A new method for identifying mechanical impedence of soft viscoelastic materials

Abstract

Following the wider use of viscoelastic materials in industries over the past few decades, being able to characterize mechanical properties of such materials experimentally has become increasing important. However, existing dynamic testing methods such as free decay method, forced vibration method, resonant method and wave propagation method share a common drawback. Each of these methods requires the use of sensors and actuators in exciting and measuring the load and deformation which renders the loading effect of sensors for small samples and the mass effect of sensors at high frequency testing inevitable. Furthermore, installation of sensors and actuators is difficult when the space is small. Samples preparation is also a major problem to overcome in applying the existing methods. Moreover, they are not suitable for in situ testing. Is this study, a new method, transduction matrix method, for testing complex modulus and loss factor of soft viscoelastic materials is designed by using Simultaneous Sensing and Actuating (SSA) technique. In transduction matrix method the actuator is described by a 2x2 transduction matrix which allows the mechanical impedance of the output port of the actuator to be detected from the electrical impedance of the input port while the actuator works as a sensor simultaneously. The detected mechanical impedance can then be analyzed to provide mechanical properties of viscoelastic materials. Transduction matrix method was first applied on electrodynamic actuators. The theoretical model of the shaker was then derived and proved to be the closed form of the four transduction functions based on some simplification on the shaker. Transduction matrix ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library 2.2.4 Wave propagation method 19 2.2.5 Comparison of the methods 22 2.3 Mechanical properties of human tissues 24 2.3.1 In vitro mechanical properties of human tissues 24 2.3.2 In vivo mechanical properties of human skin 25 2.3.3 Equipment for testing mechanical properties of human skin 28 2.4 Summary 31 3. Transduction matrix method and its application on shaker 3.1 3.4.4 Adding mass method 54 3.5 Actuating and sensing capability of shaker 64 3.5.1 Actuating capability of shaker 64 3.5.2 Sensing capability of shaker 65 3.6 Decoupling effect of the transduction matrix method 73 v