Residual stresses glass

Many authors have shown that residual stresses in glass articles can be formally considered as the thermal stresses due to a certain fictitious temperature field. In the general case... 

Theory of the fictitious temperature field allows us to analyze the problems of residual stresses in glass using the mathematical apparatus of thermoelasticity. In this part we formulate the boundary-value problem for determining the internal stresses. We will Lheretore start from the Duhamel-Neuinan relations... 

Effect of processes. The fabricating operations for plastics, steel, glass, and so forth may, and usually do, introduce unwanted stress concentrations and residual stresses if not properly processed. 

Zhang, Y., Wang, W.H. and Greer, A.L. (2006). Making metallic glasses plastic by control of residual stress. Nature Materials 5 857-860. 

Fig. 7,10. A comparison of (a) axial, (b) radial and (c) circumferential residual stresses in an E-glass-...

Tempered glasses are useful because failure normally occurs under an applied tensile stress, and failure in ceramics and glasses is almost always initiated at the surface. When a permanent compressive stress, called a residual compressive stress, is placed on a surface, either through thermal or chemical means, the applied stress must first overcome this residual compression before the surface is brought into tension under which failure cau occur (see Figure 5.53). Notice that the residual stress is compressive in nature at the surface of the plate and is tensile in the center (shaded areas). When... 

Figure 5.53 Residual stress, applied stress, and resultant stress distribution for transverse loading of a tempered glass plate. From W. D. Kingery, H. K. Bowen, and D. R. Uhknann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Plate-like particles of interest in this context include mica, aluminum flake, hammered glass, magnesium hydroxide and talc. Physical properties of composites containing these additives depend strongly on the flow-induced morphology and on the distribution of residual stresses [31]. 

Fig. 8.9. Stress fields at the end of a trench etched in a 15f

On the epoxy side of the interface, high fracture toughness and low residual stresses 72,73) are a requirement for optimum transverse strength in graphite and glass-epoxy 1A) composites. Since the adsorption of epoxy components has been shown to be probable, the local structure of the epoxy at the interphase will most likely not be the same as in the bulk. This local anisotropy caused by the interphase is a limitation in the predictive capability of micromechanical models which do not include the interphase as a component. 

This is a very useful approach, which can be recommended for practical applications. A simplified version of this general treatment may also be useful. It is possible to consider vitrification of a material as a jump-like transition from a liquid to a solid state. This idea was advanced elsewhere, where residual stresses in inorganic glasses were calculated by treating vitrification as a sequential solidification of layers of a viscous liquid on the rigid surface of a previously solidified material. In a liquid layer (not yet solidified) at T > Tg, only flow deformations can occur. In the transition through Tg, this deformation is frozen in and cannot change later on. 

In a DSC analysis of a semi-crystalline polymer, a jump in the specific heat curve, as shown in Fig. 2.22, becomes visible. The glass transition temperature, Tg, is determined at the inflection point of the specific heat curve. The release of residual stresses as a material s temperature is raised above the glass transition temperature is often observed in a DSC analysis. 

As opposed to composites reinforced with SiC(w), the residual stresses formed at the interface should be lower as the CTE of the matrix and the reinforcement phase are similar. Several modeling studies have shown that the composition of the grain/glass interface has a negligible effect on the magnitude of the residual stresses and therefore has no significant influence on the fracture behaviour.19... 

All solidity is gone. All phenomena are paper images pasted on the glass screen of consciousness. For the unprepared, or for the person whose karmic residue stresses control, the discovery of the wave-nature of all structure, the Maya revelation, is a disastrous web of uncertainty. 

Radiant cured epoxy systems also provide high glass transition temperatures, low shrinkage during cure, and relatively low residual stresses. They have no volatile emissions and excellent shelf life. The good physical and adhesive properties of conventional epoxy adhesives systems are maintained with the radiant cured systems. 

We propose to rationalize the observation by a phenomenon known as residual thermal stresses. Residual thermal stresses arise from the fact that carbon-fiber and epoxy have different thermal expansion coefficients and a quenching of the composite would conceivably produce residual stresses. Apparently, the quenching process may produce enough residual stresses to lower the toughness of the composite. In the absence of such residual stresses the free volume concept alone would predict a quenched glass to have larger amount of free volume and hence constitute a less brittle substance. 

Crack branching is a common feature in failure of ceramic components. Cracks branch at a critical velocity, which is of the order of half the speed of sound in the specific glass under study. The acceleration of crack initiation to the critical velocity depends on the energy dissipation available from the release of stored energy. The energy source can be applied stress, prestressing or residual stress. 

Before use, all the polymer pellets were dried at Ta +20 K for at least 48 h. Then, sheets of controlled thickness B were firstly compression molded under vacuum at Ta + 50 K and then cooled down slowly through the glass transition region. Next, samples of dimensions suitable for the SENB tests were cut from the sheets with a diamond saw, machined to produce the notch, and annealed once more at Ta +20 K for 72 h. The purpose of this thermal treatment is double first, it allows the elimination of the residual stresses induced by sample pressing and machining and secondly, it permits removal of eventual moisture which is known to affect dramatically the SAPA mechanical properties [12]. The pre-crack was produced in the samples just before testing. 

Quantification of residual stresses after manufacture. The build up of thermal stresses starts during fabrication of the laminate when it is cooled from the stress free temperature to room temperature. The stress free temperature in the case of an amorphous thermoplastic as used in this study is taken as the glass transition temperature [1] Tg of the Polyetherimide used is 215°C). On a fibre-matrix scale, the contraction of the matrix ( = 57 x 10 /°C) is constrained by the presence of the fibre (cif = -1 x 10 /°C for the carbon in the fibre direction). This results in residual stresses on a fibre-matrix scale (microscale). On a macroscopic scale, the properties of a unidirectional layer can be considered trans ersally isotropic. This means, in turn, that a multidirectional composite will not only contain stresses on a microscale, but also on a ply-to-ply (macroscopic) scale. 

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