PSI - Issue 25

Pedro R. da Costa et al. / Procedia Structural Integrity 25 (2020) 445–453 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Figure 8 shows SEM uniaxial tension-compression detailed images for both conventional and ultrasonic fatigue testing conditions. From both sets of images, no clear difference was found in the origin of fatigue initiation as well as for propagation marks. The propagation marks only show to have a slight spacing increase related to higher crack growth in conventional testing. In Figure 9 the multiaxial tension/torsion images using SEM are shown for both conventional and ultrasonic fatigue testing method. Total non-fatigue failure is different between both loading conditions. This happens because in ultrasonic fatigue testing total failure after fatigue crack propagation is made by a tension machine leading to tensile non-fatigue fracture, while in the conventional testing the undergoing multiaxial stress leads to total failure. Propagation marks are similar between loading conditions and indicate the existence of shear stresses. They have a laminated shape as seen in both ultrasonic and conventional higher amplification images ((C) and (D)) and the separation lines from the chipping point to the initiation zone as it can be better observed in the general view taken by SEM in the (B) images. Such lamination is linked to fatigue shear damage mechanism. When there is no shear a more random lines are generated as seen in the amplified uniaxial propagation marks ((C) and (D)) from both ultrasonic and conventional specimens. 4. Conclusions The ultrasonic fatigue testing machine working at 20 kHz and with the specimen designed according to the specific rules is adequate to perform very high cycle fatigue, under multiaxial loadings, tension/torsion or axial/axial stress under stress ratio R = -1 in a reasonable time of testing. Fatigue experiments were already conducted with fairly good results, showing the correct induction of multiaxial stresses. From the SEM images it can be observed the existence of shear stress comparing fatigue crack between multiaxial and uniaxial ultrasonic results. No clear difference was observed through SEM imaging comparison between conventional and ultrasonic fatigue cracks. The tension/torsion fatigue life stress results showed to have a higher stress to life result than expected and without any failure in the VHCF region. Such result shows to be in line with what is state in literature (Bach, Göken, and Höppel 2018), where for certain metals appliance of high frequency fatigue testing has an impact on fatigue life stress results. Other researches from different authors have had similar observations where the application of high frequency fatigue resulted in fatigue life strengthening (Guennec et al. 2015; Nonaka, Setowaki, and Ichikawa 2014; Tsutsumi, Murakami, and Doquet 2009). All mentioned researches in this paragraph worked with low carbon metals with a ferrite-perlite microstructure. Looking at the (Bach, Göken, and Höppel 2018) results, a tested C60E metal shows to be relatively similar to the material in study in terms of microstructure and carbon percentage. The presented results have a similar increase in uniaxial fatigue and no failure beyond 10 7 was reached. It is stipulated through experimental results comparison to other metals that frequency effect increases with the increase of ferrite content. It is also mentioned that the perlite matrix is the root cause for the non-growth of micro-cracks in the VHCF regime and both the material in study and the C60E have a predominant perlite phase resulting in little or no failure in the VHCF regime. A higher stress-life increase was obtained between the conducted multiaxial fatigue tests. Such increase can be associated to a higher influence of shear stress in frequency fatigue strengthening of the material. It could also be related to the torsion stress gradient or a geometry effect/volume effect. Acknowledgements The author thanks FCT, Fundação para a Ciência e Tecnologia, which through several research projects in the last 20 years has supported financially the development of these technologies of multiaxial very high cycle fatigue and namely through LAETA/IDMEC, project UID/EMS/50022/2019 and Project PTDC/EMS-PRO/5760/2014. References Bach, J., M. Göken, and Heinz-Werner Höppel. 2018. “Fatigue of Low Alloyed Carbon Steels in the HCF/VHCF-Regimes.” Fatigue of Materials at Very High Numbers of Loading Cycles : 1–23. Bathias, C. 1999. “There Is No Infinite Fatigue Life in Metallic Materials.” Fatigue and Fracture of Engineering Materials and Structures 22: 559–565.