PSI - Issue 53

Benjamin Möller et al. / Procedia Structural Integrity 53 (2024) 190–202

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Author name / Structural Integrity Procedia 00 (2023) 000–000

Fig. 14 S-N curves for von Mises stresses of specimens modelled with ideal flat, smoothed and “real” scanned surface.

4. Conclusions and outlook 4.1. Conclusions The following conclusions are drawn from the results:

- Successful fatigue testing under axial bending load with a load ratio of R F = 0 could be performed using epoxy resin adaptors for the clamping. The Wöhler ( S - N ) curve for maximum stress in the tensional loaded fiber of the ideal geometry, evaluated from linear-elastic FE load simulations, is described by  a,vM,k = 44.7 MPa, N k = 800,000, k = 3.1, k * = 22.0, and T  = 1 : 1.93 using the MLE. The scatter value is a little higher as found for welded joints, in general. This is an indication that fatigue relevant notches of the WAAM structures differ more than these of conventional welds. - For prognosis of the point of fracture and to evaluate maximum local stresses of the “real” as well as smoothed structures from FE simulations successfully, 3d scans of each specimen need to be postprocessed and imported into the FE software for meshing and simulating. A differentiation between small-scale imperfections being present in “real” scanned models and macrogeometric shape variations (smoothed models) is possible by using the smoothing function. The smoothed model still gives a good prognosis of the point of fracture and shows the importance of the consideration of the macrogeometric surface, even though the factual notch effect is not represented. However, compared with the Wöhler curve parameters of the ideal geometry, the MLE of the von Mises stresses for the smoothed model reduces the scatter to T  = 1 : 1.63. A reduction of the scatter is often used to demonstrate how scattered result merge when a structural or notch stress approach is applied. - From the “real” scanned simulation result, the stress concentration can be approximated. The fatigue notch effect with regard to the surface topography has been shown based on the numerical assessment of the fatigue resistance using von Mises stress amplitudes and evaluating the Wöhler curve parameters  a,vM,k = 89.0 MPa, N k = 800,000, k = 3.8, k * = 22.0, and T  = 1 : 1.61 from the MLE. Related to the ideal flat specimen geometry, the linear-elastic simulation results of the “real” scanned geometry of the WAAM structure results in twice as high stress amplitudes, i. e. a surface-related stress concentration factor of K t,sur = 2. The scatter of the “real” scanned geometry of T  = 1 : 1.61 is similar to the one for the smoothed specimens and, therefore, significantly lower than the scatter value from the load-controlled evaluation and stress-based evaluation of the ideal flat geometry. - The surface-related stress concentration factor for smoothed specimen models of K t,sur = 1.5 is reduced by a factor of 0.75 compared with the one for the “real” scanned geometry. This shows a huge potential for stress reduction if such a smoothing could be realized within the physical structure.

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