Acoustic measurements for determination of the materials damping using a sound source localisation system

Abstract

Noise measurements are especially a problem when the object under investigation is largely dimensioned and can only be measured from the distance. To obtain information about the location of noise generation, a measurement technique which makes sound visible in order to introduce further specific technical measures is advantageous.

The question is if the time dependence of the signal got from such a non touching and non destructive acoustic method has the potential to calculate the materials damping from. Using a small sound source localization system (Microflown™ probe), specimens of magnesium alloys with different grain sizes obtained by hydrostatic extrusion were investigated. It has been demonstrated that the fine grained microstructure led to a lower damping and its dependence on the strain was reduced.

In this context, acoustic measurements by a sound source localisation system which makes materials characterization possible could be used as a quality tool to detect changes in highly stressed components. This approach allows to promote a selective substitution of conventional materials by more innovative ones. The offshore sector is considered to be a field of application: Wind turbines produce noise emission which stems mainly from turbulences at the tips of the rotor blades and vibrations of components inside the nacelle (e.g. gearbox, generator, …). If the vibration amplitudes are too high, microcracks of components can lead to failure over the long term. The monitoring of microstructural changes using a larger type of a sound source localisation system is considered as an innovative technical approach with respect to maintenance activities.