Abstract
Viscoelastic properties of glass within molding temperatures, such as shear relaxation modulus and bulk relaxation modulus, are key factors to build successful numerical model, predict forming process, and determine optimal process parameters for precision glass molding. However, traditional uniaxial compression creep tests with large strains are very limited in obtaining high-accuracy viscoelastic data of glass, due to the declining compressive stress caused by the increasing cross-sectional area of specimen in testing process. Besides, existing calculation method has limitation in transforming creep data to viscoelasticity data, especially when Poisson's ratio is unknown at molding temperature, which further induces a block to characterize viscoelastic parameter. This study proposes a systematic acquisition method for high-precision viscoelastic data, including creep testing, viscoelasticity calculation, and finite element verification. A minimal uniaxial creep testing (MUCT) method based on thermo-mechanical analysis (TMA) instrument is first built to obtain ideal and accurate creep data, by keeping compressive stress as a constant. A new calculation method on viscoelasticity determination is then proposed to derive shear relaxation modulus without the need of knowing bulk modulus or Poisson's ratio, which, compared with traditional method, extends the application range of viscoelasticity calculation. After determination, the obtained viscoelastic data are further incorporated into a numerical simulation model of MUCT to verify the accuracy of the determined viscoelasticity. Base on the great consistence between simulated and measured results (uniaxial creep displacement), the proposed systematic acquisition method can be used as a high accuracy viscoelasticity determination method.
Highlights
Precision Glass Molding (PGM), as shown in Fig. 1, is a replicative process for mass-producing lenses of various shapes in a large volume for high end precision optical applications [1,2,3]
The proposing shear calculation method extends the application range, especially when Poisson’s ratio and bulk modulus are unknown at a certain temperature
This study proposes a new testing and calculation method for determination glass viscoelasticity, and the conclusions of this study can be drawn as follows: Firstly, a minimal uniaxial creep testing (MUCT) method based on thermo-mechanical analysis (TMA) is proposed
Summary
Precision Glass Molding (PGM), as shown in Fig. 1, is a replicative process for mass-producing lenses of various shapes in a large volume for high end precision optical applications [1,2,3]. An accurate measurement method for viscoelastic properties of glass at certain temperature is critically needed. An initial deformation is instantaneously applied to the sample and kept at the last position, the decreasing stress as a function of time is measured to calculate the modulus relaxation function [15,16]. Both dynamic and static methods can measure viscoelasticity of glass material, there are still some challenges in measuring viscoelasticity accurately, especially at high temperatures. An ideal testing and easy implemented calculation method need to be developed to have high accuracy viscoelastic data of glass material, with more convenience
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