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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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 − N f 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 0° 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 (0°) 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 inno vation programme under grant agreement No 862015. References

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