Research On The Composite And Multilayer Defensive Broadside Structure For Large Surface Ship

National Navies are the core force in maintaining the thalassocracy and maritime security. In the previous naval battles, large surface vessels have played the leading role. Nowadays, with the rapid development of the anti-ship weapons, especially the anti-ship missiles, the large surface vessels become an easy target because of its large hull, limited speed and battle effectiveness. Therefore a large surface vessel itself should own the protective structural system which is the last defense line for maintaining the life security. A good protective structure can successfully "digest and absorb" the anti-ship weapons which "luckily" penetrated the active defense system, so that the vitality and effectiveness of the large surface vessels can be retained.The composite and multilayer defensive structure can make the advantage of the large dimension of ship hull to "digest" the attack from the anti-ship weapons by the approach of "conquering the unyielding with the yielding". Such kind of structure layout has generally adopted to build the ship broadside by foreign naval power. During the World Warâ…¡, the composite and multilayer defensive structure was mainly used below the waterline to resist the attack of the mines and torpedoes. Now, such structure layout has spread above the waterline due to the increasing threat of the anti-ship missiles. Take the follow-up warship of the American Nimitz Aircraft Carrier as an example, its composite and multilayer defensive structure increases to the hangar-deck. Generally, the composite and multilayer structure is consisted of the outside board, empty cabin, liquid cabin, and defensive longitudinal bulkhead. The number of layers is usually three, four, and five and so on. However, the research such as how to determine the layer number, the protection function of each layer, the optimum design of the protective structure and the protective effects, are quite limited in china; and the related information is difficult to obtain from abroad.In this paper, by use the theoretical analysis and numerical simulation, research on the mechanism of the composite and multilayer defensive structure, which is adopted as the broadside of the large surface vessel, was carried out; the defensive effects of different broadside structure layouts have also been investigated. The research may have great significance in guiding the design of the composite and multilayer structure and improving the vitality and the battle effectiveness of the large surface vessel. The detail research contents are as follows:(1) The mechanism of the missile penetrating the target was investigated. Through numerical calculation method, the numerical simulation of the conical projectile penetrating the thin plate vertically and obliquely was realized and the calculation result shows a good agreement with the related experimental result. The mechanism of the petals formed after the projectile penetrating the target vertically and the motion characteristics of the projectile after penetration are analyzed. The dividing line of the three different failure types of the plate after penetrating by the conical projectile proposed by the Zaid and Paul is revised. The residual velocity formula of the conical projectile after penetrating the thin plate at low and high oblique angle is deduced through theoretical analysis method.(2) The anti-penetration performance of the broadside grillage structure of the ship vessel was studied. The parameters in the J-C strength model of the 945 steel used in numerical simulation were confirmed and the numerical simulation progress of the Exocet missile penetrating the broadside grillage was realized. The vertical penetrating characteristics at six typical bullet points and oblique penetrating characteristics at the stiffeners were studied for comparison. The research results show that the stiffeners can effectively reduce the residual velocity of the anti-ship missile after penetration. Increasing the thickness of the plate had little effect on the reduction of the residual velocity of the missile after warhead penetration, while brings remarkable effect to reduce the residual velocity of the missile after the wing penetration. During the vertical penetration, the residual velocity of the missile after the warhead penetration and the wing penetration decreases as the penetration angle increases. Through the comparative analysis of the research results, the design idea of the anti-penetration of the broadside plate was proposed:the first, a side longitudinal girder can be set at the height where the anti-ship missile sea-skimming attack the ship, which can increase the probability of the missile impact on the strong components. The second, when the space of the broadside is permitted, increasing the distance of the outer plate and the internal structure can make the fuze spring when the warhead hit the plate, which can make the warhead of the missile blast in the empty cabin as far as possible. The third, the oblique broadside plate should be set, or the oblique deck should be set near the waterline as far as possible.(3) The calculation formulas of the free air explosive blast wave parameters were summarized, analyzed and compared. The most appropriate formula about calculating the explosive blast wave parameters was chosen. The different solution methods on the numerical simulation shock wave propagation in the air by the Dytran and Autodyn were compared and the relatively high precision numerical simulation method was determined.(4) The time-related displacement expression of the rectangular grillage under the shock wave was deduced based on the assumption that the plate and the stiffeners of the grillage have the same deflection under the shock wave load. The response of the grillage subjected to the air non-contact explosion was simulated by Autodyn. The influence of the self-contact on the simulation results was discussed. Through the comparative analysis of the deformation of the stiffeners on the surface of the explosion and at the back surface of the explosion, the design idea that the direction of the stiffeners on the longitudinal bulkhead of the complex multi-layered defensive structure should be pointed to the ship center was proposed.(5) The composite and multilayer defensive broadside structure subjected to internal blast loading was numerically simulated by use of the general coupling algorithm by Dytran. The peak pressure values at the corner of the transverse bulkhead with and without pressure release hole were compared. The results prove that the transverse bulkhead with pressure release hole brings remarkable effects to reduce the pressure gathered at the corner. In order to reduce the occurrence of the overall tear failure at the corner of the transverse bulkhead, three types strengthened corner structure were propsed in this paper. The effects of reducing the peak pressure values at the corner and the stress of the structure were compared. The inclined plate was confirmed to be the relatively reasonable strengthened structure of the corner.(6) The response of the defensive liquid cabin at the broadside of the ship subjected to the internal blast loading was simulated by the Autodyn. Some key technologies such as the state equation of the liquid and the boundary condition of the model etc. were solved. The cavity effect of the liquid and the influence of the deformation of the inner panel on the size of the cavity were realized by numerical simulation. The results show that the liquid cabin with full liquid can defense the explosion shock waves effectively. It was found that the strengthened designing form of inner and outer plate corner structure can effectively reduce occurrence of the tear at the corner of liquid cabin. During the simulation of the random fragments formed when the missile warhead exploded by Autodyn, the method to control the quality of the fragments by controlling the size of the mesh and the length of the section was proposed. The viscous effect of the liquid was introduced to the pressure empirical formula, and then on the basis of the resistance formula, the time-distance formula of the fragment movement was derived. By using the deduced formula, the most optimal width of the liquid cabin about absorbing the fragments can be determined on the condition that the mass, the initial velocity of the fragment and the ballistic limit velocity of the inner panel were found, which can guide the design of the defensive liquid cabin.