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Impact on Laminated Glass: Post-breakage Behaviour Assessment Emmanuel Nourry – Development Engineer

Jean-Clément Nugue – Development Manager for Facade Products

CDI - Saint-Gobain Glass France Keywords

1 = Laminated glass 2 = Interrupted impact 3 = Energy dissipation 4 = Penetration resistance

Abstract

Thanks to its properties laminated glass, as used in building applications (protection for shops and commercial buildings, guarding, balustrading, etc.), is designed for many purposes (protection against risk of injury, against vandalism, burglary, etc.). Shock and penetration resistance, residual strength, reduction of fragments projection and laceration risk are evaluated by standard tests.

The European Standard EN 356 (categories P1A to P5A) defines the levels of protection against vandalism and burglary. The impact test method of this standard is a 4.1 kg steel ball drop test.

A better understanding of laminated glass fracture behaviour during this standard test will make it possible to predict its impact resistance. Laminated glass behaviour up to perforation is therefore investigated. Input kinetic energy dissipation is made through mechanical phenomena analysis. Then, using an original interrupted impact facility which discretizes damage evolution, we quantify the glazing ability to degrade impact energy versus time. The principle of this device is to vary the perforation distance of the projectile for the same input energy. Instrumentation of the latter gives us displacement evolution during the impact. Thus, the damage evolution can be described and compared with the energy dissipation in order to characterize the impact resistance of laminated glass to the standard test. Foreword

With laminated glass, glass becomes a safety product. Indeed, during a shock, if glass breaks, the interlayer retains the glass fragments thus avoiding the projection of fragments and the risk of laceration. It confers moreover on the glazing a residual rigidity guaranteeing the

stability of its processing and ensures the retention of the impactor. The combination of viscoelastic properties of polymer and brittle elastic behaviour of glass enables laminated glass to be used in many markets such as automotive (windshields), defense (armored glazings), aeronautic (cockpit windows) and building trade, as a material of protection against vandalism, burglary (shopwindows), accidental falls (guarding and balustrading), accidental shock and against the fall of objects on a glass roof.

The application fields of laminated glass, mentioned previously, evokes naturally very diverse kinds of impact: defined by input energies (mass, velocity), shapes and rigidities of contact (pointed or blunted, hard or soft), sizes of impact zones (concentrated or distributed), dimensions and shapes of glazings (flat or curved), which govern the impact behaviour of this type of material.

The intention of the European standardization authorities associated with the contribution of the industrialists was to define standards guaranteeing the properties of the laminated glass with respect to their applications and thus of a quite precise type of impact. Each segment of use of laminated glass is, in the building trade, defined by a standard characterizing their penetration resistance. The European standard, which is of interest to us, defines, in Europe, the protection against vandalism and burglary with the standard EN 356 classes P1A to P5A [1]. This standard (classes P1A to P4A) corresponds to a hard shock of a steel ball of 4.1 kg falling three times in three impact points (defined by an equilateral triangle of 130 mm side in the center of the glazing) on a laminated glass 1100 mm by 900 mm in horizontal configuration. The drop height and the configuration of the laminated glass vary according to the

class. The drop heights vary from 1.5 m to 9 m. Class P5A differs from the preceding ones by the number of impacts which is then nine for a drop height of 9 m. The penetration resistance is always the criterion of success to these standard impact tests.

To understand and predict the response of the laminated glass with respect to such types of shock, it is necessary to improve our comprehension of behaviour up to the perforation of laminated safety glass. Introduction

If, at low speed and more particularly

before damage, the impact behaviour of laminated glass can be described in a precise way by numerical modelling by coupling it with a probabilistic approach of the glass fracture [2, 3], the taking into account of mechanisms of rupture (fragile and ductile) and large viscoelastic strains is difficult. A numerical modelling of the perforating impact would require predicting the fragmentation pattern of the glass sheets and the behaviour suitable for each numerical fragment resulting from this rupture. That implies, on the one hand, a modelling by divisible elements strongly dependent on the size of the finite elements and on the other hand, to establish the interactions between the two competing mechanisms, i.e. the delamination and the large deformations of polymer interlayer, which lead to the initiation and the propagation of tears.

For all these reasons, the methodology used to understand and describe the laminated glass perforation under hard-body impact is based on an energy incremental approach. It is a question to experimentally discretize the global energy balance of the impact and to quantify the mechanical phenomena responsible for the input energy dissipation of the projectile.

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Energy balance

During an impact by a spherical steel projectile on laminated glass, a part of the input kinetic energy is transferred to the structure (laminated glass and attachment unit) then is converted into two categories of energy. This phenomenon is represented schematically on figure 1.

Fig 1 Energy balance of a hard body impact on laminated glass.

The projectile is considered

indeformable and does not damage itself during the impact.

The energies responsible for the input energy dissipation are the thermal and mechanical energies, which are coupled with regard to the mechanisms of deformation (viscoelasticity, viscoplasticity).

We consider in our energy

balance only the contribution of the mechanical energies to the dissipation of the input kinetic energy: - glass fragmentation - viscoelastic deformation of the

interlayer and its rupture - delamination at the interface

glass/interlayer - projection of glass fragments

The contribution of glass fragmentation in the energy balance of a hard shock was determined by the measurement of the surfaces created during the cracking of 44.2 laminated glass (two glass sheets 4 mm thick and an interlayer PVB 0.76 mm thick) 300 mm by 300 mm impacted by a steel ball of 4.1 kg falling 6 m. This energy represents only 1.5 % of the energy degraded during the impact.

In practice, the projection of glass fragments during the shock appears difficult to measure because it requires knowing the mass and the velocity of each glass fragment ejected. This energy can be approximate by measuring the total mass of ejected fragments and by considering a mean velocity common to each fragment, identified with a high-speed video camera. It was evaluated to be of the same order of magnitude as the energy dissipated by fragmentation.

Consequently, it is the deformation of polymer interlayer governed by the conditions of adhesion which, locally between the cracks of the glass sheets, promote the energy dissipation of the impactor. Indeed, the dynamic conditions of deformation of polymer interlayer are closely related on the fragmentation pattern of the glass sheets and thus to the adhesive characteristics of the laminated assembly [4, 5].

On the one hand, an excessively

strong adhesion would decrease local delamination at the interface glass/interlayer. This would lead to a more significant local deformation of the polymer interlayer which would promote its rupture in traction or its indentation by the glass fragments and thus the propagation of possible tears. On the other hand, a too weak adhesion limits the function of the laminated glass which is to retain the glass fragments and ensure the stability of its processing and durability.

In short, the major part of glass

fragments remaining stuck to interlayer and the value measured for the energy dissipated by fragmentation, lead us to neglect in the total energy balance of the impact these two mechanisms of dissipation. The major part of the energy dissipated during the perforation is thus by delamination and deformation and is strongly dependent on the fragmentation pattern. These two mechanisms are coupled and must thus be considered jointly to qualify the ability of the laminated glass to resist perforation with respect to a particular fragmentation pattern and thus of impact [6].

Incremental methodology

The objective of our analysis is to evaluate the evolution of the energy dissipation during the perforation and to define the conditions of initiation and evolution of the damage. The methodology generally used consists in varying the initial conditions. However, it does not make it possible to describe in a rigorous way the evolution of the damage for the same mass and the same velocity of projectile. An alternative to this technique is the use of optic devices such as high-speed video camera but the identification and the quantification of dynamic phenomena are not easy. If high-speed video camera can contribute to the global quantification of the impact, the phenomena of fragmentation and tearing initiated locally, are not easily observable and require significant technical means. We then decided to develop an interrupted impact device, in the same spirit as those defined by Lataillade et al. in static (traction) and dynamic (Hopkinson’s bars) to describe the impact behaviour of a composite material [7].

This impact device makes it possible

to stop the impactor during the perforation of the target (figure 2).

Fig 2 Schematic diagram of the interrupted impact device.

Thus, for the same input energy, this

device makes it possible to vary the penetration distance of the impactor in the target (the impactor run being controlled after contact with the laminated glass) and thus to follow the damage evolution and the variation of corresponding energy (figure 3).

The principle of this impact device is inspired by the impact device used by Bouzid et al. [8] to study the damage evolution of glass plates. It consists in propelling, using a compressed air launching tube, a cylindrical bar or projectile which strike (transfer of momentum) an impactor which impacts the laminated glass (figure 2 and 4).

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Fig 3 Principle of the experimental discretization of the damage and the evolution of the kinetic energy during an impact.

Fig 4 Photography of the interrupted impact device: buffer, projectile and velocity measurement device, instrumented impactor and attachment unit of the glazings.

The velocity of the impactor depends

on the velocity of the projectile, which we can vary from 5 with more than 15 m.s -1.

The impactor whose mass is 4 kg (identical to that of the steel ball of the standard EN356) moves on a fixed distance of 150 mm, where it encounters a buffer.

The quasi elastic shock which results from it, makes the impactor rebound, which is then stopped using hydraulic dampers. Because the impactor always stops at the same position, the penetration distance is defined by positioning, to the tenth of a millimeter, the laminated glass compared with the end of the impactor in its initial position.

The impactor is instrumented in order to measure the evolution of its displacement during the perforation. This data enables us to characterize the response of the laminated glass during the perforation.

Initially, we considered one impact of a hemispherical projectile on a 300 mm by 300 mm laminated glass in vertical configuration and fixed with circular simple supports. This type of impact makes it possible to understand and quantify the local behaviour of a laminated glass to the impact of a projectile identical to that of the European standard EN356.

Energy dissipation and damage evolution

Several 300 mm by 300 mm 44.2

glazings assembled with a polyvinyl butyral interlayer (PVB) and fixed with circular simple supports (Ø 280 mm), were impacted for an impactor velocity of 8.7 m.s-1 (+/- 0.5 %) and penetration distance varying from 0 to 70 mm every 10 mm (figure 5).

The approximate duration of the non-interrupted impact is 15 ms and the impact energy is approximately 150 J.

When the glazing is positioned 150 mm from the end of the impactor, the penetration distance is close to zero. This penetration distance makes it possible to illustrate the initiation of the damage.

For this level of damage, radial cracks initiated in the center of the glazing on the opposite side to the impact point are preponderant. Circumferential cracks are in large part present in the fixings area. For a penetration distance of 10 mm, the radial cracks are more numerous and the compact zone of fragmentation under the impact point is clearly marked. The circumferential cracks remain localized in the area of the fixings. For the three next increments (20, 30 and 40 mm), the diameter of the strongly cracked zone under the impact point does not progress any more and is followed by numerous other radial cracks. Between the

increments of 40 and 50 mm, the polymer interlayer is torn between the zone of strong fragmentation and that made up mainly of radial cracks. This tear then progresses along the radial cracks, but from the penetration distance of 60 mm, the damage does not change any more and all the impact energy is degraded by the laminated glass. Although the laminated glass is torn, the impactor does not pass through it, but rebounds.

At this stage, the interrupted impact device shows its capacity to discretize the damage evolution.

In the case of a perforating impact,

we will compare the damage evolution with the energy dissipation of the impactor.

Eight 44.2 300 mm by 300 mm

glazings assembled with PVB interlayer, are impacted for an initial velocity of approximately 9.2 m.s-1, which corresponds to an input kinetic energy of 170 J. The temperature of the glazings during the tests was approximately (20 +/- 2) °C.

The penetration distances associated with these eight tests are: 20, 40, 50, 60, 80, 100, 120 and 140 mm.

The curves of impactor displacement for each penetration distance are represented on figure 6. They represent the impactor displacement evolution after contact with the laminated glass.

The experimental curve giving the evolution of the impactor displacement after contact with the glazing, for a penetration distance of 140 mm, is derived and gives us the evolution of the impactor velocity and thus the variation of kinetic energy.

We compare this curve with the various states of damage of the laminated glass obtained during the interrupted impacts to the penetration distances: 20, 40, 50, 60, 80, 120 and 140 mm (figure 7).

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Fig 5 Experimental discretization of the damage evolution for an input energy of 150 J. The first impact corresponds to a penetration distance close to 0 mm, that is 150 mm between the impacted side of the laminated glass and the end of the impactor in initial position. For positions 50, 60 and 70, the laminated glass is torn (T), but the impactor bounces. The impact velocity is indicated for each test and is approximately 8.7 m.s -1. The temperature of the laminated glass is 17°C.

Fig 6 Curves showing the evolution of the impactor displacement after contact with the laminated glass during interrupted impact tests at various penetration distances. The impact velocity is 9.2 m.s -1 which corresponds to an input kinetic energy of 170 J. The temperature of the laminated glass during the tests is 20°C.

The laminated glass is perforated for

the penetration distance of 140 mm, the impactor retains a part of its initial kinetic energy. This part represents approximately 13 % of the initial kinetic energy.

The energy dissipation is significant in the first moments of the impact. 70 % of the kinetic energy is degraded before 9 ms of perforation. For a penetration distance of 60 mm, the interlayer is torn, whereas for a

penetration distance of 50 mm it is not. We can therefore deduce that 60 to 70 % of the initial kinetic energy of the impactor is degraded by the laminated glass before initiation of a tear in the interlayer. Thereafter, the laminated glass continues to degrade energy by deformation of interlayer and this mechanism is coupled with the propagation of the tear.

Since the part of energy represented

by the mechanisms of fragmentation and projection of fragments is small, the variation of energy estimated represents primarily the degradation of energy by deformation of interlayer and local delamination in the area of the cracks. The major part of input energy is degraded before initiation of a tear in interlayer.

Comparison between the evolution of the impactor kinetic energy and the damage evolution enables us to define two criteria, which are characteristic of the impact behaviour and penetration resistance of laminated glass.

First, the trend of the kinetic energy evolution of the impactor gives us information on the swiftness and the importance of the energy dissipation.

This trend makes it possible to quantify the severity of the response of the laminated glass with respect to the impactor and throughout all impactor/target interaction. This severity results in a significant dissipation of energy into a very short time and defines the law of deceleration of the impactor.

Secondly, we can locate the time of initiation of the tear in interlayer, which is a criterion of penetration resistance of the laminated glass and can be used to compare different kinds of assemblies.

This time of initiation, associated with its corresponding energy dissipation, defines what one can call the "global tenacity" of the laminated glass.

Fig 7 Evolution of the impactor kinetic energy after contact with the laminated glass during an interrupted impact for a penetration distance of 140 mm and fragmentation patterns associated with the various penetration distances. The impact velocity is 9.2 m.s-1, which corresponds to an initial kinetic energy of 170 J. The temperature of the laminated glass during the tests is 20°C.

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The energy dissipation and the damage evolution allow a finer comparison of the impact behaviour of laminated glass. The degradation of the energy of the impactor is quantified and confirms the importance of the interlayer deformation in the energy balance. Moreover, it makes it possible to identify the law of deceleration of the impactor.

The interrupted impacts define the criteria of penetration resistance and impact behaviour by the identification of the time of initiation of the tear in interlayer and by the quantity of energy able to be degraded before this rupture. Conclusions

The analysis of the energy balance of the hard body impact on laminated glass confirms the small part of energy degraded by fragmentation and projection of glass fragments. The interlayer deformation, governed by the properties of adhesion, dissipates the major part of the kinetic energy of the impactor.

This dissipation can be evaluated using the interrupted impact device which we developed. Compared with the damage evolution, the measurement of the energy dissipation makes it possible to characterize and quantify the impact response of the laminated glass. The interrupted impacts make it possible to describe the processes of damage and their dynamics.

This analysis can be extended to the study of soft body impact such as that defined by the European standard EN 12600 [9]. Finally, incremental methodology is a validation tool for future modellings of the impact behaviour of laminated glass.

References [1] European Standard NF EN 356. Glass in

building – Security glazing –Testing and classification of resistance againts manual attack. September 2000.

[2] Vidal B. Modelling impact on laminated architectural glass. Glass Processing Days, 1999.

[3] Vidal B. Modélisation d’impacts sur vitrages feuilletés. Ph. D. of the Ecole Nationale Supérieure des Arts et Métiers, 1998.

[4] Seshadri M., Bennison S.J., Jagota A, Saigal. S. Mechanical response of cracked laminated plates. Acta Materialia 2002 ; 50:4477-4490.

[5] Hunstberger J.R. − Adhesion of plasticized poly(vinyl butyral) to glass. J. Adhesion, 1981 ; 13:107-129.

[6] Nugue J.C., Nourry E. Toughness, Resiliency and adhesion of polyvinylbutyral (PVB) interlayers with regards to impact resistance. Glass Processing Days, 2003.

[7] Lataillade J.L., Delaet M., Collombet F., Wolff C. − Effects of the intralaminar shear loading rate on the damage of multi-ply composites. International Journal of Impact Engineering 1996 ; 18(6):679-699.

[8] Bouzid S., Nyoungue A., Azari Z., Bouaouadja N., Pluvinage G. − Fracture criterion for glass under impact loading. International Journal of Impact Engineering 2001 ; 25 :831-845.

[9] European Standard NF EN 12600. Glass in building – Pendulum test – Impact test method for flat glass and performance requirements. September 2003.