Research On PMMA Frequency-dependent Fatigue Property

Polymethyl Methacrylate(PMMA) the most transparent plastic is widely used not only in daily life, but also in key structure such as cockpit canopy, windshield, etc. PMMA is one kind of high polymer material, and it exhibits obvious viscoelastic character. The mechanical behavior of PMMA is sensitive to time and temperature. And the fatigue property of PMMA is time dependent too. With the application of PMMA in important structure, the research on the fatigue property of plastic and life prediction became more important. At present, the research on fatigue life of PMMA based on the similar methods used in metal material’s research, consideration of viscoelatic character is not enough. in this paper, time(frequency) dependence of PMMA was researched by theoretical and experimental method.Firstly, several mechanical characters of plastic material different from that of metal materials were introduced: viscoelastic, time-temperature superposition, crazing, and relevant research work. The study issue of PMMA was also reviewed, especial the frequency dependent fatigue.Creep tests at room temperature were conducted, and creep curves and creep fracture time at different stress level were obtained. Time-stress equivalent relationship was analyzed, and the time-stress shift factor was obtained, moreover, based on this, creep curve was described simply by empirical equation.Fatigue tests at different frequencies of 1Hz, 0.5Hz, 0.1Hz, 0.03 Hz were carried on, and the stress strain response was recorded. The fatigue life at different frequency was obtained. The character of mechanical response was analyzed, and a nonlinear viscoelastic model was proposed to simulate the hysteresis loop. The fracture surfaces of tension, creep, and fatigue were compared. The influence of frequency on fatigue life was analyzed based on the experimental result. Creep damage and rate sensitivity were two main elements when frequency is lower than 1Hz. A new creep-fatigue damage model was proposed, and frequency modification was made on the fatigue evolution equation. The parameters within the model were determined easily by Differential Evolution logarithm. Based on the calculated results, the pure fatigue life, pure creep life and nonlinear time life factor equation were obtained, this equation is convenient for engineer application.The comparison result showed that, the model proposed in this paper can describe and predict the fatigue life at different frequency of PMMA, and the calculated results were consistent with the experimental results. The proposed model can also be used in the condition with load holding.High polymers are often used as the matrix of one kind important structure material—fiber reinforced plastic(FRP) composited material, the mechanical property of matrix has unignorable influence on FRP. And now the viscoelastic phenomenon in the fatigue process of FRP is getting more attention. The study on viscoelastic fatigue of plastic is helpful to further research the fatigue property of FRP.