Dynamic amplification factor
D(r) = 1/√((1−r²)² + (2ζr)²)
Short definition
How many times the dynamic response exceeds the static one at frequency ratio r = ω/ωₙ.
Full definition
The factor D(r) depends only on the frequency ratio r = ω/ωₙ and damping ζ: D → 1 quasi-statically as r → 0 and peaks at 1/(2ζ) at resonance. VibroTheory uses it (config.dyn_factor) for quick working-frequency detuning checks.
Examples
r = 25/45.5 ≈ 0.55, ζ = 0.25 → D ≈ 1.27.