Blast Performance

Introduction

Aesthetic desires to provide transparency in podium levels of buildings increase the potential use of glass in its construction. These lower levels of a building are most vulnerable for nearby blast events. Blast events can create extreme loads that damage the interior as well as projecting debris towards the exterior, creating significant danger to people and property alike. Laminated glass façades are commonly used in these building structures when concerned with the potential for a blast event. There have been standards developed for sizing the geometry of the structural silicone sealant when attaching laminated glass designed to withstand blast loading. For example, ASTM F2248, Standard “Practice for Specifying an Equivalent 3-Second Duration Design Loading for Blast Resistant Glazing Fabricated with Laminated Glass” recommends to design the structural sealant joints with a thickness of at least 5mm and a width (bite) at least equal to or greater than 10 mm or the nominal thickness of the glass to which it adheres, but less than two times the nominal thickness of glass. It is, however, known that the joint geometry and material properties strongly influence the performance of bonded façades under various loads. Therefore, a numerical modelling approach is highly desirable to understand structural sealant joint performance under blast loading, which involves a combination of tension, compression and shear loading.

Silicone sealant performance was explored under blast loading through both numerical modelling and experimental testing. Glazing assemblies were fabricated and tested at a blast test facility using explosives charged in a shock tube. The first step was a comparative analysis was completed between two test samples looking at observed behavior of the glass and silicone performance versus the predictive model. Next, a collaborative effort between authors improved the alignment of the modeled behavior of the glass to the behavior that was observed. The aim of these studies is to provide a roadmap for robust modeling practices to inform design.

Blast Test for Laminated Glass Façades

The blast tests for the silicone bonded Laminated Glass façades were performed at University of Kentucky underground test laboratory, as shown in Figure 1. The silicone sealant joint was prepared in a temperature and humidity-controlled lab by bonding laminate glass onto welded metal frames of either steel or aluminum. Then, the silicone bonded laminate glass façades were assembled onto a shock tube for the blast test. The silicone structural bite of the test samples were varied to understand potential influence of bite dimensions on performance. Glass thickness was a nominal 6.35 mm heat strengthened (HS) by 2.28 mm polyvinyl-butyral (PVB) by 6.35 mm HS laminate. Explosives were charged into the interior of the shock tube and positioned a specific distance from the glazing assemblies to control the peak pressure and impulse delivered to the glazing. The blast loadings for the two shots described in this paper are summarized in Table 1.

Figure 1: Blast Test Setup for Glass and Sealant Joint Performance Evaluation at University of Kentucky

Summary of Material and Their Properties used in Blast Test

Table 1: Summary of Material and Properties Used in Blast Test

The material properties for the laminated glass were estimated from published literature on laminated glass blast tests that included modeling. Typical material properties for Glass and PVB laminate layer are summarized in Tables 2-4 [1-3].

Table 2: Glass Properties of the Laminated Glass

Table 3: PVB Properties in the Laminated Glass

Table 4: Prony Series Parameters for PVB in the Laminated Glass

The structural sealant tested in this blast test is a two-part silicone sealant, and its simple tension, planar tension, and biaxial tension property measurement are shown in Figure 2. Its fitted Hyperelastic material properties and Viscoelasticity (Prony Series) are summarized in Tables 5 and 6.

Figure 2: Simple Tension, Planar Tension and Biaxial Tension Property for the 2-Part Structural Silicone Sealant

Table 5: Material Properties and Hyperelastic Model for Structural Sealant

Table 6: Prony Series Parameter for the Structural Sealant

Development of Blast Model and Experimental Comparison with Case 1

A finite element analysis (FEA) model has been developed to predict laminated glass and silicone sealant performance under blast loading. The FEA calculation was implemented in a commercially available FEA software, LS-DYNA. The Case 1 simulation is shown in Figure 3, where a 1.219m x 1.829m laminated glass is bonded to a rigid metal frame through a 6.35mm (thickness) x 12.7mm (width) structrual sealant.

Figure 3: FEA modeling for Case 1 Laminated Glass Design and Blast Test

In the Case 1 blast analysis, 56.7kg TNT equivalent explosive is ignited at 19.2m distance away from the outer layer of the laminated glass. As shown in Figure 4, the laminated glass will reach a maximum inward deflection of 0.0597m after 0.0492s of explosion under the positive pressure wave. Then, the laminated glass deflects outward to 0.05199m due to the negative pressure wave.

Although both plies of glass crack in the laminate, no structural sealant failure is observed in the calculation. The FEA results predict the peak effective strain in the structural sealant to be 0.528 during the blast, which is less than the effective strain limit (1.26) of the sealant, as shown in Figure 5. Hence, only glass laminate will fragment in the Case 1 blast loading case, the structure sealant is expected to hold the glass laminate onto the metal frame after the blast loading.

Figure 4: Case 1 Blast Test Modeling for a 1.219m x 1.829m Laminate Glass bonded by 6.35mm x 12.7mm silicone sealant joint

Figure 5: Peak Effective Strain Prediction in the silicone sealant during Case 1 Blast Test

A blast test has been performed for the Case 1 simulation condition, as shown in Figure 6. During this blast test, the glass lite fragmented but was still retained on the frame by the silicone sealant without evidence of sealant rupture. This demonstrates that the FEA model for the blast test is reasonablely accurate for predicting glass laminate and silicone sealant performance under blast loading.

Figure 6: Still photographs at different times of blast test Case 1 of laminated glass bonded to the frame with silicone sealant

Blast Test and Analysis for Glass Bonding Configuration Case 2

With the FEA model developed for the blast test, different glass bonding conditions have been evaluated by both numerical simulation and blast test. As shown in Figure 7, the Case 2 study evaluated the performance of a 1.524m x 1.524m laminate glass under same peak postive pressure (89.135 kPa) and blast load distance (19.209m) as in Case 1. The silicone sealant joint dimensions are the same in Case 1 and Case 2 at 6.35mm x 12.7mm. The main difference between Case 2 and Case 1 is that Case 2 has a 4% larger surface area, and it has equal edge lengths for all sides.

Modeling of the Case 2 laminate glass blast test predicted that the laminated glass will crack under the blast loading, but the silicone sealant will not fail, as shown in Figure 8. Modeling indicated that slightly higher peak effective strain (0.603) will develop in the sealant joint in Case 2 compared to Case 1 (0.528) due to its larger surface area, as shown in Figure 9. As this peak effective strain in the sealant joint is lower than its strain limit of 1.26, the silicone sealant will not fail.

Figure 7: FEA modeling for Blast Test Case 2 with 1.524m x 1.524m Laminate Glass Design

Figure 8: Simulation of Case 2 Blast Test on a 1.524m x 1.524m Laminate Glass bonded by 6.35mm x 12.7mm silicone sealant joint

Figure 9: FE Analysis Depicting the Peak Effective Strain Prediction in the silicone sealant during Case 2 Blast Test

Figure 10: Blast Test for Case 2 - 1.524m x 1.524m Laminate Glass bonded by 6.35mm x 12.7mm silicone sealant

Blast Test and Analysis for Glass Bonding Configuration Case 3

The Case 3 glass laminate features the same sealant joint design as Case 2, but it is different from Case 2 in that the Blast loading in Case 3 occurs from the inner surface of the glass laminate, as shown in Figure 11. From FEA simulation, it is predicted that the glass laminate will be blown away under the blast due to silicone sealant failure, as shown in Figure 12. This indicates that silicone sealant location is critical in the façade design under blast loading. The Case 3 glass laminate layout was then tested by applying same peak postive pressure (89.135 kPa) and standoff distance (19.209m) as Case 1 and Case 2 towards the inner surface of the glass laminate. It was confirmed from test that silicone sealant did fail in the test (see Figure 13), as predicted by the FEA model.

Figure 11: Case 3 with 1.524m x 1.524m laminate Glass and 6.35mm x 12.7mm silicone sealant, Blast From Inner Surface

Figure 12: FEA modeling for Blast Test Case 3 with 1.524m x 1.524m Laminate Glass and Blast From Inner Surface

Figure 13: Still photographs at different times of blast test Case 3 of laminated glass bonded to the frame with silicone sealant

Improving the Glass Breakage Model

Utilising the properties detailed above in Table 4 and Table 5 and incorporating the cohesion properties from Aggromito et al. [4], the same approach for modelling laminated glass was assessed against the two tests completed in the experimental program at the University of Kentucky. Table 7 provides a comparison between the peak pressure and impulse from Aggromito et al. [4] and the measured loads and impulse from the testing at the University of Kentucky. Two load conditions were chosen as the one at 13 psi did not exhibit glass breakage whereas the load approaching 14 psi resulted in glass fracture. Loads for analysis were generated via curve fitting of an exponential decay curve to the peak pressure, impulse, and duration observed in the experiments including the negative phase of the blast.

Table 7: Summary of Blast Tests

Expanding on the modeling approach detailed in the sections above, the Glass layer in this section is modelled using T-shells and MAT_280 (MAT_280 is not developed for solid elements). MAT_280 was chosen for this study to try to better capture the crack patterns witnessed in the experiment. In the development of cracks for MAT_280, a crack occurs perpendicular to the maximum principal stress direction as soon as tensile failure occurs. Table 8 provides the material properties input to represent the glass in the initial assessments and are based on the values provided in Aggromito et al, [4]. The material inputs for the silicone are as detailed in Table 5 above.

Table 8: Model Parameters for Glass

Figure 14: Model Set-up

Figure 15: Lay-up Supported by the Frame

Results

Cracking was witnessed in the simulation for both pressure and impulse loadings when using the glass material properties in Table 8. As the strength of glass varies with strain rate, the scale factor for tensile strength (FTSCL) was varied until cracking was witnessed for a pressure load of 14 psi and cracking was not witnessed for a pressure loading of 13 psi, consistent with the test observations. Consequently, for this glass type and particular strain rate, an FTSCL of 2.705 was required to correlate to the experiment for cracking to occur.

Figure 16: Comparison of Cracking Dependent on Load left) 13 psi, right) 14 psi

Figure 17 presents a comparison of the displacement of the pane for the analysis and the experiment for a loading of 14 psi with the simulation results visually overlaid on stills from the high-speed video recording of the test. As is demonstrated, the displacements correlate well between the analysis and the experiment throuhgout the positive excursion and rebound of the glass.

Figure 17: Comparison of Displacement between the Analysis and Experiment at Different Time Steps.

Figure 18 presents a comparison of the crack pattern between the analysis and the experiment. While the crack patterns do not line up completely between the simulation and the experiment, there is generally good correlation between crack pattern and density in the simulated and observed results. Around the edges of the pane, the simulation predicts slightly less cracking than the experiment, however, as the crack propagates from the corners towards the center, a similar cracking pattern is witnessed between the simulation and experiment.

Figure 18: Comparison of the Cracked Pane between the Experiment and the Analysis

Conclusion

From this study, it was confirmed that laminate glass and silicone sealant performance under the blast loading can be accurately predicted by FEA model. Peak effective strain is a critical design parameter for silicone performance modeling under blast loading.

With same sealant joint design, same glass laminate stack design, larger glass surface area will cause a high strain into the sealant joint when total length of the sealant joint bite is the same.

With identical sealant joint design, glass laminate design and surface area, it is found the direction of the blast loading is critical in affecting the failure of the sealant joint. When blast is applied from the out surface of the glass, sealant is mostly under compression loading during the peak positive pressure wave, and the sealant less likely to fail. However, if the silicone sealant is applied on the same side of the glass where blast loading is applied, it is mostly undertaking tension during the initial blast wave, and it is more likely to fail and result in whole laminate glass unit detached from the building structure.

[1] Silicone structural glazing under blast loading, Valérie Hayez, Jon Kimberlain, Jie Feng, Sigurd Sitte, Mark Mirgon, Glass Struct. Eng, 2022

[2] Descamps, P., Durbecq, S., Hayez, V., Chabih, M., Van Wassenhove, G.: Dimensioning silicone joints used in bomb blast resistant facade systems. In: Proceedings of Challenging Glass 6, Conference on architectural and structural applications of glass, Belis Bos & Louter, Ghent University (2018)

[3] Aggromito, D., Tartasky, M., Wholey, W., Farley, J., Klimenko, J., Pascoe, L.: Modelling laminated glass in LS-DYNA under extreme loading conditions. In: 13th European LS-Dyna Conference, Ulm Germany (2021)

[4] Aggromito, D., Pascoe, L., Klimenko, J., Farley, J., Tatarsky, M., Wholey, W. 2022, Simulation of PVB-glass adhesion and its influence on the blast protection properties of laminated safety glass. International Journal of Impact Engineering, Vol. 170

[5] Hooper P. PhD Thesis. Imperial College London; 2011.