Materials are generally considered to be viscoelastic if they have an elastic and viscous behavior. The elastic behavior is typically rate-independent and represents the recoverable deformation due to loading, while the viscous behavior is typically rate-dependent and represents dissipative mechanisms within the material.

A wide range of materials (such as polymers, glassy materials, soils, biological tissue, and textiles) exhibit viscoelastic behavior. Viscoelastic materials exhibit viscous fluid behavior at high temperatures and solid behavior at low temperatures.

For most viscoelastic materials, the effect on the material properties caused by changes in temperature is similar to that of the effect caused by changes in the time scale. Such materials are considered to be thermorheologically simple. A general material property called the shift function can reduce the constitutive relation at a reference temperature and shifted time. The shift function can lessen the amount of experimentation needed to determine the material parameters.

The following shift functions are available for representing thermorheologically simple materials:

The shift functions reproduce the behavior of a wide range of viscoelastic materials. For special requirements, user-defined shift functions can also be defined.

The fictive temperature is the temperature at which the current microstructure of glass is in an equilibrium state. For the TN shift function with fictive temperature model, the fictive temperature is used to model materials containing an intrinsic equilibrium temperature that typically differs from the ambient temperature of the material. The fictive temperature relaxes toward the ambient temperature similar to the way that deviatoric and volumetric stiffness constants of the viscoelastic material relax toward the long time-elastic constants.

With the shift function, the evolution of the fictive temperature for any thermal history can be calculated. As the fictive temperature approaches the actual temperature, the viscoelastic material becomes more relaxed. The fictive temperature model is often used to model the melting and solidification process of viscoelastic materials such as glass and stiff polymers. This problem uses a fixed partial denture (FPD) model to determine the residual stresses due to the solidification of a glass veneer on a ceramic core material.

Metal-free ceramic materials are biocompatible, chemically durable, and aesthetically desirable. Such materials are therefore ideal for FPDs. Thermal loading during the glass layer manufacturing process causes residual stresses in FPDs. Higher residual stresses caused by thermal contraction incompatibility between the veneer and core materials can lead to failure under occlusal loading in the oral cavity.[1] The ability to determine residual stresses in an FPD subjected to thermal loading is useful for predicting the life of the FPD.