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The logarithmic decrement of a decaying free vibration is defined as the natural logarithm of the ratio of

  • A.the applied force to the resulting displacement at resonance
  • B.two successive peak amplitudes
  • C.the natural period to the damped period of the same system
  • D.the stiffness of the system to the mass of the system

Answer: B

Correct answer: two successive peak amplitudes

The logarithmic decrement is

\[ \delta = \ln\frac{u_i}{u_{i+1}} \]

taken from a free vibration record. The ratio is constant from cycle to cycle for viscous damping, which is what makes a single measurement meaningful and lets several cycles be averaged for accuracy.

It is the simplest experimental route to the damping ratio: displace the structure, release it, and measure the decay. No forcing equipment is needed, unlike the half-power bandwidth method.

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