Viscoelasticity is a mechanical phenomenon where the material modulus varies with time and temperature. Modern experimental methods can determine material properties within certain time and temperature ranges, but modeling the viscoelastic behavior remains challenging, mainly because the data processing is complex and different materials have distinct properties. Using polyetherimide as an example and based on the change in the secondary bonds of polyetherimide in different viscoelastic stages, we proposed a new shift factor model in Arrhenius format with alterable activation energy. We also used two methods based on nonlinear least squares to obtain the Maxwell model of the polyetherimide, and we then used a novel method integrated with Laplace transforms and partial fraction decomposition to convert the Maxwell model into the Voigt model. The results of our model are reliable and self-consistent, showing its potential for modeling the viscoelasticity of other materials. V C 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46102.

Amorphous thermoplastics, as a type of engineering plastic material, are used in various industrial sectors. In order to manufacture high-performance products, it is important to optimize their forming process to mitigate residual stresses. However, stress in a plate is difficult to measure, therefore, modeling provides a powerful way to investigate and understand the evolution of stress. In this study, the forming process of a polyetherimide (PEI) plate was modelled using finite element analysis, and then validated through a comparison with a warpage experiment. This study reveals that the whole forming process can be divided into three stages by the glass transition temperature Tg of the PEI. The second stage, corresponding to the plate cooling from above Tg to below Tg, contributes a large portion of the residual stress in a short time. The final residual stress, the magnitude of which is affected by the cooling rate and plate thickness, shows a parabolic distribution through the thickness of the plate. These important conclusions are beneficial for improving the quality of an amorphous thermoplastic plate, while allowing highly efficient production.

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As the thickness of a composite differs significantly from the size of a representative volume element, composite is studied at both micro- and macroscales. In this study, the synergy between the prescribed displacement boundary and massively parallel computing enables end users to model a composite described in the micro-meter scale and take into account the global influence of the forming process. After validating the software and material models, the residual stresses of a sandwich thermoplastic composite caused by the dynamic thermomechanical forming process were simulated. The results of the macro-micro simulation revealed that the micro structure of a composite consisting of continuous carbon fiber and thermoplastic that have significantly different material properties has a weak impact on temperature distribution throughout the thickness, but exerts a significant influence on the distribution of in-plane stresses. The stresses within a representative volume element at the top and bottom surfaces of the composite layer were further studied to explain the effect of the temperature gradient on the simulated stresses along with the axial and transverse directions of the fiber. The results of this study provide a practical method to reveal actual residual stresses feasibly and efficiently.