Physical Aging And Accelerated Characterization Of Long-term Mechanical Properties Of PMMA

Polymer has been widely used in many applications, such as mechanical, chemical and medical areas, due to its good mechanical strength, light quality, insulation, corrosion resistance,easy processing and long service life properties, which put forward the higher requirements to the service life and makes the life prediction methods becoming more and more necessary. In many physical and mechanical properties of polymer, the viscoelasticity and the aging rule are relatively more complex. Viscoelasticity refers to the time dependent property, namely the rheological property. Nowadays, most of engineering designs do not take the rheological properties into account,which results in the occurrence of delayed failure accidents. Polymer is often in the glassy state and physical aging also makes the mechanical property change. Thus, it is high time to take the time dependency into consideration in the process of structure design. To this end, we need to fully grasp the evolution rule of the material properties with time, especially the material physical aging law, which will help us to make the reasonable use in engineering design process and reduce engineering delay failure accidents. The time-temperature superposition,time-stress superposition, time-temperature-stress superposition, and time-aging time equivalence principle play important roles in the accelerated characterization of long-term mechanical properties,but there are still some problems to be further discussed. This paper focuses on the study of physical aging and accelerated characterization of long-term mechanical properties and the main research contents and results are as follows:(1) Based on time-temperature-stress superposition principle, a two-step shift method of master curve of the material viscoelastic properties has been developed. After the short-term creep tests, we found that the temperature and stress level had effect on the PMMA creep compliance and the creep compliance of PMMA was a function of stress and temperature, which shown that the material presents nonlinear viscoelasticity property.Based on the two-step shift method of master curve, the creep compliance master curves of a reference temperature have been constructed by use of a different shifting order and the results are in good agreement with each other. Therefore, the validity of the two-step shift method has been verified. Meanwhile, the study shown that, for the nonlinear viscoelastic material, the temperature and stress shift factors related to the reference stress and temperature, respectively.(2) By use of a long-time scale strain measurement method for PMMA long time creep test of based on digital image, there existed some differences between the measured creep compliance curve and the shifting master curve by means of the time -stress superposition principle, and we found that the physical aging in the process of testing was a main factor causing the differences. The aging shift rate was obtained after the analysis of the PMMA short-term creep tests under the conditions of different aging time. After the combination of time-temperature-stress superposition principle, the physical aging long term creep compliance master curve was obtained, which was in good agreement with the long-term creep experimental results(3) The effects of physical aging on the static mechanical properties of polymers were investigated. After the short-term creep tests, we found that the instant elastic modulus and fracture stress would increase with the increase of aging time, which could be simulated by the KWW model.(4) The effects of physical aging on the dynamic mechanical properties of polymers were investigated. The dynamic mechanical experiments of PMMA with different aging time were conducted under the same frequency and amplitude. And the results show that the dynamic storage modulus would increase with the increase of aging time. Besides, the aging time and the corresponding storage modulus satisfied the time-aging time superposition principle and there was a linear relationship between the shift factor and logarithmic aging time.(5) Based on the short-term experimental results under various aging times and stress levels, the fractional differential Maxwell model was used to analyze the effect of physical aging and stress level on the creep behavior. Meanwhile, we found that there was a power law relationship between the aging time and relaxation time and the relaxation time increased with the increase of physical aging time.(6) The influence factors of crazing damage in glassy polymers are investigated. The influence of stress and time on the crazing damage of PMMA under creep condition is investigated. A crazing damage model of crazing is developed. The effect of dynamic strain amplitude and strain rate on crazing damage of PMMA are investigated. Research shows that with the increase of dynamic strain amplitude, crazing damage more serious,leading to the storage modulus decreases with the increase of strain amplitude decreased.While the loading rate decreases, the critical strain of crazing initiation decreases, the same strain under the condition of surface crazing density increases, craze more fully.