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Abstract

The selective laser sintering (SLS) is an additive manufacturing technology with clear potential for producing high quality polymeric components of various thermoplastic polymers and elastomers which can be used in demanding engineering applications under complex service loading conditions. The prerequisite for these applications is that the stiffness and the tensile strength of the SLS specimens is in the same range as for injection molded specimens using a proper parameter set of the SLS process and qualified materials. While the tensile strength of SLS printed polymers is recently determined by many researchers, hardly any data are available about the fatigue behavior of SLS polymers on specimen level and even less on component level. Cylindrical specimens with and without round notch have been designed and additively manufactured using PA12 and TPU SLS grades in two different printing directions (vertical and horizontal). The monotonic tensile behavior was characterized over a wide loading rate range (0.1 to 100 mm/s) and the tensile strength and failure strain values were determined. The fatigue behavior was characterized under cyclic loading conditions at various stress ratios (R=0.1, -1), at a constant frequency of 5 Hz at 5 stress levels. Stress vs. cycle number-to-failure, Nf points were determined for constructing conventional S-N curves for the materials investigated. A distinct anisotropy of the tensile strength and failure strain was recognized for the SLS TPU investigated.
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Procedia Structural Integrity 34 (2021) 191–198
2452-3216
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2021 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0)
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10.1016/j.prostr.2021.12.028
10.1016/j.prostr.2021.12.028 2452-3216
© 2021 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (
https://creativecommons.org/licenses/by-nc-nd/4.0
)
Peer-review under responsibility of the scientic committee of the Esiam organisers
Available online at www.sciencedirect.com
ScienceDirect
Structural Integrity Procedia 00 (2019) 000000
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2452-3216 © 2020 The Authors. Published by ELSEVIER B.V.
This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0)
Peer-review under responsibility of the scientific committee of the Esiam organisers
The second European Conference on the Structural Integrity of Additively Manufactured
Materials
Characterization of the Fatigue Behavior of SLS
Thermoplastics
Zoltan Majora, Michael Lacknera, Anna Hössinger-Kalteisa and Thomas Lückb
a Institute of Polymer Product Engineering, Johannes Kepler University Linz, Altenberger Str 69, 4040 Linz, Austria
b Cirp GmbH, Römerstrasse 8, D-71296 Heimsheim, Germany
Abstract
The selective laser sintering (SLS) is an additive manufacturing technology with clear potential for producing high quality
polymeric components of various thermoplastic polymers and elastomers which can be used in demanding engineering applications
under complex service loading conditions. The prerequisite for these applications is that the stiffness and the tensile strength of the
SLS specimens is in the same range as for injection molded specimens using a proper parameter set of the SLS process and qualified
materials. While the tensile strength of SLS printed polymers is recently determined by many researchers, hardly any data are
available about the fatigue behavior of SLS polymers on specimen level and even less on component level. Cylindrical specimens
with and without round notch have been designed and additively manufactured using PA12 and TPU SLS grades in two different
printing directions (vertical and horizontal). The monotonic tensile behavior was characterized over a wide loading rate range (0.1
to 100 mm/s) and the tensile strength and failure strain values were determined. The fatigue behavior was characterized under
cyclic loading conditions at various stress ratios (R=0.1, -1), at a constant frequency of 5 Hz at 5 stress levels. Stress vs. cycle
number-to-failure, Nf points were determined for constructing conventional S-N curves for the materials investigated. A distinct
anisotropy of the tensile strength and failure strain was recognized for the SLS TPU investigated.
© 2020 The Authors. Published by ELSEVIER B.V.
This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0)
Peer-review under responsibility of the scientific committee of the Esiam organisers
Keywords: Type your keywords here, separated by semicolons ;
1. Introduction and Objectives
A number of polymeric components are exposed to a complex combination of external loads in engineering
applications. Due to the layered character of the processes, additively manufactured components reveal inherently
Available online at www.sciencedirect.com
ScienceDirect
Structural Integrity Procedia 00 (2019) 000000
www.elsevier.com/locate/procedia
2452-3216 © 2020 The Authors. Published by ELSEVIER B.V.
This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0)
Peer-review under responsibility of the scientific committee of the Esiam organisers
The second European Conference on the Structural Integrity of Additively Manufactured
Materials
Characterization of the Fatigue Behavior of SLS
Thermoplastics
Zoltan Majora, Michael Lacknera, Anna Hössinger-Kalteisa and Thomas Lückb
a Institute of Polymer Product Engineering, Johannes Kepler University Linz, Altenberger Str 69, 4040 Linz, Austria
b Cirp GmbH, Römerstrasse 8, D-71296 Heimsheim, Germany
Abstract
The selective laser sintering (SLS) is an additive manufacturing technology with clear potential for producing high quality
polymeric components of various thermoplastic polymers and elastomers which can be used in demanding engineering applications
under complex service loading conditions. The prerequisite for these applications is that the stiffness and the tensile strength of the
SLS specimens is in the same range as for injection molded specimens using a proper parameter set of the SLS process and qualified
materials. While the tensile strength of SLS printed polymers is recently determined by many researchers, hardly any data are
available about the fatigue behavior of SLS polymers on specimen level and even less on component level. Cylindrical specimens
with and without round notch have been designed and additively manufactured using PA12 and TPU SLS grades in two different
printing directions (vertical and horizontal). The monotonic tensile behavior was characterized over a wide loading rate range (0.1
to 100 mm/s) and the tensile strength and failure strain values were determined. The fatigue behavior was characterized under
cyclic loading conditions at various stress ratios (R=0.1, -1), at a constant frequency of 5 Hz at 5 stress levels. Stress vs. cycle
number-to-failure, Nf points were determined for constructing conventional S-N curves for the materials investigated. A distinct
anisotropy of the tensile strength and failure strain was recognized for the SLS TPU investigated.
© 2020 The Authors. Published by ELSEVIER B.V.
This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0)
Peer-review under responsibility of the scientific committee of the Esiam organisers
Keywords: Type your keywords here, separated by semicolons ;
1. Introduction and Objectives
A number of polymeric components are exposed to a complex combination of external loads in engineering
applications. Due to the layered character of the processes, additively manufactured components reveal inherently
192 Zoltan Major et al. / Procedia Structural Integrity 34 (2021) 191–198
2 Author name / Structural Integrity Procedia 00 (2019) 000000
anisotropy in the mechanical behavior. This anisotropy is reflected in the stiffness, strength and toughness. For a
number of applications, this inherent anisotropy does not really play a significant role. More demanding applications
would require, however, a more precise and reliable strength analysis considering the strength anisotropy and the
different failure modes. Furthermore, one of the main barriers to a broader engineering application of additively
manufactured polymeric components is the absence of appropriate material data for a reliable strength and durability
analysis.
While the stiffness is expected to be less influenced by the process induced anisotropy for the majority of polymeric
materials and for all additive manufacturing techniques, the strength and the fatigue strength are more sensitive. The
selection of a proper set of processing parameters for specific polymeric materials may guarantee the best achievable
material properties. A high number of monotonic and cyclic tests should be performed under controlled test conditions
using these qualified materials. Hence, the objective of our research was the characterization of the fatigue behavior
of selective laser sintered (SLS) PA12 and thermoplastic polyurethane (TPU) grades in various test configurations.
For stiff polymeric materials (e.g. PA12) the methodology to characterize the fatigue behavior is widely accepted and
is usually conducted under stress-controlled loading conditions determining Wöhler curves (S-N) using standardized
ISO multipurpose specimens (ISO 527). Whereas, for elastomers and thermoplastic elastomers the fatigue
characterization methodology is not unambiguous.
The importance of thermoplastic elastomers (TPE) and more specifically the wide range of ther moplastic
polyurethane elastomers (TPU) is obvious for many demanding industrial and consumer applications (e.g. automotive,
medical and electronic devices) [Holzweber, 2018 and Eberlein, 2019]. The trend of a steady increase in the use and
variety of formulations is likely to continue in the near future and new applications are to be expected. As more and
more thermoplastic elastomer materials become available for additive manufacturing (filaments for fused filament
fabrication (FFF) and powder for SLS), the application of these materials is increasing. TPUs are frequently exposed
to complex combinations of repeated thermomechanical bulk and surface loads in many demanding applications. The
extended applicability to additive manufacturing would support the development of novel individual products for
special application under specific loading conditions [Pan, 2020]. Hence, the proper characterization of the long-term
behavior and the prediction of the service life time along with sufficient reliability of the component is one of the key
aspects for the improvement of both recent and perspective applications. The long-term behavior is ranging from the
change of the deformation properties in terms of viscoelastic parameters up to the classical fatigue failure. It is,
however, somewhat surprising that in spite of the above-mentioned importance and in contrary to classical rubber
compounds the characterization of the fatigue behavior of thermoplastic elastomers is rather underrepresented in the
literature [Major, 2020]. Hardly any data are available for additively manufactured (FFF or SLS) thermoplastic
elastomers (e.g., TPU, TPO, TPA). The majority of these experiments are carried out only at laboratory specimen
level.
The two different methods can be used to characterize the fatigue behavior of SLS printed TPEs:
Mechanics of materials approach: Determination of Wöhler (S-N) curves
o While typical fatigue tests are performed under force (stress) controlled conditions, elastomeric
materials are tested under both displacement and force-controlled conditions. The applicability of
the force-controlled tests is rather limited to low modulus elastomers. For these soft elastomers due
to large deformations the accurate force tuning is hardly any possible and also the determination of
relevant stress values is complicated. Due to the inherent viscoelastic behavior of the TPEs different
results are expected with regards of the material grades investigated (material comparison).
o To overcome these difficulties displacement-controlled test methods were developed and applied.
An overview about the methodology used for displacement-controlled tests and for determining the
so-called local strain based Wöhler (LSWC, -Nf) curves is given in [Belkhiria, 2020 and Major,
2020].
Fracture mechanics approach: Determination of Fatigue Crack Growth Curves (da/dN-T)
o Razor blade pre-cracked planar tensile specimens can be tested under displacement-controlled
loading conditions typically at a strain ratio of R=0.1. To achieve a proper stress state the length of
the planar tensile specimen should be significantly higher than the height of the specimen. The
Zoltan Major et al. / Procedia Structural Integrity 34 (2021) 191–198 193
Author name / Structural Integrity Procedia 00 (2019) 000000 3
sequences of the loading - number of load levels, cycle numbers - were varied. The test frequency
was in the range of 1-10 Hz and the tests were frequently performed at room temperature (RT). The
local crack tip loading was characterized by tearing energy (T). The length of the typically blunted
crack was optically measured and crack propagation rate values, da/dN were calculated and
subsequently combined with the calculated tearing energy values. The tearing energy was calculated
using the deformation energy values corrected with the crack length. The basic assumption was that
for appropriate planar tensile specimens no geometry factor is needed for calculating the tearing
energy for sharply notched specimens [Mars, 2001]. While the stable part of the crack growth curves
can be used for predicting the fatigue life time, the threshold values of these curves were used to
predict the endurance strain limit, eth of model components.
The flexibility of the additive manufacturing makes the production and usage of unique non-standardized specimen
configurations easily possible. The cylindrical dumbbell specimen reveals several advantages:
o Due to the higher stiffness both displacement and force-controlled tests can be performed reliably
o Due to the higher stiffness both uniaxial tensile and compressive (without buckling) testing are
possible (variation of the R ratio)
o The cylindrical specimen can be loaded in torsion and proper shear stress-strain curves can be
determined
o Multiaxial (axial/torsional) loading is also possible
o Local stress concentrations can be introduced (notches with various radius)
The degree of inherent anisotropy of additively manufactured materials depends on a number of factors and is
typically underestimated by practical engineers. Based on some similarities with composite materials, we can consider
some basic corner points of a proper fatigue methodology. The engineer’s perception of the phenomenon of fatigue is
so closely associated with the behavior of macroscopically homogeneous, isotropic materials that there is often a
tendency to treat other materials (i.e., fiber reinforced composites or additively manufactured metals and polymers) as
they were conventionally manufactured materials. Since anisotropy is a characteristic of additively manufactured
materials that we should accept we have to take it into account in the design. Besides the need to understand the
mechanisms by which fatigue damage occurs in additively manufactured polymers, we also need methods that can
reliably predict the development and accumulation of this damage and thus the likely lifetime of the material (or
component) [Harris, 2003 and Aidy, 2018].
In recent study, we preferred the stress-based technique in order to properly compare PA12 and TPU. In our research
project we are using these two commercially available materials for manufacturing various lattice structures
[MOAMMM, Horizon 2020, grant agreement No 862015.]. The commercial availability is an important issue
regarding the quality assurance both in the process and for the component. The results generated on laboratory test
specimen level should be transferred to components containing lattice structures.
While in this paper we are focusing on the load-controlled fatigue test of SLS TPU materials, displacement-
controlled fatigue tests will also be performed and local strain-based Wöhler-curves will be determined in the next
phase, for additively manufactured (SLS) TPU. These results can then be compared with existing results of injection
molded thermoplastic polyurethane materials using similar cylindrical specimen configurations. In addition, there is
also a need for shear strength data of the interface and the multiaxial combination of axial/torsional data, which are
currently under investigation.
194 Zoltan Major et al. / Procedia Structural Integrity 34 (2021) 191–198
4 Author name / Structural Integrity Procedia 00 (2019) 000000
2. Experiments
2.1. Materials and Specimens
Two cylindrical test specimen configurations (hollow cylindrical (CH) and bulk notched cylindrical specimens
(BNC)), with approx. the same nominal cross-section, were produced at cirp GmbH (Heimsheim, Germany) using an
EOS Formiga SLS machine (EOS GmbH, Krailling, Germany). The printing orientation is vertical (parallel to the
faces and perpendicular to the symmetry axis (90°)) and horizontal (perpendicular to the faces and parallel to the
symmetry axis (0°)). The specimens are shown in both printing position in Fig. 1. Similar specimen geometry is used
for fatigue testing of rubbers [Alshuth, 2002; Abraham, 2005].
Fig. 1. Cylindrical test specimen configurations (a) printing orientation vertical (90°); (b) printing orientation parallel (0°).
These cylindrical specimens can be tested under axial (tensile/compressive) loading and under torsional loading
with different boundary conditions at the top face of the specimens. Recent investigations are focused on the axial
loading (tension/compression) but follow-up investigations will also utilize a wide complexity of multiaxial
(axial/torsional (shear)) loading conditions.
2.2. Test parameters and equipment
The following test parameters have been varied during the investigation:
Material grades; PA12 (PA 2200 - Polyamide 12) and TPU (EOS TPU 1301),
Hollow cylindrical (HC) and bulk notched cylindrical specimens (BNC),
0° (parallel) and 90° (perpendicular) printing direction relative to the symmetry axis of the specimens,
Monotonic tensile tests with testing rates 0.1 mm/s, 1 mm/s, 10 mm/s, 100 mm/s at RT;
Cyclic tensile tests with R = 0.1 (tension/tension) and R = -1 (tension/compression), at a frequency of f=5
Hz at T=23 °C and
Specimens with and without surface treatment
The monotonic tests at various loading rates have been performed on a servohydraulic testing machine (MTS
Damper Test System, MTS Systems, Minneapolis, USA), the cyclic fatigue tests were carried out on an
electromechanical testing machine (TAinstruments, ElectroForce 3500, Minneapolis, USA). The same self-designed
and home-made fixture with corresponding adapter was used for both tests.
3. Results
Monotonic tests
First, the results of the monotonic tensile tests are described and discussed. Load-displacement curves of PA12 and
TPU for 0.1 mm/s are plotted in Fig. 2. The differences in the stress-strain curve and thus in the corresponding tensile
Zoltan Major et al. / Procedia Structural Integrity 34 (2021) 191–198 195
Author name / Structural Integrity Procedia 00 (2019) 000000 5
modulus, yield stress and fracture strain are obvious. These differences are, of course, expected by everybody with
minor knowledge in polymers, and these simple data can be useful for designers and engineers for specific
applications. Furthermore, elastic-plastic and elastic-viscoplastic models for further FE simulations can also be
derived using these curves. It is notable here, however, that the SLS printed TPUs reveled a distinct semi-brittle failure
behavior for printing directions perpendicular to the loading (90°). The smooth notch effect was partly overshadowed
by the influence of the printing directions, that is, by the weak interfaces between the layers.
a b
Fig. 2. Load-displacement curves for cylindrical test specimen configurations of the polymers investigated at 0.1 mm/s loading rate and RT (a)
PA12; (b) TPU.
Stress-displacement curves of the investigated SLS printed TPU materials at various loading rates (from 0.1 mm/s
to 100 mmm/s) are shown in Figs. 3a and b. As expected, a pronounced loading rate dependence of the deformation
and failure behavior was observed. The increase of the loading rate up to 100 mm/s did not influnce the shape of the
stress-strain curves and the failure mode observed. As usual, the tensile modulus, yield strength and fracture (failure)
strain values were determined.
a b
0 5 10 15 20 25
0,0
2,5
5,0
7,5
10,0
12,5
15,0
stress, MPa
displacement, mm
SLS TPU notched (BNC)
100mm/s
10mm/s
0,1mm/s
1mm/s
0° orientation
90° orientation
020 40 60 80
0,0
2,5
5,0
7,5
10,0
12,5
15,0
stress, MPa
displacement, mm
0° orientation
90° orientation
100mms
10mms
1mms
0,1mms
SLS TPU hollow cylinder
Fig. 3. Stress-displacement curves for cylindrical test specimen configurations of the polymers investigated (a) bulk notched specimens (BNC);
(b) hollow cylindrical (HC) specimens.
0123456
0
2000
4000
6000
8000
load F, N
displacement s, mm
notched transverse
notched parallel
hollow transverse
hollow parallel
Cylindrical specimens
PA12
23 °C, 0.1 mm/s
-10 0 10 20 30 40 50 60 70 80
0
250
500
750
1000
1250
1500
load F, N
displacement s, mm
notched transverse
notched parallel
hollow transverse
hollow parallel
Cylindrical specimens
TPU
23 °C, 0.1 mm/s
196 Zoltan Major et al. / Procedia Structural Integrity 34 (2021) 191–198
6 Author name / Structural Integrity Procedia 00 (2019) 000000
To show the effect of the anisotropy, the yield stress and the failure strain values were plotted in perpendicular
(90°) and in parallel (0°) direction in one diagram for both specimen configurations in Fig. 4. The straight line
represents the perfect isotropy, the deviation of the points from this isotropy line shows the degree of anisotropy.
a b
6 8 10 12
6
8
10
12
yield stress for 90°, MPa
yield stress for 0°, MPa
hollow (HC)
notched (BNC)
SLS tensile test
cylindrical specimens
23 °C, 0.1 to 100 mm/s
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
fracture strain 90°, MPa
fracture strain 0°, MPa
notched (BNC)
hollow (HC)
SLS tensile test
cylindrical specimens
23 °C, 0.1 to 100 mm/s
Fig. 4. Comparison of printing directions (a) yield stress; (b) failure strain.
The yield stress values of notched and hollow specimens revealed similar anisotropy. The fracture strain values of
notched specimens showed the highest anisotropy.
Fatigue tests
The stress amplitude,  vs. cycle number-to-failure, Nf, values of the SLS TPU material are plotted in Fig. 5 for
both specimen configurations investigated. The corresponding monotonic tensile strength values revealed also a good
agreement with the tendency of these curves (see the monotonic tensile strength values in Fig. 5a and 5b).
100101102103104105106107
0
2
4
6
8
10
12
0° orientation, R=-1
90° orientation, R=0.1
90° orientation, R=-1
0° orientation, R=0.1
stress amplitude, MPa
cycle numer-to-failure, N
f
SLS TPU fatigue
notched cylindrical (BNC)
5 Hz, RT
t
=8.5
t
=10.5 MPa
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
2
4
6
8
10
12
90° orientation, R=0.1
0° orientation, R=-1
90° orientation, R=-1
0° orientation, R=0.1
stress amplitude, MPa
cycle number-to-failure, N
f
SLS TPU fatigue
hollow cylinder (HC)
5 Hz, RT
t
=10.2 MPa (0°)
t
=7.8 MPa (90°)
Fig. 5. Data points of Wöhler curves for the SLS TPU material; (a) bulk notched cylindrical specimens (BNC); (b) hollow cylindrical (HC).
Zoltan Major et al. / Procedia Structural Integrity 34 (2021) 191–198 197
Author name / Structural Integrity Procedia 00 (2019) 000000 7
The fatigue ratio (fatigue limit divided by tensile strength) is in the range of approx. 5 for both specimen
configurations and printing directions. That is, a significant decrease of the fatigue strength observed during the cyclic
loading. To calculate the fatigue strength over a wide cycle number range, these − Nf points were fitted with the
well-known simple Basquin model [Belkhiria, 2018].
As expected, significant differences were observed between the two printing orientations and the R values (-1, 0.1)
for both specimens (BNC and HC). The lowest fatigue strength was observed for the 90° specimens with the stress
ratio of -1 (tension/compression). The highest fatigue strength was observed for the specimens with stress ratio of
0.1. Although we have performed the tests up to 5 million cycles and in several cases no failure was observed, we
could not exactly determine the durability limit, d. It can be roughly estimated that relevant d values for the SLS
printed TPU lays in the range of 1.5-1.8 MPa. This can be interpreted as a rather low value for practical engineering
applications, with regard to the monotonic strength values of the SLS TPUs as well as compared with previous results
of injection molded TPU specimens [Eberlein, 2018 and 2019; Major, 2020]. The printing direction induced
anisotropy of the specimens is a key factor for the further strength analysis. Injection molded specimens reveal rather
small, negligible amount of anisotropy for the mechanical behavior [Van Hooreweder, 2013 and Khudiakova, 2020].
The additive manufacturing industry (in combination with material and process development efforts) has to use this
as benchmark and try to approach these values.
4. Summary and Conclusions
Two cylindrical specimen configurations - hollow cylindrical (HC) and bulk notched cylindrical (BNC) specimens
- were manufactured in the selective laser sintering process for two commercially available polymers, polyamide 12
(PA12) and thermoplastic polyurethane (TPU). These specimens were printed both parallel () and perpendicular
(90°) to the symmetry axis of the specimens. The SLS processing parameters were selected based on the experience
of the company partner cirp. The printing orientation induces a distinct anisotropy of the mechanical behavior. This
anisotropy is less pronounced for the tensile modulus values and moderate for the yield stress or tensile strength
values. The main influence was observed in the failure strain.
While the hollow cylindrical specimen represents an ideal uniaxial stress state, a smooth notch (R=1 mm) was
introduced in order to generate a weak multiaxial stress state. The influence of the notch is clearly visible in the stress-
strain curves of the TPU specimens. While a moderate influence was observed in terms of the yield/tensile strength,
the failure strain was highly affected by the macroscopic notch. The macroscopic stress concentration effect is
interacting with the anisotropy effect. The lowest monotonic tensile strength values were observed for notched
specimen printed perpendicular to the loading. Somewhat surprisingly, the hollow cylindrical specimens revealed
lower fatigue strength values in this printing direction than the notched specimens but with a minor difference.
We conclude that the most meaningful and significant material property for both the quality assurance of the SLS
process and for the strength evaluation is the failure strain. These values have shown very unambiguously the effects
of the macroscopic stress concentration and the weak interfaces between the layers perpendicular to the loading
directions. Hence, we are in the process of developing a critical strain-based fatigue model which is able to consider
also the anisotropic material behavior.
Acknowledgements
These experiments have been performed in the Multi-scale Optimisation for Additive Manufacturing of fatigue
resistant shock-absorbing MetaMaterials (MOAMMM) project. This project has received funding from the European
Union’s Horizon 2020 research and innovation programme under grant agreement No 862015.
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SYNOPSIS Fatigue tests on ethylene propylene (EPDM) and styrene-butadiene (SBR) rubber revealed physical behaviour that is not seen in stiffer less extensible materials. Uniaxial cyclic tests, using cylindrical dumbbell testpieces, with the same minimum stress of zero (smin = 0) and varying stress amplitude (sa), predictably gave decreased fatigue life with increased stress amplitude and hence maximum stress (smax). However, tensile uniaxial cyclic tests where smin was increased in successive tests whilst the stress (sa) remained constant, produced longer fatigue lives for higher values of smax. EPDM and SBR materials were chosen for the tests because they do not strain crystallise during deformation and consequently this phenomenon has no influence. The results show that for smax cannot be used as a criterion to predict fatigue life of non-strain crystallising elastomers which are filled. An evaluation of recorded data of stress against strain gave evidence that energies control the fatigue life rather than stress and strain. However, what role the dissipated energy plays remains an open question. Experimental results on filled and unfilled rubber materials are evaluated and discussed.