Estimation of complex Youngʹs modulus of non-stiff materials using a modified Oberst beam technique

The paper presents a modified Oberst beam technique to evaluate the complex Youngʹs modulus of non-stiff materials. Unconstrained layer theories and non-parametric complex modulus identification methods used for stiff materials form the basis for the method. The proposed approach has several advantages over the standard Oberst beam technique. In particular, the layer properties can be evaluated at any frequency, and the base beam need not be completely covered with the layer material. In addition, the proposed method does not require that the complex modulus vs. frequency curve for the base beam should have a flat area near analyzed resonance frequencies in order to yield accurate results. The experiments conducted on a styrene–butadiene rubber (SBR) sample using a polymethyl methacrylate (PMMA) base beam produced good results. Uncertainty analysis shows that the measurement accuracy can be improved by increasing the modulus magnitude ratio between the layer material and the base beam material, or the thickness ratio between the layer and base beam.