Introduction and General Requirements

Experimental procedures for viscoelastic materials, as in other experiments in mechanics, make use of a method for applying force or torque, a method for measuring the same, and a method for determining displacement, strain, or angular displacement of a portion of the specimen. If the experimenter intends to explore a wide range of time, and/or frequencies, the requirements for performance of the instrumentation can be severe. Elementary creep procedures are discussed first, because they are the simplest. Methods for measurement and for load application are discussed separately, since many investigators choose to assemble their own equipment from components. Many procedures in viscoelastic characterization of materials have aspects in common with other mechanical testing. Therefore, study of known standard methods [1, 2] for mechanical characterization of materials is useful. As in other forms of mechanical characterization, it is important that the stress distribution in the specimen be well-defined. End conditions in the gripping of specimens are usually not well known. The experimenter often uses elongated specimens for tension, torsion, or bending, to appeal to Saint Venant's principle in using idealized stress distributions for the purpose of analysis. The determination of viscoelastic properties as a function of frequency is at times referred to as mechanical spectroscopy.

Frequency response is an important consideration for instruments used in viscoelasticity; the transducers [3] for force generation and measurement and for measurement of deformation must respond adequately at the frequencies of interest.