Optimization of surface damping treatments for vibration control of marine structures

Abstract

Uncontrolled vibration develops into a serious of problems in machinery and structures and damping is the simplest method of limiting the amplitudes. Active damping involves complicated electronic 'gadgets and is yet to gain popularity. Structural damping is usually very small in metals but can be greatly enhanced by either adding a layer of viscoelastic (VE) material to form what is called as Free Layer Damping (FLD) or sandwiching the viscoelastic layer in between two or more metal layers, an arrangement known as Constrained Layer Damping (CLD). Different beam samples were made comprising of Mild Steel (MS) beams as well as CLD and FLD beams of varying material thickness. Their response to sinusoidal excitation was measured at different frequencies and the results plotted. These samples were also modeled using ANSYS and analyzed as per Ross, Kerwin and Ungar (RKU) method as suggested by Macioce. The loss factor was calculated for CLD as a function of layer thickness by the Oberst and Schommer approach with the help of RKU equations. From these equations optimum loss factor was determined. It was found that the results as projected by the Finite Element Analysis validate the experimental results. A CLD thickness of 1.5mm increases the loss factor from 0.107 for the base MS beam to a value of 0.123, an increment by 16% for a CLD beam. A marine structure comprising two VE layers of 1mm thickness each sandwiched between two MS layers of thickness 1.5mm and a base plate of 12mm was also taken up as a case study and the results are plotted. In conclusion both CLD and FLD improve damping characteristics significantly and between these two, CLD performs better. Hence these techniques open up an economical and simple method of controlling undesirable vibrations. © 2012 Praise Worthy Prize S.r.l.