Underwater shock loaded sandwich structures

Sammanfattning: The effects of underwater explosions on ships have beenstudied ever since it was realised that explosions under watercould be accomplished. In order to study the response of asandwich structure in water, exposed to an underwaterexplosion, we need to have knowledge about the whole chain ofevents, i.e. THE EXPLOSION, THE FLUID-STRUCTURE INTERACTION andTHE RESPONSE OF SANDWICH STRUCTURES.There are several different kinds of loads generated by anunderwater explosion that might affect a vessel in water andthey are all very complex. At an explosion we have the initialshock wave that is transmitted through the water to thestructure. This initial shock wave, will give rise toadditional loads on the structure, when reflected on the watersurface and the sea bottom. The reflection on the water surfacecauses a zone of water just under the water surface to cavitateand the water surface to rise (spray dome). At the chargelocation we have the formation of the bubble, with itspulsation and migration towards the surface. At every bubbleminimum, a shock wave is generated which rapidly decreases inmagnitude for each pulsation. The bubble is also attracted tothe sea bottom and objects in the water, but rejected by thewater surface. However this is an area that is not covered inthis thesis. When the shock wave reaches a structure, thestructure moves away, but in the next instant when the pressureis applied, the structure is moving in the same direction asthe load and therefore the load pressure is decreased. As thewave hits the structure it bounces back and may causecavitation to occur at the fluid-structure interface. All thesephenomena must be taken into account when evaluating thepressure acting on a structural surface.Finally we have the response of the sandwich structure whichdepends on the many parameters that a sandwich structure ismade up of. We have the faces with their different properties,i.e. depending on reinforcement (fibre) material, reinforcement(fibre) direction and thickness (number of fibre layers). Thecore may vary in density and thickness. This gives us a verylarge number of possible sandwich configurations. However,since we are dealing with fast loading sequences the mass andits distribution in the structure are very important.Looking locally (transverse direction) at a sandwichstructure, the front face is first compressed into the corewhen hit by a shock wave. This initiates oscillation about thecore of the faces. This may cause significant transversetension in the faces and delamination may occur. This mode offailure has not been considered before and has to beinvestigated further in the future. Going from one dimension totwo dimensional problems leads to the study of sandwich beams.The change in behaviour of a sandwich beam when the parameterschange can in many cases be predicted by the use of a simpleexpression. In order to verify this, several numerical exampleswere analysed and some important conclusions were reached. Whenthe parameters of the sandwich beam change it is not only thestiffness of the beam that changes, but also the distributionof the energy between the faces and the core of the sandwich.This sometimes makes it very difficult to predict the completeresponse. For example, stiffer faces lead not only todecreasing deformation and face stress, but also to anincreasing core stress. This is due to the redistribution ofenergy between the faces and the core in the sandwich beam. Thebehaviour of three-dimensional structures, such as a sandwichpanel, is similar to that of a sandwich beam. One can use theexample of the beam to predict the change in the response of apanel when the parameters vary.This thesis does not try to cover the latest researchconcerning all the different phenomena occurring during anunderwater explosion. For some of the phenomena there are moreadvanced methods available. This is rather an attempt toprovide engineers with methods that can be used to solve thistype of problem.