Structural Design of Laser-Welded Sandwich Panels for Marine Applications

University dissertation from Stockholm : Farkost och flyg

Abstract: In shipbuilding the replacement of stiffened platestructures by laser-welded sandwich panels offers considerableadvantages for production as well as vessel operation. In viewof reduced weight and space requirements an area of particularinterest is the incorporation of an adhesively bonded core bywhich the stiffness and strength can be substantiallyincreased, and thermal-acoustic insulation can be integrated.This thesis aims to offer practical solutions for thestructural design of laser-welded sandwich panels, that can beimplemented in design guidelines and rules of approval.n the first paper a stiffness model for laser-weldedsandwich panels is developed where the sandwich panel isrepresented by a homogeneous 2D thick orthotropic plate withequivalent stiffness properties. Equivalent bending, transverseshear and twisting stiffness expressions are derived andnumerically evaluated for a variety of core materials.Predicted stiffness and deflection values are shown to agreewell with the results from 2D and 3D finite element analysis,mechanical testing and frequency response measurements. Forcurrent sandwich configurations it is found that even the useof a relatively low modulus core material improves thecooperation between the two face sheets and thereby increasesthe shear stiffness perpendicular to the webs considerably.The next three papers examine the elastic buckling andcollapse behaviour of laser-welded sandwich panels, withparticular focus on the stability of the face plate in betweentwo webs. A simple buckling model is developed which makes itpossible to demonstrate the effect of different boundaryconditions at the laser weld and different core properties bythe variation of a clamping factor and two foundation moduli.Effectively, this model shows that for low modulus corematerials the elastic buckling load is equal to the bare platebuckling load with clamped boundary conditions, while for highmodulus core materials the solution converges towardanti-symmetrical wrinkling. As a result, the buckling stresscan reach the proportional limit of the face plate and webmaterial before elastic buckling occurs and the model istherefore extended to cover inelastic buckling and plasticityas well, giving good agreement with non-linear finite elementand experimental results.To help decide whether the shipbuilding industry can benefitfrom the continued development of cored sandwich panels and inwhat direction future research should be directed, the lastpaper investigates the possibilities for structuraloptimisation of laser-welded sandwich panels. Based on, amongothers, the stiffness and strength models developed in theforementioned papers, a structural optimisation method isimplemented and used to minimise the structural weight for atypical accomodation deck configuration. It is concluded that,within the span of production parameters and rule requirements,substantial improvements can be made, with or without corematerial and without any changes to the surroundingstructure.Keywords:Sandwich; Laser welding; Web; Core; Stiffness;Elastic Buckling; Ultimate strength; Finite element analysis;Mechanical testing; Optimisation.

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