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ESDU 09005

2009 Edition, May 1, 2009

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Introduction to Damping



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Product Details:

  • Revision: 2009 Edition, May 1, 2009
  • Published Date: May 1, 2009
  • Status: Active, Most Current
  • Document Language: English
  • Published By: Engineering Sciences Data Unit (ESDU)
  • Page Count: 48
  • ANSI Approved: No
  • DoD Adopted: No

Description / Abstract:

INTRODUCTION

This Item is intended to provide introductory and background information to a series of Items that consider some aspects of damping in more detail. For example, ESDU 07011 (Reference 43) is concerned with radiation damping and ESDU 09006 (Reference 44) presents a description of material damping. Future Items will deal with the damping of composites and the provision of a compendium of damping for built-up structures. In this Item concepts related to different measures applied to damping in materials, as well as to damping in mechanical systems, are introduced. Every structure in vibration can be approximated by a dynamically equivalent mechanical system with a finite number of degrees of freedom. The degrees of freedom refer to the number of independent coordinates necessary to describe the motion of the system. In particular situations a single degree of freedom system can be an adequate model for the real structure. Although the majority of structures cannot be treated as SDOF systems, the measures specifying damping can be defined using a SDOF model without compromising the generality of the definitions.

Damping in structures may be categorised as either intrinsic, i.e., within the material of the structure, or as the damping which depends upon other dissipative factors, such as friction in rivets or bolts, or from lubrication (Reference 22). Damping also occurs in a structure which is radiating sound, as vibrational energy is lost in the surrounding medium rather than by dissipation in the structure. Material and radiation damping can be estimated theoretically more easily than other types of damping; in the case of material damping prediction is based on the material properties and for radiation damping from the geometrical and material properties of the structure. When the damping of a structure is measured in most practical tests, it is the damping of all kinds which is determined, including the material damping and, provided there is a surrounding fluid medium, radiation damping.

The measures of damping which are most often used in dynamic analysis are: the viscous damping ratio (ζ), the specific damping capacity (ψ), the loss factor (η), the inverse quality factor (Q–1), the logarithmic decrement (δ) and, in the case of material damping, the tangent of the phase lag (ϕ). and the ratio of the loss modulus to the Young's modulus. Definitions of the measures are introduced in the Item using examples of linear mechanical models, such as Kelvin-Voigt (with viscous damper) and hysteretic damping model (with hysteretic damper), especially applicable to material damping. In ESDU Items information on damping is mostly provided in terms of the viscous damping ratio. For example, in Data Item No. 72005, ‘The estimation of r.m.s. stress in stiffened skin panels subjected to random acoustic loading' (Reference 20), nomographs are used which are based on a viscous damping ratio of 0.017, a value regarded as typical for aerospace structures, and, if a different value is used, a correction factor is applied. The relationships between the different measures of damping are summarised in the Appendix A.