STRUCTURAL PERFORMANCE OF LAMINATED GLASS MADE WITH A STIFF INTERLAYER

Transcription

1 STRUCTURAL PERFORMANCE OF LAMINATED GLASS MADE WITH A STIFF INTERLAYER S.J. Bennison, C.A. Smith, A. Van Duser & A Jagota, E.I. DuPont de Nemours & Co. Inc., Wilmington, DE , USA. ABSTRACT The growing demand for laminated glass in building facades and interiors is driving the development of new interlayers that extend the physical performance of laminated glass. In this contribution we discuss the structural performance of laminates made with such a new interlayer: SentryGlas Plus (SGP). This new interlayer is significantly stiffer, tougher and chemically more robust than traditional PVBs and provides enhanced structural performance in many applications. The mechanical properties of a stiff interlayer lead to benefits in three main areas: 1) glass strength; 2) stiffness and creep resistance, both before and after glass breakage; 3) temperature performance. Strength benefits are realized in applications where significant bending stresses develop during loading, such as: laminates with two sided support, balustrades (cantilevers), bolted glass and laminates supported on three sides. Enhanced post glass breakage performance expands the design envelope for glass in horizontal and sloped applications such as overhead glazing, glass floors and stairs. The enhanced stiffness of fragmented laminates made with SentryGlas Plus (SGP) also allows design of security glazing with ultra-high blast performance. All of these structural properties are maintained to higher temperatures as compared to conventional PVB. We review the role of interlayer properties in the aforementioned mechanical applications. INTRODUCTION Architectural laminated glass is dominated by the use of PVB interlayers, such as Butacite. This domination can be attributed to the long successful history of PVB use in the automotive industry for safety glass. The primary requirements of automotive safety glass are: 1) impact resistance from external objects, 2) sufficient compliance to ensure minimal head trauma for a passenger that strikes the windshield. This latter feature is a major reason why plasticized PVB was developed; an elastomeric interlayer with the correct elastic response to cushion passenger impact was needed. However, if we look to the requirements of architectural laminated glass we find that high compliance and an elastomeric response to be unnecessary in many applications. Indeed, a compliant interlayer can hinder structural performance in some cases such as in applications where low glazing deflection is specified. If we relax the requirement of high compliance after glass breakage, many more polymer systems now look attractive for laminated safety glass. In this contribution we consider a new polymer system for laminated glass. The polymer, SentryGlas Plus (SGP), is significantly stiffer than conventional PVB and displays elasto-plastic stress-strain behavior. It has a totally different molecular architecture to PVB and for most applications SentryGlas Plus (SGP) is below its 1 Material from

2 glass transition temperature (T g ~ 55 C). We describe several examples of the performance benefit of laminated glass made with SentryGlas Plus (SGP). This structural interlayer is particularly effective in applications where: 1) glazing is subjected to bending loads, such as 1 and 2-sided and point support conditions and/or concentrated loading; 2) post-glass breakage performance is important, e.g. in overhead glazing; 3) enhanced performance under long duration loading or at elevated temperatures is required, such as in creep loading due to self-weight. STRENGTH PERFORMANCE The benefits of enhanced interlayer stiffness for laminate strength can be readily appreciated from a simple experiment in which glass stress during loading is measured directly as a function of polymer type and thickness. Figure 1 shows the results from one such experiment. Figure 1. Measurement of glass stress as a function of uniform applied pressure for 4-sided support monolithic glass plates and several laminate plate builds. Note that for the glass design stress specified by the horizontal dashed line all laminates display greater load bearing, or strength characteristics. In this experiment a grid of 15-rosette strain gages were attached to the glass surface of a series of laminates made with either PVB (DuPont Butacite ) or SGP (DuPont SentryGlas Plus). The laminate plates were supported on four sides and loaded with uniform pressure. Maximum principal stress was measured at each gage location along with plate deflection during loading. Figure 1 shows the development of maximum principal stress in the glass during an experiment. Several features may be noted from these data. First, the upper curve consists of observations from a nominal 6 mm monolithic glass and shows non-linear stress-pressure behavior that is characteristic of large deformation of a 4-side supported plate. Second, a laminate made from 3 mm glass / 0.76 mm Butacite PVB / 3 mm glass shows significantly 2 Material from

3 less glass stress development at a specified applied pressure; i.e. the laminate is stronger than the equivalent monolithic glass plate. This result is consistent with much research demonstrating that 4-side supported, uniform pressure loaded laminated glass often displays equivalent or enhanced strength over monoliths [1-7]. Third, increasing the Butacite PVB interlayer thickness increases strength. Fourth, a laminate made from 3 mm glass / 2.29 mm SGP / 3 mm glass shows significant strengthening over monolithic glass and laminated glass made with PVB. For example, at the glass design stress denoted by the horizontal dashed line the SGPbased laminate can sustain almost 2x the applied pressure than the equivalent monolith. In this example the SGP-based laminate displays strength characteristics that are virtually equivalent to a heat-strengthened 6 mm monolith. Of course the overall laminate thickness is greater than the monolith but in many design codes around the world this laminate would be considered to be structurally inferior to the equivalent monolith. We have carried out many such experiments with SGP-based laminates and have observed that strengthening generally results due to the combined effect of using this stiffer interlayer and increasing the overall laminate thickness, compared to monolithic glass. As a guide to design with SGP-based laminates we have constructed strength charts that aid the selection of polymer type and thickness, glass thickness, for specified loading/support and rate/temperature conditions. These charts have been computed using a procedure described in detail elsewhere [8]. Briefly, the procedure consists of: 1) establishing a constitutive model for the interlayer by dynamic mechanical analysis [9]; 2) carrying out finite element analyses of glass stress development [10]; 3) validating selected analyses against controlled loading experiments [8]; 4) combining stress analyses with a statistical (Weibull) glass breakage model [11-13] that incorporates a time-dependence for glass strength [14,15]. Design charts may then be constructed for specified glass breakage probability and laminate build of interest. An example of such a design chart for an SGP-laminate is shown in figure 2. Figure 2. Strength design chart for an SGP-based laminate. The chart plots allowable applied uniform pressure contours for various plate sizes. Note the specified probability of breakage (0.008), load duration (3s) and temperature (50 C). 3 Material from

4 This chart has been constructed in ASTM E1300 [16] format using the glass strength parameters and minimum glass thickness specified in that standard [17]. The chart plots allowable pressure loading contours as a function of plate dimensions for a breakage probability of Note that the chart is constructed for a 3 mm glass / 1.52 mm SGP / 3 mm laminate under 3 s wind loading at 50 C. A series of such charts for common laminate constructions is used in an iterative fashion to select the optimum laminate build for specified wind-load conditions. Comparison of this chart with one for an equivalent 6 mm monolithic glass shows strengthening over the whole field of plate sizes. Interestingly, the degree of strengthening depends on plate size and aspect ratio with the greater strength benefits appearing at higher aspect ratios. At these higher aspect ratios a greater degree of bending deformation occurs and suggests that the benefits of SGP may be realized in loading conditions where bending stresses dominate. Accordingly a series of beam bending experiments have been carried out in which glass-stress development as a function of loading has been measured directly using attached strain gages. Figure 3 shows results of one set of experiments. Figure 3. Measurement of glass stress development for monolithic glass beams and SGP-based laminate beams tested in three-point bending. For a specified glass design stress (14 MPa) the figure plots the applied force as a function of rate. Note that the 5 mm / 3 mm SGP / 5 mm laminate demonstrates greater strength than a 10 mm monolithic glass and essentially equivalent strength to a 12 mm monolithic glass. In this figure the force required to produce a specified glass stress has been measured as a function of loading rate in a three-point bend geometry. The main feature to note from these measurements is that a beam made from a 5 mm glass / 3 mm SGP / 5 mm 4 Material from

5 laminate significantly outperforms a 10 mm monolithic glass beam in strength properties. Again the total laminate thickness is greater than the equivalent monolith but many building codes around the world neglect this and penalize laminates against equivalent monoliths. When comparing strength performance of the laminate against a 12 mm monolithic glass it can be seen that the monolith and laminate are essentially equivalent. Of course the laminate contains significantly less glass and has a weight advantage over the monolith. Note also from Figure 3 that glass stress development in the SGP-laminate is not rate sensitive at the test temperature. The lower loading rate tests take on the order of 10 4 s to run and demonstrate that for temperatures well below the polymer glass transition temperature long duration loading does not result in a strength performance penalty due to interlayer deformation. In bending-dominated loading/support conditions the interlayer shear properties play a significant role in glass stress development as pointed out by Hooper [18,19]. Under such conditions SGP-based laminates are expected to significantly outperform PVB-based laminates. POST-GLASS BREAKAGE PERFORMANCE SentryGlas Plus was originally developed to enhance post-glass breakage performance in a cycling test component of new hurricane performance standards that are set to be adopted in certain regions of the USA. We demonstrate the benefits of enhanced interlayer stiffness of SGP-based laminates in the following test. Laminate plates consisting of 3 mm heat strengthened glass / 2.29 mm interlayer / 3 mm heat strengthened glass have been dry glazed into a frame, loaded under uniform pressure to glass breakage, and then the pressure cycled to study the maximum (center) deflection response of the fragmented laminate. Figure 4 shows the results of one such test. First consider the PVB-laminate response (dotted curve). Plate deflection increases with increased applied pressure until on lite of glass breaks; further loading causes the second lite to break. Continued loading causes significant plate deflection on the order of 100 mm for 2 kpa applied pressure and significant hysteresis during pressure cycling. Basic plate analysis [20] of the experiment, assuming that the plate is acting as a pure membrane, reveals that the effective modulus of this plate is on the order of 400 MPa. Consider now the SGP-laminate (solid curve) in Figure 4. First note that significantly more pressure is required to cause glass breakage than in the PVB-based laminate case. Second, once glass breakage occurs the SGP-based laminate demonstrates significantly less deflection and hysteresis than the PVB-based laminate. Analysis of this test reveals that the laminate modulus post-glass breakage is on the order of 12 GPa. Thus the stiffness properties of SGP result in a 30-fold increase in plate stiffness post-glass fracture. This reduced plate compliance reduces the tendency for interlayer tearing during loading of a cracked laminate and diminishes in-plane motion and tendency for glazing pullout from the framing system. 5 Material from

6 Figure 4. Deflection-applied pressure behavior for two laminates tested under uniform pressure, 4-side support. The PVB-based laminate displays significantly greater compliance than an SGP-based laminate, particularly after glass breakage. It is expected that the stiffness of a cracked laminate will scale with interlayer modulus. However, other factors contribute also to the resulting laminate compliance. These other factors include fragment size, which is determined by glass strength and glass type, and glass/polymer adhesion. Qualitatively, small glass fragments or high glass strength will reduce laminate stiffness and low glass/polymer adhesion will reduce laminate stiffness. Note that the full potential stiffening effect of SGP is not realized due to these other factors and glass fragments play a significant role in plate deformation characteristics. TEMPERATURE EFFECTS The polymer structure of SentryGlas Plus results in a glass transition temperature, Tg, on the order of C, the stiffness advantage of SGP versus traditional PVBs is maintained up to and exceeding the glass transition temperature. The advantage of this higher Tg is demonstrated in a design example for an overhead glass canopy in which laminated glass is required for safety performance. The question addressed in the example concerns the long-term creep performance of the point supported overhead structure under self-weight at 40 C. We ask the question: how will the canopy deflect over time and what effect will changing the interlayer from PVB to SGP have on performance? We approach this problem using our finite element design methodology used to construct wind load design charts discussed earlier. 6 Material from

7 Figure 5. Computed deformed shape of a point-supported laminate canopy after 30 years at 40 C under self-weight. Figure 5 shows the final deformed shape of a PVB-based laminate canopy after selfweight loading for 30 years at 40 C. One advantage of our finite element based approach is that we have constructed constitutive models for our polymers that have full time-temperature superposition capabilities that allow efficient calculation of rate and temperature effects [8,9]. Figure 6 shows the predictions deflection behavior. Figure 6. Computed maximum deflection of the canopy shown in Figure 5 as a function of time under self-weight at 40 C. Note that the SGP-based laminate canopy is predicted to display essentially constant deflection over time as compared to the PVB-based laminate, which is predicted to steadily deflect over time. 7 Material from

8 Consider first the upper curve for a PVB-based laminate. It is predicted to deflect over time until reaching a steady state condition after a million seconds or so. The lower curve in Figure 6 shows the predicted response for an SGP-based laminate of the same build. Note that overall deflections are lower than those predicted for PVBlaminates and that the deflection response is essentially stable with time for these conditions. Again the enhanced stiffness of SGP versus PVB results in significant performance enhancements in deflection response over time at elevated temperatures. CONCLUSIONS We have shown that a new interlayer, DuPont SentryGlas Plus, demonstrates significant structural performance advantages over traditional PVB in many applications. The enhanced structural performance results from increased interlayer stiffness and higher glass transition temperature compared to conventional PVB. The primary attributes resulting from a stiffer interlayer include: 1) enhanced strength, particularly where bending stress states dominate laminate deformation; 2) enhanced laminate stiffness both in pre and post-glass breakage conditions; 3) enhanced temperature performance. Such performance attributes present architects and engineers with more design options for optimum performance glass facades and structures. It is expected that these attributes will also extend the design possibilities for laminated glass. References 1. Behr, R.A., Minor, J.E., Linden, M.P., Vallabhan, C.V.G., Laminated Glass Units Under Uniform Lateral Pressure, Journal of Structural Engineering, 111[5] (1985). 2. Behr, R.A., and Linden, M.P., Load Duration and Interlayer Thickness Effects on Laminated Glass, Journal of Structural Engineering, 112[6] (1986). 3. Vallabhan, C.V.G., Minor J.E., and Nagalla, S., Stresses in Layered Glass Units and Monolithic Glass Plates, Journal of Structural Engineering, 113[1] (1987). 4. Das, Y.C. and Vallabhan, C.V.G., A Mathematical Model for Nonlinear Stress Analysis of Sandwich Plate Units, Mathematical Comput. Modelling, (1988). 5. Behr, R.A., and Norville, H.S., Structural Behavior of Architectural Laminated Glass, Journal of Structural Engineering, 119[1] (1993). 6. Vallabhan, C.V.G., Das, Y.C., Magdi, M., Asik, M., Bailey, J.R., Analysis of laminated glass units, Journal of Structural Engineering, 119[5] (1993). 7. Van Duser, A., Jagota, A., Bennison, S.J. Analysis of Glass/Polyvinyl Butyral (Butacite ) Laminates Subjected to Uniform Pressure Journal of Engineering Mechanics, ASCE, 125[4] (1999). 8. Bennison S.J., Davies P.S., Jagota A., Van Duser A., Smith C.A., Foss R.V., Structural Performance of Laminated Safety Glass, presented at Glass Tech Asia 2000, Singapore. 8 Material from

9 9. Ferry, J.D., Viscoelastic Properties of Polymers, 3 rd edition, John Wiley & Sons, (1980). 10. ABAQUS version 5.8, (1998) Hibbit, Karlsson & Sorensen, Inc., Pawtucket, R.I Weibull, W., A Statistical Distribution Function of wide Applicability, J. Appl. Mech., (1951). 12. Davidge, R.W., Mechanical Behavior of Ceramics, chpt 9, p , Cambridge Solid State Science Series, Cambridge Beason, W.L., Morgan, J.R., Glass Failure Prediction Model, Journal of Structural Engineering, 110 [2] (1984). 14. Brown W.G., A Practicable Formulation for the Strength of Glass and its Special Application to Large Plates, Publication number NRC 14372, National Research Council of Canada, Ottawa (1974). 15. Reed D.A., Fuller E.R. (Jr.), Glass Strength Degradation Under Fluctuating Loads, Journal of Structural Engineering, 111 [7] (1984). 16. ASTM Standard E Determining Load Resistance of Glass in Buildings in 1997 ASTM Annual Book of Standards, American Society for Testing and Materials, West Conshohocken, PA. 17. Norville, H.S., Minor, J.E., Strength of Weathered Window Glass, American Ceramic Society Bulletin, 64 [11] (1985). 18. Hooper, J.A., On the Bending of Architectural Laminated Glass, International Journal of Mechanical Sciences, (1973). 19. Norville, H. S., King, K. W., Swofford, J.L., Behavior and Strength of Laminated Glass, Journal of Engineering Mechanics (ASCE), 124 [1] (1998). 20. Timoshenko, S., Woinowsky-Krieger, S., Theory of Plates and Shells, McGraw-Hill, New York (1959). 9 Material from