a Institute for Polymer Product Engineering, Johannes Kb Institute for Polymeric Materials and Testing, JohannUniversity of Leoben, Leoben, AustriacPolymer Competence Center Leoben GmbH, Leoben, A

a r t i c l e i n f o

Article history:Received 5 January 2009

upon the sudden release of the gas pressure in a brittle manner. The rapid gas decompression failure of elastomer seals is animportant issue for the oil exploration industry and since the mid of the seventies is the objective of an intensive theoreticaland practical research. The process is relative complex and includes the gas permeation, solution in the bulk rubber, theextension of the cavitations in the rubber matrix and the occurrence of crack initiation and crack growth under non-isother-

1350-6307/$ - see front matter 2009 Elsevier Ltd. All rights reserved.

* Corresponding author. Address: Institute for Polymer Product Engineering, Johannes Kepler University Linz, Polymer Competence Center Leoben GmbH,Altenberger STr 69, Linz, Austria. Tel.: +43 732 2468 1651.

E-mail addresses: zoltan.major@jku.at (Z. Major), reinhold.lang@jku.at (R.W. Lang).

Engineering Failure Analysis 17 (2010) 701711

Contents lists available at ScienceDirect

Engineering Failure Analysisdoi:10.1016/j.engfailanal.2009.08.004tance curves in terms of tearing energy (T Dc) and crack tip opening angle (CTOA Dc)functions were derived. While a continuous crack growth was observed for HNBR at allloading rates, a discontinuous crack extension was observed for FKM. Finally a phenome-nological model was deduced for describing the rapid monotonic crack extension processfor these elastomeric compounds.

2009 Elsevier Ltd. All rights reserved.

1. Introduction and objectives

A phenomenon termed rapid gas decompression (RGD) damage occurs if elastomer seals exposed to high gas pressure failReceived in revised form 6 August 2009Accepted 17 August 2009Available online 21 August 2009

Keywords:Rapid gas decompression failureCrack initiation and propagationFracture toughness of rubberTearing energyContinuous and discontinuous crack growthepler University Linz, Austriaes Kepler University Linz, A, Former Institute of Materials Science and Testing of Plastics,

ustria

a b s t r a c t

The monotonic fracture behaviour of two elastomer compounds (HNBR and FKM) wascharacterized in this study. These elastomeric materials are frequently used in many oil-eld applications (e.g., seals and hoses) and are exposed to a complex combination ofmechanical, thermal and environmental loads. Furthermore, rapid gas decompression fail-ure of these elastomeric materials was observed under real service conditions. A phenom-enon termed rapid gas decompression (RGD) damage occurs when elastomer seals exposedto high gas pressure fail in a brittle manner upon the sudden release of the gas pressure.Hence, to support both material development and design efforts, it is of prime theoreticaland practical importance to characterize the fracture behaviour of these materials. Fracturemechanics tests using a faint waist pure shear specimen (FWPS) conguration were per-formed over a wide loading rate range. First, global force and displacement values weremeasured and single parameter fracture mechanics values in terms of peak tearing energy,Tp values were calculated. Furthermore, the crack growth process and the local crack tipdeformation were characterized by non-contact optical devices. The digital image correla-tion technique used allows for the determination of displacement and full-eld strain up toa high strain and crack growth rate. Based on above measurements, crack growth resis-Characterization of the fracture behavior of NBR and FKM grade elastomersfor oileld applications

Z. Major a,c,*, R.W. Lang b,c

journal homepage: www.elsevier .com/locate /engfai lanal

develofailureeffect

mance in the rst experiments [11,12].

In the second part of the study fracture tests were run on a high rate servohydraulic test system (MTS 831.59 Polymer TestSystem, Berlin, Germany). A single edge notched pure shear specimen conguration with a faint waist in the mid-section

702 Z. Major, R.W. Lang / Engineering Failure Analysis 17 (2010) 701711(SEN-FWPS specimen) with a nominal width,W of 200 mm and with a nominal thickness, B of 2 mm was used in this study.The FWPS specimen conguration along with the test set-up used are shown in Fig. 3. The force was measured by the loadcell of the testing machine and the global displacement was measured by the LVDT integrated in the actuator. For more de-tails to the specimen geometry and the test procedure refers to Feichter [15]. Single parameter critical tearing energy values,Tc. were calculated based on the loaddisplacement or nominal stressstrain curves of FWPS specimens as follows.

T Wh0 R smax0 Fds

BW a 1

where W is the strain energy density, B is the specimen thickness and (W a) is the remaining ligament length. The criticaltearing energy, Tc was dened as the peak load tearing energy, Tp and it was determined at the peak load value. The loadingrate was dened as the velocity of the actuator and a nominal strain rate (far eld) was calculated using this loading ratedivided by the nominal gage length of the specimen (h0 = 16 mm).

In addition to the conventional global forcedisplacement measurement, optical, non-contact devices (Aramis, GOM,Braunschweig, Germany) and an IR-thermography testing system (CEDIP Infrared Systems, Paris, France) were also appliedfor characterizing the crack tip and the ligament deformation behaviour. The complete test system (testing machine, Aramis2.2. Test methods and test set-ups

In the rst part of the study pressurization/depressurization experiments were performed in a high pressure autoclave (SI-TEC, Sieber Engineering AG, Zrich, Switzerland) using cylindrical specimens with 10 mm nominal diameter. In these exper-iments the pressure and temperature change of the chamber and the volume expansion of the test specimens werecontinuously measured and recorded. The specimen expansion was measured by a CCD camera, the images were storedand the expansion was determined using a software tool developed for a video extensometer. The various stages of defor-mation during the pressurization/depressurization phase are seen in Fig. 2. In the pressurization phase a moderate increaseof material volume was observed up to the saturation (compare Fig. 2a and b). After the pressure release a sudden increase ofthe volume up to very high volumetric strain was observed in a short time (compare Fig. 2b and c). Finally, an instability ofthe material and crack appearance on the specimen surface was recognized (see Fig. 2d). More detailed information aboutthe experimental set-up, data recording and reduction procedure and the material performance can be fund in [11,12].and Thompson [2], Lakes [13], and Medri and Strozzi [14]. In spite of these signicant efforts, hardly any accurate descriptionof the fast crack growth process was found and no material parameters in terms of fracture toughness (Gc, Jc or CTOD) areavailable for both material development and for design efforts. Hence, the objective of this paper is to characterize the crackinitiation and crack growth behaviour of two selected RGD relevant elastomer grades over a wide deformation rate rangeusing pure shear fracture specimen and to derive appropriate fracture toughness parameters.

2. Experimental

2.1. Materials and test specimens

Two elastomer types, a hydrogenated nitrile rubber (HNBR) and a uorelastomer type (FKM) were selected for theseinvestigations. The compounds were produced with a nominal hardness of 85 5 SHA by the company partner (SKF Econo-mos GmbH, Judenburg, Austria) and provided for the investigations as cylindrical specimens with a nominal diameter of10 mm and height of 25 mm and as faint waist pure shear (FWPS) test specimens. Furthermore, the nominal stressstraindiagrams of both materials both for an unloaded case and after pressurization are shown in Fig. 1a and b. Moreover, scanningelectron microscopy images of both materials are shown in Fig. 1c and d. It is clearly seen that in spite of the exactly samenominal hardness values, both the microstructure and the macroscopic deformation behavior of these compounds are sig-nicantly different. More controversially, both materials revealed a very similar rapid gas decompression failure perfor-ped by the authors group and several elastomer grades were characterized regarding to their rapid gas decompressionbehavior [11,12]. Moreover, it was early recognized that fracture mechanics concepts and methods can provide an

ive tool for characterizing the rapid gas decompression failure process as described by Briscoe and Savvas [1], Derhammal and non-isobar test conditions. The overall RGD process is described by Briscoe et al. [1], Derham and Thompson [2],Stewenson and Morgan [3] and Seregely and Schunck [4] for elastomers and by Lorge et al. [5] for a thermoplastic polymerwith many different aspects. More specically, the gas permeability and diffusivity problems are described and discussed byKlopfer et al. and Filus et al., [6,7] and the cavitation process analyzed along with a simple solution among others by Chaoand Gent [8]. Recently, a novel model was proposed by Volokh [9] and simulations were performed on cavities in hydrostatictension applying the softening hyperelasticity theory and a coupled approach for modeling thermo-mechanical loading ingaseous environment was developed by Rambert and Grandidier [10]. Furthermore, a novel instrumented test was recently

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c dand thermo-camera system) is shown in Fig. 4. The Aramis system uses the digital image correlation technique and both thelocal deformations can be measured and 2 or 3-dimensional full-eld strain analysis can also be performed. While an excel-lent overview of the image correlation technique is given by Sutton and co-workers [16] the details of the procedure used inour experiments are described by Feichter et al. [17] and by Jerabek et al. [18]. As the digital image correlation system ap-plied here has a maximal image acquisition rate of 8000 frames/s with reduced resolution (512 256) and about 500frames/s with full (1024 1024) resolution, the rapid crack propagation can be followed and recorded with sufcient qualityfor further analysis.

Furthermore, multiple parameter crack growth resistance curves (T Dc) for various elastomer types under monotonictest conditions were determined over a wide loading rate range. The crack length, c was measured by the Aramis systemfrom the edge of the specimen to the crack tip in the crack plane and the actual crack growth rate, dc/dt was also calculated.Due to the continuous measurement of the crack length the actual cross section can be calculated. Hence, the tearing energywas calculated based on true (actual cross-section) stress values. Moreover, the crack tip opening angle; CTOAwas dened at1 mm distance from the actual crack tip to the specimen edge between two secant lines on the crack contour. The results ofthe fracture experiments are also analysed based on the crack growth kinetics (dc/dt c curves) and crack tip opening anglevs. crack extension (CTOA Dc) curves.

Finally, to gain more insight into the real local material behavior under complex loading conditions full-eld strain andtemperature measurements were performed both in the near crack tip and in the far eld in the specimen ligament and thestrain distribution is depicted in Fig. 5a. Although, the full-eld strain analysis did not always work perfectly on the contouredge of the crack, the maximal crack tip strain, ectmax was realistically estimated. In addition to the tearing energy derived fromglobal forcedisplacement data, the knowledge of local strain makes the determination of local strain or local strain energybased fracture parameters possible [17].

Fig. 1. Structureproperty relationship for the two elastomer grades investigated; Nominal stressstrain curves of virgin (unloaded) and pressurized/depressurized O-rings, (a) HNBR and (b) FKM; SEM images with a magnication of 5000; (c) HNBR and (d) FKM.

704 Z. Major, R.W. Lang / Engineering Failure Analysis 17 (2010) 701711The temperature and temperature distribution across the specimen was measured by the thermo camera. The crack tiptemperature of FWPS specimen is shown at the moment of maximum load and at about 1 s1 nominal strain rate in Fig. 5bfor HNBR. As expected, high strain and temperature gradient was observed in the vicinity of the crack tip. These strain andtemperature gradients depend on the crack length and reveal also pronounced loading rate dependence. In addition to thegradient a higher temperature crack layer was also observed. Further details to this phenomenon and about the possiblecharacterization methods can be found at Chudnovsky and Moet [19] and at Persson and Brener [20] and will also be de-scribed in a future work by the authors.

3. Results and discussion

The change of the test parameters (pressure, p and temperature, T) and the volumetric deformation, DV behaviour of acylindrical test specimen is shown in Fig. 6 during an instrumented RGD test. While the overall process is shown inFig. 6a, the pressurization process is shown in Fig. 6b and the depressurization in Fig. 6c in an enlarged time scale. In thepressurization phase a moderate increase of the material volume was observed up to the saturation (up to about 40% volume

Fig. 2. 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.

Fig. 3. Specimen conguration and test set-up for bulk elastomer fatigue tests; (a) test system, (b) xture and (c) FWPS test specimen with initial notch.

Z. Major, R.W. Lang / Engineering Failure Analysis 17 (2010) 701711 705expansion) until 250300 min. After the pressure release a sudden increase of the specimen volume up to very high volu-metric strain was observed in a short time (from 40% to 500% in 30 s, compare Figs. 1c and 6c). Furthermore, the due tothe crack initiation the volume increase was stop and a sudden decrease of the volume was detected (compare Figs. 1d

Fig. 4. Test set-up with full-eld strain and temperature analysis systems.

Fig. 5. Strain and temperature distribution of a FWPS specimen at the crack tip and in the ligament; (a) the vertical strain, y component and (b) thetemperature prole is depicted.

temp V,

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aand 6c after 1360 min). Moreover, the chamber temperature was not constant but rst decreased with the pressure and thanincreased after the crack initiation. This is associated with the adiabatic heating during the fast deformation process.

Based on these diagrams and images depicted above (see Fig. 1) the entire process can be analyzed in detail. Due to therapid 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 simultaneouslydecreasing chamber temperature was recorded. The possible locations of the crack initiation are; (i) rubber matrix/llerinterface and (ii) the space between particles where the matrix is highly constrained. A detailed comparison of RGD behav-iour of the elastomers along with scanning electron microscopy images of the fracture surfaces after RGD test is provided bySchwarz et al. [12].

As the autoclave experiments are very time consuming and expensive, efforts were made to characterize the elastomersby fracture mechanics methods on the laboratory specimen scale.

3.1. Determination of single fracture parameters using loaddisplacement data (global measurements)

Global loaddisplacement curves at two testing rates are shown in Fig. 7a for the HNBR compound. First, single parameterfracture toughness values in terms of tearing energy were determined at the peak load, Tp. Both compounds revealed distinctrate dependence, with increasing loading rate continuously increasing peak tearing energy values, Tp were observed for bothelastomer grades (see Fig. 7b) with moderate date scatter (about 15%). Furthermore, as it is seen in Fig. 7b no signicant dif-ference between HNBR and FKM was obtained. This is may be due to the fact that at about 100% strain similar stress valueswere observed in the nominal stressstrain diagrams for both compounds (compare Fig. 1a and b). Hence, further experi-ments were performed and local parameters related to the crack tip deformation and crack growth were derived and dis-cussed below.

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Fig. 6. 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.

3.2. Determination of single fracture parameters using local measurements

The nominal strain rate dependence of the maximal strain at the vicinity of the crack tip, ectmax is shown in Fig. 8a. A mate-rial dependent decrease of the ectmax values was observed with increasing loading rate and FKM revealed signicantly higherstrain rate dependence. Contrary to the ectmax values a signicant increase of the maximum crack tip temperature, Tmax wasobserved with increasing loading rate and with similar manner for both elastomers as it is seen in Fig. 8b. In good agreementwith the nearly same stiffness and the similar hysteretic behaviour, the crack tip temperatures are in the same range for bothelastomers with somewhat lower values for FKM.

3.3. Determination of multiple fracture parameters using local data

In addition to the crack tip strain and temperature distribution and to the peak strain and peak temperature values at thepeak load, the crack growth was also measured. Crack resistance (R curves) in terms of tearing energy and crack extension,T Dc were constructed and plotted in Fig. 9a and b for both materials. To gain more information about the crack growthkinetics and to compare with single point values the locations of peak load values, Tp are plotted in the diagrams. As ex-pected, highly non-linear R curves were observed for both materials for all loading rates. However, some minor differencesin terms of the non-linear slope were recognized. While HNBR revealed a continuous increase of the slope, a small but sig-nicant rate dependent decrease of the slope was observed for FKM. Furthermore, the location of Tp values uniformly in-creases with the crack length (from 10 to 30 mm) for increasing loading rate for HNBR. In contrary, Tp values were

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Z. Major, R.W. Lang / Engineering Failure Analysis 17 (2010) 701711 70710-2 10-1 100 101 102 1030

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observed at signicantly lower crack length values (from 5 to 15 mm) for FKM. This difference is assumed to be associatedwith the different crack tip deformation and crack growth initiation process for the two elastomer grades.

Moreover, the kinetics of the crack growth was also analysed and the crack growth rate, dc/dtwas calculated based on thevideo sequences. The crack length and loading rate dependence of the crack growth rate is shown in Fig. 10a for HNBR and inFig. 10b for FKM. While a constant increase of the crack growth rate with increasing crack length with similar slope at allloading rates was observed for HNBR, signicant changes are seen in the same curves for FKM. It is assumed that the crackgrowth is continuous in the pure shear specimen for HNBR and contrary to this, it is discontinuous for FKM with crack decel-eration and acceleration phases. Latter phenomenon (crack jumping) is associated with the crack tip blunting and sharpen-ing process. Considering the real RGD loading situation where the gas expansion induces a stress controlled loading withhigh loading rates, this could be a disadvantageous property for the FKM grade.

Furthermore, the crack tip blunting was also analyzed during the crack growth. A crack tip opening angle value, CTOAwasdened and CTOA values were determined for all test conditions. The loading rate dependence of CTOA values is shown inFig. 11a for HNBR and in Fig. 11b for FKM. While the CTOA value remains nearly constant during the crack growth processand reveals only small rate dependence for HNBR, signicant and with increasing loading rate increasing changes of the CTOAvalues were observed for FKM (see Fig. 11b). This higher strain rate dependence of FKM is in good agreement with the obser-vation shown in Fig. 8a.

The nominal strain rate dependence of the CTOAmax values at the maximal crack length are plotted in Fig. 12 for both elas-tomer grades. While the CTOAmax values revealed a small decrease with increasing strain rate for HNBR (about 20%) a sig-nicant decrease was observed for FKM (more than 100%). Furthermore, the rate dependence of CTOAmax is opposite as itwas observed for peak load tearing energy, Tp values (compare Fig. 12 with Fig. 7b). It was assumed, however, that this ten-dency corresponds to and describe more realistic the crack initiation process observed in the autoclave experiments.

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708 Z. Major, R.W. Lang / Engineering Failure Analysis 17 (2010) 701711Fig. 9. Loading rate dependence of crack growth resistance curves and the location of peak load tearing energy, Tp values for; (a) HNBR and (b) FKM.

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Fig. 10. Crack length and loading rate dependence of crack growth rate, dc/dt for; (a) HNBR and (b) FKM.

These crack tip deformation processes are schematically summarized in Fig. 13 for both elastomers investigated. Thecrack tip blunting is uniform during the entire crack growth process and remains the same over the loading rate range inves-

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Z. Major, R.W. Lang / Engineering Failure Analysis 17 (2010) 701711 709Fig. 13. Schematic representation of the crack tip deformation process with increasing crack length for both elastomer grades (FKM and HNBR)investigated.

two opposite tendencies is a highly non-uniform temperature prole in the bulk material during the decompression

710 Z. Major, R.W. Lang / Engineering Failure Analysis 17 (2010) 701711process.As overall conclusion it is stated that simple fracture experiments on laboratory specimen scale does not sufciently re-

ect the very complex thermo-mechanical loading situation in rapid gas decompression and hence, can not substitute thecomponent tests in autoclave. However, these laboratory fracture experiments provide useful information both about thefracture behavior as well as provide input data for further modeling and simulation work.tigated for HNBR elastomer. This results in a continuous crack growth and in a uniform crack growth rate prole. In contraryto this, the degree of the crack tip blunting changes during the crack growth process for FKM grade elastomer. The variationof blunted and sharpened crack tip results in continuous and discontinuous crack growth for FKM. Finally, the kinetics of thiscontinuous and discontinuous process highly depends on the global loading rate of the specimen.

4. Summary and conclusions

A comprehensive characterization of the rapid gas decompression behavior of these two elastomer grades was performedusing both standardized tests and using the novel method by our group [11] and by the researchers of the company partner[12]. It was found in both test series that in spite of the different microstructure (polymer type, cross-link density, ller typeand size, processing) and macroscopic deformation behavior (stressstrain curves) and the different volume change in boththe pressurization (5% HNBR and 15% FKM) and depressurization phase (100% HNBR and 300% FKM) hardly any differencewas observed between the two materials. Microscopic observations revealed, however, some differences regarding the crackinitiation site and the internal crack surface arisen during the crack growth process. Furthermore, it was stated in [12] thatno clear relationship between the basic mechanical properties and the RGD failure behavior was found. It was also specu-lated, that the quality of the polymer network and the processing quality (dispersion of the ller particles) may signicantlyaffect the RGD behavior.

Due to the high volume deformation induced by the expansion of cavitations inside the materials, crack initiation andcrack growth during the RGD process may occur in the specimen. These cracks frequently initiate at the vicinity existingimperfections (e.g., particle agglomerate, a single large particle or void). It was proposed by many authors [15] that thisprocess can adequately be characterized by fracture mechanics concepts and methods. To support material development ef-forts by comparison of materials or by the denition a target toughness value in terms of a relevant fracture parameter, reli-able fracture mechanics parameters should be determined. Furthermore, the appropriate modeling and simulation of theRGD process necessitates the determination and use of relevant material models along with proper material parameters.While novel extended hyperelastic material models [7] may successfully be used for modeling the cavitation and expansionprocess, computational fracture mechanics tools and models are needed to characterize the crack growth process. The pre-requisite of the applicability of these models is the existence of adequate fracture mechanics parameters (single geometryindependent fracture toughness values) or parameter functions (i.e., cohesive zone model, crack resistance curve approach).

Hence, single andmultiple fracture toughness values were determined in this paper and the ndings of these experimentsare summarized below:

To characterize the deformation and failure behavior of elastomers under complex loading conditions, experimentswere performed both on component level under near service conditions and on laboratory test specimen level over awide loading rate range using two model elastomer grades selected (HNBR and FKM). Crack initiation and rapid crackpropagation was observed during the instrumented autoclave tests in the cylindrical specimens during the rapid gasdecompression phase. It was assumed that the crack growths from the bulk (from an imperfection located in the inside)to the surface. Furthermore, fracture experiments on laboratory specimen level applying FWPS specimens have also beencarried out for both elastomer grades. First, conventional fracture analysis was conducted and peak tearing energy valuesdetermined based on global loaddisplacement data. Moreover, to gain more insight into the material response, novelnon-contact, full-eld strain and temperature analysis methods and techniques were also applied. The kinetics of theloading process and the material response was measured, recorded and analyzed in terms of directly measured quantitiesas local crack tip strain, crack length, crack tip opening angle and crack tip temperature. Multiple fracture mechanicsparameters were derived in terms of crack resistance curves, T Dc and crack growth rate, dc/dt c curves.

Material grade and loading rate dependent distinct crack tip blunting was observed in the FWPS test specimens. A uni-form blunting process goes hand in hand with a continuous crack growth and in contrary to this, for a non-uniform bluntingprocess a discontinuous crack growth was observed. While an increase of global peak tearing energy, Tp values with increas-ing loading rate was observed, the local deformation based fracture parameters, maximal crack tip strain, ectmax and the cracktip opening angle, CTOA were found to clearly decrease with increasing loading rates for both elastomer grades investigated.It is expected, however, that due to the particularly high loading rates in the RGD experiments, the material resistanceagainst crack initiation and crack growth signicantly decreases. The tearing energy values based on global forcedisplace-ment curves do not reect this reduced material resistance against crack growth.

Moreover, high material temperatures were measured in the specimens during the fracture process and with increas-ing loading rate increasing maximum crack tip temperatures were observed. It is speculated that the same material tem-perature increase would be observed in the cylindrical specimen in the extension phase. On the other hand a decreasingtemperature was measured in the pressure chamber during the decompression. It is assumed that the resultant of these

Acknowledgement

The research work for this paper was performed at the Polymer Competence Center Leoben GmbH (PCCL, Austria) withinthe framework of the Kplus-programs of the Austrian Ministry of Trafc, Innovation and Technology with the contributions ofthe University of Leoben as scientic partner and the SKF Economos (Judenburg, Austria) as company partner. The PCCL isfounded be the Austrian Government and the State Governments of Styria and Upper Austria. Special appreciation goesto Mr. Klaus Lederer for performing the majority of these tests and data reduction in his M.Sc. Thesis and for efciently sup-porting us during the project work. The additional technical support from Dr. T. Schwarz, M. Moitzi and M. Mitterhuber (SKFEconomos) is also gratefully acknowledged.

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Z. Major, R.W. Lang / Engineering Failure Analysis 17 (2010) 701711 711

Characterization of the fracture behavior of NBR and FKM grade elastomers for oilfield applicationsIntroduction and objectivesExperimentalMaterials and test specimensTest methods and test set-ups

Results and discussionDetermination of single fracture parameters using loaddisplacement data (global measurements)Determination of single fracture parameters using local measurementsDetermination of multiple fracture parameters using local data

Summary and conclusionsAcknowledgementReferences