LIGO Document P0900261-v1

Observations of a new dissipation regime in metals

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We have observed a low frequency transition, in metal flexures, from viscous-like dissipation, dominated by individual dislocations at high frequency, to a less predictable dissipation mode at low frequency, dominated by collective dislocation effects, governed by Self Organized Criticality statistics. The threshold frequency is determined by the material characteristics, but likely also by the dimension of the flexures. In the newly observed mode sudden instabilities are observed, including spontaneous change of the filter equilibrium point, collapse in mechanical oscillators, hysteresis and anomalous 1/f transfer functions. The evidence is that a non-negligible fraction of a metal’s Young’s modulus is contributed by entangled dislocations. Dislocation disentanglement leads to avalanches, during which the material Young’s modulus is reduced and the viscous-like dissipation is enhanced. The observed results will influence the design of the seismic isolation chains of third generation Gravitational Wave Detectors, and likely affect the operation of the second generation ones, presently under construction.
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