Blast Mitigation Strategies for Marine Sandwich Structures

Abstract

Blast mitigation is being researched from the time of world war II. There were full field blast tests and analytical studies on the performance of monolithic structures to mitigate blast load. Sandwich structures were also used during that time to make the body of warplanes owing to their property of high stiffness at low weight cost. Sandwich construction at present is also extensively being used in the construction of war ships. It is very important that they are designed to resist underwater explosion. To build sandwich structures capable of resisting underwater explosion loading, it is necessary to understand the underwater explosion (UNDEX) phenomena, structural modeling and the interaction between the fluid and the structure. In the present work, attempt has been made in sandwich structure modeling, Fluid-Stucture Interaction (FSI) modeling and finally proposing blast mitigation design strategies.

Spectral element method (SEM) for a higher order sandwich beam incorporating flexible core behaviour is formulated for a thick multi-layered and multi-oriented composite laminate sandwich beam. To incorporate the core compliance, a high order sandwich theory known as Extended High-order Sandwich Panel Theory (EHSAPT) is adopted. The efficiency of the SEM based computational model to capture the dynamic behaviour of sandwich structure for a high frequency loading in terms of convergence and computational cost is demonstrated in comparison with commercially available software based on FEM. The SEM based computation model is further used in conjunction with Hayman's model for FSI in order to capture the effect of UNDEX loading on the sandwich structure. The difference in the fluid pressure calculations and the FSI due to the presence of the core is shown, thus bringing out the importance of core in the design of sandwich structure to UNDEX loads. The apparent observation from the studies in literature that a soft core is beneficial in reducing the fluid pressure coming on the structure is studied. Effect of core softness in reducing the fluid pressure due to an UNDEX loading is illustrated and at the same time the trade off with structural stiffness are also brought out. A dual (hard-soft) core design configuration is proposed to overcome this trade off, and its effectiveness is illustrated.

Finally, blast mitigation strategies based on the understanding of the effect of rise-time to peak pressure of the load and the Response spectrum of the structure for a given UNDEX loading are proposed. The two strategies being prolonging the rise-time by providing a contrivance before the structure and the other is, designing the structure such that the response does not fall in the rigid body response region of the Response spectrum. The effectiveness of these strategies is illustrated using the developed spectral element model in conjunction with FSI model.