Divergence of the third harmonic stress response to oscillatory strain approaching the glass transition

The leading nonlinear stress response in a periodically strained concentrated colloidal dispersion is studied experimentally and by theory. A thermosensitive microgel dispersion serves as well-characterized glass-forming model, where the stress response at the first higher harmonic frequency (3 omega for strain at frequency omega) is investigated in the limit of small amplitude. The intrinsic nonlinearity at the third harmonic exhibits a scaling behavior which has a maximum in an intermediate frequency window and diverges when approaching the glass transition. It captures the (in-) stability of the transient elastic structure. Elastic stresses in-phase with the third power of the strain dominate the scaling. Our results qualitatively differ from previously derived scaling behavior in dielectric spectroscopy of supercooled molecular liquids. This might indicate a dependence of the nonlinear response on the symmetry of the external driving under time reversal

ISO.Wind - A monitoring system for wind parks using passive radar

Intelligent wind park monitoring systems may allow cutting the levelized cost of wind-generated electricity by deploying maintenance personnel more efficiently. The non-contacting passive radar technology and advanced sensing technologies on the plant side offer significant potential for such monitoring systems. The goal of the 3-year-long project ISO.Wind is to identify the most cost-efficient sensing technologies to detect maintenance-relevant damages and to use them for a wind park monitoring system. For this purpose a commercial 3MW wind turbine is instrumented with strain gauges following IEC standard 61400-13 and a network of accelerometers. It is also monitored by passive radar technology. A learning algorithm is developed and fed with available data from the sensor systems and operational data from the instrumented wind turbine. The algorithm is capable of detecting operational patterns and damage cases of the wind turbine. A graphic user interface illustrates these conditions in a comprehensible way. First field measurements show the suitability of the passive radar technology to detect the damage-relevant dynamics of the instrumented wind turbine. Validated simulations of typical damage cases prove that both instrumentation on the plant side and the passive radar sensing technology allow reliable damage detection for the examined wind turbine