A dynamic indentation method for characterizing soft incompressible viscoelastic materials

This paper presents a new method for characterizing soft incompressible viscoelastic materials. A small electromagnetic shaker together with an indenter fixed at its output end forms an electromechanical actuator which takes in ac electrical power and exerts dynamic indentation to the material specimen under testing. Voltage and current are variables at the electrical input port while force and velocity are variables at the mechanical output port of the actuator. A 2×2 transduction matrix relating these input and output variables is first calibrated over a range of frequency through a series of experimental tests in which various known mechanical loads are imposed at the output port. Since this transduction matrix neatly relates the mechanical impedance of the material with the electrical impedance measured at the input port of the actuator, the electromechanical transducer senses impedance simultaneously when actuating. From the detected mechanical impedance, the complex modulus of testing specimen is then deduced using a mathematical model by Oestreicher. Experimental results from the proposed method on rubber materials show good agreement in a specific frequency range with those obtained by dynamic mechanical analyzer if the material is soft. Because no direct measurement of motion and force are required, this method possesses advantages in testing miniature samples in situ and development into a low cost handy tool for characterization of soft tissues, such as human skin is in progress.