a b s t r a c tThe paper is concerned with the accurate numerical simulation of localized deformation that can develop into necking and failure, induced by combined bending and tension in shell structures. The study is motivated by the need to establish the onset and evolution of such failures in imploding underwater structures. Such localized zones of deformation are shown to develop under controlled conditions in experiments on Al-6061-T6 cylindrical shells crushed laterally by rigid punches. The crushing induces gradually developing local depressions in the shells, at radially constrained locations. As the crushing progresses, the depressions with a width of the order of the shell wall thickness, deepen, increase their span, become neck-like and develop inclined failures. In the experimental set-up used, the crushing was terminated when the first of four such depressions that develop ruptured. The shell was sliced along the principal plane of crushing and the most deformed cross sections of the necks were measured. The crushing experiments were simulated numerically using solid FE models. The material was modeled as a finitely deforming elastic-plastic solid that hardens isotropically using the von Mises, the non-quadratic isotropic Hosford and anisotropic Yld04-3D yield functions suitably calibrated. While the overall structural response was reproduced well by all models, differences were observed in the evolution of localization in the depressions. For the von Mises yield function, the localized deformation was significantly milder than in the experiments. The isotropic Hosford yield function produced necks that were closer to the experimental ones, while Yld04-3D produced results that were very close to the measurements. Clearly, and in concert with other applications, the adoption of a non-quadratic yield function is necessary for reproduction of localized and other challenging deformation histories in Al alloys. The addition of anisotropy in such models improves further the predictions.

This brief note reports the results of a set of biaxial experiments on Al-6061-T6 tubes tested to rupture under radial stress paths of combined internal pressure and axial load. The experiments are then simulated with shell-type finite element models, in which several yield functions are calibrated and implemented and their performance evaluated against the experimental results.

The interest in the numerical modeling of a deepwater pipe-in-pipe system (PiP) at a free span location is growing. In a free span, the inner pipe bends substantially due to internal pressure and temperature and possibly contacts the outer pipe. Concerns arise how the interaction between them affects the PiP behavior under thermal expansion and contraction. The study is motivated by the need to understand how a PiP at a free span can be modeled adequately. Different finite element (FE) approaches using Abaqus have been practiced. The pipes were simulated with pipe, elbow, and shell elements. The interaction between the inner and outer pipes was simulated with multi-point constraints (MPC), tube-to-tube contact elements (ITT), and contact surfaces. FE model accuracy and cost effectiveness were compared in a case study. The conclusions in this paper would also be applied for PiP sections with bending, such as for lateral buckling of a PiP laid on the seabed and upheaval buckling of a buried PiP. Introduction A PiP insulates the inner pipe using an outer pipe as shown in Figure 1(a). In a free span, Figure 1(b), the pipes bend and the gap between them allow for relative displacement due to different effective tension and weight of the pipes. A shell FE model of a PiP at a span was developed to investigate the behavior during operating and cool down conditions. In addition, pipe FE models were developed and evaluated against the shell element model. It is shown that while the shell FE model simulates the motion of the inner pipe relative to the outer pipe and the contact between them, it requires a lot of computational effort. Contrary, a pipe FE model provides global deformation and pipe stresses close to those of the shell FE model and is cost effective. Table 1 and Table 2 summarize the PiP properties, span under investigation, and loading conditions for each pipe.