This work investigates some of the challenges concerning the numerical simulations of structural failure in case of ship structures, inciuding events like ship collisions. It was highlighted that the large finite element models employed for computations with advanced nonlinear finite element solvers can provide a fairly accurate image of the structural behavior well beyond into the domain of large deformations and large plastic strains. However, there are some specific issues in respect to the traditional modeling of ship hulls, particularly concerning the implementation of failure criteria related to the ductile cracking of large plate fields. Shell finite elements are mandatory to be used in such models, due to aseries of well-known reasons based on the size and complexity of ship hulls. These shell elements are unable to detect the plate thickness necking themselves, especially when a displacement driven boundary condition is used. As a workaround, thickness necking is related to the level of plastic strain by the analyst. There is a common view that the element size is critical in setting the right failure criteria. In contact problems, where plastic strain concentrations frequently occur due to friction, a false image may arise concerning the element size sensitivity of the failure criteria. In many cases, this sensitivity is associated with the model convergence in respect to the plastic flow. In such cases, the element size sensitivity criteria should be applied to the plastic flow field and not directly to the thickness necking. For a force driven boundary condition and a fairly simple investigation, like a tensile test, an indicator for the thickness necking might be the stiffness matrix becoming negatively definite. For practical use trus is not very helpful, as post-failure behavior is searched for in most of the cases. If an explicit finite element solver is used, there is not even the need to assemble the global stiffness matrix, so there is DO signal to be read from the conditioning of trus matrix.
On the collapse behavior of ship structures
2009
105 Seiten, Bilder, Tabellen, 33 Quellen
Hochschulschrift
Englisch
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