Compression creep tests (CCTs) have been widely used in phenomenological characterization of viscoelastic materials such as glasses. However, disturbed by specimen‐tool interface friction, the real stress‐strain data regarding the pure viscoelastic deformation are frequently misestimated in conventional CCTs, causing decreased accuracies of the derived viscoelastic parameters. This study proposes a comprehensive CCT‐based approach to develop a viscoelastic model with weakened frictional disturbance and enhanced predictive accuracy. An integrated calculation procedure is first built to mathematically characterize the frictional and viscoelastic behaviors of glass during compression. Uniaxial CCTs of a typical borosilicate glass (L‐BAL42) are then performed at varied frictional conditions. The quantified coefficients of interface friction indicate that a minor frictional disturbance is achieved when Nickel foils are used as interfacial layers, whereby a more realistic viscoelastic constitutive relation of the glass is derived. The obtained frictional and viscoelastic constants are further incorporated into computational modeling of the CCT and precision molding processes. The demonstrated consistencies between the simulated and measured results (creep displacement and molding force) suggest that, by technically slashing the interface friction and theoretically correcting the friction‐involved stress in CCTs, the frictional disturbance to experimental stress‐strain data can be effectively weakened, and a viscoelastic model of enhanced predictive accuracy can be thus developed.
Ultrasonic‐assisted glass molding process (UGMP) has gained a promising start in microptics fabrication in recent years. To further understand the microforming process and comprehensively evaluate the microformability of the glass in UGMP, theoretical considerations and numerical simulations are performed in this study. A generalized dynamic viscoelastic model of glass in UGMP is first reformulated based on the ultrasonic thermo‐mechanical effects. Correspondingly, a multiphysics numerical model of UGMP is developed to dynamically observe the thermo‐mechanical rheological behaviors of the L‐BAL42 glass inside the microscale mold cavities. The stress distributions and deformation features of the formed micro‐V‐grooves in different molding processes are further employed to fully investigate the ultrasonic mechanical and thermal effects on glass molding. The results show that, as a product of the combined ultrasonic mechanical and thermal effects, the filling rate of the glass in UGMP is observably increased and homogenized, while its maximum forming stress is reduced by 57.6% compared with the conventional molding process (GMP). It is also found that, the ultrasonic thermal effect is dominant in lowering the forming stress of the glass, while the ultrasonic mechanical effect plays a leading role in homogenizing the filling rate of the glass. This study will provide both theoretical and technical supports for high‐efficiency and high‐precision fabrication of surface‐relief microptical elements.
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