Viscoelastic-plastic Materials: Parameter Estimate and Numerical Simulation of Experimental Tests

This paper is focused on a generalized Voigt model applicable to three-dimensional stress states and suitable for the numerical analysis of structural systems made of bituminous mixtures. In this context, it is noted that viscoelastic-plastic models, in principle, can be easily and effectively utilized in the field of pavement analysis and design, since they require limited computational effort. In fact, modern computers and well tested algorithms are definitely adequate for the nonlinear analysis of structural systems, although elastic discrete models are obviously easier to handle, because the response to external actions is obtained by solving a single linear system of equations. Instead, even in the case of quasi-static loading conditions, viscous materials require an incremental analysis, which is carried out by subdividing the load history into a finite number of time-steps. In addition, when plastic deformations are considered, an iterative algorithm is needed to find the non-reversible strains during each step. The main objective of the paper is the estimate of the mechanical properties, since any material model can be used if it is possible to determine its parameters, preferably by means of simple testing procedures. Therefore, a linear elastic analysis (when reasonably applicable) represents the most appealing option, thanks to the additional advantage provided by the easy estimate of elastic properties, while the parameters that govern the response of viscous materials inevitably imply more difficult challenges—and further efforts are required in the presence of plastic strains. However, it is shown that the parameters concerned with viscoelastic-plastic materials can actually be estimated on the basis of traditional, simple compression tests on cylindrical specimens. Optimal values can be found by using classical system identification procedures, but we preferred to give attention to some special features of the material model discussed here and eventually implemented a trial and error algorithm specifically designed for viscoelastic-plastic specimens. Numerical simulations do suggest that the proposed approach is suitable and effective for the estimate of the parameters, which are needed to characterize these materials.