PSI - Issue 34

Zoltan Major et al. / Procedia Structural Integrity 34 (2021) 191–198 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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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 thermoplastic 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,  -N f ) 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