Equations of state of the viscoelasticity of polymethyl methacrylate

The results of studying creep of polymethyl methacrylate (PMMA) in the temperature range from 0 to +30°C at a strain rate from 0.02 to 2% per minute and holding for up to 100 h under stress values within a range of 48 – 72 MPa are presented. The viscoelastic behavior of PMMA is analyzed under normal operating conditions before the onset of the material damage. A unified power dependence of the creep deformation on time was obtained for the entire holding period, without any division into the stages of the unsteady and steady creep. Formulas to be used for approximating the results of isothermal tests of samples at a constant strain rate and holding under a constant load are proposed. The dependences of the approximation parameters on the strain rate, stress level, and temperature of PMMA tests are obtained. A comparison of the creep strain diagrams for the same holding stress after deformation at different rates showed that the diagrams lie on a single curve with a time shift. This indicates the possibility of describing the totality of the experimental data obtained by a single equation of state linking the creep rate, stress and temperature. Differentiation of the approximating formulas made it possible to reveal the regularities of changes in the creep rate during testing and repeated differentiation allowed us to obtain an equation for the creep acceleration upon deformation at a constant rate and to exclude the time variable from it. Similarly, the time variable was also excluded from the creep deceleration equation obtained for holding under constant stress. In this form, these two equations can be considered special cases of the equation of state of a viscoelastic material which behavior is independent on the loading prehistory. Creep under continuous deformation is a superposition of two processes: creep acceleration due to the stress growth and creep deceleration with time. On this basis, a unified equation of state for a viscoelastic material was derived for a process with an arbitrary law of the strain and stress growth. The parameters of this equation are the temperature, creep velocity and acceleration, stress and the rate of stress change. The accumulated creep strain is not a parameter of equation. The applicability of this equation under more complex conditions of a nonmonotonic thermopower loading of materials requires additional experimental justification, as well as identification of the equation parameters.