Stress Relaxation and Refractive Index Change of As₂S₃ in Compression Molding

Abstract

As 2 S 3 is one of the chalcogenide glasses that have attracted increasing interests for compression molding applications. This paper is aimed to evaluate the stress relaxation behavior of As 2 S 3 above its glass transition temperature and calculate its refractive index change during cooling. First of all, creep tests were conducted with cylinder glass specimens at three different temperatures, in order to deduce the shear stress relaxation function by using the relationship with creep compliance function. In addition, the shift factor for thermo-rheological simplicity using Williams-Landel-Ferry equation was obtained. Then, finite element simulation was implemented to verify the calculated shear stress relaxation function. The acquired shear stress relaxation function needs to be modified to compensate the influence of friction on the thickness change in the experiments, so that the simulation results using the modified shear stress relaxation would match the experiments better. Finally, the refractive index changes of As 2 S 3 at different cooling rates were modeled by using the Tool-Narayanaswamy-Moynihan model for structural relaxation behavior. It is confirmed that the slower the cooling rate is, the less the refractive index drop will be. It was also demonstrated that the refractive index drop is strictly dependent on the cooling rate logarithmically by using Tool-Narayanaswamy-Moynihan model. In summary, the results presented in this paper can provide reliable references for viscoelastic characterization of As 2 S 3 glass, crucial for compression molding or similar applications. K E Y W O R D SAs 2 S 3 , chalcogenide glass, compression molding, refractive index drop, stress and structural relaxation, viscoelastic

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

“…[20][21][22][23] Among them the most commonly used is CCT that can be handily implemented on hightemperature testing machines without complex specimen or fixture preparations. 9,16,24 These parameters were then fitted by phenomenological models and incorporated in finite element (FE) calculations to predict the molding force, 24 profile deviation, 11 and birefringence distribution 18 for industrial purposes. 9,16,24 These parameters were then fitted by phenomenological models and incorporated in finite element (FE) calculations to predict the molding force, 24 profile deviation, 11 and birefringence distribution 18 for industrial purposes.…”

“…16,25 It was revealed by Avitzur that the interface friction during mental disc forging contributed considerably to the increase in the required forging force. 9 A practical method for merely achieving this displacement agreement is to adjust the interfacial friction coefficients in FE simulations (nearly zero, typically). For another, Joshi et al revealed that during constant-load compression the creep displacement of the glass specimen varied greatly with the interface friction.…”

“…26 This is true for compression tests of glass. 9,25 However, such settings may deviate from the actual frictional conditions, causing decreased predictive accuracies in frictional-induced heat, residual stress, and bulging profile of the compressed specimen, as demonstrated in references 12,13,17 . 14 As a result, the stress-strain relations (constitutive models), exacted from the uncorrected force-displacement data (friction-involved), could be of friction sensitivity, raising uncertainties in further numerical validations concerning the agreement between the simulated and tested compression displacements.…”

“…16,25 During compression the interface friction between the specimen and loading platens will yield partially constrained material deformation at both contact interfaces. 9 Unfortunately, to the best knowledge of the authors, there have been no reports on the analytical correction of stress-strain relation in viscoelastic characterization of glass. That is, during CCTs the deformation of glass is not purely viscoelastic but friction-involved, causing considerable uncertainties in the obtained stress-strain data (as aforementioned).…”

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

“…At such high temperature, in fact, the glass becomes liquid‐like where its relaxation time is extremely fast ( τ «seconds) so that the data obtained from the stress relaxation experiment is not sufficient and reliable for postprocessing. Based on the creep approach, the mold displacements are acquired and used to compute creep compliances . The drawback of this approach is that the temperature‐dependent viscoelasticity is not accomplished.…”