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Research Papers

J. Appl. Mech. 2018;86(2):021001-021001-11. doi:10.1115/1.4041765.

The improvement of the accuracy and efficiency of microforming process of polymers is of great significance to meet the miniaturization of polymeric components. When the nonuniform deformation is reduced to the microscopic scale, however, the mechanics of polymers shows a strong reinforcement behavior. Traditional theoretical models of polymers which have not considered material feature lengths are difficult to describe the size effect in micron scale, and the process simulation models based on the traditional theory could not provide effective and precise guidance for polymer microfabrication techniques. The work reported here proposed strategies to simulate size effect behaviors of glassy polymers in microforming process. First, the strain gradient elastoviscoplastic model was derived to describe the size affected behaviors of glassy polymers. Based on the proposed constitutive model, an eight-node finite element with the consideration of nodes' rotation was developed. Then, the proposed finite element method was verified by comparisons between experiments and simulations for both uniaxial compression and microbending. Finally, based on the FE model, under the consideration of the effect of rotation gradient, the strain distribution, the deformation energy, and the processing load were discussed. These strategies are immediately applicable to other wide-ranging classes of microforming process of glassy polymers, thereby foreshadowing their use in process optimizations of microfabrication of polymer components.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Appl. Mech. 2018;86(2):024501-024501-6. doi:10.1115/1.4041824.

This paper presents an investigation on the free vibration of an oscillator containing a viscoelastic damping modeled by fractional derivative (FD). Based on the fact that the vibration has slowly changing decay rate and frequency, we present an approach to analytically obtain the initial decay rate and frequency. In addition, ordinary differential equations governing the decay rate and frequency are deduced, according to which accurate approximation is obtained for the free vibration. Numerical examples are presented to validate the accuracy and effectiveness of the presented approach. Based on the obtained results, we analyze the decay rate and the frequency of the free vibration. Emphasis is put on their time-dependence, indicating that the decay rate decreases but the frequency increases with time increasing.

Commentary by Dr. Valentin Fuster

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