ETH Zürich Insitut für Baustoffe

Glass and natural stone in construction

Post-breakage behavior of laminated

Manuel Zimmermann

December 18, 2015 Supervisors: Dr. F. K. Wittel and Dr. T. Wangler

Contents

1 Introduction 2

2 Laminated safety glass 2 2.1 Glas types ...... 3 2.2 Interlayer materials ...... 3 2.3 Assessment (Testing) ...... 3

3 Post breakage behavior of LSG 4 3.1 Failure of LSG ...... 6

4 Conclusion 6

References 7

1 1 Introduction Manuel Zimmermann

1 Introduction

Laminated glass was first introduced in the automobile sector and means basically as- semblies of two or more flat glass sheets with a polymer interlayer. The technological developments specially in terms of flatness of glass made it also applicable for architectural purpose. products conquered the structural building and call for safety glazing units. This application as so called structural glazing call for a safe design concept which engineers are used from other materials. With the technological development of polymer materials, a variety of glass products with different safety features where introduced. The difficulty is that laminated glass properties do not only depend on the used materials but also on temperature, rate of loading and support type.

Nowadays the newly build famous buildings couldn’t be possible without Laminated Safety Glass (LSG). Think of the Prime Tower in Zürich, the Skywalk in Longgang and many others. Glass is also widely used in overhead and flooring applications which call for a clear estimation of residual load-bearing capacity. For latter one it does not seem to be enough that the glass unit can carry its own dead load but also some supplementary load for a certain time.

In this work the term laminated (safety) glass is explained and the basic assessment concepts are discussed. Starting from the different material properties of the used glass types and polymer interlayers. In a second step the performance and safety of broken laminated glass is explained and the most resent approaches for calculation and design are shown.

2 Laminated safety glass

Laminated glass was needed with the technological which improved the quality and flatness of flat glass. It consists of at least 2 glass sheets with organic interlayer. The term laminated safety glass refers to glass made from float glass or toughened glass where in case of breakage the interlayer serves to retain the glass fragments and offers residual resistance. The thickness of the glass layers usually vary between 4 to 6mm. The interlayer has a thickness of 0, 38mm and its multiples. The products are covered by a European harmonized standard released in 2005 by the CEN. With this configuration the harder glass protects the interlayer which leads to a product with interesting features. Namely the familiar transparency of glass combined with the ductile behavior expected from building materials.

Before breakage LSG shows load bearing behavior like a laminate. The main con- tribution of the interlayer is the shear transfer between the glass layers. This can be described by a sandwich model according to Kott [1] where the interlayer has linear elastic properties and the deformations are small. The glass plies are then working as a beam, with tension on one and compression on the other side as seen in figure 3 stage I. For

2 2 Laminated safety glass Manuel Zimmermann the global stiffness of the laminate, the thickness of the interlayer and its shear stiffness are important. The tensile strength of the glass is the determining factor for strength. For design purposes the glass sheets are assumed to carry all the load and the interlayers contribution is usually neglected.

Breakage of toughened glass layer occurs when a crack reaches the tension zone in- side the glass. In therms of floor glazing this can happen either from bending, where the tension on the bottom of the beam let the tension zone grow toward the edge where it eventually meets surface cracks. Due to vandalism or also accidentally a hard body impact could introduce a crack on the top layer that reaches the tension zone. This are also the manly used assessment methods for LSG testing as seen in section 2.3.

2.1 Glas types For LSG application annealed, heat strengthened and is used. Annealed glass is not pre-stressed, has therefore a low bending capacity and forms very big and sharp pieces when ist breaks. This does not make it very suitable for structural glazing applications. Instead pre-stressed glass which shows initially a better residual stress state and thus to higher bending capacity is used. The initial stress state is provided by heat treatment and cooling of the glass surface. The temperature gradient causes compression stresses at the surface and tension in the core, which makes the glass less sensitive to surface flaws. Further one can distinguish between heat strengthened and tempered glass. First one shows larger fragments as tempered glass because its moderate cooling rate and residual stresses. The bending strength of heat strengthened glass is about 70MPa and its pre-stressed to 50MPa in compression. This is lower as the bending strength of tempered glass which is around 120MPa. Tempered glass breaks into very small fragments because of its high elastic energy due to pre-stressing up to 120MPa in compression.

2.2 Interlayer materials The mainly used interlayer for LSG are (PVB) and Ionomers (SG) further Ethylene vinyl acetate (EVA) and Thermoplastic polyurethanes (TPU) are used [4]. In the past mainly PVB was used, which offers good binding between the glass panels, optical clarity, thoroughness and flexibility. But is very soft compared to SG as seen in figure 1. They both show temperature and time dependent viscoelastic properties. The testing of the polymers is mostly done by uniaxial tensile test with a dog bone specimen. The typical results obtained are shown in figure 1.

2.3 Assessment (Testing) Due to the lack of predictive models the assessment of LSG and its residual load carrying capacity is generally provided by experiments. Where the individual support conditions have to be taken into account [3]. Kott and Delincè[1, 2] used the pendulum as well as the hard body drop test (Fig 2). In both cases, breakage of the constitutive glass sheets is allowed, but not supposed to lead to an overall failure or collapse of the tested

3 A quantitative comparison of these different results is not straightforward, because of the different geometries of the test specimens. Whereas of test results within a series (Figure III.16 to Figure III.18) seems within reasonable limits, comparison of loading curves corresponding to in principle identical tests shows a noticeable difference in yield stress, with a still more noticeable difference between the post-yield part of the curves (between Figure III.17 and Figure III.18, tests on SG-specimens at 100 mm/min; difference of about a factor 1.3 between measured tensile strength). These differences can be explained by different experimental issues or differences in samples.

3 Post breakage behavior of LSG Manuel Zimmermann

Figure 1:FigureTypical III.12 stress-strain – Typical nominal curves stress-strain for uniaxial curve tensiles obtained test from for conventional specimens ofuniaxial PVB-films tensile tests on dog-bone specimens cut out of PVB-films and SG-sheets. and SG-sheets taken from [2]. Above pictures show the deformation pattern along the loading curve for SG-specimen.

element, at least not within specified time. This tests only provide a pass of fail criteria, no quantitative value of post-fracture resistance for design purposes. The assessment is relatively complex because of its multi-scale characteristics and its in mechanical properties completely different components. Furthermore one type of safety performance does not provide characteristic values usable for design. The assessment does not evaluate the contribution of the individual components of laminated glass. For non vertical glazing 134 Chapter III units with other supporting conditions and loads the testing may has to be different. Questions in terms of staying in place of the element as well as to quantify the performance of a broken glass unit have to be taken into account.

3 Post breakage behavior of LSG

The post breakage of LSG defines the state when at least one glass sheer is broken and the glass pieces are still bonded to the interlayer [3]. In most cases this happens under bending. According to Kott [1] one can distinguish between three different stages where in Stage I both glass layers are intact. In Stage II one and in Stage III both of the glass layers are broken (Fig. 3). Specially with the use of floor glazing the load-bearing capacity of fractured elements is getting much more important. Specially time-temperature depen- dency as well as the mechanical behavior of fractured units (they may have to perform for a longer time, as it was known from overhead and façade applications). The fragmentation response and post-breakage behavior of LSG seem more to be an element property than a product or material one. Furthermore, impact resistance can not be defined at the level of cross-section, but as a combination of element and loading configurations. And to conclude the critical failure mode can change due to damage progression [2].

In Stage II where only one glass sheet is broken usually does not occur when a healthy glass sample is loaded because of the chain reaction caused by the reduction in cross-section. But it is encountered if a hard body impacts and causes damage. Which makes this stage very important for residual strength of structural glazing applications. The failure stress level is approximately 60% of that in stage I. Because the cross-section is less than 60% this means that the interlayer is able to transfer stresses between the both pieces of

4 Verbundsicherheitsglas Verbundsicherheitsglas

Bei betretbarer Überkopfverglasung darf die Glasscheibe nur zu Wartungs- und Reini- Bei betretbarer Überkopfverglasung darf die Glasscheibe nur zu Wartungs- und Reini- gungszwecken betreten werden, deshalb muss der Anprall von Personen nicht berück- gungszwecken betreten werden, deshalb muss der Anprall von Personen nicht berück- sichtigtsichtigt werden. werden. a)a) b) b)

Verbundsicherheitsglas Bild 2.33Bild Pendelschlagversuch 2.33 Pendelschlagversuch für die Simulation für die Simulation des weichen des Stosses weichen [N 23];Stosses (a) [NVersuchsaufbau 23]; (a) Versuchsaufbau für für den weichenden Stoss,weichen (b) Stoss,gebrochene (b) gebrochene VSG-Sche ibeVSG-Sche nach nichtibe nachbestandener nicht bestandener Pendelschlagprü- Pendelschlagprü- fung. fung. Bei betretbarer Überkopfverglasung darf die Glasscheibe nurVerbundsicherheitsglas zu Wartungs- und Reini- gungszweckenHarter HarterStoss betreten Stoss werden, deshalb muss der Anprall von Personen nicht berück- Bei betretbarer Überkopfverglasung darf die Glasscheibe nur zu Wartungs- und Reini- 3 Post breakage behavior of LSG Manuel Zimmermann sichtigtSowohlgungszwecken werden. bei betretbaren betreten als werden, auch begehbardeshalb mussen Verglasungen der Anprall von ist Pers mitonen einem nicht so berück- genannten Sowohl bei betretbaren als auch begehbaren Verglasungen ist mit einem so genannten hartensichtigt Stoss werden. zu rechnen. Man spricht vom harten Stoss, wenn die gesamte aufgebrachte a)harten Stoss zu rechnen. Man spricht vom harten Stoss, wenn b) die gesamte aufgebrachte Energiea) beim Stoss durch die VSG-Scheibe aufgenommen b) wird. Die Einwirkungen, Energie beim Stoss durch die VSG-Scheibe aufgenommen wird. Die Einwirkungen, meistens hervorgerufen durch Vandalismus, entstehen durch Bewurf der VSG-Scheiben meistens hervorgerufen durch Vandalismus, entstehen durch Bewurf der VSG-Scheiben mit harten Gegenständen oder durch direktes Anschlagen der Glasoberflächen. Der mit harten Gegenständen oder durch direktes Anschlagen der Glasoberflächen. Der Nachweis der Tragfähigkeit unter einem harten Stoss wird mit einem zylindrischen FallkörperNachweis der Masse der Tragfähigkeit40 kg [F 42] oder unt erdurch einem eine harten Stahlkugel Stoss dewirrd Masse mit einem 4.1 kg zylindrischen er- bracht. FallkörperDie Fallversuche der Masse könn en40 je kg nach [F 42]Anforderung oder durch ve einerändert Stahlkugel werden. Der der VersuchMasse 4.1 kg er- gilt als bracht.bestanden, Die wenn Fallversuche der Stosskörper können di jee Glasscheibenach Anforderung nicht vol velständigrändert durchstossen werden. Der Versuch hat undgilt die alsVerglasung bestanden, nicht wenn von der der StosskörperAuflagerung di rutschte Glasscheibe sowie keine nicht Bruchstücke vollständig her- durchstossen abfallen,hat die und grösser die Verglasung sind als in nichtDIN 1249-12 von der angegebenAuflagerung [N rutscht 4]. sowie keine Bruchstücke her- Bild abfallen,2.33 Pendelschlagversuch die grösser sind für die als Simulation in DIN des 1249-12 weichen Stosses angegeben [N 23]; (a)[N Versuchsaufbau 4]. für a)den weichen b) Stoss, (b) gebrochene VSG-Scheibe nach c) nicht bestandener Pendelschlagprü- fung. Bild 2.33a) Pendelschlagversuch für b) die Simulation des weichen c)Stosses [N 23]; (a) Versuchsaufbau für 40 kg 4.1 kg Harterden Stossweichen Stoss, (b) gebrochene VSG-Scheibe nach nicht bestandener Pendelschlagprü- 40 kg 4.1 kg fung.

Sowohlh bei betretbaren als auchh begehbaren Verglasungen ist mit einem so genannten harten Stoss zu rechnen. Man spricht vom harten Stoss, wenn die gesamte aufgebrachte h h HarterEnergie Stoss beim Stoss durch die VSG-Scheibe aufgenommen wird. Die Einwirkungen, meistens hervorgerufen durch Vandalismus, entstehen durch Bewurf der VSG-Scheiben Sowohlmit bei harten betretbaren Gegenständen als oder auch durch begehbar direktesen Anschlagen Verglasungen der Glasoberflächen. ist mit einem Der so genannten harten NachweisStoss zu der rechnen. Tragfähigkeit Man unt sprichter einem vom harten harten Stoss Stoss,wird mit wenn einem die zylindrischen gesamte aufgebrachte Fallkörper der Masse 40 kg [F 42] oder durch eine Stahlkugel der Masse 4.1 kg er- EnergieBild 2.34 beim Stossversuche Stoss durch für die Simulationdie VSG-Scheib des harten Stosses;e aufgenommen (a) mit zylindrischem wird. Fallkörper Die Einwirkungen,(b) mit bracht. Stahlkugel,Die FallversucheFigure (c) 2:durchPendulum könn Stahlkugelen je test nachgebrochene (top) Anforderung andVSG-Scheibe. hard verändert body dropwerden. test Der (bottom) Versuch for the assessment of meistensgilt Bhervorgerufenalsild bestanden, 2.34 Stossversuche wenn durchlaminated der Stosskörper für Vandalismus, die glass Simulation di productse Glasscheibe des entstehen harten modified nichtStosses; durchvol from (a)lständig [1].mit Bewurf zylindrischem durchstossen der VSG-ScheibenFallkörper (b) mit Stahlkugel, (c) durch Stahlkugel gebrochene VSG-Scheibe. mit hartenhat und Gegenständen1868 die Verglasung nicht oder von derdurch AuflagerJ. Belisdirekt etung al. / esrutscht Anschlagen Failuresowie Analysis keine 16 Bruchstückeder (2009) 1866–1875Glasoberflächen. her- Der Nachweisabfallen, der die Tragfähigkeit grösser sind als in unt DINer 1249-12 einem angegeben harten [NStoss 4]. wird mit einem zylindrischen C C C Fallkörper der Masse 40 kg [F 42] oderglass durch eine Stahlkugel der Masse 4.1 kg er- a) b)T c) T 49 bracht. Die Fallversuche können je nach interlayerAnforderung verändert werden. Der Versuch 40 kg C 4.1 kg C glass 49 gilt als bestanden, wenn derT Stosskörper die GlasscheibeT nicht vollständig durchstossen hat und die Verglasung nicht von der Auflagerung rutscht sowie keine Bruchstücke her- h Stah ge I Stage II Stage III abfallen, dieFig. grösser 2. Three stages sind in the als failure in process DIN of a1249-12 laminated plate angegeben composed of two glass[N sheets 4]. and one interlayer (in the example shown, the upper sheet broke first, e.g., dueFigure to a hard 3: body The impact). three stages of laminated glass during failure taken from [5].

a) b) c) reference marks

Bild 2.34 Stossversuchebroken40 kg glass für die as Simulation shown12.5 des in ± 1 [5].harten4.1 Stosses; kg (a) mit zylindrischem Fallkörper (b)4 ± mi 0.1t Stahlkugel, (c) durch Stahlkugel gebrochene VSG-Scheibe. Stage III is very complex, because many20 ± 0.5 influencing factors mainly the size of the broken glass pieces, the crack pattern and75 local delamination effect. The residual load h carrying capacity getsh much lower and the deformation increases as seen in figure 4. The parametersFig. 3. Basic shape that and are main influencing sizes of SGP samples thepost-breakage used for uniaxial tensile behavior tests (mm) are,(for more the details, mechanical see EN ISO 572-2 properties[11]). 49

Table 2 Overview of laminated glass test samples

Name Mean height Glass thickness, a Glass thickness, a Mean interlayer Failure stages (mm) (front plate) (mm) (back plate)5 (mm) thickness, t (mm) studied (–) 120_1 120.78 5.95 5.97 1.71 I and III Bild 2.34 Stossversuche120_2 120.79 für die Simulation5.97 des harten Stosses;5.94 (a) mit zylindrischem1.70 Fallkörper (b) mit 120_3 120.32 5.96 5.95 1.64 Stahlkugel,120_4 120.29 (c) durch Stahlkugel5.95 gebrochene VSG-Scheibe.5.97 1.61 120_5 120.27 5.94 5.82 1.72 120_6 120.30 5.96 5.80 1.76 120_7 119.45 5.96 5.94 1.72 120_8 119.30 5.95 5.94 1.74 150_1 149.94 5.95 5.95 1.72 I and III 150_2 149.97 5.93 5.95 1.75 150_3 150.01 5.95 5.94 1.71 150_4 150.02 5.94 5.96 1.73 150_5 150.26 5.85 5.84 1.75 150_6 150.21 5.89 5.90 1.64 49 150_7 150.04 5.90 5.90 1.66 150_8 149.91 5.84 5.82 1.78 200_1 199.79 5.97 5.98 1.66 II and III 200_2 199.76 5.95 5.96 1.71 200_3 199.84 5.96 5.95 1.70 200_4 199.74 5.95 5.95 1.68 200_5 200.47 5.98 5.97 1.69 200_6 200.55 5.98 5.97 1.69 200_7 199.81 5.95 5.94 1.77 200_8 199.88 5.96 5.94 1.76

2.1. Interlayer

Initially, 25 T-bone shaped samples have been extracted from a sheet of SGP.1 According to EN ISO 527-2 [11], as illustrated in Fig. 3. The nominal thickness of the sheet was 1.52 mm and the mean measured thickness was 1.67 mm.

2.2. Laminates

In addition, 24 laminated glass pieces with a constant nominal length of 1100 mm have been tested, divided in three ser- ies according to their nominal height (h) (120 mm, 150 mm and 200 mm). Each test specimen consisted of two glass sheets

1 SGP 2000 was used for the tests. However, at the time of writing a more recent version called SGP 5000 had been released [12]. 1872 J. Belis et al. / Engineering Failure Analysis 16 (2009) 1866–1875

abc F F F

laminating tolerance

Fig. 8. Principle of unequal load transfer to both glass sheets due to laminating tolerances: (a) unloaded situation; (b) intermediate situation with load transfer through interlayer; and (c) final situation.

of the beam. To investigate this stage, a single crack at mid span was made in one glass sheet of each of the 200 mm high beams prior to the tests. Again, a linear load–deflection relationship was observed until brittle fracture occured. Crack initiations did always ap- pear in the centre of the beam span, corresponding to the location where the initial single crack in the other glass sheet had been made: the initial weakening of the specimens seemed to be significant enough to determine the position of the fracture. The corresponding failure stress level in this stage was approximately 60% of that expected in stage I. In other words, the failure stress level was approximately 20% higher than the expected stress level for one glass sheet. Consequently, the SGP interlayer seemed to be able to transfer (at least partially) the compressive stresses that appeared between both pieces of the originally broken glass sheet, increasing the residual resistance of the damaged laminate. Again, the measured deflections were slightly underestimated by elastic theory.

4.4. Stage III

Stage III, studied for all test specimens, is very complex and ambiguous due to the large variety of crack origins, crack patterns and local delamination effects. After breakage of both glass sheets the load decreased to a relatively low level (typ- ically between 2 kN and 3 kN) before the broken glass pieces and interlayer started again to build up compressive and tensile stresses, respectively. Subsequently, the load slightly increased again and after reaching a (sometimes barely noticeable) maximum, it decreased significantly (to less then 0.3 kN) (see Fig. 9). Finally, an A-shaped gap appeared at the bottom where the SGP was fissured and large deflections were measured, as illustrated in Fig. 10. This was the case for the specimens with a height of 150 mm and 200 mm. However, when testing the 120 mm high beams, the opening immediately appeared4 Conclusion after breakage of the second Manuel sheet. Zimmermann Again, low load levels caused large deflections. As deformations increased, all beams finally failed due to tearing of the foil over its entire height. As a result

of the interlayer, the adhesion between glass and interlayer and the size of glass fragments.

F [kN] 14 glass breakage 12 10 8 6 stress equilibrium between broken glass pieces and interlayer 4 rupture of interlayer 2 w [mm] 0 0510152025303540

Fig. 9. Typical load (F)–deflection (w) curve in stage III (sample 150_4). Figure 4: Load deflection behavior of LSG in stage III taken from [5].

3.1 Failure of LSG The post-breakage safety and performance of LSG is linked to the remaining load carrying capacity which means how much load can be added until it collapses. For this to happens one can distinguish between a cross-sectional level and an element level. First one is ruled by either stretching up to failure of the interlayer or delamination between glass and interlayer. On the element level the different boundary conditions as well as the used materials have an effect. Depending on the different type of pre-stressing as mentioned in section 2.1 the glass shows different crack patterns. For heat strengthened glass bigger pieces are formed and failure can occur by forming a mechanism as known from plate theory. Furthermore, one has also to be aware that the glass unit can slide from the supports which would also mean failure. Tempered glass forms very small fragments when it breaks. This can cause big deformation after breakage and the sheet behaves like a membrane. Failure can happen because of either sliding from the support for line supports or by tearing out of the support in case of point support.

4 Conclusion

On the basis of this work the answer to the safety and post-breakage performance of LSG seems to be difficult. Assessment of LSG with different tests seems to remain the only possibility to characterize post-breakage behavior of LSG. To gain further in- formation testing methods should include type of failure mode as well as temperature and loading rate. The contribution of the interlayer should also be assessed and test

6 References Manuel Zimmermann methods to investigate the performance of the interlayer under real conditions have to be found. The mechanical properties of the interlayer are mainly contribution the load transfer mechanisms of broken LSG. Further the level of adhesion and the stiffness of the interlayer should be tested separately and some recommendations should be published.

One of the possibilities to improve safety of LSG seems to be the use of better in- terlayer materials, where in case of failure the residual strength is only provided by the interlayer and the critical failure mode is known. To achieve these different materials could be used, for example carbon or glass fibers. This would permit the design based on load cases and ultimate strength.

References

[1] Alexander Kott. “Trag- und Resttragverhalten von Verbundsicherheitsglas”. In: IBK Bericht 299. vdf Hochschulverlag, ETH Zürich, 2007. [2] Didier Delincé. “Experimental approaches for assessing time and temperature de- pendent performances of fractured laminated safety glass”. eng. PhD thesis. Ghent University, 2014, pp. XXVIII, 279. isbn: 9789085786702. [3] Didier Delincé et al. “Post-breakage behaviour of laminated glass in structural applications”. In: Conference on Architectural and Structural Applications of Glass. IOS Press. 2008, pp. 459–467. [4] Johannes Kuntsche, Johannes Franz, and Jens Schneider. “Untersuchungen zum Resttragverhalten von Verbundglas”. In: Glasbau 84 (2015), pp. 383–395. [5] Jan Belis et al. “Failure mechanisms and residual capacity of annealed glass/SGP laminated beams at room temperature”. In: Engineering Failure Analysis 16.6 (2009), pp. 1866–1875.

7