Rene Gy

Stress corrosion of silicate glass: a review

In the first section of the present paper, some examples, from the field, of the manifestations or consequences of the fatigue of silicate glass are briefly presented. In the second section, the interpretation of fatigue in terms of stress corrosion is reviewed: the role of ambient molecular water is well established. Whatever the details of the mechanism of action of water, it takes place very efficiently in the highly strained material close to the tip of a surface crack. This enables its sub-critical growth to be explained. But it does not explain many other observed effects of the environment on the mechanical behavior of silicate glass, and, in the last section, questions and issues are presented, which would still need to be investigated.

The role of surface and volume defects in the fracture of glass under quasi-static and dynamic loadings

In this paper, the role of surface and volume defects on fracture in soda-lime glass is analyzed when samples are submitted to quasi-static or dynamic loadings. To investigate fracture, different experiments are carried out, namely, quasi-static compression of glass spheres and edge-on-impacts. The first test series aims at studying crack initiation. Different surface treatments are performed to study their influence on the failure load, from which it is concluded that initiation of cracks occurs in the vicinity of the contact surface. The second series is concerned with the examination of crack patterns under dynamic loadings with two different strikers (i.e., soft/flat and hard/perforating projectiles). Crack initiation under dynamic load histories is investigated near and far from the impact zone and it is concluded that it cannot take place within the volume, except in a very small zone close to the impact point. Conversely, initiation of damage from the surface, at a location far from the impact point, is possible and clearly present with a soft projectile.

New insights into the viscoelasticity of glass

Abstract The behaviour of a linear viscoelastic liquid with finite compressibility is investigated theoretically and numerically. Some conditions for the generalized Maxwell model to give a consistent description of the different features of its phenomenology are given. Creep-recovery experiments are shown to be very valuable in evaluating the long-term part of the relaxation and the compressibility effect. These effects are investigated numerically in the case of soda-lime-silica glass in the transformation range. The possibility of modelling the stress relaxation in this material with the Kohlrausch-Williams-Watt function is questioned.

Generalized Maxwell model for the viscoelastic behavior of a soda-lime-silica glass under low frequency shear loading

The linear viscoelastic behavior of a soda-lime-silica glass under low frequency shear loading is investigated in the glass transition range. Using the time-temperature superposition technique, the master curves of the shear dynamic relaxation moduli are obtained at a reference temperature of 566°C. A method to determine the viscoelastic constants from dynamic relaxation moduli is proposed. However, some viscoelastic constants cannot be directly measured from the experimental curves and others cannot be precisely obtained due to non-linearity effects at very low frequencies. The generalized Maxwell model is investigated from the experimental dynamic moduli without fixing the viscoelastic constants. A set of parameters is shown to be in good agreement with the experimental dynamic relaxation moduli, but does not give the correct values of the viscoelastic constants of the investigated glass. The soda-lime-silica glass exhibits a non-linear viscoelastic behavior at very low stress level which is usually observed for organic glasses. This non-linear behavior is questioned.

Viscoelastic behavior of a soda-lime-silica glass: inadequacy of the KWW function

Abstract The viscoelastic behavior of a soda-lime-silica glass has been experimentally investigated in the glass transition range using shear and uniaxial creep-recovery experiments. No departure of linearity and thermorheological simplicity is found in the investigated stress and temperature ranges, respectively. The shear and uniaxial viscoelastic constants and retardation functions have been precisely measured from the recovery part. An attempt to model the shear and uniaxial viscoelastic behaviors with KWW functions is made and it is shown that this model cannot be a good representation of the linear viscoelastic behavior of the investigated glass.

Viscoelastic constants of a soda-lime-silica glass

Abstract A complete development of the viscoelastic constants is given in the framework of the viscoelastic theory using the definition of Tschoegl and the notion of moment introduced by Gy et al. for the linear viscoelasticity. Viscoelastic constants were considered as ‘constant’ because these dimensionless parameters do not change with loading level for a linear viscoelastic material. In addition, for a thermorheologically simple viscoelastic material, some of them are temperature-independent, whereas others follow the temperature-dependence of the Poisson ratio. Since some of them have a simple physical meaning, all models have to take them into account for simulating mechanical behavior of viscoelastic materials. Experimental measurements for some of viscoelastic constants under shear and uniaxial loadings allow determination of bulk equilibrium elastic modulus and other viscoelastic constants, used for describing hydrostatic pressure and biaxial behavior.

On the (de)stabilization of draw resonance due to cooling

We study the drawing of a Newtonian viscous sheet under the influence of cooling with temperature dependence of the viscosity. Classically this problem has an instability called draw resonance, when the draw ratio Dr , which is the ratio of the outlet velocity relative to the inlet velocity, is beyond a critical value Dr c . The heat transfer from the surface compared to the bulk energy advection is conveniently measured by the Stanton number St . Usual descriptions of the problem are one-dimensional and rigorously apply for St ≤ 1. The model presented here accounts for variations of the temperature across the sheet and has therefore no restriction on St . Stability analysis of the model shows two different cooling regimes: the ‘advection-dominated’ cooling for St ≪ 1 and the ‘transfer-dominated’ cooling for St ≫ 1. The transition between those two regimes occurs at St = O (1) where the stabilizing effect due to cooling is most efficient, and for which we propose a mechanism for stabilization, based on phase shifts between the tension and axial-averaged flow quantities. Away from this transition, the sheet is always shown to be unstable at smaller draw ratios. Additionally, in the limit of St → ∞, the heat exchange is such that the temperature of the fluid obtains the far-field temperature, which hence corresponds to a ‘prescribed temperature’ regime. This dynamical situation is comparable to the isothermal regime in the sense that the temperature perturbation has no effect on the stability properties. Nevertheless, in this regime, the critical draw ratio for draw resonance can be below the classical value of Dr c = 20.218 obtained in isothermal conditions.

On the equilibrium isothermal compressibility of soda-lime silicate glass

Abstract An estimate of the equilibrium isothermal compressibility in the transformation range is obtained for soda-lime silicate glass. This work is based on previously published uniaxial and pure shear static experimental data and on specific theoretical derivations. It is shown that a more accurate determination of this quantity could be performed by measuring the total elastic recovery in two experiments of pure shear and uniaxial stress on glass samples in the transformation range.

Minimal Glass Deformations with Rapid Thermal Annealing Control

A critical step in the fabrication of polycrystalline silicon thin-film transistors (TFTs) is to crystallize the amorphous silicon layer deposited on glass. This paper describes a complete rapid thermal annealing (RTA) process applied to the crystallization of the amorphous layer. The RTA heating and cooling rates have been optimized to minimize thermal gradients and to improve glass substrate stability. Numerical simulations taking into account the lamp characteristics, the annealing chamber configuration and the sample properties have been developed and allow elucidation of thermal nonuniformities in the sample during the RTA process. Although the transition point of a glass (T g) is often considered as an upper limit in usual polysilicon TFTs processes, this paper shows that if the cooling rate is accurately controlled, annealing temperatures can be above T g without thermal damage to the glass.

Sur le dimensionnement des structures en verre trempé

ABSTRACT This work deals with the use of glass as a real structural material, and concerns more particularly the detailed study of the potentiality of a connection by insertion of a steel dowel in a hole of a glass plate, in the aim of the making of long beams or beams with large inertia. The determination of the optimal geometry for such a connection passes first by the superposition of two stress states (the first one is generated by the thermal tempering and the second is due to the loading of the metallic connector in the plane of the glass plate). Then a design method ensuring the durability of the structure is proposed.