Public defence: Sigbjørn Tveit
Sigbjørn Tveit defended his thesis “Forming and fatigue in floating photovoltaic structures. Approaches based on continuum mechanics” for the degree of PhD in Engineering Science.
Sigbjørn Tveit defended his thesis “Forming and fatigue in floating photovoltaic structures. Approaches based on continuum mechanics” for the degree of PhD in Engineering Science.
Yonas Zewdu Ayele, Head of Department, Department of Built Environment, OsloMet, Norway
The research presented in this thesis is related to the structural analyses of a floating photovoltaic (FPV) concept developed by Sunlit Sea AS. They use a metallic buoyant float structure that acts as a thermal bridge between the water body and the photovoltaic panels.
For the solution to be commercially viable, the structure must be efficient in terms of heat transportation and material use. This has motivated the company to explore innovative structural designs, where the current solution is a shallow-draft shell structure made up of two formed aluminum alloy sheet components. As the platform must maintain its structural integrity throughout the intended lifetime, fatigue failure due to ocean wave-induced wave loads is a concern.
This thesis presents new models and approaches for the analysis of such structures. One of the key contributions is a submodeling technique that incorporates sheet metal forming (SMF) effects in the structural analysis of formed components. For a critical component region, sheet metal forming simulations are conducted in LS-DYNA to obtain realistic geometric configurations and distributions of residual stresses and material thinning.
Then, a simplified service load analysis of the complete structure is performed to obtain boundary conditions for the LS-DYNA model by using a mapping technique based on normalized segment length interpolation. It is shown how a continuum mechanics-based fatigue model can be combined with SMF submodeling to estimate the structure’s fatigue life under high-cycle loading.
It is revealed that the continuum mechanics fatigue model cannot correctly predict shear stress- dominated fatigue failure for materials that display brittle or very ductile fatigue behavior. This results in two proposed modifications to enhance the model’s efficacy.
The first modification adopts a Hershey-Hosford function to generalize the endurance surface formulation. This solves the issue for isotropic ductile metallic materials where the relationship between the fatigue limits in fully reversed shear stress and fully reversed normal stress varies from 1/2 to 0.585. The second modification proposes a conditionally convex endurance surface that allows the model to take on any relationship between the shear and normal stress fatigue limits by considering the effect of the Lode angle. This is of significant interest for exploring float designs based on additive manufacturing, since these materials reportedly display highly brittle fatigue behavior.