A Study On The Variations Of Vibration And Acoustic Radiation Characteristics Of A Damaged Stiffened Plate

As ship structure is in multi-excitation condition, researching the characteristics of vibration and acoustics is very important to improve the warship's vitality and combat capacity.Structure vibration and acoustic radiation problems are focused on in engineering always There are resolution method and numerical method for resolving the problems. Former is applicable to the regular elastic structures with simple geometry, simple material characteristic and simple boundary conditions and the latter is applicable to the complicated elastic structures. FEM is the common numerical method for resolving vibration problems with discretional complicated surface in engineering.The damage is inescapable for structures during the process of bearing the weigh of all kinds of loads and damaged structures have different mechanics characteristics compared with health structures. The changes of mechanics characteristics will bring the changes of acoustic radiation characteristics as well. Using dynamical changes, damages of structures can be identified and be evaluated. The changes can also be used to prognosticate damaged structures' life and remained intension and to research variances of acoustic radiations. In this paper, the stiffened plates are choiced as research objects that are common structures in navel and ocean engineering. After studying basic theories of damage mechanics, finite element method and boundary element method, the damage stiffened plates' acoustic radiation characteristics are investigated.The paper is concerned with assessing the influences of the damages present in a stiffened plate on its acoustic radiation properties. Acoustic modes, radiation power and directivity are three indices, which are able to fully describe the inherence acoustic radiation properties of a linear structure. The sound modes, power and directivity pattern of a stiffened plate under different damaged conditions are thus investigated. Using Mindlin theory, we have built the four-node health and damage shell finite elements, and then applied the finite element method to calculating the dynamic response of a structure. Isotropic damage elements are built by global and isotropical weakening of the stiffness. Anisotropic damage elements are built by using reduction coefficients for elasticity damages in x and y directions based on Kachanov's theory. Sound pressure of structure radiating outwards can be calculated through Rayleigh's integral formula, by use of which the radiated power and directivity can then be obtained. Influences of various types of damages on natural frequencies, vibration modes, acoustic radiation power and directivity of sound are studied. Finally, a computational method is given to analyze acoustic behavior of a damaged structure, demonstrated by some typical examples.