Helium as good as vacuum: the poor manʹs approach for accurate material damping measurements

In this paper a relatively simple method is presented which allows the performance of accurate material damping experiments in a frequency range where usually the internal losses are masked by sound radiation. Damping measurements performed on beam specimen vibrating in bending are renown to suffer from such problems, in particular when they regard lightweight materials with small internal losses such as for instance aluminium alloys. Instead of turning to complex measurements in vacuo, it is shown that a plastic bag filled with helium in most cases provides a satisfactory reduction of the sound emissions. Relatively simple expressions for the radiation losses of cylindrical beams are included and alternative closed form expressions for the radiation damping well below coincidence are derived. These expressions allow the design of the beam specimen in such a way as to limit the radiation damping below a desired level in the frequency range of interest for the experiments. The helium method furthermore allows the performance of damping experiments on the (eventually large) actual objects, allowing to study their internal losses for particular vibration modes. The results of material damping measurements performed on a beam specimen and on a combustion engine oil sump are presented.