PSI - Issue 53

Benjamin Möller et al. / Procedia Structural Integrity 53 (2024) 190–202 Author name / Structural Integrity Procedia 00 (2023) 000–000

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3. Stress-based fatigue assessment 3.1. Scan to FE-model

For a stress-based fatigue assessment based on the test campaign described above, a procedure is needed to derive equivalent stresses from the load amplitudes. A stress-based approach using load simulations of finite element (FE) models was chosen. In general, this kind of approach simulates the testing on a nominal (design) geometry. The results of this simulation are used to transpose the load amplitudes from the force-controlled testing into stress amplitudes. Since WAAM process is still very challenging to control, the nominal geometry can differ significantly from the actual additively manufactured structures. This becomes obvious by looking at the surface finishing and at the cross-sections of the handlebar. To take in account the effects of these imperfections, a FE approach has been chosen that uses the 3D scanning technique (Keyence VL500) as input data in order to create FE element models to be simulated. This process showed two main issues:  First, due to the narrow shape of the handlebar and visual accessibility of inner surfaces, it was impossible to scan the inside of the structure resulting in a scanned part that was not a solid geometry but just an envelope of the outer surface.  Second, the scanner output delivered a very big stl-file (shell models made of approximately 5 ∙ 10 6 triangles) which was impossible to be used directly as an FE model. Due to these reasons, it was necessary to modify the data set of the scanned parts and postprocess it prior to the meshing and FE simulation. This process, shown in Fig. 7, was done using the 3d graphics software Blender and the following steps to obtain the FE model from the scanned geometry, ready to be simulated, : 1. The degree of details of the 3d scan is reduced to finalize the stl-data (processable data),  in Fig. 7. 2. The stl-file from the scanner is imported into the 3d graphics software Blender, from  to  in Fig. 7. 3. The upper and the lower end of the handlebar were cut, so that the WAAM structure between the two clampings is left, which is relevant for the FE simulation,  in Fig. 7. 4. The number of elements that describe the outer surface is reduced,  in Fig. 7. 5. The internal surface is created in order to transform the model into a 3D solid geometry. The internal surface is obtained as a homothetic scaling of the end of the profile that is then extruded along the longitudinal axis of the handlebar,  in Fig. 7. 6. The stl-file is imported into Abaqus as a geometry, from  to  in Fig. 7. 7. The meshing is performed using tetrahedral brick elements: the mesh is created to be identical to the blender file on the outer surface (for a better representation) while on the inside is free and the elements are bigger (to reduce the number of elements),  in Fig. 7.

Fig. 7. Process from 3d scan to FE-model.

Once it was possible to obtain a working FE model, there were two more modelling aspects to be considered:  The definition of a constant thickness for the section of the structure  The choice of the element size and number of elements for the meshing