Annealed Float Glass Research Articles

Non-destructive strength testing of microindented float glass by a nonlinear acoustic method

The present paper describes a method for non-destructive testing of the glass strength. Square 10 × 10 cm2 samples of annealed float glass was inflicted with a controlled defect in the centre of the atmospheric side using Vickers microindentation-induced cracking with a force of 2 N, 5 N and 10 N and compared to an un-indented reference. The samples were non-destructively tested using a nonlinear acoustic wave method resulting in defect values. The average of the defect values was found to linearly correlate to the indentation force in a log–log relationship. The samples were subsequently tested in a ring-on-ring setup that allows for an equibiaxial stress state. The indentation-induced cracking gave practically realistic strength values in the range of 45 to 110 MPa. The individual sample values for failure stress as a function of normalized defect value show linear trends with approximately half of the data within 95% confidence limit. In summary, this study provides an initial proof-of-concept for a non-destructive testing of the strength of glass.

Structural behaviour and micro-structural characteristics of coloured kilned glass panels

In recent years, the architecture, engineering and construction industry is increasingly embracing digital design approaches and robotic production methods. This is as well noticeable in the façade design sector, especially when it comes to architectural design tasks dealing with complex geometries or adaptive components. Although glass is mainly used in facades due to its transparency, it often determines significantly the aesthetic appearance of a building and has to control the transmitted light and the visibility between exterior and interior. Recently, a novel method for producing polychromatic glass panels by fusing granular glass powder of different colours onto the surface of annealed float glass in a kilning process was developed at ETH Zurich. By using a multi-channel tool attached to a robotic arm for placing the granular powder onto the glass surface, digitally designed patterns can be realised precisely and repetitively. This paper focuses on the structural behaviour and the micro-structural characteristics of this novel type of coloured glass panels for façade applications. The bending strength of such glass panels was determined based on four-point-bending tests and was compared to that of annealed float glass. A non-negligible decrease in the bending strength was observed. Furthermore, the reasons for this reduction in structural performance were analysed based on microscopic investigations. It was observed that both the kilning process and the density of deposited granular powder had an influence on the surface microstructure. The presented results are essential for the production of polychromatic kilned glass as well as for future applications of this type of glass panel in façades.

Mechanical performance of polyhedral hollow glass units under compression

This research presents an experimental program and a numerical analysis executed to understand the strength and stiffness properties of polyhedral hollow glass units (HGU) that are intended for use in modular construction of all-glass, compression-dominant, shell type structures. The proposed compression-dominant geometric form has been developed using the form finding methods of 3D/polyhedral graphical statics. This research takes the first steps towards a new construction methodology for glass structures by exploiting the high compression strength of glass. The test matrix includes four HGUs, two each fabricated with 1 mm and 2 mm thick adhesive tape where the glass plates are all 10 mm thick annealed float glass cut using a 5-axis abrasive waterjet. Testing was done with the HGU oriented such that load was introduced on the short side edges of the two deck plates, resulting in an asymmetric load-support condition. All samples failed explosively by flexural buckling. Strain and deformation data clearly show the presence of second order behavior resulting from bending deformation. In general, linear axial behavior transitions to nonlinear second order behavior, with increasing rates in deflection and strain growth ultimately ending in glass fracture on the tension surfaces of the buckled deck plates. Failure resulted in complete disintegration of the deck plates, but with no observable cracking in any of the side plates and a secure connection on all adhesive tape. A companion finite element analysis was performed to validate the experimental results of this study. The results of the experimental and numerical programs clearly demonstrate the feasibility of using HGUs for modular construction of compression-dominant, all-glass shell type structures.

An experimental study on the structural behaviour of laminated glass members under combined axial compression and in-plane bending

Laminated glass (LG) is increasingly being used as load carrying components in modern buildings thanks to its relatively good safety performance and aesthetic benefits. This paper presents an experimental study into the applicational scenario of structural LG beam-columns subjected to combined axial compression and in-plane bending, such as LG fins supporting glass facade. The investigated LG specimens consisted of annealed float glass plies bonded together by SentryGlas interlayer. A total of 17 tests were carried out, from which the structural behaviour in terms of load carrying capacity, failure mode, post-breakage strength and deformability of the LG beam-columns were investigated in detail. Through the tests, the failure modes and corresponding mechanisms were identified; the influence of axial compression, laminate number and slenderness ratio on the structural behaviour were assessed; and the major stress components resulting in fracture of glass were analyzed. Based on the test and analytical results, a design N–M interaction domain was proposed to facilitate strength checking of LG beam-columns, and recommendations were made to improve the post-breakage performance. The results obtained are expected to provide supplementary information that is currently lacking in existing literature.

Probabilistic considerations about the strength of laminated annealed float glass

Laminated structures are parallel (or redundant) systems, i.e. failure occurs when all the elements (glass plies) reach, in cascade, the ultimate limit state. Following the failure mode approach, the reliability analysis is consequent to the identification of all possible rupture modes of the glass plies, where each mode is identified by the sequence of collapse, synthetically schematized as an event-tree. The event “structural failure” is the union of all the possible failure modes. The static theorem of limit analysis guarantees that the more the structure is divided into load bearing elements acting in parallel, the safer it is, but this conclusion holds only for ideal ductile systems. For brittle glass it is often assumed that lamination gives a beneficial contribution in all cases, but glass strength is affected by a size effect in terms of area, because surface micro-cracks govern the overall capacity of the material. Taking this into account, we show through the failure mode approach, under some simplifying assumptions, that lamination can decrease the strength of a plate made of annealed glass, since the higher the number of plies is, the larger is the surface area under tensile stress. This finding focalizes the attention on the importance of an accurate characterization of the size effect in glass strength.

Cut edge of annealed float glass: crack system and possibilities to increase the edge strength by adjusting the cutting process

The edge strength of annealed float glass is an essential aspect in engineering design. A common problem in engineering applications is the failure of the edge due to thermally induced stresses. The edge strength of annealed glass is affected by cutting, further processing and handling. Thermally treated glass exhibits higher edge strengths, but may show optically negative effects such as anisotropy and roller waves. In case of covered edges, e.g. insulating glass covered by framing, an enhanced strength of the cut edge is desirable. This paper presents results showing that enhanced strength of the cut edge is feasible and reproducible using regular cutting technology. A large part of this contribution deals with the crack system resulting from the cutting process. A deep understanding of this is essential for the relationship between cutting parameters, damage and edge strength. In addition to optical microscopic analyses, confocal microscopy and indentation tests have been carried out. The influence of different cutting process parameters on the edge strength was investigated within the scope of extensive analyses by the Fachverband Konstruktiver Glasbau e.V. This allowed to adjust selected process parameters so that the edge strength could be reproducibly enhanced. In this contribution the latest results of these investigations are presented. In addition, the correlation between microscopic optical characteristics and the mechanical strength as expressed by the macroscopic fracture stress is described.

Fracture and fragmentation of blast-loaded laminated glass: An experimental and numerical study

In this study, we use the explicit finite element method in combination with higher order elements and 3D node splitting to simulate fracture and fragmentation of blast-loaded laminated glass. Node splitting is a modelling technique where elements are separated instead of being eroded when a fracture criterion is reached. The resulting FE simulations are thus capable of describing behaviours such as fragmentation without loss of mass or momentum, fine cracking of the glass plates, and delamination and separation between the glass and the polymer interlayer. The simulations are compared to blast experiments conducted in a shock tube. In total, 15 laminated glass specimens (consisting of annealed float glass plates and PVB) were tested at five different pressure levels. The time and position of fracture initiation in the glass plates varied, which in turn resulted in varying post-fracture behaviour within the different pressure levels. The simulations were in good agreement with the blast tests, revealing the potential of the selected numerical method. Additional simulations of monolithic (i.e., non-laminated) glass plates were conducted and compared to experiments that were presented in an earlier study. Again, these simulations displayed a highly comparable response to the experiments, and were able to describe crack branching, formation of large glass splinters and free-flying fragments.

On the Dynamic Response of Laminated Glass Exposed to Impact Before Blast Loading

In this study, the effect of fragment or bullet impact before blast loading on laminated glass is studied experimentally. First, laminated windows consisting of two 3.8 mm thick annealed float glass plates and a 1.52 mm thick PVB interlayer were blast loaded in a shock tube with various pressures as a reference. In these tests, the blast loading was successively increased until fracture occurred not only in the glass plates but also in the PVB interlayer. Second, a diamond drill was used to make a 5 mm diameter centrally placed hole in some windows before they were blast loaded with the same pressures as those used for the undamaged windows. Third, windows were impacted by 7.62 mm AP bullets, both with and without the brass jacket, before they were blast loaded. Such bullets may have similar mass and velocity to typical primary fragments from an explosive detonation. The results are finally compared with each other and discussed with respect to the blast protection offered. It is found that the capacity is significantly reduced if the laminated glass is perforated by a fragment or a bullet before it is blast loaded and that such impacts should be considered in the design of blast-resistant windows.

Simultaneous chemical vapor deposition and thermal strengthening of glass

In the current paper we present a concept combining metal organic chemical vapor deposition with thermal strengthening process of flat glass. As the flat glass is heated to be thermally strengthened, which takes up to 20 minutes, there is an opportunity for performing a surface modification. We describe the application of transparent and amorphous Al2O3 thin films during the thermal strengthening process. Al2O3 was chosen due to the following desirable properties: increased surface mechanical properties and increased chemical durability, the latter has not been investigated in the current paper. The residual surface compressive stresses after performed strengthening of the coated glasses were quantified to be in the range of 80–110 MPa. The Al2O3 content in the surface was measured using the Surface Ablation Cell employed with Inductively Coupled Plasma Atomic Emission Spectroscopy and found to be at least doubled at the surface and having an increased Al2O3 content at least 0.5 μm underneath the glass surface. During the surface reaction, sodium is migrating to the surface giving a hazy salt layer on the glass which can easily be washed off with water. The applied coatings are transparent and provide increased surface hardness and crack resistance at low indentation loads. At higher indentation loads the interaction volume is larger and displays the same effect on the surface mechanical properties as for thermally strengthened glass. The contact angle with water compared to annealed float glass is significantly increased from 5° to 45° due to the different surface chemistry and surface topography.

Testing and modelling of annealed float glass under quasi-static and dynamic loading

In recent years, a considerable number of studies has been carried out to analyse the behaviour of laminated glass plates under blast loading by the use of the finite element method. This has proven to be quite challenging, as the response of the laminated glass is complex. The fracture strength of the glass layers govern much of the total response; however, a limited effort is often made to selecting this value in the analyses. The current work aims to identify the probabilistic fracture strength of the glass alone as a function of its geometry, boundary conditions and loading situation by the use of a newly proposed strength prediction model. It should be noted that the current study focuses on the initiation of fracture in glass plates, and no effort has been put into the description of crack propagation. To facilitate the validation of the model, three different experimental test series were carried out on annealed float glass. This included quasi-static four point bending tests on relatively small glass specimens, and quasi-static and blast pressure tests on larger glass plates. The experimental work demonstrated that the fracture strength of glass exhibits a large scatter within the same test setup. It also revealed that the fracture strength and its scatter were dependent on the geometry, and the boundary and loading conditions. The strength prediction model was able to successfully capture many of the trends observed in the quasi-static tests. Regarding the blast tests, the model was able to reproduce the experimental results reasonably well.

Shock Tube Testing and Modelling of Annealed Float Glass

Failure of glass is dominantly brittle, and is caused by microscopic flaws randomly distributed on the surface. Fracture mainly initiates in these flaws, and this leads to a high variability in the glass strength, which depends on geometry, boundary conditions and loading situation. Consequently, the identification of the fracture strength, in e.g. finite element analyses, is not straightforward. For rapid loading conditions, as for blast loading situations, the glass strength is generally increased because flaws need time to grow into cracks. The current study aims to identify the probabilistic fracture strength of glass plates under blast loading as a function of the plate?s boundary conditions, geometry and loading by using a newly proposed strength prediction model. To validate this model in some measure, 12 blast tests on annealed float glass were performed in a shock tube. As expected, the tests showed a large scatter in fracture strength. The strength prediction model captured the main trends found in the experimental tests, but a closer investigation of the strain rate sensitivity of glass was deemed necessary. Finally, the results from the strength prediction model were used as input in a simulation of annealed float glass under blast pressure in the finite element program IMPETUS Afea Solver. By use of a node splitting technique, the simulations captured the behaviour displayed in the experimental tests to a great extent.

Performance of Standard Statistical Distributions for Modeling Glass Fracture

Experimental data on the strength of new annealed float glass tested in an ambient environment was collected. A comparison was made between four standard distributions, the normal, lognormal, Gumbel and Weibull, with respect to the performance in modelling the strength. The Weibull distribution outperformed the normal and lognormal distributions when the data contained edge only failure origins. When the data was selected to contain surface only failure origins it is indicated that the extreme value distributions performed poorly. The Weibull model is known to have a basis in a failure-mechanism concept based on the weakest-link principle. The Gumbel distribution can also be derived from failure-based mechanics and be associated with certain types of flaw size distribution. The Weibull model, however, is a better choice for a failure model of glass edge strength compared to the normal and lognormal distributions and at least as good as a Gumbel distribution. The surface strength is complicated to model and none of the standard distributions which were examined are capable of producing a proper model. The sample size also has a profound impact on the performance of the surface strength models.

Performance of statically indeterminate reinforced glass beams – Experimental comparison with determinate systems and effect of a discontinuous glass section

A well-performing concept is the reinforced glass beam, developed in analogy to reinforced concrete. This concept proved its feasibility both in statically determinate and indeterminate systems, for a variety of parameters. However, the advantages of indeterminacy over determinacy in terms of structural performance and safety are not evaluated for this concept. This paper presents experimental results of similar statically determinate and indeterminate bending tests (i.e. with the same free span length and load point positions) on stainless steel reinforced laminated glass beams, made of annealed float glass (ANG). The interlayer is used to bond the reinforcement. With these results, a comparison is made to quantify the structural efficiency of the latter system, at two test temperatures and for two reinforcement percentages. In addition, statically indeterminate bending test results on ‘discontinuous’ glass beams are presented and compared to those of normal beams to evaluate the effect of local reduced beam stiffness on the system’s load-carrying behaviour. The first investigation led to the conclusion that a 25% increase in ultimate capacity can be reached when applying a statically indeterminate system, provided that the reinforcement-to-glass bond is thoroughly designed for the specific conditions (enabling the beam to form all necessary plastic hinges). Furthermore, when the latter condition is fulfilled, a locally reduced beam stiffness has a limited effect on the overall load-carrying behaviour of the statically indeterminate beam system.

Failure behavior of annealed glass for building windows

The failure strength of annealed float glass is difficult to predict accurately due to several factors that influence the glass’ mechanical behavior, including stress rate, environmental conditions, fabrication process, glass thickness and the presence of surface flaws. Some of these parameters strongly depend on the construction practices of each country. This study aimed to assess the failure strength of annealed glass commonly used in the construction industry in Mexico, based on experimental tests. It is also determined experimental data dispersion as function of the glass thickness. The approach employs destructive tests on annealed glass plates subjected to out-of-plane loading in a coaxial double ring (CDR) test setup and analytical linear and a geometric non-linear finite element analyses. The mechanical behavior determined by the numerical model agrees with the failure strength and deflection obtained experimentally. The results show the mean failure strength of glass specimens with several thicknesses typically used in Mexico, and the analysis of data dispersion is also presented. The statistical analyses of glass failure strengths with thickness range 3–19mm indicate that all samples cannot be considered coming from the same population and data dispersion depended on glass thickness. Finally, the use of the experimental results for evaluating mean failure strength of glass panels and a comparison with expected wind-induced pressure in buildings located in two sites in Mexico is exposed.

The statistical interpretation of the strength of float glass for structural applications

The characteristic value of the strength of annealed float glass, to be used in structural calculations, is assessed by standards on the basis of a classical experimental campaign using the Coaxial Double Ring (CDR) test with additional overpressure. Experimental data were regressed according to the 2-parameter Weibull distribution, assuming that the induced state of stress is equibiaxial in practice. We show that, by splitting the data in two categories according to the surface under tensile stress (either the “tin” or the “air” side), a more accurate statistical interpretation can be obtained. Comparisons with the normal and log-normal distributions are made with the chi-square goodness-of-fit test. Moreover, we observe that the calibration curve suggested by the test standard is not precise, and that the stress state in the testing configuration is not equibiaxial. Therefore the rough data need to be further corrected and re-scaled to a common reference condition, according to a criterion of equal failure probability, by determining the effective area of the loaded specimen. Doing so, the considered statical distribution are able to fit to the data with much more accuracy. Considering that for very low failure-probabilities the tails of the statistics are the most important, and observing that the 2-parameters Weibull function fails to interpret a lower bound for the strength of glass that can be inferred from experiments, the use of a 3-parameter Weibull distribution is proposed. After having derived the corresponding expressions for the effective area, we present a new statistical characterization of glass strength that provides the best fit with the experimental data, at least for the air-side.

Experimental testing of load-bearing timber–glass composite shear walls and beams

The paper presents results from the experimental testing of load-bearing timber–glass composite shear walls and beams.Shear wall specimens measuring 1200 × 2400 mm2 manufactured with three adhesives of varying stiffness were tested. Twelve specimens with a single 10 mm thick glass pane and one specimen with an additional insulating glass unit were produced. The testing procedures involved various loading conditions: pure vertical load and different combinations of shear and vertical loading. The test results showed that the adhesive had only a minor influence on the buckling load which was the main failure mechanism.240 mm high and 4800 mm long timber–glass beams manufactured with adhesives of different stiffness were tested. For the webs, two types of glass were used: annealed float and heat-strengthened glass, in both cases 8 mm thick panes were used. In total, 12 beams were tested in four-point bending until failure. Despite the considerable difference in adhesive stiffness, beam bending stiffness was similar. Concerning load-bearing capacity, the beams with heat-strengthened glass were approximately 50% stronger than the beams made using annealed float glass.

Dynamic material model of annealed soda-lime glass

Glass is an omnipresent material which is widely used as façade in buildings. Damage of glass windows and the associated glass fragments induced by impact and blast loads impose great threats to people in the vicinity. Much effort has been directed towards understanding glass material properties, and modeling of glass window responses to impact and blast loads. For reliable predictions of glass structure performances under dynamic loadings, an accurate dynamic constitutive model of annealed float glass, which is commonly used for glass windows, is therefore needed. In current practice, the Johnson-Holmquist Ceramic (JH2) model is most commonly used in simulating glass plate responses to impact and blast loads. In this study, the accuracy of the JH2 model in modeling annealed float glass material, especially at high strain rate is examined in detail. Static compressive tests and dynamic compressive tests using Split Hopkinson Pressure Bar (SHPB) are carried out on soda-lime glass specimens sampled from commercially used annealed float glass panes. These testing results are used together with the authors' previous testing data and data reported by other researchers in the literature to determine the constitutive constants for the JH2 model, including Equation of State (EOS), strength criterion and strain-rate effect. The JH2 model with new material constants is then programmed in commercial code LS-DYNA. To verify the model, it is used to simulate a SHPB compressive test on a 15 mm by 15 mm (diameter by length) glass specimen, a field blasting test on a laminated glass window of 1.5 m by 1.2 m in dimension, and a full-scale laboratory windborne debris impact test on a laminated glass window. The simulation results demonstrate that the JH2 model with the new material constants for annealed glass gives good predictions of glass material and glass window responses to impact and blast loads.

Glass, timber and adhesive joints – Innovative load bearing building components

Structural glass–timber composite beams and shear wall elements were investigated in terms of their mechanical behaviour, energy performance and their LCA performance. The load bearing components were manufactured using annealed float glass which was adhesively bonded to the timber with different adhesives. The results show, among other things, that is is possible to join the two materials glass and timber and obtaining a non-brittle failure of the beams. The shear wall elements have the potential of being used as stabilising elements and load bearing walls in buildings of up to 4 storeys height. It is possible to combine glass and timber in a load bearing shear wall without loss of energy performance of a building or without loosing LCA performance. In addition to these benefits, the timber glass composite wall has, of course the benefit of being transparent.

Unusual Variation of Thickness Within a Pane of Annealed Float Glass

The thickness of a 183 cm x 122 cm pane of annealed, float glass of 6 mm nominal thickness was examined. A thickness variation of 0.13 mm over a distance of approximately 30 cm and an overall variation of 0.14 mm within the entire pane were noted. These findings support the exclusion criterion of greater than 0.15 mm for float glass thickness comparisons, regardless of the dimensions of the pane.

Laboratory Test on Dynamic Material Properties of Annealed Float Glass

In this study, laboratory tests were conducted to investigate the dynamic material properties of annealed float glass, which is widely used in building applications. The influence of strain rate effect on glass strength and Young's modulus is studied. Quasi-static tests were performed first to determine the glass static strength and Young's modulus; then dynamic compressive tests were carried out at the strain rates from 98/s to 376/s using a modified Split Hopkinson Pressure Bar. Tensile tests were performed in the strain rate range of 35/s to 990/s through splitting tensile test (Brazilian test). Test results reveal that the compressive and tensile strengths of annealed glass are very sensitive to strain rate. Dynamic increment on glass compressive strength is found more significant than its tensile strength, a phenomenon different from other brittle materials such as concrete. The Young's modulus is found relatively insensitive to strain rate in the testing range, and is slightly larger in compressive tests than in tensile tests. Based on the test data compressive and tensile dynamic increment factors (DIF) of annealed glass with respect to strain rate are formulated. The glass fracture process is also investigated in this paper based on the images taken by high-speed camera during the tests. The fracture images and glass fragments are discussed and used to explain the testing results.