Is the assumption of linear elasticity within prosthodontics valid for polymers? —An exemplary study of possible problems

Abstract

As shown in previous studies within other scientific fields, the material behavior of polymethyl methacrylate (PMMA) is viscoelasticviscoplastic. However, in dental biomaterial science it is mostly considered as linear elastic or elastic-plastic. The aim of the present study was to evaluate, whether the assumption of elastic or elastic-plastic material behavior for PMMA is a practicable simplification or a potential source of error, especially considering clinical loading conditions. Telio-CAD was tested in three-point bending tests with different test velocities to examine the material behavior at different initial loading rates. Additionally, a dynamic-mechanicalthermal-analysis at different frequencies and temperatures was used. Here, a significant influence of loading rate and temperature as well as stress relaxation and creep were observed. To describe the rate-dependency of the elastic modulus, a new model was created, from which the elastic modulus can be calculated with a given strain rate. This model was validated using linear elastic finite element analysis.

Finite element simulation of fixed dental prostheses made from PMMA —Part II: Material modeling and nonlinear finite element analysis

“…as an adaptation of the relationship proposed by Schmidt et al 9) . Here, Eqs is the elastic modulus from the quasistatic 3PB test, Edyn is the elastic modulus at the maximum loading rate, and αE and βE are the fit parameters.…”

“…In preliminary investigations, the cross-linked PMMA material with percentage weight 99.5%, <1% pigments with no further fillers (Telio CAD, shade A3, Ivoclar Vivadent, Schaan, Lichtenstein) [13][14][15] was tested via three-point bending (3PB) tests and dynamic mechanical technical analysis (DMTA) 9) . A 3PB simulation model was set up to determine the material parameters and validate the material cards resulting from different experiments that were performed in the past 9) . The computational study was conducted via FEA using pre-and postprocessor LS-PrePost and the implicit finite element solver LS-DYNA (LSTC, Livermore, CA, USA).…”

“…The contact between the specimen and fins was modeled using a two-sided automatic surface-to-surface mortar contact. The specimen geometry, the distance of the bearing and the loading velocity of the bending fin were input to the model according to the 3PB test setups performed by Schmidt et al 9) . To simulate the material behavior of Telio CAD, a material card based on the semi-analytical model for polymers (SAMP) was created 16) .…”

“…(Formula 2) 1−c2ɛp with the yield stress σy, and the two hardening parameters c1 and c2. The parameters were optimized to fit the 3PB test results of Schmidt et al 9) . Furthermore, to accompany stress relaxation, the viscoelastic decay coefficient was determined via a relaxation test.…”

“…On the other hand, however, simplifying assumptions can lead to a limited validity for the calculated results. For this reason, the material properties of the material used (Telio CAD) were first examined in a previous work 9) . In this study it was shown that the material behavior of the used PMMA is strongly nonlinear, because the material exhibits stress relaxation, plastic deformation and a rate-dependent elastic modulus.…”