3.2 Damped free vibrations

Damped free vibrations are a crucial aspect of mechanical systems, affecting everything from car suspensions to building structures. This topic explores how energy dissipation through damping impacts the motion of objects, introducing key concepts like damping ratio and natural frequency.

Understanding damped vibrations is essential for engineers designing stable and efficient systems. We'll examine different damping types, their effects on system behavior, and how to analyze and predict the response of damped systems using mathematical models and real-world applications.

Equation of Motion for Damped Vibrations

Derivation and Components

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• Equation of motion for damped free vibration system stems from Newton's Second Law of Motion
• Incorporates mass, damping, and stiffness components
• General form $mẍ + cẋ + kx = 0$
  • m represents mass
  • c signifies damping coefficient
  • k denotes spring stiffness
  • x indicates displacement
• Damping force modeled as proportional to velocity (viscous damping)
  • Represented by term $cẋ$ in the equation
• Free body diagram includes:
  • Inertial force ($mẍ$)
  • Damping force ($cẋ$)
  • Spring force ($kx$)

Normalization and Interpretation

• Normalize equation by dividing all terms by mass
• Results in $ẍ + 2ζωnẋ + ωn²x = 0$
  • ζ represents damping ratio
  • ωn denotes natural frequency
• Physical meaning of each term crucial for:
  • Interpreting system behavior
  • Designing vibration control strategies
• Equation applies to various mechanical systems (automotive suspensions, building structures)

Damping Types and Effects

Classification of Damping

• Three main types of damping in mechanical systems:
  • Underdamped (0 < ζ < 1)
  • Critically damped (ζ = 1)
  • Overdamped (ζ > 1)
• Damping ratio ζ determines damping type
  • Defined as ratio of actual damping to critical damping
• Additional damping forms in real systems:
  • Coulomb damping (dry friction)
  • Hysteretic damping (internal material damping)
• Logarithmic decrement method used to experimentally determine damping ratio
  • Measures decay of free vibrations in underdamped systems

System Response Characteristics

• Underdamped systems:
  • Exhibit oscillatory behavior
  • Amplitude decreases over time
  • Overshoot equilibrium position
  • Examples: lightly damped pendulums, guitar strings
• Critically damped systems:
  • Return to equilibrium in shortest time without oscillation
  • Ideal for many engineering applications (door closers, some vehicle suspensions)
• Overdamped systems:
  • Approach equilibrium without oscillation
  • Slower than critically damped systems
  • Examples: heavily damped shock absorbers, some electrical circuits

Damped Natural Frequency and Decay Rate

Solution to Damped Free Vibration Equation

• Find roots of characteristic equation $s² + 2ζωns + ωn² = 0$
• For underdamped systems:
  • Solution form $x(t) = Ae^(-ζωnt)cos(ωdt + φ)$
  • ωd represents damped natural frequency
• Damped natural frequency related to undamped natural frequency:
  • $ωd = ωn√(1 - ζ²)$
• Decay rate given by real part of complex roots:
  • $σ = ζωn$
  • Determines vibration diminishment rate
• Critically damped and overdamped systems:
  • Solution involves exponential functions without oscillatory terms

Frequency and Decay Relationships

• General solution combined with initial conditions determines specific system motion
• Crucial relationships for predicting system behavior:
  • Damping ratio
  • Natural frequency
  • Damped natural frequency
• Examples of applications:
  • Tuning musical instruments (adjusting decay rate)
  • Designing earthquake-resistant structures (optimizing damping)

Response Analysis of Damped Systems

Initial Conditions and Solution Parameters

• Typical initial conditions:
  • Initial displacement x(0)
  • Initial velocity ẋ(0)
• Amplitude A and phase angle φ in solution determined by initial conditions
• Decay envelope described by exponential term $e^(-ζωnt)$
• Time constant τ = 1/(ζωn):
  • Measures response decay rate
  • Amplitude reduces to ~37% of initial value after one time constant

Response Characteristics and Measurements

• Calculate cycles required for amplitude decrease using logarithmic decrement
• Underdamped systems:
  • Period of oscillation increased by damping
  • $Td = 2π/ωd$ longer than undamped period $T = 2π/ωn$
• Settling time:
  • Time for oscillations to reduce to 2% of initial amplitude
  • Important parameter in system design
  • Related to damping ratio
• Examples of response analysis applications:
  • Optimizing suspension systems in vehicles
  • Designing vibration isolation for sensitive equipment

Critical Damping and Vibration Control

Critical Damping Concept

• Occurs when damping ratio ζ = 1
• Represents boundary between oscillatory and non-oscillatory behavior
• Critical damping coefficient $cc = 2mωn = 2√(km)$
  • m represents mass
  • k denotes stiffness
• Critically damped systems:
  • Return to equilibrium in shortest time without oscillation
  • Ideal for quick stabilization applications

Applications and Design Considerations

• Many systems designed to be slightly underdamped (ζ ≈ 0.7):
  • Balances quick response with minimal overshoot
• Critical damping crucial in design of:
  • Shock absorbers
  • Door closers
  • Other vibration control devices
• Response of critically damped system under initial displacement:
  • No oscillation
  • Approaches zero asymptotically
• Tuning system parameters achieves desired transient response:
  • Elevator braking systems
  • Robotic arm positioning
  • High-precision measuring instruments