# LIGO Document G1000660-v1

- It was long understood that the presence of dislocations reduces the material Young’s modulus by several percent. It was also well understood that for a given dislocation density in the material the “modulus defect” depends strongly from the average free length of each dislocation between pinning points.

It was well known that dislocation can entangle and effectively pin each other (work hardening is an example).

Nobody realized that during slow transients dislocation dis-entanglement can occur.

Disentanglement of dislocations leads to exotic behaviors like temporary change of resonant frequency, random walk of equilibrium position, in extreme cases spontaneous collapse, very large dissipation while dislocations re-arrange.

We observed a phase transition at low frequency with dislocations acting collectively at low frequency (avalanching) and independently at high frequency.

Several measurements illustrating the above scenario will be shown.

The exotic effect observed have profound implications in the design of seismic attenuation for low frequency GW observatories and for all sort of inertial sensors.

These effects may also affect the present GW detectors. Ideas on how to mitigate these effects are discussed at the end.

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