Impact response of laminated glass with varying interlayer materials

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Impact Response of Laminated Glass with Varying Interlayer Materials Xiaowen Zhang , Haibao Liu , Chris Maharaj , Mengyao Zheng , Iman Mohagheghian , Guanli Zhang , Yue Yan , John P. Dear PII: DOI: Reference:

S0734-743X(19)30983-2 https://doi.org/10.1016/j.ijimpeng.2020.103505 IE 103505

To appear in:

International Journal of Impact Engineering

Received date: Revised date: Accepted date:

9 September 2019 15 January 2020 15 January 2020

Please cite this article as: Xiaowen Zhang , Haibao Liu , Chris Maharaj , Mengyao Zheng , Iman Mohagheghian , Guanli Zhang , Yue Yan , John P. Dear , Impact Response of Laminated Glass with Varying Interlayer Materials, International Journal of Impact Engineering (2020), doi: https://doi.org/10.1016/j.ijimpeng.2020.103505

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Highlights 

This study investigates the influence of the interlayer materials on the low velocity impact performance of laminated glass. The effect of impact energy levels (3, 5, 10 and 15J) has been explored on the impact resistance of laminated glass and the failure mechanisms have been assessed. The bullet points can be drawn as follows:



The interlayer materials have a great influence on the impact performance of the laminated glass. The laminated glass with TPU or PVB interlayer exhibited better impact resistant properties than laminated glass with the TPU/SGP/TPU hybrid interlayer or the SGP interlayer when impacted at the lower 3 and 5J impact energy levels, thought to be caused by the different modulus of the interlayer materials.



The laminated glass with SGP or TPU/SGP/TPU hybrid interlayer showed better load carrying capacity and anti-deformation capability at the higher 10 and 15J impact energy levels. This is because the mechanical properties of SGP at room temperature lead to a higher strength and a higher stiffness of the entire laminated glass structure.



The ratios of the absorbed energy (Ea) to the incident energy (Ei) of the impactor for different laminated glass structures depend on the impact energies. The glass layers dominate the impact response of the laminated glass structures and lead to lower Ea/Ei values when subjected to the low energy impact e.g. 3 and 5J. At the higher impact energy levels such as 10 and 15J, the rebound of the impactor was affected by the viscoelasticity of the interlayers. Different rate-dependent properties of the SGP, TPU and PVB interlayers lead to the different values of Ea/Ei that are obtained for the studied laminated glass structures.

1

Impact Response of Laminated Glass with Varying Interlayer Materials Xiaowen Zhanga, Haibao Liub, Chris Maharajc, Mengyao Zhenga, Iman Mohagheghianb,d, Guanli Zhanga, Yue Yana, John P. Dearb

a

Beijing Institute of Aeronautical Materials, AECC, Beijing Engineering Research

Centre of Advanced Structural Transparencies for the Modern Traffic System, Beijing 100095, China b

Department of Mechanical Engineering, Imperial College London, London SW7 2AZ,

United Kingdom c

Department of Mechanical and Manufacturing Engineering, The University of the

West Indies, St. Augustine, Trinidad and Tobago d

Department of Mechanical Engineering Sciences, University of Surrey, Guildford,

Surrey GU2 7XH, United Kingdom

Abstract This study investigates the influence of the interlayer materials on the low velocity impact performance of laminated glass. By varying the impact velocity, with a drop-weight, the effect of impact energy levels (3, 5, 10 and 15J) has been explored on the impact resistance of laminated glass and the failure mechanisms have been assessed. The four interlayer materials investigated were: SGP–Ionoplast as employed in Sentry Glas® Plus, TPU-Thermoplastic polyurethane, PVB-Polyvinyl butyral and a TPU/SGP/TPU hybrid interlayer. The drop weight method has been employed to obtain the energy dissipation, loading and deformation of the laminated glass. The low velocity impact results indicate that both the type of the interlayer materials and the impact energy have great influence on the impact performance of the laminated glass. The laminated glass with TPU and PVB interlayer exhibited better impact resistance than the laminated glass with SGP and TPU/SGP/TPU hybrid interlayer, when impacted at energies of 3 and 5J 

Corresponding author. E-mail address: [email protected] Corresponding author. E-mail address: [email protected]  Corresponding author. E-mail address: [email protected] 

2

(corresponding to impact velocities of 1.71 and 2.21 ms-1 respectively). However, the laminated glass with SGP and TPU/SGP/TPU hybrid interlayers has better load carrying capacity and better anti-deformation property than the TPU and PVB interlayers at the higher impact energies of 10 and 15J (impact velocities of 3.13 and 3.83 ms-1 respectively). The results are thought to be attributed to the differences in the viscoelastic properties of the interlayer materials with strain rates.

Keywords: laminated glass, impact performance, interlayer material, impact energy

1. Introduction Laminated glass is widely adopted in many engineering applications for its excellent safety properties and is now finding wider application as a structural component [1, 2]. Laminated glass usually consists of two or more layers of transparent material plates, such as an inorganic glass, polycarbonate and acrylic material which are bonded together with a transparent, thermoplastic elastomeric polymer interlayer. The transparent material plates give the structural component whilst the polymer interlayer plays an important role in absorbing impact energy. For example, after impact and fracture, the glass shards will be held by the polymer interlayer, owing to its good ductility and ability to deform as a continuous membrane. Also, the viscoelasticity of the interlayer materials can help absorb impact energy by nonlinear large-scale deformation [3, 4].

Polymer interlayers, including for example, polyvinyl butyral (PVB), thermoplastic polyurethane (TPU), ethylene-vinyl acetate (EVA) and ionoplast Sentry Glas® Plus (SGP), have been widely used as interlayers in laminated glass. The mechanical properties of these interlayer materials have a great influence on the structural performance of the laminated glass. The polymers PVB, TPU and EVA perform as a soft elastomer, having lower glass transition temperatures and lower stiffness at room temperature. After glass breakage, the residual static load carrying capacity and stiffness of the laminated glass is poor [5]. However, SGP shows a significantly 3

stiffer response at room temperature [6 ,7], which can offer higher bending strength and better post-breakage strength of laminated glass and thus is recommended for use in structural applications. TPU has better flexible properties at lower temperature because of its lower glass transition temperature

[8]

, which benefits to

the impact performance of the laminated glass at lower temperatures Mohagheghian et al.

[11, 12]

[9, 10]

.

performed high velocity soft impact tests on the

laminated glass using different glass and interlayer types. The effects of type of glass, order of the glass layers and polymer type on the structural performance have been investigated. It was concluded that the polymer interlayer type (including PVB, TPU, SGP and the multi-layering polymer interlayer TPU/SGP/TPU) had the most significant effect on both the deformation and the failure of the laminated glass windows at room temperature. There are distinct advantages from multi-layering the polymer interlayer to optimise the performance of laminated glass windows and this concept is likely to attract much industrial interest. Speranziniet al. [13] investigated the structural behaviour of laminated glass plates made with different types of interlayer materials: PVB, SGP, EVA and XLAB (Polyethylene terephthalate + Ethylene Vinyl Acetate) when subjected to four-point bending tests to study the structural behaviour in both elastic and post-breaking phases. It was concluded that the glass/SGP laminates showed a greater load-carrying capacity in the elastic phase and better post-breakage resistance than that corresponding to the EVA and PVB laminated plates and the XLAB-glass laminates provided the highest initial-breakage strength values but without further improving the post-failure safety properties.

Impact performance of laminated glass is of great importance for transport applications and has attracted much research interest

[2, 14, 15]

. The impact

performance of laminated glass is essentially different from that of monolithic glass and is determined by many factors, such as the hardness of impactors, boundary conditions, striking velocity, material and geometrical properties [16-18]. Cantwell et al. [19]

have investigated the effect of varying the thickness of the individual glass layers

on the impact resistance of the laminated glass. The critical velocity for damage 4

initiation, Vcrit, was employed to evaluate the impact behaviour of the laminated glass. The results indicated that the thickness of the outer glass is the primary parameter determining Vcrit whereas the inner thickness has a secondary influence on this threshold. Zhang et al.

[20]

conducted laboratory tests and numerical

simulations on the vulnerability of laminated glass windows subjected to windborne wooden block impact. The effects of wooden debris with different weights and the PVB interlayers with different thicknesses on the impact behavior of the laminated glass windows were investigated. It has been found that interlayer thickness plays a dominant role in the penetration resistance capacity of the laminated glass windows subjected to windborne debris impact.

An extensive range of literature has studied the impact performance and post breaking behaviours of laminated glass [20-22]. The basic laminated glass architectures, functional layers and transparent materials and a set of design and material selection guidelines were analysed and proposed based on the experimental and numerical simulation process

[23, 24]

. However, for the low velocity impact, the impact energy

might greatly affect the impact performance of the laminated glass for different kinds of interlayer materials, as different interlayer materials tend to show different mechanical response with the changing loading rates

[9, 10, 25]

, especially under the

dynamic loading conditions.

This current study investigates the low velocity impact performance of laminated glass windows using various polymer interlayer systems at different impact velocities and associated impact energies. The focus will be on the effects of the interlayer structure and the effect of impact energies, as related to impact velocity, on impact resistance as well as damage development of the laminated glass windows.

2. Experiments 2.1 Materials The laminated glass specimens used in this study were manufactured using two 5

layers of silicate float glass bonded with one or three layers of different kinds of interlayers. The monolithic glass plates, used for the preparation of laminated glass specimens, were manufactured with the dimensions of 100 mm × 150 mm and a thickness of 2 mm. The tin side of the glass plate was employed for bonding with the polymer interlayer and thoroughly cleaned before lamination. In this study, three types of polymer interlayer materials were employed: Ionoplast Sentry Glas® Plus (SGP) (from DuPont), Thermoplastic Polyurethane (TPU) -PF2300 (from PPG company) and Polyvinyl Butyral (PVB) (from DuPont). The laminated glass specimens were prepared in an autoclave under the following processing conditions: the temperature was raised to 137°C in 30 mins whilst the pressure was raised to 10 bar, then both the temperature and pressure were held for 210 mins, before lowering the temperature to 49°C by a rate of 2.5°C/min and releasing the pressure naturally. Four different laminated glass configurations were manufactured for each type of interlayer material (SGP, TPU, PVB – Case 1, 3 and 4) as well as a hybrid multilayer interlayer (TPU/SGP/TPU - Case 2). The paper aims to disclose the effect of adding two thin layers of TPU on either side of SGP in Case 2 compared with Case 1 of pure SGP interlayer, the thickness of SGP interlayer has been kept fixed. The influence of the polymer interlayer thickness on the flexural stiffness and low velocity impact performance of laminated glass plates has been investigated previously

[9, 17]

and

showed negligible effect. The details of each configuration are listed in Table 1.

Table 1 The interlayer configurations for samples investigated in this study. Configuration

Glass and polymer layers

Total plate thickness (mm)

Case 1

2.0 mm glass/1.52 mm SGP/2.0 mm glass

5.52

Case 2

2.0 mm glass/0.38mm TPU/1.52mm

6.28

SGP/0.38mm TPU/2.0 mm glass Case 3

2.0 mm glass/1.52 mm TPU/2.0 mm glass

5.52

Case 4

2.0 mm glass/1.52 mm PVB/2.0 mm glass

5.52

6

2.2 Measurements and characterizations 2.2.1 Low velocity impact tests The impact experiments were performed with an Instron CEAST 9350 drop weight equipment. The testing machine and experimental setup are shown in Fig. 1. Experiments were conducted at four different energy levels of 3, 5, 10 and 15J respectively (corresponding to impact velocities of 1.71, 2.21, 3.13 and 3.83 ms-1 respectively for a total dropping mass of 2.048 kg including the carriage mass of 1.3 kg and the impactor mass of 0.748 kg). The impact velocities and energies were achieved through changing the height of the impactor. A rubber gasket was used as the buffer between the specimens and the base. Three specimens of each configuration were impacted at the aforementioned impact energies. All the experiments were conducted under test temperature of 23°C.

Fig. 1. Experimental setup used for low velocity impact tests with the Instron CEAST 9350 drop tower and the temperature-controllable chamber.

2.2.2 Crack patterns analyses Crack patterns of the post-impacted laminated glass specimens were obtained using a digital camera. For the post-impacted specimens with only one fractured layer of 7

glass, the patterns of the cracks were captured in the natural light room. For the post-impacted specimens with two fractured layers of glass, the patterns of these two fractured surfaces were captured in either natural light rooms or black rooms in order to obtain the best contrast for the crack patterns.

2.2.3 Tensile tests The tests at different strain rates were conducted using an SANS universal test machine (CMT 4204). Type 1A dog-bone shaped tensile specimens were cut from the polymer sheets according to the ISO 37-2011 [26]. The specimens had a gauge length of 20 mm and width 4 mm. The thickness of the specimens varied between the different polymers. The engineering stress, σ = F/A0, is computed from the force, F, measured by the load cell, and A0 is the initial central cross section of the specimen. The engineering tensile strain ε= (l-l0)/l0, where l0 = 20mm is the distance between the initial displacement extensometer fixture, and l is the actual distance between the extensometer fixture.

3. Results 3.1 Different types of interlayer materials 3.1.1 Impact energy of 3J Force versus deformation and energy versus time curves for all the tested laminated glass specimens, with various types of interlayer materials, are shown in Fig. 2 and Fig. 3, respectively. Three samples (S1 to S3) were tested for each configuration (Case 1 to Case 4). All the experiments have been conducted using an impact energy of 3J (associated with an impact velocity of 1.71 m s-1). The values for absorbed energy (Ea), elastic energy (Ee), the first peak force and corresponding deformation (Ffp and Dfp), the second peak force and corresponding deformation (Fsp and Dsp) (if available in the curves), the maximal deformation (Dmax) and the fracture description of the specimens are presented in Table 2. The Ffp value is relevant to the response of the back layer of glass whereas Fsp is determined by the response of the front layer of glass. The results showed that Ffp is higher than Fsp because the back layer of glass 8

absorbed most of the energy through cracking and deformation and the bending stress imposed on the front layer glass becomes less than that on the back glass layer. The deformation still kept increasing a small value after Fsp was reached. This can be attributed to the compressibility of the interlayer material. Then the deformation would decrease corresponding to the rebounding of the impactor after impact, with almost linear relevance to the force. It should be noted that only the back layer of the laminated glass fractured for these cases whilst the front layer remains intact. It has been reported that cracks on impacted layer (front layer) always appear later than those on the non-impacted layer (back layer) [27, 28].

Fig. 2 shows the typical loading versus deformation curves for the specimens. For specimens without any breakage after impact, such as curves S1 and S3 of Case 3 and curves S2 and S3 of Case 4, only one peak force exists on each curve and the deformation, as expected, does not increase much further after the peak force. This is because no fracture occurred in the Case 3 and Case 4 and the laminated glass samples are still in elastic stage, in which the impactor is rebounded directly from the laminated glass after the maximum loading force. Then both the deformation and force decrease and the curves behave in almost a linear manner until the force is reduced to about zero at which time point the impactor is separated with the laminated glass specimen. After the force became almost zero, the machine stopped capturing the data. In these cases, the laminated glass specimens are strong enough to withstand the impact loading.

From Figs. 2a-2d, the gradient of the force-deformation curves is similar to each other for the three specimens with the same structure before the first peak force is reached, due to their similar elastic bending strength. Similarly, at the rebound stage, the gradient of the force-deformation curves for the intact laminated glass specimens is observably higher than that with one fractured layer, for the higher elastic bending strength of the intact laminated glass specimens after impact. For Case 2, 3 and 4, the intact specimens after impact showed higher Ffp and Dfp but 9

lower Dmax values compared with the specimens in which one layer of glass fractured after impact. This is because the laminated glass, which keeps intact after impact, has better loading carrying capacity than the others. As a result, the intact laminated glass specimens after impact present a higher Ffp value, which can cause higher Dfp value. However, the specimens that experienced glass fracture after impact dissipated impact energy through further glass deformation and interlayer compression after the breakage of the glass layer, which induces higher Dmax and Ea values for the laminated glass specimens. Therefore, it is also apparent in Figs. 3b-3d and Table 2 that the absorbed energy is relatively higher for the specimens with one layer of fractured glass, than those intact specimens after impact. Accordingly, it is reasonable that the higher Ee values will induce higher reverse velocity of the impactor after impact as shown in the velocity-time curves in Fig. 4a-4b. Here, Case 2 and Case 3 have been chosen to elaborate on this phenomenon.

10

Fig. 2. Force versus deformation curves for the studied laminated glass specimens with different types of interlayer materials with impactor energy of 3J (impact velocity of 1.71 m s-1). a) Case 1: SGP, b) Case 2: TPU/SGP/TPU, c) Case 3: TPU, d) Case 4: PVB. S1, S2 and S3 denote the three different specimens for each testing.

11

Fig. 3. Energy versus time curves for the studied laminated glass specimens with different types of interlayer materials with impactor energy of 3 J (impact velocity of 1.71m s-1). a) Case 1: SGP, b) Case 2: TPU/SGP/TPU, c) Case 3: TPU, d) Case 4: PVB. S1, S2 and S3 denote the three different specimens for each testing.

Fig. 4. Velocity versus time curves for the laminated glass specimens with different types of interlayer materials with impactor energy of 3 J (impact velocity of 1.71m s-1). a) Case 2: TPU/SGP/TPU, b) Case 3: TPU. S1, S2 and S3 denote the three different specimens for each testing.

From Table 2, it can be concluded that Case 3 and Case 4, with TPU and PVB as the interlayer respectively, have better impact resistance properties than Case 2. Case 1 12

is the worst case when subjected to the relatively low impact energy of 3J. These differences are attributed to the varying modulus of the interlayer materials. SGP material shows higher stiffness and storage modulus (E’) than those of TPU and PVB material both under dynamic loading and quasi-static loading conditions at low strain rates. The dynamic mechanical analysis results are shown in Fig. 5 from Ref 12 by the authors. Also, a series of uniaxial tensile experiments were conducted on the SGP, TPU and PVB interlayer specimens at different strain rates ranging from 0.238 to 23.8 s−1, as shown in Fig. 6. Higher stiffness and storage modulus (E’) of the SGP interlayer material can impart a higher bulk modulus to the laminated glass. Higher bulk modulus means bulk waves can be transmitted more easily, without any dissipation, for a low energy impact

[29]

. This will cause higher bending stress in the

back layer of the laminated glass, which coincides with the results from Table 2 showing that SGP laminated glass has highest Ffp values among the four laminated glass. Consequently, the laminated glass with SGP interlayer is considered more fragile than the other three structures when striking at 3J energy, as all three SGP laminated glass specimens show fracture after the experiments. This conclusion can also be proved by the crack patterns on the back layer of glass. As shown in Fig. 7, denser radial cracks have been observed from the tested SGP laminated glass specimen, compared to the other three structures.

13

Fig. 5. The dynamic mechanical analysis results of the storage modulus (E’) plotted against temperature for SGP, TPU and PVB interlayers [12].

14

Fig. 6. Uniaxial tensile properties of (a) SGP, (b) TPU and (c) PVB polymer interlayers at strain rates ranging from 0.238 to 23.8 s −1.

Table 2. Low velocity impact of laminated glass with different interlayers (SGP – Case 1, TPU/SGP/TPU - Case 2, TPU - Case 3, PVB - Case 4) with a 3J impact energy. Ea(J)

Ee(J)

Ffp(kN)

Fsp(kN)

Dfp(mm)

Dsp(mm)

Dmax(mm)

Case 1

Specimen

1.41, 1.30, 1.78

1.59, 1.70, 1.22

4.03, 3.55, 4.25

2.46, 2.15, 2.11

1.36, 1.27, 1.85

1.40, 1.56, 2.02

1.42, 1.90, 2.06

Three: one layer fractured;

Case 2

1.81, 1.48, 1.20

1.19, 1.52, 1.80

2.42, 1.93, 2.67

1.29, 1.14, -

1.57, 1.29, 1.78

1.94, 1.85, -

2.29, 2.82, 1.80

Case 3

0.88, 1.03, 1.50

2.12, 1.97, 1.50

2.56, 2.53, 2.17

-, -, 1.22

2.01, 1.95, 1.23

-, -, 2.20

2.04, 1.97, 2.64

Case 4

1.03, 1.16, 1.50

1.97, 1.84, 1.50

2.97, 2.95, 2.43

-, -, 1.20

1.66, 1.69, 1.37

-, -, 1.84

1.71, 1.73, 2.59

Two: one layer fractured; one: non fracture One: one layer fractured; two: non fracture One: one layer fractured; two: non fracture

15

Fractured description

Fig. 7. Crack patterns on the fractured face of the laminated glass at impactor energy of 3J (impact velocity of 1.71m s-1) for: (a) Case 1 – SGP, (b) Case 2 – SGP/TPU/SGP, (c) Case 3 – TPU, (d) Case 4 – PVB.

Fig. 8. Typical loading response during the impact for the four structures with impactor energy of 3J (impact velocity of 1.71m s-1), SGP – Case 1, TPU/SGP/TPU Case 2, TPU - Case 3, PVB - Case 4.

16

The loading response obtained from one of the three tested samples has been chosen as the typical example for each case as shown Fig. 8. The gradients of the force-deformation curves between Ffp and Dfp are different from each other after the force is higher than about 0.6 kN. In detail, the gradient in descending order can be drawn as: Case 1 (SGP) > Case 4 (PVB) > Case 2 (hybrid) > Case 3 (TPU). This is coincident with the conclusion drawn previously, the higher modulus of the interlayer material results in a higher elastic bending strength in the laminated glass specimens. For Case 2 with a hybrid multilayer interlayer (TPU-SGP-TPU), the gradient of the force-deformation curves between Ffp and Dfp over 0.6 kN is, as expected, smaller than that of Case 1 but higher than that of Case 3. The reason is Case 2 used one layer of TPU as a transition layer between SGP and glass, with the shear transfer between glass and SGP being affected by TPU interlayer. It should be noted that, despite the different interlayer materials, the responses of all four cases are the same up to approximately 0.6 kN, which corresponded to the duration of the impactor being in contact with the glass before the impact force reached the interlayer material [10].

3.1.2 Impact energy of 5J In Fig. 9, force versus deformation curves are presented for all the studied laminated glass specimens with various types of interlayer materials at an impact energy of 5J (impact velocity of 2.21m s-1). The values for Ea, Ee, Ffp, Dfp, Fsp, Dsp, Dmax and the fracture description of the specimens are presented in Table 3. The typical force-deformation curve for the specimen with two fractured glass layers, subjected to an impact energy of 5J, is shown as the S1 and S2 curves in Fig. 9a. These reveal two distinct force peaks. The Ffp corresponds to the fracturing of the back layer of glass and Fsp corresponds to the fracture of the front layer of glass. The deformation continuously reaches the maximal deformation, Dmax, before the impactor rebounds back due to the residual strength of the laminated glass after fracture. The response of the laminated glass in that rebounding stage, which is mainly controlled by the strength of polymer interlayer, shows a very small gradient of the force-deformation 17

curves caused by the low residual strength of the interlayer.

The typical force-deformation curves for the specimens, fractured only in the back layer of glass, are demonstrated as the S3 curve in Fig. 9a and all the three curves in Fig. 9b, which have the similar character as that in Section 3.1.1. For Case 1, compared with the specimen fractured in both layers of glass, Ffp and Dfp values are much higher for the specimen fractured in only one layer of glass. It can be deduced that the back layer of glass dissipated most of the energy absorbed by the laminated glass structure, and then protected the front layer of glass from being destroyed. Here it is comprehensible that the Dmax and Ea values for the specimen fractured in only one layer of glass are smaller than those presented by the specimen fractured in both layers of glass.

For Case 2, all the three tested specimens are fractured in one layer of glass after 5J impact and show very similar Ea and Ee values (Table 3). Case 3 and Case 4 show a similar fractured description after impact with two specimens fractured in the back face and one specimen that keeps intact after impact. The description and analysis for these three structures are similar to those in Section 3.1.1 with the main difference being an increased likelihood of back glass layer fracture due to the increased impact energy.

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Fig. 9. Force versus deformation curves for the studied laminated glass specimens with different types of interlayer materials subjected to an impact energy of 5 J (impact velocity of 2.21 m s-1). a) Case 1 - SGP, b) Case 2 - TPU/SGP/TPU, c) Case 3 -TPU, d) Case 4 - PVB. S1, S2 and S3 denote the three different specimens for each testing.

As shown in Table 3, although Case 1 shows the similar Ea and Ee values to Case 2, Case 2 has better impact resistance properties than Case 1 at 5J impact energy level. The reason is all the three specimens have been fractured in one layer of glass for Case 2, but two of the three specimens have been fractured in both layers of glass for Case 1. This might be because the Ee value is influenced mainly by the elastic properties of interlayer material when the glass layers keep the same to each other, leading to the similar elastic properties of Case 1 and Case 2 as both of them used SGP as the interlayer. However, in Case 2, besides SGP, one layer of TPU has been used as a transition layer between SGP and glass, which can transfer the shear stress between glass and SGP and also dissipate more energy, consequently, the energy dissipated by the glass layer and the stress imposed on the glass layer is relatively reduced compared with that of Case 1.

Similar to the results obtained in Section 3.1.1, Case 3 and Case 4 with TPU and PVB as the interlayer show better impact resistant properties than Case 2. Furthermore, 19

Case 1 is the worst when subjected to 5J impact energy for the same reason as that in Section 3.1.1. This conclusion is also supported by the observation from the crack patterns of the laminated glass specimens after impact, as shown in Fig. 10. Case 1 shows more serious damage on both faces of the laminated glass than the other three structures. It can also be seen that the circumferential cracks have been created on the front face of Case 1, while no circumferential cracks can be seen from the other three structures with only one layer of glass being fractured after impact. This is because circumferential cracks are created after the full growth of the radial cracks and could continue to dissipate impact energy after the full-growth of radial cracking for Case 1. However, Table 3 shows that Case 4 has the highest Ea value among the four structures. Theoretically, higher Ea value would cause more damage to the specimens, which is contrary to the actual results. It can be deduced that PVB interlayer dissipates the absorbed energy and transferred the shear stress between the two layers of glass better than the other three types of interlayer at 5J impact energy.

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Table 3. Low velocity impact of laminated glass with different interlayers (SGP – Case 1, TPU/SGP/TPU - Case 2, TPU - Case 3, PVB - Case 4) with a 5J impact energy. Specimen

Ea(J)

Ee(J)

Ffp(kN)

Case 1

1.84, 1.63, 1.40

3.16, 3.37, 3.60

1.77, 1.70, 4.34

Case 2

1.54, 1.63, 1.72

3.46, 3.37, 3.28

Case 3

1.90, 1.39, 1.11

3.10, 3.61, 3.89

Case 4

2.10, 2.49, 3.63

2.90, 2.51, 1.37

Fsp(kN)

Dfp(mm)

Dsp(mm)

Dmax(mm)

2.46,2.01,1.92

0.65, 0.38, 1.49

3.40, 3.04, 1.98

4.03, 4.89, 2.60

2.26, 3.28, 3.18

1.41, 1.60, 1.74

1.27, 2.32, 1.58

2.03, 2.61, 2.29

3.49, 2.65, 3.15

2.95, 1.96, 3.40

1.56, 1.36, -

2.03,1.54 , 2.55

2.63, 2.49, -

3.28, 3.95, 2.57

3.73, 3.13, 3.27

-, 1.54, 1.65

2.15, 1.66, 2.28

-, 2.30, 2.62

2.22, 3.30, 2.72

21

Fractured description Two: two faces fractured; One: one face fractured Three: one face fractured; Two: one face fractured; One: non fractured Two: one face fractured; One: non fractured

Fig. 10. Crack patterns on the fractured face of the laminated glass subjected to an impact energy of 5 J (impact velocity of 2.21 m s-1), for: (a-F) front layer of glass for Case 1 – SGP, (a-B) back layer of glass for Case 1 – SGP, (b) Case 2 – SGP/TPU/SGP, (c) Case 3 – TPU, (d) Case 4 – PVB.

3.1.3 Impact energy of 10J In Fig. 11, typical force versus deformation, energy versus time and velocity versus time curves for all the studied laminated glass structures are presented for an impact energy of 10J (impact velocity of 3.31 m s-1). The values for Ea, Ee, Ffp, Dfp, Fsp, Dsp, Dmax of the specimens are presented in Table 4. All the structures fractured in both faces when subjected to 10J impact. Therefore, one of the three curves for each structure has been chosen here and the average values are presented in Table 4. 22

As shown in Fig. 11a and Table 4, Case 1 shows the highest Ffp value but the lowest Dmax in the four structures. Due to the stiffest property of SGP among the three kinds of interlayer materials, Case 1 shows the best anti-deformation and load carrying capacity after the fracture of glass layers. This can also be further elaborated by the rebound curves in Fig. 11a, the ultimate deformation values on the force-deformation curves for Case 3 and Case 4 are much higher than those for Case 1 and Case 2. The maximal reverse velocity of the impactor after impact for Case 3 and Case 4 are lower than those of Case 1 and Case 2 as seen in Fig. 11c, meaning that Case 1 and Case 2 with SGP interlayers have better deformation recovery properties than the other two cases.

As shown in Fig. 11b and Table 4, Case 1 (SGP) shows the highest elastic energy among these four structures followed by Case 2 (TPU/SGP/TPU). Case 3 (TPU) and Case 4 (PVB) show the lowest elastic energies, which is different from the results of 3 and 5J conditions. When impacted at 10 J energy, both layers of glass are fractured after impact, and the rebounding of impactor is greatly influenced by the elastic contributions of the interlayer. The mechanical properties of SGP material at room temperature contribute to the higher strength, stiffness and creep resistance of the whole laminated glass compared with TPU and PVB interlayers, both before and after glass breakage [12, 30, 31], which can further be understood from Fig. 5 and Fig. 6.

23

Fig. 11. Effects of interlayer structure on the impact responses of laminated glass subjected to 10J impact energy (impact velocity of 3.31 m s-1). (SGP – Case 1, TPU/SGP/TPU – Case 2, TPU– Case 3, PVB– Case 4) (a) force versus displacement curves, (b) energy versus time curves, (c) velocity versus time curves.

Although Case 1 with SGP as the interlayer provides minimum Ea value among the four structures, a highest Ffp value has been exhibited by the Case 1. The highest Ffp value caused more serious damage to the back layer of glass, which is shown as the radial crack patterns on the back layer of glass of laminated glass specimens after impact in Fig. 12. It seems that the density of the radial cracks for Case 1 (SGP) and Case 2 (TPU/SGP/TPU) caused by the bending strength of the impactor is higher than that of Case 3 (TPU) and Case 4 (PVB). 24

For the front layer glass of the specimens, the crack patterns are shown in Fig. 13. Although the Ea values of Case 1 and Case 2 is lower than those of Case 3 and Case 4, the density and number of the circumferential cracks for Case 1 (SGP) and Case 2 (TPU/SGP/TPU) is higher than that of Case 3 (TPU) and Case 4 (PVB). The reason is the circumferential cracks are initiated under the influence of Rayleigh waves [27, 28], bulk waves can be more easily transmitted with less dissipation for the SGP interlayer with higher stiffness compared with the other two interlayers. Therefore, it is reasonable to deduce that Case 1 and Case 2 with SGP as the interlayer mainly dissipate the impact energy through the fracturing of the glass layers, while Case 3 and Case 4 with TPU and PVB as the interlayer mainly dissipate the energy through the deformation of the glass layers and the interlayer materials. The reason is that Case 3 and Case 4 show much higher Dmax values than Case 1 and Case 2. These results are coincident with the conclusions drawn in Sections 3.1.1 and 3.1.2. From Fig 12 and Fig 13, compared with the 3J and 5J conditions, more serious damage has been caused in all the four structures subjected to 10J impact energy, as more energy has been absorbed by the laminated glass in this higher impact energy level.

Table 4. Low velocity impact of laminated glass with different interlayers (SGP – Case 1, TPU/SGP/TPU - Case 2, TPU - Case 3, PVB - Case 4) with a 10 J impact energy. Specimen

Ea(J)

Ee(J)

Ffp(kN)

Fsp(kN)

Dfp(mm)

Dsp(mm)

Case 1

7.02±0.70

2.98±0.70

6.22±0.24

2.34±0.85

2.15±0.22

2.84±0.20

4.56±0.50

Case 2

7.98±0.36

2.02±0.36

4.31±0.96

2.64±0.86

2.05±0.59

3.50±0.53

5.93±1.14

Case 3

9.50±0.18

0.50±0.18

2.23±0.77

2.28±0.36

1.58±0.63

3.87±0.42

13.37±0.90

Case 4

9.14±0.15

0.86±0.15

1.85±0.50

2.87±0.41

1.11±0.37

4.07±0.62

11.72±1.21

25

Dmax(mm)

Fig. 12. Crack patterns on the back face of the laminated glass at impact energy of 10 J, for: (a) Case 1 – SGP, (b) Case 2 – SGP/TPU/SGP, (c) Case 3 – TPU, (d) Case 4 – PVB.

Fig. 13. Crack patterns on the front face of the laminated glass at impact energy of 10 J, 26

for: (a) Case 1 – SGP, (b) Case 2 – SGP/TPU/SGP, (c) Case 3 – TPU, (d) Case 4 – PVB.

3.1.4 Influence of the rubber gasket with an impact energy of 15J When impacted with 15J energy (impact velocity of 3.83 m s-1), all the structures are fractured in both faces. The values for Ea, Ee, Ffp, Dfp, Fsp, Dsp, Dmax of the specimens are presented in Table 5. Similar conclusions can be drawn as those in Section 3.1.3. This section will focus on the effect of the rubber gasket instead of the interlayer structure.

Table 5. Low velocity impact of laminated glass with different interlayers (SGP – Case 1, TPU/SGP/TPU - Case 2, TPU - Case 3, PVB - Case 4) with a 15J impact energy. Specimen

Ea(J)

Ee(J)

Ffp(kN)

Fsp(kN)

Dfp(mm)

Dsp(mm)

Dmax(mm)

Case 1

8.05±0.34

6.95±0.34

4.68±1.93

2.18±0.69

1.47±0.39

3.70±0.69

9.71±0.57

Case 2

9.10±0.41

5.90±0.41

2.33±0.29

2.25±0.15

1.32±0.39

3.78±0.22

10.97±0.28

Case 3

13.82±0.31

1.18±0.31

1.83±0.92

2.36±1.09

1.72±0.45

4.40±0.24

19.71±2.73

Case 4

13.89±0.24

1.11±0.24

2.01±0.79

1.95±0.54

1.95±0.96

3.47±0.40

18.77±3.74

When the impact experiments were conducted at 15J without the rubber gasket around the specimen, as seen in Table 6, it indicated that the Ee value decreased but Ea value increased compared with Table 5, which was more obvious for Case 1 and Case 2 that used SGP as the interlayer. This is because the relatively stiff SGP layer can give the laminated glass a resistance to bending and transfer the impact energy to the rubber gasket more efficiently, which gives the laminated glass better elastic properties. However, for the flexible TPU and PVB interlayer materials, because of the lower elastic modulus, the interlayer would deform easily after the fracture of the glass layers. In this case, little energy will be transferred to the rubber gasket, resulting in a lesser influence on the Ea and Ee values after introducing the rubber gasket. It should also be noted that Dfl and Dsl values increase due to the addition of the rubber gasket around the specimen. This might be caused by the compressibility of the rubber gasket. For Case 1 and Case 2, the Fsp value also obviously decreased after introducing the rubber gasket around the specimen. It has been reported that 27

the laminated glass panel with clamped edges (the rubber gasket has been used) requires more energy to trigger glass breakage and has an evident improvement in pre-breakage stiffness for the SGP interlayer laminated glass, which can add benefits to the critical breakage velocity [14].

Table 6. Low velocity impact on laminated glass with different interlayers (SGP – Case 1, TPU/SGP/TPU - Case 2, TPU - Case 3, PVB - Case 4) with a 15 J impact energy without the rubber gasket around the specimen [10] Specimen

Ea(J)

Ee(J)

Ffp(kN)

Fsp(kN)

Dfp(mm)

Dsp(mm)

Dmax(mm)

Case 1

11.57±0.12

3.31±0.13

4.98±0.46

4.46±0.51

1.13±0.05

2.34±0.23

10.04±0.2

Case 2

12.06±0.05

2.82±0.08

2.56±0.16

4.54±0.29

1.16±0.23

3.55±0.12

10.88±0.21

Case 3

14.01±0.85

0.99±0.85

2.10±0.24

2.70±0.43

1.03±0.24

3.32±0.46

20.69±1.53

Case 4

14.09±0.20

1.00±0.23

2.32±0.31

2.76±0.22

1.06±0.28

3.02±0.17

25.80±0.78

3.2 The effect of different impact energies In Fig. 14, force versus deformation curves for all four laminated glass structures at different impact energies are shown. It can be seen that the gradient of the force-deformation curves between Ffp and Dfp is similar for the same structure, when the impact energy changes from 3 to 15J. This result confirms that the discrepancy of the gradient of the force-deformation curves between Ffp and Dfp for the different laminated glasses is caused by the different properties of the interlayer materials, which is coincident with the former conclusions. Also, as the impact energy increases, permanent deformation might be caused to the laminated glass as the end points of the deformation for some curves are not zero, such as all the cases impacted with 15J energy and Case 3 and Case 4 impacted with 10J energy.

28

Fig. 14. Force versus deformation curves obtained from the laminated glass specimens with different types of interlayer materials, impacted at different impact energy levels: Case 1: SGP, Case 2: TPU/SGP/TPU, Case 3: TPU, Case 4: PVB.

From the above analysis, it can be concluded that the properties of interlayers play an important role in the impact performance of the laminated glass. To make the analysis more explicit, the ratios of the absorbed energy (Ea) to the impact energy (Ei) are also shown in Fig. 15. It can be found that for the low energy impact like 3J, Case 1 and Case 2 show similar Ea/Ei values, which are higher than those of Case 3 and Case 4, which is coincident with the conclusions drawn in Section 3.1.1 and more serious damage has been caused in Case 1 (SGP) and Case 2 (TPU/SGP/TPU).

For the Ea/Ei values obtained at 5J energy level, there is a little difference between 29

the Ea/Ei values obtained from Case 1, Case 2 and Case 3. In Section 3.1.2, the damage caused in Case 1 is more serious than Case 2 and Case 3, meaning that the TPU interlayer can dissipate the absorbed energy more expediently than SGP interlayer at 5J impact condition, due to the better flexible properties of the TPU interlayer. However, Case 4 with PVB as the interlayer absorbed more energy when impacted at 5J energy compared with the above three cases while no serious damage occurred, indicating that PVB exhibited the best energy dissipation capability among the three kinds of interlayers.

Absolutely different from the Ea/Ei values obtained at the 3 and 5J conditions, Case 1 (SGP) exhibited lower Ea/Ei values than Case 2 (TPU/SGP/TPU), and much lower than Case 3 (TPU) and Case 4 (PVB) at the 10 and 15J conditions. This can be explained as when the specimens were impacted at 10 and 15J energy levels, fracture would be caused to both layers of the glass, and the rebounding of impactors and the elastic energy (Ee) values are greatly influenced by the modulus and stiffness of the interlayer materials [31]. From the experimental results in Fig. 6, it can be found that, among the three types of interlayer, SGP specimens showed the highest stiffness and modulus at both the lower and higher stain rates. Moreover, at the lower strain rates, the TPU and PVB specimens presented similar stiffness. Accordingly, it can be inferred that the glass layers dominated the impact response of laminated glass structures when subjected to the low energy impact like 3 and 5J impact. The absorbed energy is dissipated mainly through the deformation and cracking of the glass layers. The glass layers play an important role in rebounding the impactor, which results in the lower Ea/Ei values, compared with those obtained at the 10 and 15J conditions, this conclusion is applicable for all the four structures.

Additionally, Case 1 and Case 2 showed higher Ea/Ei values when struck at 10J energy level than those of 15J energy level while Case 3 and Case 4 showed similar Ea/Ei values for these two impact energy levels. This might be caused by the different rate-dependent properties of the three types of interlayers. When loaded at low 30

strain rates, the TPU and PVB materials behave as the rubbery state and the mechanical responses are not very sensitive to the strain-rate changes

[4, 32]

.

Differently, SGP exhibits elasto-plastic material properties and considerable sensitivity to the strain-rate, at low strain rate range [9, 33]. These can be elucidated by the engineering stress versus engineering strain curves in Fig. 6. The yield strength of SGP interlayer increases obviously with growing strain rates, which induces higher Ee/Ei value for 15J impact energy than that for 10J. However, TPU and PVB interlayers show a little change in the engineering stress versus engineering strain curves as the strain rates change from 0.238 to 23.8 s −1.

Fig. 15. Plots of Ea/Ei ratios for the laminated glass with different types of interlayers impacted at different impact energy levels

Conclusions In this paper, low velocity hard impact performance and the impact resistant properties of laminated glass with four different types of interlayers were evaluated. The effects of impact energies of 3, 5, 10 and 15J corresponding to impact velocities of 1.71, 2.21, 3.13 and 3.83 m s-1 (for an impactor mass of 2.048 kg) respectively, have been analysed. Through a systematic study, the effect of the interlayer materials and impact energies on the energy dissipation, loading response and stiffness of the 31

laminated glass structures were measured and analysed. The following conclusions can be drawn:

 The interlayer materials have a great influence on the impact performance of the laminated glass. The laminated glass with TPU or PVB interlayer exhibited better impact resistant properties than laminated glass with the TPU/SGP/TPU hybrid interlayer or the SGP interlayer when impacted at the lower 3 and 5J impact energy levels. This difference in impact resistance is thought to be caused by the different modulus of the interlayer materials. SGP interlayer, which has a relatively higher stiffness, can impart higher bulk modulus to the laminated glass. Higher modulus means a stiffer material in which bulk waves can more easily transmit without significant dissipation at a relatively low impact energy level. This causes higher bending stress in the back layer of the laminated glass.

 The laminated glass with SGP or TPU/SGP/TPU hybrid interlayer showed better load carrying capacity and anti-deformation capability at the higher 10 and 15J impact energy levels. In these two conditions, both layers of glass have been fractured after impact, and the rebound of the impactor is significantly influenced by the elastic contributions of the interlayers. The dynamic mechanical properties of SGP at room temperature lead to a higher strength and a higher stiffness of the entire laminated glass structure.

 The ratios of the absorbed energy (Ea) to the impact energy (Ei) of the impactor for different laminated glass structures depend on the impact energies. The glass layers dominate the impact response of the laminated glass structures and lead to lower Ea/Ei values when subjected to the low energy impact e.g. 3 and 5J, corresponding to impact velocities of 1.71 and 2.21 m s-1 respectively. At the higher impact energy levels such as 10 and 15J, corresponding to the higher impact velocities of 3.13 and 3.83 m s-1, the rebound of the impactor was affected by the viscoelasticity of the interlayers. Different rate-dependent 32

properties of the SGP, TPU and PVB interlayers lead to the different values of Ea/Ei that are obtained for the studied laminated glass structures.

Acknowledgement This research was performed in collaboration with the BIAM Centre for Materials Characterisation, Processing and Modelling at Imperial College London. This research is financially supported by the Innovation Foundation Programme of Beijing. Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author statement Xiaowen Zhang: Conceptualization, Investigation, Data curation, Writing- Original draft preparation. Haibao Liu: Writing- Reviewing and Editing, Visualization. Chris Maharaj: Writing- Reviewing and Editing, Visualization. Mengyao Zheng: Investigation, Validation. Iman Mohagheghian: Writing- Reviewing and Editing, Formal analysis. Guanli Zhang: Project administration. Yue Yan: Conceptualization, The corresponding author to ensure that the descriptions are accurate and agreed by all authors, Supervision. John P. Dear: The corresponding author to ensure that the descriptions are accurate and agreed by all authors, Writing- Reviewing and Editing, Validation.

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