Resonance & Structural Analysis Flashcards

What is operational deflection shape (ODS) analysis, and how does it differ from modal analysis?

Answer: ODS shows actual deformation under real operating forces at specific frequencies; modal analysis shows theoretical mode shapes from controlled excitation tests

ODS shows how a structure actually moves under its real operating loads at specific frequencies or time instants, while modal analysis isolates the intrinsic dynamic properties (natural frequencies, mode shapes, damping) from controlled excitation tests.

What is structural resonance a common cause of in rotating machinery?

Answer: Excessive vibration at a running speed that excites a structural natural frequency, resulting in damage or fatigue

When a machine's running speed or a harmonic of it coincides with a structural natural frequency (of the baseplate, piping, or casing), resonance amplifies vibration to levels that can cause fatigue cracking or accelerated bearing wear.

What technique is commonly used to excite structural resonances for impact testing?

Answer: Impact from an instrumented hammer with a force transducer built into the tip

An instrumented impact hammer with a built-in force transducer simultaneously measures the applied force and triggers the response measurement, providing the force spectrum needed to calculate the FRF.

What does the half-power (3 dB) bandwidth method measure?

Answer: The bandwidth around a resonant peak used to estimate the damping ratio of the system

The half-power bandwidth method measures the frequency width at 0.707 (−3 dB) of the peak amplitude in the FRF; the damping ratio is approximately ζ = Δf/(2fn), where Δf is the bandwidth and fn is the natural frequency.

What is transmissibility, and why is it important for vibration isolation design?

Answer: The ratio of output vibration to input vibration across a vibration isolator as a function of frequency

Transmissibility (T = Xout/Xin) describes how much vibration passes through an isolator; it is greater than 1 near isolator resonance and less than 1 above √2 × the isolator's natural frequency, defining the effective isolation region.

What is the primary purpose of adding mass or stiffness to a structure to address a resonance problem?

Answer: To shift the natural frequency away from the excitation frequency to detune the resonance

Adding stiffness increases the natural frequency (fn ∝ √k) while adding mass decreases it (fn ∝ 1/√m); either change shifts the natural frequency away from the troublesome excitation frequency.