One-dimensional Mechanical Behavior Research And Molecular Dynamics Simulation Of Polymethyl Methacrylate

Given its good diaphaneity, light weight, high capability to break, ageing resistance, and good mechanical properties, PMMA was widely used in the areas of aviation, architecture, biomedical field and so on. Especially in the field of aviation, the canopy and the wind shield of aircraft made by PMMA were threatened by bird strike in a long term, and such accidents had still happened sometimes in recent years. What’s more, the wind shield of aircraft might need to face the test from the penetration of bullet. Pilots had to break the canopy by explosive cutting. Therefore, the mechanical properties of PMMA were significant to be studied. The research on one-dimensional constitutive relationship not only explained the mechanical characteristics under one-dimentional stress state, but also constituted the theoretical foundation of the three-dimensional combined stress. It was particularly important to make the research.This paper had combined the results of polymer researches which were represented by PMMA researches. Theories, experiments and numerical simulations were combined to conduct a research on the viscoelastic plasticitic constitutive models of PMMA under one-dimensional compression, and the mechanical differences among PMMA with different conformations or plasticizers in performance. On one hand, the researches complemented the relative constitutive theories on mechanical properties of PMMA so far, on the other hand, the application of molecular dynamics simulation in the mechanical research of PMMA were explored and promoted. The mainly researching methods were contained as follows:(1) Quasi-static and dynamic compression tests under different strain rates (ranges from0.001s-1to3000s-1) were performed on the MDYB-3PMMA in the same temperature, to obtain the stress-strain curves and the final morphologies, also to analyze the strain rate effect;(2) Multi-Maxwell model had been modified for a better description of viscoelastic behavior of PMMA. And it was compared with ZWT model and original multi-Maxwell model.(3) Ree-Eyring yield model and Cooperative yield model had been modified to describe the effect of oriented stretching on the yield behavior of PMMA. The test results were also tried to be described with Johnson-Cook viscoplastic model.(4) With the help of molecular dynamics simulation method, glass transition temperatures and mechanical constants were calculated and compared among PMMA with different conformations and plasticized PMMA.Five results had been obtained as follows:(1) The experiment results indicated that PMMA had obvious strain rate effect. With increasing strain rate, the yield strength and the initial elastic modulus increased. This outcome was related to the internal viscosity of PMMA.(2) In describing the viscoelastic behavior of PMMA, ZWT model had few parameters, and was convenient to fit, which made it flexible and easy to use. Meanwhile, parameters in modified multi-Maxwell model could adjust to the test data from more states in different strain rates, which made this model more accurate.(3) In describing the yield behavior of PMMA, fitting results in modified Ree-Eyring model were smaller than the results of experiments. Modified Cooperative model was more accurate than modified Ree-Eyring model in the description of quasi-static yield stress. Through the contrast results between yield model and experiment, peak stress in dynamic compression was found to be failure stress. Samples with the strain rate above1500s-1failed before achieving yield stress.(4) In description of viscoplasticity after yield, Johnson-Cook model fit well with a single stress-strain curve. But this model could not accurately describe more curves in different strain rates, which meant it could not represent the strain rate relativity of PMMA.(5) In the calculating results of molecular dynamics simulation, the glass transition temperature of syndiotactic PMMA was the highest, followed by that of syndiotactic PMMA, while isotactic PMMA had the lowest value. After plastification, the glass transition temperature of PMMA had decreased. As large free volume was contained in internal structure, the glass transition temperature of isotactic PMMA was lower, and greater toughness was presented. Plasticizer was found to be conducive to reducing the glass transition temperature. And DOP was more effective than DBP. The density, mechanical constants and the radial distribution function by calculation were similar to corresponding reference data, which demonstrated that molecular dynamics simulation was valid in the microscopic scale calculation of PMMA.Finally current research results of PMMA were summarized. Some problems and insufficiency were presented and analyzed. What’s more, the developing direction in mechanical researching of PMMA had been indicated.