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SECTION 5

DAMPING OVERVIEW

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DAMPING IN DYNAMIC ANALYSIS Damping is present in all oscillatory systems

Damping removes energy from a system

Possible energy dissipation via:

Heat

Sound waves

Fluid motion

Resulting effect:

Free Vibration gives decay in amplitude

Mechanisms include:

Internal Molecular friction - raw material (structural damping)

Sliding Friction - joints, plies

Fluid Resistance - dampers, air or water environment (viscous damping)

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DAMPING IN DYNAMIC ANALYSIS (Cont.) What levels of damping should I use?

There is no one good answer to this question!

For a particular structural configuration and material look at:

Test results

Industry standards

Company experience

Remember in General lowest levels of damping are most conservative:

Use a range of damping values in presenting results

Peak response is difficult to capture

Typical values (NOT TO BE QUOTED!):

NC machined components – 2% to 5% critical

Fabricated metal components – 4% to 10% critical

Composites – 6% to 20% critical

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Damping Model – Discrete Dashpots

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Damping Model – Viscous Material Damping

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Damping Model – Modal Damping

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Damping Model – Modal Damping

Damping in Modal transient Analysis

Direct Damping

• Allows definition of damping as a fraction of critical damping.

• Typical value is between 1% and 10% of the critical damping.

• The same damping values is applied to different modes.

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Damping Model – Rayleigh Damping

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Damping Model – Rayleigh DampingHarmonic Response

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Damping Model – Rayleigh DampingHarmonic Response

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Damping Model – Structural Damping

Part of this energy dissipation is caused by internal friction in the material.

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Damping Model – Structural DampingHarmonic Response

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Damping Model – Structural DampingHarmonic Response

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Damping Model – Structural DampingHarmonic Response

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Damping Model – Numerical DampingTransient Response