Constitutive Discription On Time-Dependent Ratcheting Of Polymer Matirials

Time-dependent ratcheting of polymer materials is of great importance in the safety design and life assessment of structures. In recent years, the cyclic deformation of polymer materials including ratcheting behavior presented during the asymmetrical stress-controlled cyclic loading was extensively studied by many researchers experimentally, and the theoretical models were constructed. However, as for the obtained experimental and theoretical results, there are still two shortcomings. First, there are few researches on the ratcheting behavior of polymer materials, especially for the time-dependent ratcheting. Second, the new models for the time-dependent ratcheting of polymer materials need to be further constructed and the existed models also should be extended.Therefore, there are three aspects of contributions in this work as follows:1) The time-dependent ratcheting of a Polyetherimide (PEI) resin is experimentally investigated under the uniaxial stress-controlled cyclic loading with non-zero mean stress at room temperature. The main attention is paid on the effects of different mean stresses, stress amplititudes, stress rates and holding-times at peak stress on the ratcheting of the PEI polymer. Some significant conclusions useful to construct a cyclic constitutive model of ratcheting are obtained.2) Based on the Schapery model, the uniaxial time-dependent ratcheting of the Polyetherimide (PEI) resin was simulated with different mean stresses, stress amplititudes, stress rates and peak-holdings. It is shown that the Schapery model can describe the stress-strain curve and predict the evolution trend of the uniaxial time-dependent ratcheting of the PEI polymer. Meanwhile, the applications of the Schapery model are extended to the prediction of ratcheting.3) In order to assess the capability of the Schapery model to predict the time-dependent ratcheting of the Polyetherimide (PEI) resin, the comparison between the simulated results obtained under the tension-tension and tension-compression cyclic loading was provided. It is shown that the Schapery model can't simulate the ratcheting well under the tension-tension cyclic loading. After changing the parameter function of the original Schapery model, the modified Schapery model can simulate the ratcheting under the tension-tension and tension-compression cyclic loading simultaneously and the results are in good agreement with the experiments.