Experimental Study On Hull Damping Under Underwater Explosion Load

At present, the mechanism of damping is not completely clear. The ship trial which is a complicated system engineering is one of the most effective methods to study the hull vibration damping. A reasonable test plan is the guarantee to complete the test target, and the basic to accuratel measure structural response. Another key problem of experimental studying on damping is how to identify the structural damping ratio from the measured structural responses in time domain signal. There are many kinds of time domain methods to identify damping, however, the general weakness are how to eliminate false mode. Choosing an appropriate damping ratio is the foundation to improve calculation accuracy of hull structure response. The existing test data is quite limited,which is difficult to meet the needs of the project. Some damping models such as Rayleigh damping model are often used to decouple the dynamic equations, and the real physical meaning of damping is neglected, and the applicability of these models have not been analyzed further. The existing formulas of the total vibration damping ratio of the ship hull are based on the test data obtained from the ship navigation condition, yet some factors are neglected, such as amplitude, structural form,boundary conditions and so on. Whether these empirical formulas are suitable for underwater explosion and other strong impact load is needed to be verified. This paper mainly focus on that how to properly test the vibration response of different ship hull structures, how to accurately identify the structural damping ratio, the change of different hull structural damping ratio with the modal frequencies is got, the influence of different factors on the damping ratio is analyzed,a hull damping ratio empirical formula is presented suiting for ship hull under the action of the underwater explosion loads. The specific contents of this paper are as following:Firstly, Least Squares Complex Exponent method (LSCE) is optimized based on Empirical Mode Decomposition(EMD) de-noising and stability diagram. For rejecting false modes and solving order problem which commonly found in the time domain identification methods, the noise mode in the signal is removed based on EMD de-noising method,including EMD’s scale de-noising method and EMD’s threshold de-noising method. The de-noised signal is used to identify modal parameter using LSCE,then modal parameters are obtained including a small amount of noise modes and a lot of computing false modes due to the estimated system order bigger than actual value. Stability diagram is used to exclude false modal then obtain system physical modal parameters. Finally, the optimized modal parameter identification process is given and its accuracy is verifiedSecondly, the test schemes for vibration damping are designed, including test scheme of local hull plate frame damping and the whipping motion damping test scheme. Model design,point layout and numbering system, testing instruments and testing main process which are important parts of the test scheme are described.Thirdly, the hull typical location vibration damping ratio is analyzed. Modal damping ratio trends with the modal frequencies of free boundary condition hull typical board planes and different ship hull plate frame are obtained using optimized LSCE method. Rayleigh damping model is referenced, which is frequently used in engineering, and Rayleigh damping coefficients of different structure are obtained. The influence of different factors on the damping ratio are discussed in terms of different pallet forms, different impact strength,different boundary conditions and different ship hull structures, etc.Finally, low order total hull vibration damping is analyzed. Effect of the impact strength on the ship damping ratio is discussed based on comparing the work from damping force and energy from structural damping. Low order vibration damping of the underwater cylindrical beam and two different ship are calculated. Damping empirical estimation formula is given which is applied to ship low order total vibration underwater explosion depending on the existing damping experience estimation formula.