![]() Here we define the stress relaxation as follows. Schematic drawing of the stress response according to many experimental results is shown in Fig. Then, in the case of viscoelastic materials, the response is expected to have both characteristics of elasticity and viscosity. Materials that exhibit viscoelastic properties can be considered as having both elastic and viscous components. 3(b) and (c), which is also expectable from the Newtonian constitutive equation. If the same strain excitation is applied to an idealistically viscous material, the response will be those as shown in the Fig. ![]() This response can be easily expected from the Hookean constitutive equation. When a step strain excitation is applied to an idealistically elastic material, the response of the material will be that shown in Fig. As measuring method varies depending on the modulus value of the specimens, the various methods used in studying viscoelastic properties of biological materials will be illustrated. In some cases, a mechanistic model for the viscoelasticity will be presented. In this chapter, some examples of viscoelastic nature of biological materials and then their relevance to the structure would be presented. Such a study will contribute to the construction of molecular theory for the viscoelasticity in amorphous materials. Making use of the information, it will be possible to investigate the viscoelastic properties of biological materials on the basis of their structure. Contrary to these materials, for biological tissues, many structural investigations have been made and as a result detailed structural information is available. Constructing mechanistic images, on the other hand, of the viscoelasticity of polymeric materials had been difficult because of dearth of the materials structural information. For long, main sample material for investigating the viscoelasticity has been amorphous polymeric materials, which brought about the remarkable development in phenomenological theories. Even hard tissues have been shown to be viscoelastic. This is because constituents of tissues cells, extracellular matrices, structural proteins, and so on are viscoelastic. Almost all of biological tissues are viscoelastic and their viscoelastic mechanical properties are important in their characteristic functions.
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