ETH ZürichInsitut für Baustoffe

Glass and natural stone in construction

Post-breakage behavior of laminatedglass

Manuel Zimmermann

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

Contents

1 Introduction 2

2 Laminated safety glass 22.1 Glas types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Interlayer materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Assessment (Testing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Post breakage behavior of LSG 43.1 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 technologicaldevelopments specially in terms of flatness of glass made it also applicable for architecturalpurpose. Laminated glass products conquered the structural building industry and callfor safety glazing units. This application as so called structural glazing call for a safedesign concept which engineers are used from other materials. With the technologicaldevelopment of polymer materials, a variety of glass products with different safety featureswhere introduced. The difficulty is that laminated glass properties do not only depend onthe used materials but also on temperature, rate of loading and support type.

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

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

2 Laminated safety glass

Laminated glass was needed with the technological innovation which improved the qualityand 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 glasswhere in case of breakage the interlayer serves to retain the glass fragments and offersresidual resistance. The thickness of the glass layers usually vary between 4 to 6mm. Theinterlayer has a thickness of 0, 38mm and its multiples. The products are covered by aEuropean harmonized standard released in 2005 by the CEN. With this configuration theharder glass protects the interlayer which leads to a product with interesting features.Namely the familiar transparency of glass combined with the ductile behavior expectedfrom 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 bedescribed by a sandwich model according to Kott [1] where the interlayer has linear elasticproperties 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 stiffnessare 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 interlayerscontribution 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, wherethe tension on the bottom of the beam let the tension zone grow toward the edge whereit eventually meets surface cracks. Due to vandalism or also accidentally a hard bodyimpact could introduce a crack on the top layer that reaches the tension zone. This arealso 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 tempered glass is used. Annealedglass is not pre-stressed, has therefore a low bending capacity and forms very big andsharp pieces when ist breaks. This does not make it very suitable for structural glazingapplications. Instead pre-stressed glass which shows initially a better residual stress stateand thus to higher bending capacity is used. The initial stress state is provided by heattreatment and cooling of the glass surface. The temperature gradient causes compressionstresses at the surface and tension in the core, which makes the glass less sensitive tosurface flaws. Further one can distinguish between heat strengthened and tempered glass.First one shows larger fragments as tempered glass because its moderate cooling rate andresidual stresses. The bending strength of heat strengthened glass is about 70MPa and itspre-stressed to 50MPa in compression. This is lower as the bending strength of temperedglass which is around 120MPa. Tempered glass breaks into very small fragments becauseof its high elastic energy due to pre-stressing up to 120MPa in compression.

2.2 Interlayer materials

The mainly used interlayer for LSG are Polyvinyl butyral (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 infigure 1. They both show temperature and time dependent viscoelastic properties. Thetesting 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 carryingcapacity is generally provided by experiments. Where the individual support conditionshave to be taken into account [3]. Kott and Delincè[1, 2] used the pendulum as wellas the hard body drop test (Fig 2). In both cases, breakage of the constitutive glasssheets is allowed, but not supposed to lead to an overall failure or collapse of the tested

3

3 Post breakage behavior of LSG Manuel Zimmermann

134 Chapter III

A quantitative comparison of these different results is not straightforward, because of the different geometries of the test specimens. Whereas dispersion 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.

Figure III.12 – Typical nominal stress-strain curves obtained from conventional uniaxial tensile tests on dog-bone specimens cut out of PVB-films and SG-sheets.

Above pictures show the deformation pattern along the loading curve for SG-specimen.

Figure 1: Typical stress-strain curves for uniaxial tensile test for specimens of PVB-filmsand SG-sheets taken from [2].

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 assessmentis relatively complex because of its multi-scale characteristics and its in mechanicalproperties completely different components. Furthermore one type of safety performancedoes not provide characteristic values usable for design. The assessment does not evaluatethe contribution of the individual components of laminated glass. For non vertical glazingunits 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 performanceof 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 andthe glass pieces are still bonded to the interlayer [3]. In most cases this happens underbending. According to Kott [1] one can distinguish between three different stages wherein Stage I both glass layers are intact. In Stage II one and in Stage III both of the glasslayers are broken (Fig. 3). Specially with the use of floor glazing the load-bearing capacityof 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 fora longer time, as it was known from overhead and façade applications). The fragmentationresponse and post-breakage behavior of LSG seem more to be an element property thana product or material one. Furthermore, impact resistance can not be defined at the levelof cross-section, but as a combination of element and loading configurations. And toconclude 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 glasssample 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 thisstage very important for residual strength of structural glazing applications. The failurestress level is approximately 60% of that in stage I. Because the cross-section is less than60% this means that the interlayer is able to transfer stresses between the both pieces of

4

3 Post breakage behavior of LSG Manuel Zimmermann

49

Verbundsicherheitsglas

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

Harter Stoss

Sowohl bei betretbaren als auch begehbaren Verglasungen ist mit einem so genanntenharten Stoss zu rechnen. Man spricht vom harten Stoss, wenn die gesamte aufgebrachteEnergie beim Stoss durch die VSG-Scheibe aufgenommen wird. Die Einwirkungen,meistens hervorgerufen durch Vandalismus, entstehen durch Bewurf der VSG-Scheibenmit harten Gegenständen oder durch direktes Anschlagen der Glasoberflächen. DerNachweis der Tragfähigkeit unter einem harten Stoss wird mit einem zylindrischenFallkörper der Masse 40 kg [F 42] oder durch eine Stahlkugel der Masse 4.1 kg er-bracht. Die Fallversuche können je nach Anforderung verändert werden. Der Versuchgilt als bestanden, wenn der Stosskörper die Glasscheibe nicht vollständig durchstossenhat und die Verglasung nicht von der Auflagerung rutscht sowie keine Bruchstücke her-abfallen, die grösser sind als in DIN 1249-12 angegeben [N 4].

Bild 2.33 Pendelschlagversuch für die Simulation des weichen Stosses [N 23]; (a) Versuchsaufbau fürden weichen Stoss, (b) gebrochene VSG-Scheibe nach nicht bestandener Pendelschlagprü-fung.

a) b)

40 kg

hh

4.1 kg

Bild 2.34 Stossversuche für die Simulation des harten Stosses; (a) mit zylindrischem Fallkörper (b) mitStahlkugel, (c) durch Stahlkugel gebrochene VSG-Scheibe.

a) b) c) 49

Verbundsicherheitsglas

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

Harter Stoss

Sowohl bei betretbaren als auch begehbaren Verglasungen ist mit einem so genanntenharten Stoss zu rechnen. Man spricht vom harten Stoss, wenn die gesamte aufgebrachteEnergie beim Stoss durch die VSG-Scheibe aufgenommen wird. Die Einwirkungen,meistens hervorgerufen durch Vandalismus, entstehen durch Bewurf der VSG-Scheibenmit harten Gegenständen oder durch direktes Anschlagen der Glasoberflächen. DerNachweis der Tragfähigkeit unter einem harten Stoss wird mit einem zylindrischenFallkörper der Masse 40 kg [F 42] oder durch eine Stahlkugel der Masse 4.1 kg er-bracht. Die Fallversuche können je nach Anforderung verändert werden. Der Versuchgilt als bestanden, wenn der Stosskörper die Glasscheibe nicht vollständig durchstossenhat und die Verglasung nicht von der Auflagerung rutscht sowie keine Bruchstücke her-abfallen, die grösser sind als in DIN 1249-12 angegeben [N 4].

Bild 2.33 Pendelschlagversuch für die Simulation des weichen Stosses [N 23]; (a) Versuchsaufbau fürden weichen Stoss, (b) gebrochene VSG-Scheibe nach nicht bestandener Pendelschlagprü-fung.

a) b)

40 kg

hh

4.1 kg

Bild 2.34 Stossversuche für die Simulation des harten Stosses; (a) mit zylindrischem Fallkörper (b) mitStahlkugel, (c) durch Stahlkugel gebrochene VSG-Scheibe.

a) b) c)

49

Verbundsicherheitsglas

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

Harter Stoss

Sowohl bei betretbaren als auch begehbaren Verglasungen ist mit einem so genanntenharten Stoss zu rechnen. Man spricht vom harten Stoss, wenn die gesamte aufgebrachteEnergie beim Stoss durch die VSG-Scheibe aufgenommen wird. Die Einwirkungen,meistens hervorgerufen durch Vandalismus, entstehen durch Bewurf der VSG-Scheibenmit harten Gegenständen oder durch direktes Anschlagen der Glasoberflächen. DerNachweis der Tragfähigkeit unter einem harten Stoss wird mit einem zylindrischenFallkörper der Masse 40 kg [F 42] oder durch eine Stahlkugel der Masse 4.1 kg er-bracht. Die Fallversuche können je nach Anforderung verändert werden. Der Versuchgilt als bestanden, wenn der Stosskörper die Glasscheibe nicht vollständig durchstossenhat und die Verglasung nicht von der Auflagerung rutscht sowie keine Bruchstücke her-abfallen, die grösser sind als in DIN 1249-12 angegeben [N 4].

Bild 2.33 Pendelschlagversuch für die Simulation des weichen Stosses [N 23]; (a) Versuchsaufbau fürden weichen Stoss, (b) gebrochene VSG-Scheibe nach nicht bestandener Pendelschlagprü-fung.

a) b)

40 kg

hh

4.1 kg

Bild 2.34 Stossversuche für die Simulation des harten Stosses; (a) mit zylindrischem Fallkörper (b) mitStahlkugel, (c) durch Stahlkugel gebrochene VSG-Scheibe.

a) b) c)

49

Verbundsicherheitsglas

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

Harter Stoss

Sowohl bei betretbaren als auch begehbaren Verglasungen ist mit einem so genanntenharten Stoss zu rechnen. Man spricht vom harten Stoss, wenn die gesamte aufgebrachteEnergie beim Stoss durch die VSG-Scheibe aufgenommen wird. Die Einwirkungen,meistens hervorgerufen durch Vandalismus, entstehen durch Bewurf der VSG-Scheibenmit harten Gegenständen oder durch direktes Anschlagen der Glasoberflächen. DerNachweis der Tragfähigkeit unter einem harten Stoss wird mit einem zylindrischenFallkörper der Masse 40 kg [F 42] oder durch eine Stahlkugel der Masse 4.1 kg er-bracht. Die Fallversuche können je nach Anforderung verändert werden. Der Versuchgilt als bestanden, wenn der Stosskörper die Glasscheibe nicht vollständig durchstossenhat und die Verglasung nicht von der Auflagerung rutscht sowie keine Bruchstücke her-abfallen, die grösser sind als in DIN 1249-12 angegeben [N 4].

Bild 2.33 Pendelschlagversuch für die Simulation des weichen Stosses [N 23]; (a) Versuchsaufbau fürden weichen Stoss, (b) gebrochene VSG-Scheibe nach nicht bestandener Pendelschlagprü-fung.

a) b)

40 kg

hh

4.1 kg

Bild 2.34 Stossversuche für die Simulation des harten Stosses; (a) mit zylindrischem Fallkörper (b) mitStahlkugel, (c) durch Stahlkugel gebrochene VSG-Scheibe.

a) b) c)

Figure 2: Pendulum test (top) and hard body drop test (bottom) for the assessment oflaminated glass products modified from [1].

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 illustratedin 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].

reference marks

12.5 ± 1

20 ± 0.5

75

4 ± 0.1

Fig. 3. Basic shape and main sizes of SGP samples used for uniaxial tensile tests (mm) (for more details, see EN ISO 572-2 [11]).

Table 2Overview of laminated glass test samples

Name Mean height(mm)

Glass thickness, a(front plate) (mm)

Glass thickness, a(back plate) (mm)

Mean interlayerthickness, t (mm)

Failure stagesstudied (–)

120_1 120.78 5.95 5.97 1.71 I and III120_2 120.79 5.97 5.94 1.70120_3 120.32 5.96 5.95 1.64120_4 120.29 5.95 5.97 1.61120_5 120.27 5.94 5.82 1.72120_6 120.30 5.96 5.80 1.76120_7 119.45 5.96 5.94 1.72120_8 119.30 5.95 5.94 1.74

150_1 149.94 5.95 5.95 1.72 I and III150_2 149.97 5.93 5.95 1.75150_3 150.01 5.95 5.94 1.71150_4 150.02 5.94 5.96 1.73150_5 150.26 5.85 5.84 1.75150_6 150.21 5.89 5.90 1.64150_7 150.04 5.90 5.90 1.66150_8 149.91 5.84 5.82 1.78

200_1 199.79 5.97 5.98 1.66 II and III200_2 199.76 5.95 5.96 1.71200_3 199.84 5.96 5.95 1.70200_4 199.74 5.95 5.95 1.68200_5 200.47 5.98 5.97 1.69200_6 200.55 5.98 5.97 1.69200_7 199.81 5.95 5.94 1.77200_8 199.88 5.96 5.94 1.76

Stage I Stage II Stage III

C

C

C

C

C

T

T T

Tglass

glass

interlayer

Fig. 2. Three stages in the failure process of a laminated plate composed of two glass sheets and one interlayer (in the example shown, the upper sheetbroke first, e.g., due to a hard body impact).

1868 J. Belis et al. / Engineering Failure Analysis 16 (2009) 1866–1875

Figure 3: The three stages of laminated glass during failure taken from [5].

broken glass as shown in [5].

Stage III is very complex, because many influencing factors mainly the size of thebroken glass pieces, the crack pattern and local delamination effect. The residual loadcarrying capacity gets much lower and the deformation increases as seen in figure 4. Theparameters that are influencing the post-breakage behavior are, the mechanical properties

5

4 Conclusion Manuel Zimmermann

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

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 highbeams 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 sheethad been made: the initial weakening of the specimens seemed to be significant enough to determine the position of thefracture.

The corresponding failure stress level in this stage was approximately 60% of that expected in stage I. In other words, thefailure stress level was approximately 20% higher than the expected stress level for one glass sheet. Consequently, the SGPinterlayer seemed to be able to transfer (at least partially) the compressive stresses that appeared between both pieces of theoriginally broken glass sheet, increasing the residual resistance of the damaged laminate. Again, the measured deflectionswere 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, crackpatterns 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 tensilestresses, 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, asillustrated in Fig. 10. This was the case for the specimens with a height of 150 mm and 200 mm. However, when testingthe 120 mm high beams, the opening immediately appeared after breakage of the second sheet. Again, low load levels causedlarge deflections. As deformations increased, all beams finally failed due to tearing of the foil over its entire height. As a result

F

FF

laminatingtolerance

a b c

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

0

2

4

6

8

10

12

14

0 5 10 15 20 25 30 35 40

w [mm]

F [kN]

glass breakage

stress equilibrium between broken glass pieces and interlayer

rupture of interlayer

Fig. 9. Typical load (F)–deflection (w) curve in stage III (sample 150_4).

1872 J. Belis et al. / Engineering Failure Analysis 16 (2009) 1866–1875

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 carryingcapacity which means how much load can be added until it collapses. For this to happensone can distinguish between a cross-sectional level and an element level. First one isruled by either stretching up to failure of the interlayer or delamination between glassand interlayer. On the element level the different boundary conditions as well as the usedmaterials have an effect. Depending on the different type of pre-stressing as mentioned insection 2.1 the glass shows different crack patterns. For heat strengthened glass biggerpieces are formed and failure can occur by forming a mechanism as known from platetheory. Furthermore, one has also to be aware that the glass unit can slide from thesupports which would also mean failure. Tempered glass forms very small fragmentswhen it breaks. This can cause big deformation after breakage and the sheet behaveslike a membrane. Failure can happen because of either sliding from the support for linesupports 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 ofLSG seems to be difficult. Assessment of LSG with different tests seems to remainthe 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 temperatureand 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 tobe found. The mechanical properties of the interlayer are mainly contribution the loadtransfer mechanisms of broken LSG. Further the level of adhesion and the stiffness ofthe 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 theinterlayer and the critical failure mode is known. To achieve these different materialscould be used, for example carbon or glass fibers. This would permit the design based onload cases and ultimate strength.

References

[1] Alexander Kott. “Trag- und Resttragverhalten von Verbundsicherheitsglas”. In: IBKBericht 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. GhentUniversity, 2014, pp. XXVIII, 279. isbn: 9789085786702.

[3] Didier Delincé et al. “Post-breakage behaviour of laminated glass in structuralapplications”. In: Conference on Architectural and Structural Applications of Glass.IOS Press. 2008, pp. 459–467.

[4] Johannes Kuntsche, Johannes Franz, and Jens Schneider. “Untersuchungen zumResttragverhalten von Verbundglas”. In: Glasbau 84 (2015), pp. 383–395.

[5] Jan Belis et al. “Failure mechanisms and residual capacity of annealed glass/SGPlaminated beams at room temperature”. In: Engineering Failure Analysis 16.6 (2009),pp. 1866–1875.

7