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Erschienen in:

2014 | OriginalPaper | Buchkapitel

1. Fundamental Aspects of Viscoelastic Response

verfasst von : Danton Gutierrez-Lemini

Erschienen in: Engineering Viscoelasticity

Verlag: Springer US

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Abstract

This chapter describes the molecular structure of amorphous polymers, whose mechanical response to loads combines the features of elastic solids and viscous fluids. Materials that respond in such manner are called viscoelastic, and their mechanical properties have an intrinsic dependence on the time and temperature at which the response is measured. To put this into context, the chapter compares the nature of the response of elastic, viscous, and viscoelastic materials to several types of loading programs, examining the physical nature of their mechanical properties, their behavior regarding energy conservation, and the phenomenon of aging. The topics treated in this chapter provide the neophyte and casual reader with a good understanding of what viscoelastic materials are all about.

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Nächstes Kapitel Constitutive Equations in Hereditary Integral Form

Although polymers may be classified as crystalline and amorphous, the mechanical response of crystalline polymers cannot be described by the theory in this book; thus, only amorphous polymers are treated here.

A dashpot is a mechanical device which resists motion by viscous friction. The resisting force is directly proportional to the velocity; and, acting in the opposite direction, slows the motion and absorbs energy.

The values of E and η of real materials typically depend on temperature, among other things.

By definition, the relaxation modulus of linear viscoelastic substances is assumed independent of strain level. Thus, the magnitude of the prescribed displacement to use in a relaxation modulus test should not be important. However, the relaxation modulus of materials capable of large strains typically depends on strain level, and the selection of the test displacement should be based on the end use of the material.

At this point we avoid the temptation to describe viscoelastic response to sustained stress using the reciprocal of the relaxation modulus, as, say: ε(t) = [1/M(t)]σ _o. Suffice it to say that doing so would simply define viscoelastic behavior as little more than elastic.

By definition, the creep compliance of linear viscoelastic materials is assumed independent of stress level. Thus, the magnitude of the stress to use in a creep test should not be important. However, the compliance of nonlinear materials depends on stress level, so that the selection of the test stress level should be made carefully.

This means that if the external agents that put work into an elastic solid were completely removed, the internal energy stored in the solid could be used to perform an amount of work—on another system, say—equal to the work that was put into the solid in the first place.

In general, the loading and unloading paths do not have to be the same, as long as the test piece is returned to its original position.

Assuming it were possible to produce exactly the same non-ageing material every time—without lot-to-lot variability—this result would hold true even if the specimens came from material lots of different age.

A.N. Gent (ed.), Engineering with Rubber, How to Design Rubber Components (Jos. C. Huber KG, Diessen/Ammersee, 2002), pp. 38–40

L.E. Malvern, Introduction to the Mechanics of a Continuous Medium (Prentice-Hall Inc., 1969), pp. 306–327

J.J. Aklonis, Introduction to Polymer Viscoelasticity (Wiley, 1983), pp. 44–52

A.C. Pipkin, Lectures on Viscoelasticity Theory (Springer, 1986), pp. 14–15

H. Morawetz, Polymers: The Origins and Growth of a Science (Dover, 1985), pp. 35–36

A.D. Drosdov, Finite elasticity and Viscoelasticity, A course in the Nonlinear Mechanics of Solids (World Scientific, Singapore, 1996), pp. 229–234

Titel: Fundamental Aspects of Viscoelastic Response
verfasst von: Danton Gutierrez-Lemini
Verlag: Springer US
Buch: Engineering Viscoelasticity
Print ISBN: 978-1-4614-8138-6

Electronic ISBN: 978-1-4614-8139-3

Copyright-Jahr: 2014
DOI: https://doi.org/10.1007/978-1-4614-8139-3_1

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