Cyclic viscoelasticity of polymethyl methacrylate

The results of a study of the creep of polymethyl methacrylate (PMMA) under cyclic loading are presented. The viscoelastic behavior of PMMA was analyzed under normal operating conditions, before the onset of material damage processes. Creep during continuous deformation is a superposition of two processes: the creep acceleration due to a stress increase and its deceleration over time. During an instantaneous increase of load, only the first process takes place, and during an exposure under constant load, only the second. For each of them, equations of state for viscoelasticity are obtained that relate the acceleration of viscous deformation to the rates of elastic and viscous deformation and to the current level of elastic deformation. These equations are applicable in the stress range from the creep limit to the forced elastic limit, as well as during recovery after complete unloading. Since the equations do not explicitly include time and accumulated viscous strain, they can be used for a process with an arbitrary law of growth of strain or stress. Based on the results of cyclic tests with different rates of deformation at the stages of loading and unloading, equations of state were obtained for various combinations of values and directions of rates of elastic and viscous deformation. These equations were used to model the viscous deformation of PMMA according to a given law of the elastic deformation change. Comparison with experiment confirmed the high accuracy of the modelling. A number of successive stages of viscoelastic deformation have been identified depending on the load level: elastic at stress below the creep limit, instantaneously viscous, viscous and viscoplastic. At the first stage, viscous deformation does not increase, at the second stage it increases only with increasing load, at the third stage it also increases during the holding. When reaching the viscoplastic stage, irreversible deformation accumulates, which persists after the return is completed.