Ŕ periodica polytechnica Temperature dependent flexural Civil Engineering stiffness of load bearing laminated 54/2 (2010) 117–126 doi: 10.3311/pp.ci.2010-2.07 glass panes web: http://www.pp.bme.hu/ci c Periodica Polytechnica 2010 Kinga Pankhardt RESEARCH ARTICLE Received 2009-12-17, revised 2010-05-26, accepted 2010-06-23 Abstract 1 Introduction Load bearing glasses are used not only in interior but also in The discovery of glass lamination happened accidentally. exterior applications. The flexural stiffness of laminated glass While Benedictus (1910) heated a solution of nitrocellulose and is influenced by temperature. With increasing temperature the accidentally dropped it on a glass pane, he found that when the shear stiffness rapidly decreases and creep becomes more sig- glass fractured it did not shatter. Benedictus patented (British nificant (Krüger, 1998). According to Wölfel’s (1987) calcula- Patent 1.790) laminated glass in 1910. The production of lami- tions, the shear modulus of the interlayer material has the most nated glass started in 1912 in Great Britain. influence on the strength and deflection. In the case of thin or Nowadays laminated glass consists of two or more glass lay- large size glasses, where the deformations (deflections) are con- ers with one or more plastic layers between the glass panes. siderable, temperature dependent flexural stiffness of the overall Joining of the glass layers with foil can take place in a pres- laminate is more significant. This paper deals with the influence surised vessel, called an autoclave. In the autoclave, under si- of temperature on flexural stiffness of laminated glasses. Based multaneous heating of the already processed layers of glass and on the laboratory results, the Author modified the definition of special plastic, lamination occurs. In the case of cast in place the coupling parameter, which represents the bond of the glass process, liquid resin is cast between two sheets of glass layers layers by the interlayer and was originally defined by Krüger and then the liquid resin is polymerised with UV radiation or by (1998). catalysis. As the resin is liquid, it perfectly fills the space be- tween the glass layers, hence it is ideal for use with imperfectly Keywords smooth glass surfaces, such as tempered and textured glass or glass laminated glass flexural stiffness EVA resin non-parallel sheets, such as bent glass. Therefore, the cast in · · · · place process is mostly used for non-standard dimensions of Acknowledgement laminated glass. The author would like to express thanks to Prof. Gy. L. When laminated glass began to be used in significant quanti- BALÁZS for the support and guidance throughout her PhD ties for the architectural glazing industry in the 1960s, building work. The author would like to express thanks to Rákosy GLASS codes defined a strength factor of 0.6 relative to monolithic sin- Ltd. for providing the specimens and for the support to DSC gle glass of the same thickness [3]. The basis to define this factor analysis of plastics to Prof. Dr. V. Vargha and R. C. Bende, numerically is unclear, although in bending tests the load bear- Sz. Máté, BME Department of Plastics and Rubber Technology. ing capacity (Fmax ) of a layered two-ply laminate without an The authors would like to thank to Dr. S. G. Nehme for his intel- interlayer is 0.5 times a monolithic pane of the same thickness lectual support and would also like to thank to Dr. S. Fehérvári, [4]. Nowadays, there is a general consensus to increase the glass A. Eipl, M. Varga, D. Diriczi, G. Kovács and P. Tisza for their strength factor. Strength factor SF of laminated glass is defined technical support at Department of Construction Materials and by Eq. (1): Engineering Geology, BME. maximal resisting force of laminated glass SF (1) = maximal resisting force of monolithic single glass The interlayer has two functions: (i) to keep in place the glass Kinga Pankhardt splinters during the fracture process of a glass pane to reduce the Department of Civil Engineering, Debrecen University, H-4028 Debrecen, Hun- risk of injury and (ii) to increase residual load bearing capacity gary (Fig. 1). Fig. 1 indicates a fractured overhead glazing of a store e-mail: [email protected] Temperature dependent flexural stiffness of load bearing laminated glass panes 2010 54 2 117 in Vienna. The multilayer laminated glass remained in position even after fracturing and did not fall onto the pedestrian. Fig. 3. Results of bending tests under uniform loading at room temperatures, where specimens were supported along two edges. Squares indicate monolithic specimens with thickness of 6 mm, diamonds: two glass layers with thickness of 3 mm laminated with 0.76 mm PVB interlayer, triangles: specimens laminated with 2.28 mm of PVB interlayer thickness [5]. The load bearing capacity of laminated glass can be increased with increase of thickness of interlayer material in the case of appropriate bond. Research results of [5–7] indicated that with increase of PVB interlayer thickness the load bearing capacity Fig. 1. Fractured overhead glazing, Humanic store, Vienna, 2007. of laminated glass increases at room temperature. Depending on the shear stiffness and thickness of the inter- Fig. 3 indicates also that with an increase of the area and (or) layer, the laminate can exceed the strength of the equivalent number of glass layers, therefore, the increase of number of de- monolith glass having the same total glass thickness. How- fects on the glass surface, affects the load bearing capacity of ever, this effect is only valid as long as the interlayer material the laminate. In the case of relatively large glass layer area, the remains stiff. The strength of laminated glass is influenced by strength is considerably influenced by the size effect. The rea- the strength of the glass layers and the shear transfer of the inter- son for the size effect is the stochastic distribution of the defects layer. Without appropriate shear transfer capacity, the interlayer in the glass pane (Weibull type size effect, otherwise called sta- functions as a spacer between the glass layers, therefore, it can tistical size effect) [8]. increase the moment of inertia of the overall laminate, but can The flexural stiffness of laminated glass is influenced by tem- not decrease the deflections efficiently. perature. With increasing temperature the shear stiffness rapidly Increase of temperature results in a decrease of the stiffness decreases and creep [2] becomes more significant, therefore, the of the laminate, which also affects the strength factor. effect of temperature should be studied. To study the behaviour The strength in the glass panes is influenced by the shear of the laminated glass, it is important to know the physical prop- transfer of the interlayer [1]. When the bond is efficient and the erties of glass layers and also the physical and chemical proper- strain of the interlayer is small, the composite behaves almost ties of commonly used interlayer materials. The temperature [9] monolithic (Fig. 2). and the rate of loading [10, 11] influence the properties of the polymers which will affect also the behaviour of laminate. The main phase transition temperature of polymers is the glass transition temperature, Tg. At this temperature, the amorphous polymer or the amorphous component of the semi- crystalline polymer changes from a glassy state to a rubbery state and the material softens considerably. Beyond this tem- perature, the purely amorphous polymer does not show further Fig. 2. Rigidly bonded glass layers. (The figure is non proportional in trans- verse direction.) transition and the material is in a liquid state. However, a semi- crystalline polymer exhibits another transition at a higher tem- In the case of the so-called “rigid bond”, the load bearing perature than the glass transition temperature. This is the melt- capacity of laminated glass having same thickness compared to ing temperature, Tm. (Note that there is no melting behaviour a single layer glass can be over-estimated [4]. in the amorphous polymer.) Physical and mechanical properties Norville (1997) [5] published experimental strength data of polymers (including thermal expansion coefficient, heat ca- on laminated glass specimens manufactured with a 2.28 mm pacity and refractive index) will change at the glass transition polyvinyl butyral interlayer which exceeded the strength of lam- temperature, Tg [12]. inated glass specimens having the same dimensions but which used a 0.76 mm thick interlayer, as well as the strength of mono- lithic glass samples having a comparable thickness (Fig. 3). 118 Per. Pol. Civil Eng. Kinga Pankhardt . Fig. 4. Test method for four-point bending [13] where, 1. specimen insulation (40 mm thick), Ls : 1000 mm, Lb: 200 mm, h: thickness of the spec- 1100 360 mm, 2. bending roller, 3. supporting roller, 4. rubber strips (3 mm imen (6 mm, 12 mm, 19 mm or 2 6, 3 6 mm) [4] × × × thick, according to ISO 48 [14]), 5. self-designed transducer, 6. custom-made 2 Materials and experimental procedure ther specimens were heated to +60 ˚C or cooled to -20 ˚C. The The effect of temperature of -20 ˚C, +23 ˚C and +60 ˚C were temperature of the specimens and the room temperature were investigated on bending characteristics of laminated glasses. continuously measured during the tests. The specimens were mounted as shown in Fig. 4 (a) and (b). The deflection at mid- 2.1 Test parameters and test programme span of the glass panes were measured with displacement trans- Specimens were manufactured from soda-lime silicate float ducer in all tests. glass. All glass specimens with a constant span of Ls= 1000 mm The temperature was kept constant during the test with 1 ˚C and supported at a width of b 360 mm were tested in four- in order to avoid the development of thermal stresses.