Background: Investigation of the time-dependent behavior of dental composite materials is of importance because cumulative effects of unrecovered viscoelastic strains during cyclic loading may contribute to premature catastrophic failure.  Objective: The present study aims to develop a rheological model representing the essential features of time dependent behavior of a dental composite. Method: Data from creep and stress relaxation experiments were used in accordance with Boltzmann's Superposition Principle to modify a viscoelastic Zener model. Varying levels of stress and strain were imposed to study both viscoelastic and viscoplastic characteristics of the composite.  Sample specimens were subjected to 6 loading-unloading cycles to experimentally observe viscoelastic-plastic behavior.  Results:  Elastic behavior is evident below axial loading of 135 kg; viscoelastic behavior is observed below 275 kg; and the composite materials exhibits viscoelastic-plasticity at loads greater than 365 kg. The resulting Elastic Rheological Constitutive Model (equation 1) and Plastic Rheological Constitutive Model (equation 2) for this particle-reinforced dental composite material are given as follows:

(1)

(2)

The models were verified using data from a theoretical loading-unloading test created from these equations and compared to actual experimental data.  Results showed close agreement of the model predictions with experimental data despite the model's simplicity. Conclusions:  The model's equations to quantitatively predict viscoelastic-plastic behavior of dental composites can be used to more clearly characterize the material properties.  Modifying these equations to reduce residual strains during cyclic loading may be useful indicators for the improvement of dental composites. Supported by NIDCR grant DE07979.

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