# The Damage Sum--gRMS Dichotomy

#### Steve Smithson

Traditional frequency domain vibration metrics statistically represent Gaussian and stationary random time histories but don’t quantify end-use environments in terms of damage. A fatigue damage spectrum addresses cumulative damage for all excitations and its scalar Damage Sum is analogous to the scalar gRMS characterization of a PSD. The ∑FDS represents a volume integral of cycle-counted damage over a frequency bandwidth and allows interpretation of the relative severity of different excitations and UUT responses. Examples and applications will be offered