Vibration Fatigue and Simulation of Damage on Shaker Table Tests: The Influence of Clipping the Random Drive Signal

Suppliers of shaker controllers often use a technique called ‘sigma clipping’ to prevent the amplifier from encountering too high amplitude peaks. Sigma clipping is most commonly used to make sure a shaker system will be sufficiently powerful to reproduce the required random spectrum. The level of clipping is measured via the crest factor, which is expressed as a ratio between the absolute maximum of the signal and its root-mean-square (RMS) value.Such clipped drive signals are therefore no longer Gaussian, since their probability distribution functions (PDF) are truncated at high values. The clipped drive signal is then amplified and filtered by a system made up of the shaker itself followed by the unit under test. Although the local stress and strain responses do not show any obvious clipping, their statistics may be affected. This can feed through to fatigue damage, which is known to be exponentially related to strain amplitudes. A legitimate question is therefore “how does clipping affect damage?”This paper studies and quantifies the influence of clipping experimentally, numerically and theoretically. There are many objectives and underlying questions to consider. For the test technicians, what level of clipping to choose in order to represent the damage due a Gaussian environment? Whereas for the simulation engineers, how to enhance damage estimations by taking into account the level of clipping?