Thermal Tempering Research Articles

Ultrasonic evaluation of residual stresses in flat glass tempering by an original double interferometric detection

In industrial thermal tempering of glass, the knowledge of the homogeneity of compressive residual stress field on the glass product is fundamental to guarantee the quality of the tempered glass product. In this paper, we use the acoustoelasticity phenomenon in order to estimate the residual stress distribution by using acoustic surface wave. We present an experimental setup based on a double interferometric detection in which an aspheric lens is associated with a beam splitter and a YAG laser whose power is 100 mW. This relative high power enables us to carry out measurements on surface flat glass although optical reflection coefficient is typically weak (< 10%). Using these two points of detection, the evolution of relative surface wave velocity is obtained with a good accuracy. At last, a comparison between the numerical modeling and experimental results shows the potentiality of an ultrasonic method to estimate stress distribution in flat glass tempering.

Corrosion resistance of AA2219 aluminium alloy: electrochemical polarisation and impedance study

The addition of copper increases the strength of aluminium, but decreases the localised corrosion resistance of the metal to seawater as the copper containing intermetallics exhibit different electrochemical properties from the matrix. The main requirement of corrosion resistance could be met through the surface modification by removing the copper rich phase. The present work involves investigations of the influence of prior copper removal treatment on the pitting and general corrosion resistance of AA2219 alloy containing a high copper content in different initial thermal tempers. Significant improvement in the pitting and general corrosion resistance after surface copper removal treatment was evident from the results of electrochemical polarisation and the impedance tests. Attempts were also was made to modify the surface further, with a conversion layer of cerium as an alternative to the chromate coating.

Microstructure and pitting corrosion of partially melted zones of Al alloy GTA welds

The corrosion behaviour of partially melted zones and heat affected zones of gas tungsten arc (GTA) welds in AA2219 and AA6061 alloys with different initial thermal tempers has been studied. Continuous and pulsed current GTA welding techniques were used. Potentiodynamic polarisation testing was carried out to determine the corrosion resistance. Optical and scanning electron microscopy studies were carried out to determine the corrosion mechanism. The partially melted zone of the welds was found to be attacked severely. A pulsing technique was found to decrease the severity of corrosion damage in the partially melted zone. The prior thermal temper of the alloys was found to influence the corrosion of heat affected zones of welds.

Partially melted zone in Al–Mg–Si alloy gas tungsten arc welds: effect of techniques and prior thermal temper

Influence of prior thermal temper and welding techniques on microstructure changes in the partially melted zone (PMZ) of AA6061 alloy gas tungsten arc (GTA) welds was investigated. Welds of AA6061 in naturally aged (T4) and artificially aged (T6) thermal tempers were made by continuous current (CC) and pulsed current (PC) techniques of GTA welding. Optical, scanning electron microscopy and microhardness testing were carried out to study the PMZ in welds. Susceptibility to liquation was high in the T6 temper as compared to the T4 temper and was attributed to a greater amount of silicon rich eutectic at the grain boundaries of PMZ. The PC technique was found to improve the resistance to susceptibility to liquation in the PMZ of AA6061 alloy GTA welds and was due to the possible reduction in the peak temperature.

Effect of modified AA4043 filler on partially melted zone cracking of Al-alloy gas tungsten arc welds

The effect of grain refiners (zirconium, Tibor and scandium) added to the fusion zone through AA4043 filler on the partially melted zone (PMZ) cracking in gas tungsten arc (GTA) welded AA6061 alloy was investigated. Welds of AA6061 in thermal tempers of artificially aged condition (T6) were made with continuous current and pulsed current techniques. Varestraint testing was carried out to study PMZ cracking in welds, and optical SEM examination performed to evaluate it. Addition of grain refiners to the fusion zone improved the PMZ cracking resistance very significantly. Pulsed current technique was also found to improve the resistance to PMZ cracking, as a result of the possible reduction in the strain in the PMZ owing to the ductile fine grained fusion zone. Severe PMZ cracking in the welds of AA4043 filler without grain refiners was attributed to a greater amount of silicon rich eutectic at the grain boundaries of the PMZ.

Residual Stresses in Bulk Metallic Glasses due to Thermal Tempering

Bulk metallic glasses (BMGs) are attractive structural materials that develop residual stresses during processing due to thermal tempering. In this process, compressive surface stresses are generated within thin layers balanced with tension in the interior. In the present study, stress generation was analyzed experimentally and theoretically in a BMG plate and a cylinder. The residual stresses were measured using the crack compliance method. It was shown that high stresses can be attained in metallic glasses due to thermal tempering: over −300 MPa compression on the surface balanced by +150 MPa tension in the middle. The experimental data were then compared to the predictions of a viscoelastic model that took into account the equilibrium viscosity of BMG as a function of temperature. The model was shown to be accurate within 5 to 25% of the experimental stress data. It is therefore a powerful tool for estimating processing-induced residual stresses in bulk metallic glasses.

Residual stresses in a bulk metallic glass–stainless steel composite

Bulk metallic glasses (BMGs) are new structural materials with impressive mechanical properties. They can now be cast into large dimensions, which can lead to significant residual stress generation due to thermal tempering. In this process, a surface compression develops balanced with tension in the interior. To evaluate this phenomenon non-destructively, a model cylindrical stainless steel (SS)–BMG composite was prepared and studied using neutron diffraction and finite element (FE) modeling. The residual strain data from the SS obtained by diffraction were used in modeling calculations to show that significant tempering could be achieved in the composite (about −200 MPa surface compression in the SS). The strong bond between the SS and BMG allowed efficient load transfer and facilitated stress generation. The final values of the residual stresses were seen to be relatively insensitive to the high temperature constitutive behavior of the SS due to the physics of the thermal tempering in BMGs. The approach presented here constitutes an effective means to study non-destructively thermal tempering in BMGs.

Fracture mechanics analysis of thermally tempered glass plate: fracture induced by an embedded crack

This paper presents a fracture mechanics analysis of a thermally tempered glass plate. The fracture is induced by an embedded penny-shaped crack. The analysis shows that the existence of a penny-shaped crack will reduce the strength of tempered glass. The impact and fatigue resistance of the glass is related to the position and size of the penny-shaped crack. When the tempering intensity reached to a certain level, thermally tempered glass with a penny-shaped crack could experience spontaneous fracture. The damage of a central crack on glass is more severe than a surface crack. With surface compression, thermal tempering will increase the critical applied stress of the glass if the surface penny-shaped crack size is in the range of 0 < a/d < 0.27, where a is the crack size, d is the half thickness of the glass plate. For a small surface crack with the size of a/d ≤ 0.09, the tempering can hinder its extension. However, if there is a central penny-shaped crack, the critical applied stress of the tempered glass will decrease with the intensity of tempering.

Calcul des contraintes résiduelles dans les zones d'assemblage de plaques en verre trempé

Ce travail porte sur l'utilisation du verre comme véritable matériau de structure et concerne plus particulièrement le calcul des contraintes résiduelles de trempe autour des zones d'assemblage par insertion d'un connecteur métallique au sein de plaques trouées et chanfreinées pour la prédiction de la résistance de tels assemblages. Des simulations numériques par la méthode des éléments finis du procédé de trempe sont menées pour le calcul des contraintes transitoires et résiduelles près des bords de plaques de verre (calculs 2D) et près de trous chanfreinés (calculs 3D). Le verre est considéré comme un matériau viscoélastique, le modèle de Narayanaswamy est utilisé, il prend en compte les phénomènes de relaxation de contraintes d'origine visqueuse et structurale. Une difficulté provient de la description correcte des transferts de chaleur pendant la trempe qui est d'importance majeure pour la prédiction des contraintes résiduelles. La convection forcée due au soufflage de l'air et le rayonnement sont modélisés, le caractère semi-transparent du verre est pris en compte. Les coefficients de convection forcée dans les zones bords sont identifiés expérimentalement. Les résultats de prédiction des contraintes résiduelles de trempe sont comparés à des résultats de mesures photo-élastiques. Ce travail permet l'optimisation de la géométrie de la plaque de verre pour un renforcement maximal dans la zone d'assemblage.

Evaluation of Crack Growth Characteristics in Float Glass and Thermally Tempered Glass Based on the Crack Length Measurement Using Au Ion Sputtered Film

Thermal tempered glass is made by rapidly cooling after heating float glass to near the softening point. Compressive residual stresses at the surface layer of thermally tempered glass strengthen the material, and tensile residual stresses are also generated at the same time in the interior of the material to balance with the surface compressive residual stresses. It is considered that the fatigue crack growth characteristics of thermally tempered glass are remarkably different from that of float glass, due to the existence of those residual stresses. In this study, Au film made by ion sputtering method and an optical microscope are used to measure the crack length of the thermally tempered glass and float glass, and these fatigue crack growth characteristics are investigated. As a result, it is confirmed that the crack length measurement with an Au film has high accuracy of measurement. Furthermore, it is found that the higher stress intensity factor is needed to cause the crack growth in thermally tempered glas than in flat glass.

Procedure for residual stress profile determination by curvature measurements

A procedure is presented for the measurement of residual stress profile in regular geometry bodies such as plates and disks. The experimental work is based on the sole measurement of the sample curvature deriving from progressively etching one of its surfaces. A relationship is obtained to correlate curvature data as function of etching depth to the original residual stress profile. The sensitivity of the method and the influence of possible experimental errors are analyzed. The technique is then applied to strengthened glass samples produced by both thermal tempering and ion exchange and obtained results are compared to data obtained by available alternative technique to demonstrate the validity of the proposed approach.

Residual stresses in a bulk metallic glass cylinder induced by thermal tempering

Bulk metallic glasses are attractive structural materials that develop residual stresses during processing due to thermal tempering. In the present study, stress generation is analyzed experimentally and theoretically in a bulk metallic glass cylinder. By using a stainless steel tube as mold, thermal contact is assured throughout casting and quenching to better define the boundary conditions of the problem. The resulting residual stresses are measured using the crack compliance method. It is shown that high stresses can be attained in metallic glasses due to thermal tempering: about −300 MPa compression on the surface balanced by +150 MPa tension in the middle. The finite element method is then employed to deduce the convection heat transfer coefficient during quenching and to model the residual stress generation. The latter analysis is performed with a previously developed viscoelastic model. This model is shown to be accurate within 15–30% of the experimental stress data. It is therefore a powerful tool for estimating processing-induced residual stresses in bulk metallic glasses.

Load Bearing Capacity of Connections in Tempered Glass Structures

In the design of high load bearing capacity beams made of tempered flat glass, connections cannot be avoided when large-span or high-stiffness beams are considered. The aim of this paper is to contribute to a method to calculate the load bearing capacity that can help the design of dowel type joints in tempered glass structures in buildings. The technology presented is derived from the one used for hanging glass in facades. In a first step, residual stresses due to thermal tempering are modelled with the Finite Element Method. Narayanas-wamy’s model is used to describe the thermo-mechanical behaviour of glass. The different heat transfers are identified and modelled for a given tempering process. In a second step, the state of stresses due to the loading of the metallic connector is analysed. The numerical prediction of stresses is validated by means of experiments. The superposition of these two stress states is combined to get a design criterion for holed tempered glass plates loaded by dowel type joints.

Simulated Thermal Tempering versus Sudden Temperature Change and Short-Term Survival of Fingerling Rainbow Trout

We examined the effects of thermal shock on poststocking survival of fingerling rainbow trout Oncorhynchus mykiss (mean total length, 99 mm; mean weight, 10 g), some of which were thermally tempered at stocking and others not. In one experiment we assessed survival when fish experienced increases in water temperature from 8°C to 24°C at two tempering rates (4°C/h and 8°C/h) and by an immediate transfer. In a second experiment we assessed survival when water temperatures were cooled to 0.3°C for 2 h and fish were subsequently transferred into 8°C and 24°C water without tempering. Both experiments were conducted with controls at 8°C that were subjected to the same handling procedures without any changes in water temperature. Fingerling rainbow trout were found to be very tolerant of all treatments, with few or no mortalities during the treatments or during the 2 weeks following treatment. Our experiments suggest that rapid changes to high but sublethal temperatures will not directly cause high mortalities of fingerling rainbow trout stocked into waters for sport fisheries, but short-term behavior anomalies or other stressors may affect poststocking survival in the wild.

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.

Modeling and measurement of residual stresses in a bulk metallic glass plate

The recent advent of multi-component alloys with exceptional glass forming ability has allowed the processing of large metallic specimens with amorphous structure. The possibility of formation of thermal tempering stresses during the processing of these bulk metallic glass (BMG) specimens was investigated using two models: (i) instant freezing model, and (ii) viscoelastic model. The first one assumed a sudden transition between liquid and elastic solid at the glass transition temperature. The second model considered the equilibrium viscosity of BMG. Both models yielded similar results although from vastly different approaches. It was shown that convective cooling of Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 plates with high heat transfer coefficients could potentially generate significant compressive stresses on the surfaces balanced with mid-plane tension. The crack compliance (slitting) method was then employed to measure the stress profiles in a BMG plate that was cast in a copper mold. These profiles were roughly parabolic suggesting that thermal tempering was indeed the dominant residual stress generation mechanism. However, the magnitude of the measured stresses (with peak values of only about 1.5% of the yield strength) was significantly lower than the modeling predictions. Possible reasons for this discrepancy are described in relation to the actual casting process and material properties. The extremely low residual stresses measured in these BMG specimens, combined with their high strength and toughness, serve to further increase the advantages of BMGs over their crystalline metal counterparts.

Residual Stresses in Bulk Metallic Glasses – II: Measurement

New multi-component metallic alloys with exceptional glass forming ability have recently been developed at Caltech. These alloys allow the processing of large amorphous specimens. The possibility of formation of thermal-tempering-induced residual stresses during the processing of these bulk metallic glass (BMG) specimens was investigated. The crack compliance method was used to measure the stress profiles in a BMG plate that was cast in a copper mold. The measured profiles were roughly parabolic suggesting that thermal tempering was the dominant residual stress generation mechanism. However, the magnitude of the measured stresses was significantly lower than modeling predictions. Possible reasons for this discrepancy are presented in relation to the actual casting process and material properties.

Residual Stresses Near Holes in Tempered Glass Plates

This work presents numerical simulation results of the thermal tempering by the Finite Element Method in order to calculate transient and residual stresses near edges of a glass plate (2D calculation) and near holes (3D calculation). During thermal tempering, glass ιs considered as a viscoelastic material. The Narayanaswamy's model is used. It takes into account the structural relaxation phenomena. The particular difficulty is the correct modelling of heat transfers since transient and residual stresses strongly depend on the history of temperature within the plate, close to the edge and the hole. Both the forced convection due to the blowing of air and the radiative heat transfer are modelled numerically. The semi-transparency of glass in the near infrared range is considered. The convective heat transfer coefficients on the edge and hole walls are identified thanks to a specific experimental set-up and validated from simulations of heat transfer tests. The computed residual stresses are checked against photo-elastic measurements.

Residual Stresses in Bulk Metallic Glasses – I: Modeling

The recent advent of multi-component alloys with exceptional glass forming ability has allowed the processing of large metallic specimens with amorphous structure. The possibility of formation of thermal tempering stresses during the processing of these bulk metallic glass (BMG) specimens was first investigated using an instant freezing model. It was shown that convective cooling of Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 plates with high heat transfer coefficients could potentially generate significant compressive stresses balanced with mid-plane tension. The predictions of the instant freezing model were then compared to calculations that consider the viscoelastic properties of BMGs.

Effects of Missile Size and Glass Type on Impact Resistance of “Sacrificial Ply” Laminated Glass

Experiments were performed as part of a larger study to develop the “sacrificial ply” design concept for laminated architectural glass. This concept allows windborne debris impacts in severe windstorm environments to break the outer laminated architectural glass ply while the inner ply is preserved in order to carry the design wind pressure. Steel ball size and inner/outer glass ply type (level of thermal tempering) were varied to determine their effects on the impact resistance of the inner glass ply of laminated architectural glass when impacted on the outer glass ply. A mean minimum breakage velocity (MMBV) was determined for each variation in steel ball size and glass temper level, which defines the mean debris impact velocity on the outer glass ply that causes breakage in the inner glass ply. A 46% reduction in MMBV was observed for an increase in steel ball size from 2 g (7.9 mm diameter) to 8.4 g (12.7 mm diameter), and a 65% reduction in MMBV was observed for an increase in steel ball size from 2 g (7.9 mm diameter) to 28.2 g (19.1 mm diameter). Laminated architectural glass constructed with heat-strengthened or fully tempered inner glass plies, regardless of outer-glass-ply type, was found to have a significantly higher MMBV than laminated architectural glass constructed with annealed glass plies. In contrast, changing the outer glass ply from annealed to fully tempered glass was found to reduce the MMBV, regardless of the inner glass ply type. Relating these results to those in a previous impact study by Kaiser et al. suggests that the order of importance for design variables that most influence the inner glass ply impact resistance of sacrificial ply laminated architectural glass is the following, starting with the most important: (1) inner glass ply type; (2) inner glass ply thickness; (3) polyvinyl butyral (PVB) interlayer thickness; and (4) outer glass ply thickness.