Rheological Damage And Accelerated Characterization Of Long-term Mechanical Properties Of Solid Polymers

Polymeric materials are widely used in the fields of automotive engineering, mechanical engineering, aerospace engineering and so on, for their rich sources, easy processing and good performance. With increasingly extensive application of polymers, people have higher requirements to their mechanical properties and service life. Therefore, it is very important and necessary for understanding the rheological damage and long-term mechanical properties of polymers. The time-temperature superposition principle(TTSP), the time-stress superposition principle(TSSP) and the time-temperature-stress superposition principle(TTSSP) provide an accelerated characterization technique to evaluate the long-term properties of polymers. The accelerated characterization technique is a very important way to study long-term mechanical properties of polymers, which is a significant subject and have captured extensive attention of researchers in polymer material science and polymer mechanics. This paper tries to do some researches on rheological damage and accelerated characterization of long-term mechanical properties of polymers. The main contents and results are as follows.1. The rheological damage of solid polymers was investigated in the paper. On one hand, it was discussed on time-dependent and stress-dependent crazing damage of polymethyl methacrylate(PMMA). The tensile creep compliance of PMMA under high stress level was successfully predicted with growing damage, based on the experimental compliance data under low stress level. Then, it was experimentally studied on strain rate-dependence and strain-amplitude-dependence on crazing damage of PMMA. The results show that crazing damage is rate-dependent. Smaller strain rate causes the decrease of the critical strain, and fully developed crazes. Under dynamic loading, crazing damage of PMMA is strain-amplitude-dependent, which makes the storage modulus decline.On the other hand, the acoustic emission(AE) events, caused by irreversible damage, were monitored in various loading histories on polyvinylchloride(PVC) material to investigate its AE behavior, damage characteristics, and damage evolution based on probabilistic entropy. The results show that “Kaiser effect” remembers “maximum strain” more exactly than “maximum stress”, in case that the material is a type of “strain-softening” solid polymers. Thus, we suggest that it may be better to use “maximum strain” to describe “Kaiser effect”. In addition, an interesting phenomenon, named here as the “AE hysteresis”, was found in both the multi-step creep tests and multi-step stress relaxation tests, which referred to the viscoelasticity of polymers and time-dependent damage.2. The compressive tests were conducted on PMMA rods to explore the effect of long-term physical aging on mechanical properties of solid polymer. The PMMA rods experienced physical aging time for near 32 years, which were cut into cylindrical specimens and divided into two groups. One group remained, that’s to say, nothing done on them before compressive tests, called the aged PMMA. The other group of specimens was rejuvenated through heat treatment at a constant temperature of 105℃ for 24 h, viewed as rejuvenated PMMA. The results show that physical aging leads to an increase in elastic modulus, a decrease in yield strain, a decrement for 10%~20% in creep strain, and a more obvious strain softening phenomenon. Moreover, Findley model was employed to model the creep behavior of aged PMMA and rejuvenated PMMA under low stress level and the result is in good agreement with the test data. Besides, under high stress level, aged PMMA develops an creep acceleration in short time. The experienced time from the start to creep acceleration presents exponential decrease with the increase of stress level.3. It was studied on accelerated characterization methods of long-term mechanical properties of solid polymers, and a novel approach for constructing master curves was presented in the paper. The approach conducts a calculation formula of the shift factor, which is determined based on the minimization of the area of the overlapping region between the shift curve and the reference curve. All the parameters of the calculation formula come from the experimental data, that is to say, we can calculate the shift factor without interpolation or fitting. Thus, it makes the uniqueness and accuracy, and avoids the uncertainty or error in manual shifting and other numerical shifting methods. Finally, the novel approach was verified through three typical experiment examples of different solid polymers, referring to their creep compliance master curves, relaxation modulus master curves and dynamic viscoelasticity master curves.4. The strain measurement method was investigated and the paper presented an innovative strain measurement method using for the long-term mechanical test and developed the corresponding measurement system. The method has three main advantages, including “interruptible measurement”, “adjustable types of data acquisition interval” and “adaptive image windows”, which make the proposed strain measurement method suitable to the long-term mechanical tests of polymers. Several experiment examples were carried on to illastrate the feasibility of the proposed method. Finally, the presented method was applied to the long-term tensile creep test of PMMA under a low stress level and acquired the tensile creep rupture curve, lasting for about 31 days.