3b and 3c). Finally, an instability of the
material and crack appearance on the specimen surface was recognized (see Figures 3d).
Based on the diagrams and images depicted above the entire process can be analyzed in detail. Due to the rapid
pressure decrease the gas solved in rubber expands and tries to move from the bulk to the free surface of the
rubber. During this process a very high degree of volumetric deformation of the elastomer was observed and
simultaneously decreasing chamber temperature was recorded. The possible locations of the crack growth
initiation are; (i) rubber matrix/filler interface and (ii) the space between particles where the matrix is highly
constrained. More detailed analysis of this process can be found in [1-4].
(a)
(b)
(c)
(d)
Figure 3. Volume change of round bar test (RBT) specimen and the appearance of the crack in autoclave;
(a) original state, (b) compression phase (saturated), (c) decompression phase (maximal volume strain) and
(d) crack appearance
As the autoclave experiments are very time consuming and expensive, efforts were made to characterize the
elastomers by fracture mechanics methods on the laboratory specimen scale. Hence, the correlation between the
failure processes in the elastomer grades in the autoclave due to the gas expansion and at the crack tip due to
pure mechanical loading was investigated. First, RBT specimens after different degree of uniaxial compression
from a strain level 0 to 25 % corresponding to deformations 0 mm to 6 mm were investigated. The load-time
sequence is shown in Figure 4a and the stress-strain curves are shown in Figure 4b for HNBR. No clear influence
of the degree of pre-compression on the fracture behaviour was observed for both elastomer grades investigated.
Phase I
Compression
2000
Phase II
Relaxation
25
Phase III
Tensile load
HNBR
Dumbbel Tests
20
1600
2
v = 100 mm/s
800
15
p = 0 mm
p = 2 mm
p = 4 mm
p = 6 mm
σ,
10
400
Stress
Load
F,
N
N/mm
1200
0
5
0
-400
-5
-800
0
2
4
6
8
Time
10
t,
12
s
14
16
18
20
0
100
200
Strain
ε,
300
400
%
Figure 4. Monotonic tests of RBT (Dumbbell) specimens for HNBR; (a) loading sequence of the tests and (b)
Nominal stress-strain curves with increasing degree of pre-compression.
Moreover, the thermo-mechanical effects that take place in the materials during deformation or crack growth
respectively were monitored during monotonic tests with unnotched and pre-cracked dumbbell specimens and
also using SEN-FWPS specimens. The crack tip temperature of FWPS specimen is shown at the moment of
maximum load in Figure 5 for HNBR. Simultaneously, the strain distribution was also measured and is depicted in
Figure 5a. Although, the strain analysis did not work well on the contour edge of the crack, the maximal crack tip
strain, εmaxct was estimated. As expected, high strain and temperature gradient was observed in the vicinity of the
crack tip. These strain and temperature gradients depend on the crack length on one hand and reveal also
pronounced loading rate dependence. In addition to the gradient a higher temperature “crack layer” was also
observed.
Figure 5. Strain and temperature distribution of a FWPS specimen at the crack tip and in the ligament; (a) the
vertical strain (εy) component and (b) the temperature profile is depicted.
Load-time-temperature curves of a FWPS specimen at moderate loading rate are seen in Figure 6. The maximum
temperature observed at the crack tip region increases simultaneously with the load up to the peak load and than
decreases.
2400
50
Load F
Temperature T
Fmax
2000
45
N
F,
Load
40
1200
Tmax
35
800
30
T,
∆T
400
-400
-1
0
1
2
3
4
Time
5
6
t,
7
8
9
10
°C
25
T0
0
Cracktip Temperature
1600
20
11
s
Figure 6. Load-time-temperature curves of a FWPS specimen for HNBR.
ct
The nominal strain rate dependence of the maximal strain in the vicinity of the crack tip, εmax is shown in Figure
ct
7a. A material dependent decrease of the εmax values was observed with clear difference between FKM and
HNBR. For comparison, based on the global load-displacement curves tearing energy values at the peak load, Tp
were calculated and are shown in Figure 7b along with their nominal strain rate dependence. Contrary to the εmaxct
values a significant increase of Tp was observed with increasing strain rates.
60
N/mm
TP,
150
100
50
HNBR
FKM
Peak load tearing energy
Max. strain at crack tip
εmax, %
200
0
50
40
30
20
10
HNBR
FKM
0
0,1
1
10
Nominal strain rate
dε/dt,
100
0,1
-1
s
1
10
Nominal strain rate
(a)
dε/dt,
s
100
-1
(b)
ct
εmax
and (b) the peak load
Figure 7. Nominal strain rate dependence of (a) the maximal strain at the crack tip,
tearing energy, Tp values for both materials investigated using FWPS specimens.
The kinetics of the crack growth was also analysed and the crack growth rate, da/dt was calculated based on the
video sequences. The loading rate dependence of the crack growth rate is shown in Figure 8a for HNBR and in
Figure 8b for FKM. While a constant increase of the crack growth rate with increasing crack length with similar
slope at all loading rates was observed for HNBR, significant changes are seen in the same curves for FKM. It is
assumed that the crack growth is continuous in the FWPS specimen for HNBR and contrary to this, is
discontinuous for FKM with crack deceleration and acceleration phases. Latter phenomenon is associated with
the crack tip blunting process.
6
100000
crack growth rate
dc/dt,
4
10
3
10
2
10
1
10
0
10
FKM
FKM
FKM
FKM
10000
dc/dt,
5
10
mm/s
HNBR (v= 1 mm/s)
HNBR (v= 10 mm/s)
HNBR (v= 100 mm/s)
HNBR (v= 500 mm/s)
1000
Crack growth rate
mm/s
10
100
(v= 1 mm/s)
(v= 10 mm/s)
(v= 100 mm/s)
(v= 500 mm/s)
10
1
-1
10
0,1
1
10
crack length c,
(a)
100
mm
1000
0.1
1
10
Crack length c,
100
1000
mm
(b)
Figure 8: Loading rate dependence of crack growth kinetics of; (a) HNBR and (b) FKM.
Hence the crack tip blunting was also analyzed in one specimen during the crack growth. A crack tip opening
angle value, CTOA was defined and CTOA values were determined for all specimens. The loading rate
dependence of CTOA values is shown in Figure 9a for HNBR and in Figure 9b for FKM. While the CTOA value
remains nearly constant during the crack growth process and reveals only small rate dependence for HNBR,
significant and with the loading rate increasing changes were observed for FKM (see Figure 10).
140
FKM
HNBR
v+
80
60
40
v= 1 mm/s
v= 10 mm/s
v= 100 mm/s
v= 500 mm/s
20
0
0
10
20
Crack length
30
c,
40
°
100
120
CTOA,
120
100
Crack tip opening angle
Crack tip opening angle
CTOA,
°
140
80
60
40
20
v=
v=
v=
v=
0
0
50
10
20
30
Crack length
mm
a)
a,
1 mm/s
10 mm/s
100 mm/s
500 mm/s
40
50
mm
(b)
Figure 9: Loading rate dependence of the crack tip opening angle for; (a) HNBR and (b) FKM.
CTOA,
°
120
100
Crack tip opening angle
80
60
40
20
FK M
HNBR
0
0,1
1
N om inal strain rate
10
d ε /dt,
100
s
-1
Figure 10: Nominal strain rate dependence of the crack tip opening angle for materials investigated.
These crack tip deformation processes are schematically summarized in Figure 11. The crack tip blunting is
uniform during the entire crack growth process and remains the same over the loading rate range investigated for
HNBR elastomer. This results in a continuous crack growth and in a uniform crack growth rate profile.
In contrary to this, the degree crack tip blunting changes during the crack growth process. The variation of blunted
and sharper crack tip results in continuous and discontinuous crack growth for FKM grade elastomer. The kinetics
of this process highly depends on the global (nominal) loading rate of the specimen.
Figure 11: Schematic presentation of the crack tip deformation process with increasing crack length for both
elastomer types investigated.
Summary and Conclusions
To characterize the deformation and failure behavior of elastomers under complex loading conditions,
experiments were performed both on component level under near service conditions and on laboratory test
specimen level using two model elastomer grades selected. To gain more insight into the material response novel
non-contact full-field strain and temperature analysis methods and techniques were applied. The kinetics of the
loading process and the material response was measured, recorded and analyzed in terms of directly measured
quantities (pressure, temperature, force, strain). Moreover, based on these measurements global and local
fracture mechanics parameters were also calculated and the materials were compared.
The findings of these experiments are summarized below:
•
The crack initiation and crack growth in RGD process occurs due to the high volume deformation and
are favored by the highly multiaxial stress state both on a macroscopic and on a microscopic scale.
•
High material temperatures were measured in the specimens during the fracture process and with
increasing loading rate increasing maximum temperatures were observed. On the other hand a
decreasing temperature was measured in the pressure chamber during the decompression. It is
assumed that the resultant of these two opposite tendencies is a highly non-uniform temperature profile
in the bulk material during the RGD process.
•
Material grade and loading rate dependent distinct crack tip blunting was observed in the test
specimens. A uniform blunting process goes hand in hand with a continuous crack growth and in
contrary with a non-uniform blunting process a discontinuous crack growth was observed.
•
While an increase of global peak tearing energy, Tp values with increasing loading rate was observed,
ct
the local deformation based fracture parameters, maximal crack tip strain, εmax and the crack tip
opening angle, CTOA was found to clearly decrease with increasing loading rates for both materials.
•
As overall conclusion it is stated that simple fracture experiments on laboratory specimen scale does not
sufficiently reflect the very complex loading situation in RGD and can not yield adequate material
parameters. However, these experiments provide useful information both about the fracture behavior as
well as input data for further modeling and simulation work.
Acknowledgement
The research work for this paper was performed at the Polymer Competence Center Leoben GmbH (PCCL, A)
within the framework of the Kplus-programs of the Austrian Ministry of Traffic, Innovation and Technology with the
contributions of the University of Leoben as scientific partner and the Economos Austria as company partner. The
PCCL is founded be the Austrian Government and the State Governments of Styria and Upper Austria.
References
[1]
[2]
[3]
[4]
[5]
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3-4 November, London, 2003, 7-28.
Z. Major et al., Development of a test and failure analysis methodology for elastomeric seals exposed to
th
explosive decompression, 5 Oilfield Engineering with Polymers, MERL, 29-30 March, London, 2006,
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