David Marschall,
"Lightweight design of thin-walled, lattice based sandwich structures for directly manufactured components"
, 8-2020
Original Titel:
Lightweight design of thin-walled, lattice based sandwich structures for directly manufactured components
Sprache des Titels:
Englisch
Original Kurzfassung:
Additive manufacturing (AM) technologies, such as laser sintering (LS), multijet fusion (MJF) and hot lithography (HL) enable the production of complex thin-walled lattice-based lightweight structures. Thus, AM provides the possibility of directly manufacturing personalized, structural motorcycle components for motor sports with a reduced lead time. Detailed knowledge of the performance of AM polymers, regarding their static and dynamic mechanical behavior is essential for the design and development of additively manufactured polymer components. Therefore, the lattice-based components require a high quality and a reliable design approach for subsequent racetrack testing. In instrumented static and dynamic three-point bending (3PB) and puncture tests, the impact behaviors of polyamide (LS and MJF process) and methacrylate-based photopolymer (HL process) test specimens were compared. Fractography was performed using stereo light and scanning electron microscopy. To study the process-related effects on the mechanical properties of 450 tensile test specimens in z-direction, the build areas of two LS systems were screened and a detailed wall thickness investigation was conducted. In addition, dynamic mechanical analysis, differential scanning calorimetry, and scanning electron microscopy for several wall thicknesses similar to the spot size were carried out. Based on a stiffness evaluation, the design approach for large, laser sintered, multi-curved sandwich structures was investigated. Therefore, unit cell (UC) and sandwich beams with lattice cell cores were modeled, simulated, manufactured, and tested. A simulation of the sandwich beams was conducted using continuum and forward homogenization (FH) models with simple and complex elastoplastic fitted material models. The test specimens were printed, and 3PB tests were conducted. Multi-curved, sandwich structure based demonstrators were designed, numerically analyzed and experimentally tested with boundary forming conformal lattice cells. The truss- and surface-based non-periodic lattice cells of the demonstrator's sandwich core were computationally efficient simplified by the FH processes. To represent the stiffnesses of the top and bottom face sheet two simulation approaches were compared. In a first simulation approach (FEA C) constant material properties per thickness were used. In a second approach (FEA M) transversely isotropic, thickness and orientation depended material properties were taken into account for every element of the top and bottom face sheet. The stiffness of each FEA approach was validated by an experimental test setup including a digital image correlation system to determine the displacements of the multi-curved demonstrators. The dimensional accuracy of the test specimens, sub-components and the full scale component was evaluated. The polymers produced by LS, MJF and HL revealed comparable force{displacement behaviors in a static 3PB test, but their impact behaviors differed considerably. The obtained results highlight the impact performance of AM polymers as an essential design variable for race track components. The detailed investigation of two LS systems showed that the Young's moduli and ultimate tensile strengths of the produced specimens are similar and evenly distributed across the build area. Furthermore, structures with a thickness below 0.8mm showed distinctive losses in stiffness, ultimate tensile strength, and elongation at break. On sub-component level the stiffness prediction of the FEA M approach was more accurate than the FEA C approach and underestimated with -6.5% the test results at 1mm detection. Conducting a process parameter optimization was important, especially when producing space consuming lattice-based sandwich structures. Finally, by using a full scale front fairing of a racing motorcycle the applicability of the design and simulation approach including the manufacturing process was successfully demonstrated.