Blast Impact Resistance Laminated Glass Structures

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Blast and Impact Resistance of Laminated Glass Structures

Mr P. Hooper, Mr H. Arora and Dr J. P. DearDepartment of Mechanical Engineering, Imperial College London, SW7 2AZ, UK

[email protected]

Abstract

Glass fragments produced by explosions pose a signicant risk of injury to those close by. Laminated glass canhelp mitigate these risks. Full scale blast testing of laminate windows was performed with charge sizes from 15-500kg at ranges of 10-30 m. Full-eld deection measurements of the window pane were obtained using high-speed3D digital image correlation (DIC) along with load measurements at the joint. A high-speed servo-hydraulic machine

was used to replicate the high strain-rates seen under blast loading. Cracked laminated glass was loaded in tensionat varying rates to determine the stress-strain response. Delamination between the interlayer to glass interface wasobserved using high-speed photoelasticity. These experiments can provide input data for models of blast responseof laminated beyond the fracture of the glass plies. Assumptions made in current design standards were found tobe not valid in some cases. Interlayers of thickness less than 1.52 mm were found to fail prematurely and shouldtherefore be avoided in blast resistance designs.

1 Introduction

Buildings with prominent glazed facades make ideal targets for terrorists aiming to maximise human casualties

and perceived damage. Annealed window glass is a brittle material that offers little resistance to the shock wavesproduced by explosions and creates sharp fragments that can travel at high velocity if it fractures. Historically, themajority of injuries from bomb blasts have been from glass fragments[1]. To mitigate this laminated glass is usedto protect building occupants by retaining fragmented glass to a polyvinyl butyral (PVB) interlayer. After the glassplies fracture, the PVB interlayer between the cracked glass continues to offer resistance to the blast wave. To beeffective the laminated glass needs to be well held to a supporting structure, usually with a structural silicone joint. Ifthe joint is not strong enough, the pane could detach from the structure and y into the building, injuring occupants.The work presented in this paper is part of a research project at Imperial College London initiated by Arup SecurityConsulting. The aim of this research is improve the understanding of the behaviour of cracked laminated glass andloading on the joint so that current design standards can be improved, helping structural engineers optimise theirglazing designs for a specic blast threat.

2 Background

Three current standards address the design of glazing to reduce to hazards in a blast, UFC 4-010-01 and ASTM F2248 in the US and the Glazing Hazard Guide in the UK. The UFC standard prescribes a minimum laminatedglass make-up of a 0.75 mm PVB interlayer between two 3 mm annealed glass layers and a structural siliconebite depth of 9.5 mm[2]. This guidance is said to be valid for blast loading up to a 33 kPa peak pressure with a

Proceedings of the IMPLAST 2010 ConferenceOctober 12-14 2010 Providence, Rhode Island USA

2010 Society for Experimental Mechanics, Inc.


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(a) High-speed camera setup.

Steel subframe

Silicone joint

Laminated glass

(b) Frame cross-section showing strain gauge pair.

Figure 2: Details of instrumentation.

3.1 Image correlation

High-speed 3D digital image correlation (DIC) was used to track the full rear-surface position of the window at 1 msintervals during each blast test. Two synchronised high-speed cameras with a resolution of 1024 1024 pixels weremounted inside the test cubicle at a working distance s w from the test window and centred on the window centrepoint. The camera setup used is shown in Figure 2a. A high-contrast speckle pattern was applied to the rear ofthe window using acrylic paint to allow correlation of position between the two cameras. Paint was also applied tothe front of the window to block out light from the explosion. The system was then calibrated using a calibrationgrid before the test. After the test the captured images of the deformation were imported into the ARAMIS imagecorrelation software (produced by GOM mbH) to compute 3D position and strain of the window.

3.2 Edge reaction forces

Pairs of foil strain gauges were bonded to a steel window frame at the midpoint of each frame edge to measureedge reaction forces. The position of the gauges on the subframe cross-section is shown in Figure 2b. The strainreadings from each gauge can be used to calculate the tension in the cracked laminate, F , at an angle of pull, , atthe joint by considering the subframe as a built-in cantilever beam. Measurements for the angle of pull were madefrom analysis of the DIC results to allow the direct calculation of tension in the PVB.

3.3 Results from a 7.52 mm laminated pane

The example results presented here are from a test on a 1.5 1.2 m laminated pane using a charge weight of 30 kg(TNT equivalent) at 14 m. This charge weight and range created a peak reected pressure and reected impulseof 127 kPa and 413 kPa-ms respectively. The laminate used was constructed from two 3 mm annealed glass pliesand a 1.52 mm PVB interlayer. A 20 mm deep single sided structural silicone joint was used the bond the glass toa steel window frame. Traces of central deection, velocity and acceleration vs time are shown in Figure 3a. Thepane began to move at 19 ms and rapidly accelerated up to a velocity of 29 m/s before failure of the joint at 26ms. A peak acceleration of approximately 6 km/s 2 was recorded in this rst period. A displacement of 140 mmwas recorded at the time of joint failure. No tearing of the PVB interlayer was observed. The pane deected over250 mm before the DIC could no longer track it due to excess light entering around the failed joint. After this pointthe pane continued to travel inwards at approximately 30 m/s until it impacted a frame protecting the high-speed


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Figure 3: Results from a 30 kg charge at 14 m.

camera equipment. Figure 3b shows the angle of pull at the frame and the the tension in the cracked derived fromthe strain gauge readings. It shows the laminate forms a 30 angle with the frame edge at the time of failure and

that tearing of the joint started at approximately 25 ms with an edge load of about 20 kN/m. Tension in the laminatevaried between 20-30 kN per unit width, corresponding to a stress in the PVB of between 13-20 MPa.

Figure 3c shows a cross-sections of displacement taken horizontally across the centre of the window. Each line isplotted at 2 ms intervals ending with the line of largest deection at 30 ms. The lines clearly show a relatively atcentral region deecting into the cubicle and deformed curved regions close to the edges. As the pane deectsfurther the at central region becomes smaller until the whole prole is curved. This is due to the restraint at theedges causing transverse waves to propagate inwards towards the centre from each edge. The same effect isseen in the image sequences presented in Figure 4. The contour lines on the out-of-plane deection plots areapproximately rectangular in shape and are spaced tighter close to the window edges. This indicates that thedeformed areas are concentrated around the window edges and that the centre region of the window is largelyat and undeformed. The maximum principal strain plots show how the strain was concentrated near the edges,reaching about 8% in the corners and 5-6% near the edges. Maximum strain-rates were also calculated and were

in the order of 15 s 1

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Under impulsive blast loading the window pane rapidly accelerates and quickly acquires an approximately uniformvelocity eld across its surface. If the blast wave duration is short the subsequent deection occurs almost entirelydue to the momentum of the pane. The restraint at the edge causes a transverse deceleration wave to propagateinwards from each edge towards the centre. The ratio between the transverse wave speed and the inward velocityis crucial to the response of the window. A fast inward velocity and slow transverse wave speed will cause a largeundeformed central region, with strain and curvature concentrated near to the edges. Strain-rate in this region willalso be high and could to lead to tearing of the PVB around the edges. A slow inward velocity and fast transversewave speed will allow the strain and curvature to develop over a larger area and will be lower in magnitude.


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4 Tensile testing of cracked laminated glass

Tension in the cracked laminated glass, as observed in the blast tests, was reproduced in the laboratory using ahigh-rate servo-hydraulic tensile test machine. Test samples were prepared from 150 60 mm laminated glass stripswith 3 mm thick glass plies and PVB interlayer thicknesses ranging from 0.38 mm to 2.28 mm. The glass plies in thesample were fractured before testing to create fragments that were similar to the crazed glass seen in a blast test.This was achieved by scoring the glass at regular intervals with a purpose built jig and initiating cracks along thescore line by gently tapping with a hammer. Using this method it was possible to produce a regular and controlledpattern, enabling the effect of fragment size to be investigated. Figure 5 shows an edge-on view of the crackedlaminate. Under tension the PVB interlayer delaminates from the glass fragments and forms a ligament that bridgesthe gap between glass fragments.

Figure 5: Edge-on view of cracked laminated glass under tension.

A force vs extension curve for a cracked glass sample with a 1.52 mm interlayer is shown in Figure 6a. The glassfragment size in the sample was 10 mm and the test was conducted at 3 m/s, given a nominal strain-rate of 20 s 1 .It can be seen from the graph that the tension rises quickly to a value of about 24 kN/m as the end of the samplewas displaced. The tension was then relatively constant until the sample tore at a strain of 120%. This sharp rise

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Figure 6: Results of high-rate tension tests on cracked laminated glass with a 1.52 mm PVB interlayer.


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Figure 7: Face-on view of delamination in a 1.52 PVB cracked laminate sample at 30 s 1 .

Figure 8: Delamination and tearing of a 0.76 mm PVB interlayer at 2 m/s.

followed by a nominally constant force was typical of most tests. The initial sharp rise is due to the elastic extensionof the PVB bridging cracks in the glass plies. Here, the effective length of the sample is very small, and the strainin the PVB is concentrated around the cracks, giving local strain-rates that are much higher than the nominal strain-rate. Delamination of the PVB from the glass changes the length PVB that is able to extend and nominal forceis reached when the force required to progress the delamination is reached. Figure 6b shows the nominal forcereached as the strain-rate is increased. In the blast test shown in Section 3.3 a strain-rate of 15 s 1 was observed.The nominal force recorded using this test method was between 20-25 kN/m at that strain-rate and agrees well withthe tension values calculated from the blast test.

PVB is birefringent and a colour high-speed camera combined with a polariscope was used to observe the delamin-ation of the PVB from the glass fragments. Figure 7 shows a cracked laminate sample with a 10 mm fragmentspacing and a 1.52 mm PVB interlayer tested at 30 s 1 . The sequence shows a face-on view of the glass frag-ments, the progression of delaminated area around the cracks and the area of the fragment that is still bondedto the PVB. In this test the delamination fronts progressed far enough so that the glass fragments were no longerbonded to the PVB before the PVB failed. This is clearly not a desirable case in a blast event, defeating one re-quirement of the interlayer, and therefore the maximum nominal strain reached in any model needs to be limited.Complete debonding of the was not seen as often in interlayers thinner than 1.52 mm. Figure 8 shows a crack ona laminate sample with a 0.76 mm PVB interlayer. The sequence is focused on a single crack and shows how the


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delamination front does not propagate far into the fragments before the the PVB tears. The reason for this is that thedelamination front propagates slower due to the reduced force exerted by the thinner interlayer for a given strain.The length of PVB that is able to extend does not increase quickly enough to relieve the build up of strain in thebridging ligaments. Therefore the PVB reaches its failure strain quickly and tears. In a blast event this would resultin the laminate tearing near the edges and the whole laminate would enter the building as one piece at high velocity.For this reason PVB interlayers below a thickness of 1.52 mm should be avoided in blast resistant designs.

5 Conclusions

In this paper selected results from a research project into the post-fracture behaviour of laminated glass underblast loading have been presented. It was shown that some of the assumptions made in current design standardsare not valid in some cases. Specically the deection prole under blast loading can differ signicantly from theassumed static deection prole. Measurements of the deection prole of a window during a blast test, made usinghigh-speed digital image correlation, showed that the window was undeformed across the centre with deformationand strain concentrated near the edges. Maximum principal strain and strain-rate reached approximately 8% and15 s 1 . Tension values between 20-30 kN/m were measured in the cracked laminate during the blast. High-ratetension tests on cracked laminates recorded a nominal tension value of 24 kN/m at similar strain rates. It wasshown that thin interlayers can fail a low extensions due to a concentration of strain in the interlayer bridge betweenfragments. This was caused by a slow moving delamination front between the PVB and glass fragments. Use ofPVB interlayers below 1.52 mm in thickness should therefore be avoided because of this effect.

Acknowledgements

We thank the Engineering and Physical Sciences Research Council (EPSRC) and Arup Security Consulting (MrD. Hadden, Mr D. Smith and Mr R. Sukhram) for supporting Mr P. Hooper and Ofce of Naval Research (Dr Y.Rajapakse) for supporting Mr H. Arora.

References

[1] Smith, D. Glazing for injury alleviation under blast loading: United Kingdom practice. In Glass Processing Days Conference Proceedings , 335340 (Tampere, Finland, 2001).

[2] Department of Defense. Unied Facilities Criteria: DoD minimum antiterrorism standards for buildings. UFC 4-010-01 (2003).

[3] Norville, H. S. & Conrath, E. J. Blast-resistant glazing design. Journal of Architectural Engineering 12 , 129136(2006).

[4] ASTM. Standard practice for specifying an equivalent 3-second duration design loading for blast resistant glazing

fabricated with laminated glass. F2248-09 (2009).

[5] ASTM. Standard practice for determining load resistance of glass in buildings. E1300-09a (2009).

[6] Security Facilities Executive Special Services Group - Explosion Protection. Glazing hazard guide (RESTRIC-TED). Cabinet Ofce, London (1997).

[7] Biggs, J. M. Introduction to structural dynamics (McGraw-Hill, 1964).

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Blast Impact Resistance Laminated Glass Structures