Comparison of frequency and strain-rate domain mechanical characterization

Indentation is becoming increasingly popular to test soft tissues and (bio) materials. Each material exhibits an unknown intrinsic "mechanical behaviour". However, limited consensus on its "mechanical properties" (i.e. quantitative descriptors of mechanical behaviour) is generally present in the literature due to a number of factors, which include sample preparation, testing method and analysis model chosen. Viscoelastic characterisation - critical in applications subjected to dynamic loading conditions can be performed in either the time-or frequency-domain. It is thus important to selectively investigate whether the testing domain affects the mechanical results or not. We recently presented an optomechanical indentation tool which enables both strain-rate (nano-epsilon M) and frequency domain (DMA) measurements while keeping the sample under the same physical conditions and eliminating any other variability factor. In this study, a poly(dimethylsiloxane) sample was characterised with our system. The DMA data were inverted to the time-domain through integral transformations and then directly related to nano-epsilon M strain-rate dependent results, showing that, even though the data do not perfectly overlap, there is an excellent correlation between them. This approach indicates that one can convert an oscillatory measurement into a strain-rate one and still capture the trend of the "mechanical behaviour" of the sample investigated.