An experimental dynamic material property identification technique is presented that is based on the theoretical formulations of a vibrating pre-stressed beam. The technique determines the viscoelastic material properties of tissue mimicking materials that govern their dynamic behavior. Results are presented for a silicone based material (CF11-2188 Nusil Silicone Technology, Carpinteria, CA) that is in the form of a thick string held between fixed supports under tensile pre-stress.