Failure of thinwalled structures under impact loading

Licentiate thesis, 2013

Thinwalled structures are widely used for different applications, e.g. maritime structures, vehicles, off-shore structures, aircraft fuselage, ship panels etc., and due to their application they might be exposed to high strain rate loads as blast loads. Obviously, thinwalled structures must be capable of withstanding these loads to a certain degree. In order to investigate how failure takes place and to design structures resistant to blast loads, advanced numerical modelling techniques are required. This work is intended to develop a framework capable of analysing ductile fracture in terms of prediction of crack initiation and propagation applicable to thinwalled steel structures exposed to high strain rates. Of particular interest is the application to large scale structures for which an effcient numerical procedure is required to obtain the accurate response of the structure. In the first paper, dynamic crack propagation in elastoplastic thinwalled structures is explored. In this development, a hypoelastic-inelastic modelling framework is employed where the Johnson and Cook phenomenological model is incorporated. Thereby the in uence of the temperature and the plastic strain rate dependencies are accounted for, which is of signiffcant importance for analyses of the thinwalled structure exposed to blast loading. The shell kinematics are represented utilising a 7-parameter shell formulation including extensible directors and a second order inhomogeneous thickness deformation. In order to describe the through-the-thickness displacement discontinuity, the shifted version of XFEM is employed when localisation is predicted. Furthermore, the resisting force of the process zone is represented by a damage-viscoplastic cohesive zone model. In order to verify the model, different examples are carried out and compared against experimental tests. In the second paper, the objective is to enhance details of the crack path without the incorporation of standard remeshing procedures. Due to dealing with large thinwalled structures, it is of particular importance to represent the crack tip and kink inside the cracked element. To this end, in addition to the macroscopic continuous and discontinuous displacement fields, a discontinuous uctuation field with a Dirichlet boundary condition is employed at the element subscale level. This subscale enrichments provides an enhanced representation of the discontinuous kinematics within the crack tip element. One advantage of the current method is that a local model reduction is carried out, using dynamic condensation, so that the spatial discretisation of the domain remains intact. In order to verify the new methodology, different examples are carried out and compared against the standard XFEM enrichment.