Studies On Strain Rate Dependence Of Mechanical Behavior Of Amorphouis Polymer

Amorphous polymer, which possesses the properties of time-dependence and temperature-dependence, is a class of typical viscoelastic material. Time-dependence is also manifested in the strain rate dependence. Under conditions of low strain rate for various values, primary transition motions play a main role because secondary transition motions are relatively free, while in the cases of dynamic high strain rates, secondary transition motions also play an important role and the coupling effects between primary and secondary transition motions are responsible for the mechanical response for materials because the secondary transition motions are restricted. Compared to metals, ceramics and other materials, the structures of amorphous polymers are more complex. Therefore, it is a complex and difficult work to study the strain rate dependence of mechanical behavior of amorphous polymer. Firstly, due to the polymeric material's poor thermal conductivity, especially in the case of high strain rate, its deformation belongs to adiabatic one, which results in the sharp rise in temperature, so the softening phenomenon is very prominent. In this case, strain softening and thermal softening coupling. Secondly, the theoretical results predicted by the current constitutive models are not well fitted with the experiment data as the strain rate varies from low values to high ones. there are a lot of work needed to be done to fully understand the relationship between amorphous polymers structure and mechanical properties.In this work, strain rate dependences of two kinds of amorphous polymers, poly methyl methacrylate (PMMA) and polycarbonate (PC), are analyzed and discussed. The main research contents and conclusions are as following:1. The properties, applications and molecular motions of amorphous polymers are introduced, and then the research situation and advances in the strain rate dependence of amorphous polymer are analyzed. Finally, the thermal rheological simplicity and complexity, the intrinsic deformation behavior and molecular analysis, which are closely related to strain rate dependence of polymer, are reviewed.2. The DMA tests of PMMA are carried out on a GABO EPLEXOR. The basic theory of stress and strain in the dynamic experiments is introduced, and the relation between frequency and equivalent strain rate is constructed. The influence of frequency on glass transition temperature, and the effects of heating rate and frequency on dynamic mechanical properties of PMMA are analyzed. It is showed that the glass transition temperature Tg increases with increasing frequency (equivalent strain rate) and heating rate. The value of glass transition temperature Tg definited by the peak temperature of the loss modulus E' curve is smaller than that definited by the peak temperature of the loss factor tanδcurve. Storage modulus E increases slowly with increasing frequency. As the temperature increased, the value of frequency corresponding to the peak of tanδcurve increases, the tanδcurve is shifted from low frequency to high one.3. A series of uniaxial compressive experiments for PC and PMMA specimens are implemented. The strain rate dependence of mechanical behavior and molecular motions at various strain rates are analyzed and discussed. It is showed that the yield stress, which increases with increasing strain rate, is approximatively linear with logarithmic strain rate, but the hardening modulus of PC and PMMA firstly increases and then decreases with increasing strain rate. The calculated results of non-elastic behavior index I of PC and PMMA are not completely coincidence with Halary's theory, while the strain-softening amplitude (SSA) is nearly consistent with Halary's theory.4. The key problems of the split Hopkinson pressure bar (SHPB) test technique in the polymeric materials are summarized. Under dynamic high strain rates, it is found from the study on the mechanical response of PMMA that the strain rate sensitivity of elastic modulus of polymeric material increases at high strain rates, and an obvious strain-softening phenomenon exists at the post-yield stage. Due to the thermal softening caused by the adiabatic deformation at high strain rates, no strain harding stage is observed in the stress-strain curve.