A comprehensive study on frictional dependence and predictive accuracy of viscoelastic model for optical glass using compression creep test

Abstract

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.

Modeling of thermo‐viscoelastic material behavior of glass over a wide temperature range in glass compression molding

“…While two data have a good match within the first relaxation response, ie, fast relaxation time ( τ 1 ≈10 −5 − 10 −2 seconds), an obvious distinction can be recognized when one looks a bit closer on the long‐term relaxation response ( τ 2 ≈10 −2 – 10 3 seconds) as indicated in the zoomed plot (Figure B). Similar findings can be seen in the work of Zhou et al and Yu et al; however, impacts of such nonidentical relaxation response in the long term on the creep displacements have not been studied. It is obvious that the first relaxation period occurs extremely short (<0.01 second) compared to the total pressing time.…”

“…It is demonstrated that the accuracy of the obtained viscoelastic parameters can be enhanced if the friction between the glass and mold is sufficiently small, 23 as approached in this research, or the error due to the friction can be dismissed from the displacement data. 28 The authors agree that the friction at the glass-mold interface, which cannot be avoided by the compression setup, may violate the pure creep deformation due to viscoelastic property of an investigating material. In this study, however, another reason that crucially results in the disagreement between the experiment and simulation results is elucidated.…”

“…Several studies investigated the influence of the friction on the viscoelastic constants using the creep compression setup. It is demonstrated that the accuracy of the obtained viscoelastic parameters can be enhanced if the friction between the glass and mold is sufficiently small, as approached in this research, or the error due to the friction can be dismissed from the displacement data …”

“…31 Such ultraprecision mold surface enables a small friction coefficient at the glass-mold interface, below 0.1. 32 As indicated in the previous works, 5,26,28 the low friction coefficient is essential to minimize the error that leads to the impurity of the viscoelastic properties derived from the creep experiments due to the frictional deformation of the glass. Figure 1 exhibits the polished glassy-carbon molds and the glass specimen used for the creep experiments.…”

“…Nevertheless, satisfactory verifications of the creep displacement cannot be obtained by varying the friction coefficients. A recent study by Yu et al shows that if the frictional disturbance can be diminished from the creep data so that mere purity of the viscoelastic property is secured by creep experiments, good agreement can be found …”