An instrument to measure mechanical up-conversion phenomena in metals in the elastic regime
Crystalline materials, such as metals, are known to exhibit deviation from a simple linear relation between strain and stress when the latter exceeds the yield stress. In addition, it has been shown that metals respond to varying external stress in a discontinuous way in this regime, exhibiting discrete releases of energy. This crackling noise has been extensively studied both experimentally and theoretically when the metals are operating in the plastic regime. In our study, we focus on the behavior of metals in the elastic regime, where the stresses are well below the yield stress. We describe an instrument that aims to characterize non-linear mechanical noise in metals when stressed in the elastic regime. In macroscopic systems, this phenomenon is expected to manifest as a non-stationary noise modulated by external disturbances applied to the material, a form of mechanical up-conversion of noise. The main motivation for this work is for the case of maraging steel components (cantilevers and wires) in the suspension systems of terrestrial gravitational wave detectors. Such instruments are planned to reach very ambitious displacement sensitivities, and therefore mechanical noise in the cantilevers could prove to be a limiting factor for the detectors' final sensitivities, mainly due to non-linear up-conversion of low frequency residual seismic motion to the frequencies of interest for the gravitational wave observations. We describe here the experimental setup, with a target sensitivity of 10^(−15) m/√Hz in the frequency range of 10–1000 Hz, a simple phenomenological model of the non-linear mechanical noise, and the analysis method that is inspired by this model.
© 2016 AIP Publishing. Received 24 March 2016; accepted 19 May 2016; published online 10 June 2016. We would like to thank the members from the Seismic Isolation and Suspension Working Group in the LIGO Scientific Collaboration for many enriching discussions. Also many thanks to Seiji Kawamura, Yuri Levin, and Rai Weiss for valuable contributions to the initial experiment, and to Federico Paoletti for help in the design of the electronics. LIGO was constructed by the California Institute of Technology and Massachusetts Institute of Technology with funding from the National Science Foundation, and operates under cooperative agreement PHY-0757058. Advanced LIGO was built under Award No. PHY-0823459. This paper carries LIGO Document No. LIGO-P1500272. This manuscript has an internal LIGO project designation of P1500272. J.R.G. gratefully acknowledges the financial support of NSF No. DMR-1204864.
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