Temperature effects on the low velocity impact response of
laminated glass with different types of interlayer materials
Xiaowen Zhanga, Idris K. Mohammedb, Mengyao Zhenga, Nan Wua,
Iman Mohagheghianb,c, Guanli Zhanga, Yue Yana[footnoteRef:2],
John P. Dearb[footnoteRef:3] [2: Corresponding author. E-mail
address: [email protected]] [3: Corresponding author. E-mail
address: [email protected]]
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,
South Kensington Campus, London, SW7 2AZ, United Kingdom
c Department of Mechanical Engineering Sciences, University of
Surrey, Guildford, Surrey, GU2 7XH, United Kingdom
Abstract
This paper investigates the influence of the interlayer material
on the low velocity impact performance of laminated glass. The
effect of temperature (50°C, 23°C, 0°C and -30°C) has been explored
to observe damage mechanisms and the associated impact resistant
properties of the laminated glass. The four interlayer materials
investigated were: SGP–Ionoplast as employed in Sentry Glas® Plus,
TPU- Thermo-plastic polyurethane, PVB-Polyvinyl butyral and a
TPU/SGP/TPU hybrid interlayer. The impact resistance was measured
in terms of load peak, absorbed energy, ultimate deformation and
crack patterns. The low velocity impact results indicated that both
the type of the interlayer materials and testing temperature have
great influence on the impact resistant properties of the laminated
glass. The laminated glass with SGP interlayer exhibited best
impact resistant properties amongst the four structures at room
temperature. However, as the temperature was varied, the
TPU/SGP/TPU hybrid interlayer performed the best over the entire
range of temperatures tested, which can better ensure the safety of
the occupants in the vehicle. This is because the elastic and
viscous properties of the interlayer materials greatly changes with
the temperature caused by the different glass transition
temperatures of the interlayer materials.
Keywords: laminated glass, impact properties, interlayer
material, temperature
1. Introduction
Laminated glass has been widely used in many engineering
applications such as aircraft and automobile windshields as well as
architectural and security glazing due to its excellent safety
properties. Laminated glass normally consists of two or more layers
of glass plates which are bonded together with a transparent,
thermoplastic elastomeric interlayer. Monolithic glass has good
structural strength, but can fail dramatically when subjected to
crash or impact. However, the polymer interlayer can maintain the
structural integrity of the component after breakage of the glass
and protect the passengers from flying fragments. Additionally, the
polymer interlayer is responsible for absorbing the remaining
impact energy after glass breakage. This is a vital design
consideration especially in transport industry to ensure the safety
of vehicle occupants.
Nowadays, several polymer interlayers have been used for the
laminated glass including polyvinyl butyral (PVB), thermo-plastic
polyurethane (TPU), ethylene-vinyl acetate (EVA) and ionoplast
Sentry Glas® Plus (SGP). PVB, TPU and EVA perform as the soft
interlayer materials, which have lower stiffnesses at room
temperature; compared to SGP, which has shown a significantly
stiffer response at room temperature (e.g. the elastic modulus of
SGP is two orders of magnitude greater than PVB [1-3]). The
structural performance of a laminated glass is significantly
influenced by the properties of the interlayer material [3-5].
El-Shami et al. [4] investigated the stress development in
laminated glass with different interlayer materials including
regular polyvinyl butyral (PVB) as well as a stiffer formulation of
PVB and found that the laminated glass with the stiffer formulation
of PVB resulted in a significantly larger load resistance. PVB has
been widely used as a general interlayer material in the car and
architecture laminated glass field and is of much research interest
[6, 7]. SGP has the advantage of higher mechanical strength and
higher modulus and is effective in improving the bending strength
of laminated glass. As a result, SGP is the recommended polymer
interlayer for anti-hurricane architectural glazing[8, 9]. Another
interlayer of interest is TPU which has good adhesion to inorganic
glass and polymeric plates, good low temperature flexibility and
good anti-aging properties [10, 11].
Impact performance of laminated glass is of great importance for
transport applications and has attracted much research interest
[12-16]. The failure behaviour of laminated glass subjected to
dynamic impact varies greatly from monolithic glass, due to the
contribution of both elastic properties and energy-absorption
capacity of the interlayer [15, 17]. Low and high velocity impact
performance of the laminated glass have been studied using
different experimental methods, such as drop weight impact machine,
pendulum impactor and gas-gun tests. Mohagheghian et al. [4, 5]
performed low and high velocity impact tests on the laminated glass
using different materials and types of constructions. The effects
of interlayer thickness, type of glass and polymer type and
multi-layering the polymer interlayer on the structural performance
have been investigated. It has been concluded that increasing the
polymer interlayer thickness in the laminated glass configurations
has little effect on the stiffness of the structure, but the type
of polymer interlayer has significant effect on the stiffness and
strength of laminated glass plates. Chen et al. [18, 19] conducted
a drop-weight test in combination with high-speed photography to
investigate the in-plane crack propagation behavior in laminated
glass plates. Velocity-time history curves along with the
force-time history curves of both radial and circular crack were
calculated and analyzed. It showed that the cracks on impacted
plate appeared long after the full growth of those on backing
plate. The cracks on the backing layer were motivated by in-plane
stress concentration while the cracks on impacted layer were caused
by stress concentration in depth direction.
The impact resistance performance of the laminated glass was
determined by many factors, with most research focusing on the
influence of glass type and thickness, interlayer type and
thickness, type of impactor, boundary conditions, impact velocity
and dimensions of the glass plates [20-22]. Zhang et al. [21]
conducted laboratory tests and numerical simulations on the
vulnerability of laminated glass windows subjected to windborne
wooden block impact. Different weights of wooden debris and
different thickness PVB interlayer were investigated. It has been
found that interlayer thickness plays a dominating role in the
penetration resistance capacity of the laminated glass windows
subjected to windborne debris impact.
As can be understood from above, there is an extensive range of
studies available in the literatures considering the impact
performance of laminated glass windows. These studies mainly
focused on laminated glass using conventional polymer interlayer
materials tested at the room temperature, especially PVB as it is
widely used in automotive and construction industries. However,
firstly for the high-tech applications such as laminated windshield
glass of aircraft and high-speed rail, the laminated glass windows
can experience large temperature changing. It can range between
-50°C in the high altitude flight and 70°C during its taking off
and landing process for the aircraft use. For the high speed rail
laminated glass, it also suffers from the high and low temperature
application when used in the tropical and gelid regions. This
temperature variation greatly affects the properties of polymer
interlayer material and its suitability especially in terms of
impact performance of the whole laminated glass structure [23].
In this paper, we aim to investigate the low velocity impact
performance of laminated glass windows using various polymer
interlayer systems. The focus will be on the effect of test
temperature on the perforation resistance as well as damage
development and fracture patterns of laminated glass windows
employing various polymer interlayer systems. At temperature
controllable chamber will be used for this purpose allowing tests
at temperatures between -30°C and 50°C. The force-displacement,
energy absorption and loading response of the laminated glass were
monitored and analysed during the impact.
2. Experiments
2.1 Materials
The glass plates used in this study were manufactured using
silicate float glass. The monolithic glass plates were manufactured
in Beijing Institute of Aeronautical Materials (BIAM) with the
dimensions of 100×150 mm and a thickness of 2.0mm. Three types of
polymer interlayer material were employed: Ionoplast Sentry Glas®
Plus (SGP) (from DuPont), Thermoplastic Polyurethane (TPU)-PF2300
(from PPG company) and Polyvinyl Butyral (PVB) (from DuPont). The
lamination was conducted in an autoclave (in BIAM) by placing one
layer (interlayer with single material) or three layers
(multi-material interlayer) of polymer between two layers of glass.
For lamination always the tin side of the glass was used, which was
cleaned prior to lamination using analytical reagent ethanol. The
lamination was conducted under the following processing conditions:
the temperature and the pressure were raised to 137 °C and 10 bar
respectively in 30 mins, then both were held for 210mins before
lowering the temperature at a rate of 2.5 °C/min to 49°C and
releasing the pressure naturally. Four different laminated glass
configurations were manufactured. The details of each configuration
are listed in Table 1, for each type of material (SGP, TPU, PVB –
Case 1, 3 and 4); including a hybrid multilayer interlayer
(TPU/SGP/TPU - Case 2) for comparison. Here, Case 1 has been taken
as a reference case. It should be noted that thickness of Case 2,
which has multi-interlayer, is different from the other cases here.
The intention here is to see the effect of adding two thin layers
of TPU on either side of SGP. The influence of the polymer
interlayer thickness on the flexural stiffness and low velocity
impact performance of laminated glass plates has been investigated
previously (ref 3 and 22) and showed to have negligible effect.
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 SGP/0.38mm TPU/2.0 mm glass
6.28
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
2.2 Measurement and characterization
2.2.1 Low velocity impact tests
The impact tests were performed with an Instron CEAST 9350 drop
weight equipped with a temperature-controllable chamber. The
picture of the machine and the relative parts are shown in Fig. 1.
Liquid nitrogen was used to lower the temperature in the chamber,
for the low temperature testing conducted at 0°C and -30°C. The
impactor used for the impact tests was hemispherical in shape with
a diameter of 12.7 mm. The total dropping mass is approximately
2.048 kg including the carriage mass of 1.3 kg and the impactor
mass of 0.748 kg. To measure the impact force history, a
piezoelectric force transducer is mounted on the bottom side of a
steel cross-bar. The laminated glass specimens were fixed inside
the chamber using the clamping system which consists of four cuffs
to properly fasten the specimen. The cuffs are made of steel and
with a rubber cap at the head of the cuffs. The clamps have a
minimum holding capacity of 1100 N. In this way it can help to
prevent any plate movement during the impact test and also to
ensure the same boundary conditions for all the experiments. The
tips of the clamps are covered with the rubber caps with a
durometer of Shore A 70-80. The cut-out in the plate is 75±1mm by
125±1 mm here. Three guiding pins are located such that the
specimen shall be centrally positioned over the cut-out. The
fixture shall be aligned to a rigid base using bolts or clamps. The
schematic of clamping system is shown in Fig. 2[24].
For this machine, an anti-rebound system was also used to
prevent a secondary impact on the laminated glass specimens after
the first impact. The instrument is equipped with aninfrared
sensor, whose position is adjustable by means of a measuring
system. This sensor measures the displacement. Before starting the
test the sensor must be set, depending on the position of the
impactor. The data of time and force can be obtained from the
machine directly. According to the ASTM D7136[24], the impact
velocity (vi) can be obtained according to the data of the
displacement sensor using the Eq. 1
Where vi =impact velocity, m/s; W12 = distance between leading
edges of the first (lower) and second (upper) flag prongs, m; t1=
time first (lower) flag prong passes detector; t2= time second
(upper) flag prong passes detector; ti= time of initial contact
obtained by the machine, s.
The time and force data obtained from the machine used to
calculate the velocity of the impactor during the contact with the
structure as follow:
Where v(t) is the impactor velocity at time t (t = 0 is the time
when the impactor initially contacts the specimen), F(t) is the
measured impactor contact force at time t, g is the gravity
acceleration constant equal to 9.81 m s-2 and m is the total mass
of the impactor. It should be noted that the vi is the initial
velocity upon impact and can be calculated using equation 1.
Subsequently, the impactor displacement as a function of time can
be calculated as:
Where di is the impactor displacement from reference location
(here is the surface of the specimens) at time t = 0. Based on
equations 2 and 3, the absorbed energy transferred to the
structure, Ea, during impact can be calculated by:
Fig. 1 Experimental setup used for low velocity impact tests
with the Instron CEAST 9350 drop tower and the
temperature-controllable chamber.
Fig. 2 The schematic of the clamping system[24]
The impact tests were conducted at an impact energy of 15J,
which corresponds to an impact velocity of 3.83 m/s, and at four
different temperatures of -30°C, 0°C, 23°C (i.e. room temperature)
and 50°C. All the specimens were kept in the chamber for 20mins to
achieve the desired equilibrium temperature before the impact was
conducted (except at 23°C). The thickness of specimen was measured
using a micrometer before the testing and the data was input into
the system, thus the initial displacement (di) of the impactor from
the specimens can be calculated automatically by the system. If the
impact energy is higher than the largest gravitational potential
energy of the impactor up to the limitation of the machine, an
initial velocity will be produced by the system, but 15J is not a
higher energy than the largest gravitational potential energy of
the impactor, so the initial velocity of the impactor is zero here.
Three specimens of each configuration were impacted at each
temperature to ensure repeatability of the test results.
2.2.2 Crack patterns analyses
Crack patterns of the laminated glass specimens after impact
were obtained using a digital camera. Since both glass layers were
broken after impact it was hard to distinguish between the crack
pattern in the front and rear glass plates. In order to take the
picture of the fracture pattern in the front glass layer, the rear
glass layer was painted in black prior to take pictures. This is
firstly, to mask the cracks occurred in the rear glass layer and
secondly, to make the cracks on the front impacted face more
visible by creating a larger contrast using a black background. The
black paint finish was obtained by dissolving black dye in alcohol,
the black paint was then poured onto the surface of the specimens
instead of spraying. In this way, the black paint can infiltrate
the cracks reducing the reflection of the cracks. Another series of
photographs were also taken to capture the fracture pattern in the
rear glass layer. This time the fracture pattern in the front glass
layer was masked by black paint. Although a series of other samples
were used to capture the fracture pattern in either sides of the
glass layer, the shape and density of the crack were quite
repeatable. It should be noted that three repeated tests were
conducted for each configuration at the particular temperature.
Take the energy versus time curves as the typical representation,
the repeated tests for each configurations are plotted together at
room temperature to show the repeatability of the method, which
will be shown in Section 3.1 Fig 6.
2.2.3 Dynamic mechanical analysis (DMA)
Dynamic Mechanical Analysis (DMA) was used to obtain the storage
modulus (E’) and loss modulus (E’’) of the three interlayers. The
tests were conducted in tension mode using a TA Instruments Q800
DMA machine at the frequency of 1 Hz with oscillation amplitude of
15 μm. The specimens had a rectangular geometry with the width of
6.17 mm. The free length of each specimen was measured individually
after fixing the specimen in the clamp. The experiments were
conducted under temperature sweep testing mode, the specimens were
left at -100°C for 10 min to achieve thermal equilibrium, before
heating up to 80°C with a heating rate of 2°C min-1.
When the interlayer materials are subjected to an oscillating
load, there will be a phase hysteresis between the applied stress
and the measured strain. This is caused by the hysteresis response
of the viscous component in the material. As a result, the modulus
of the material can be expressed as Eq. 2:
where E’ and E’’ are the storage and loss modulus respectively.
The ratio of the storage over the loss modulus (E’/E’’) is defined
as tangent of the phase angle (tan delta). In the present research,
the temperature corresponds to the peak value of tan delta is
considered as the glass tan delta transition temperature (Tg). It
should be noted that the glass transition temperature measured here
is for the testing frequency of 1 Hz. If the testing frequency
changes, there will be a shift in the glass transition temperature
for all three polymer interlayer materials.
3. Results
3.1 Different types of interlayer material
Typical force-deformation and energy-time curves obtained during
impact testing of Case 1 (SGP) at room temperature (23°C) are shown
in Fig. 3a and Fig. 3b respectively. The force versus deformation
plot, as shown in Fig. 3a, can be divided into three stages: in
Stage I, an approximately monotonic increase of the force over time
until the first peak force, Ffl, at a deformation of, Dfl, which
corresponds to the fracturing of the first layer of glass. Then the
force decreases dramatically to almost zero, this phase is due to
the compressibility of the interlayer material after the fracturing
of the first layer of glass. In Stage II, the force increases over
time until a second peak, Fsl, at a deformation of, Dsl, resulting
in the fracturing of the second layer of glass. Beyond Fsl, the
significant drop in force was due to crack propagation in the
laminated glass. In Stage III, the deformation continues reaching
the ultimate deformation, Dul, before the impactor rebounds due to
the residual strength of the laminated glass after fracturing. The
response in this region is mainly controlled by the strength of
polymer interlayer.
The energy versus time plot (Fig. 3b) shows the initial impact
energy that the machine imposed on the specimen with a value of
approximately 15J. The absorbed energy (Ea) can be defined as the
value of the plateau part of the energy curve, after the initial
impact. The elastic energy (Ee) is then given by the difference
between the initial impact energy and absorbed energy [25],
corresponding to the rebound energy on the impactor after striking
caused by the rebound of the laminated glass. However, in the case
when the impactor completely perforates through the laminated glass
plate, the peak energy will never reach 15 J. In this situation,
the impactor carries a residual velocity, vr, and consequently
residual kinetic energy of Er= account for the remaining energy.
Small portion of energy might also be dissipated through friction
and bending of the intact remaining parts as the impactor continues
its movement.
Fig. 3 Typical impact response curves for laminated glass Case 1
(SGP), for an impact energy of 15 J at room temperature: (a) Force
versus deformation curve, (b) Energy versus time plot.
In Fig. 4, both force versus deformation and energy versus time
curves for all the studied laminated glass specimens with various
types of interlayer materials are presented. All the experiments
have been conducted at a test temperature of 23°C and an impact
energy of 15 J (Cases 1 to 4). The values for absorbed energy (Ea),
elastic energy (Ee), force and deformation for first glass layer
failure (Ffl and Dfl), force and deformation for second glass layer
failure (Fsl and Dsl) and the ultimate deformation (Dul) are
reported in Table 2 respectively.
As shown in Fig. 4c and Table 2, Case 1 (SGP) showed the highest
elastic energy followed by Case 2 (TPU/SGP/TPU) (with only slightly
lower elastic energy). Case 3 (TPU) and Case 4 (PVB) showed very
low elastic energies. The rebounding of impactors was greatly
influenced by the viscoelastic properties of the interlayer
materials. Viscoelasticity in the polymer interlayers involves both
“elastic” and “viscous” contributions. The “elastic” part is
responsible to the deformation recovery while its “viscous” part
leads to a delay effect in strain recovery upon impact unloading
[26]. Therefore, the above results were caused by the different
mechanical properties of the interlayer materials, SGP possessing
the highest modulus and yield strength at room temperature compared
to the TPU and PVB interlayers [1, 2, 10]. This can be clearly seen
in the DMA results in Fig. 5a where at room temperature the storage
modulus (E’) of SGP is highest amongst the three interlayer
materials used. The superior mechanical properties of SGP at room
temperature also led to benefits in the higher strength, stiffness
and creep resistance of the whole laminated glass, both before and
after glass breakage [27].
All the above mechanisms contribute to the loading response
during the impact are shown in Fig 4a. The initial part of the
force-deformation curve is cropped in Fig. 4b to make the curves
more distinct for the reader. The gradient of the force-deformation
curves between Ffl and Dfl are different from each other after the
force is higher than about 1 KN, with Case 1 (SGP) being higher
than that of Case 2, and then Case 3 (TPU) and Case 4 (PVB),
indicating that the higher modulus of the interlayer material
resulted in a higher elastic bending strength of the laminated
glass specimens (as shown in Fig 5a). It should be noted that,
despite the different interlayer materials, the response in all
four Cases was the same up to approximately 1 kN, which
corresponded to the duration of the impactor being in contact with
the glass before the impact force reached the interlayer material.
For Case 2 with a hybrid multilayer interlayer (TPU-SGP-TPU), the
gradient of the force-deformation curves between Ffl and Dfl over
1kN was, as expected, smaller than that of Case 1 but higher than
that of Case 3. It might be speculated that this result was caused
by the larger thickness of Case 2 interlayer, but it has been
reported that the interlayer thickness effects on laminated glass
unit structural response were minimal[3, 22]. Therefore, the true
reason should be 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 interlayers, which contributed to the
lower Ffl value in Case 2 compared to that of Case 1.
After the fracture of both the glass layers, the residual
loading must be carried by the interlayer materials. The lower
ultimate deformation (Dul) of the specimen means higher loading
carrying capacity and also better rebound capacity of the
interlayer materials. Accordingly, Case 1 had the best loading
carrying capacity after fracture of the glass plates, indicated by
the smallest Dul value in Table 2 and the highest reverse velocity
of the impactor in the velocity-time curve in Fig. 4d. The load
carrying capacity of Case 2 was slightly less than that of Case 1,
but its Dul value was almost half that of Case 3 and Case 4, which
indicated that the addition of an SGP layer lead to an improved
loading carrying performance of the laminated glass after impact.
The results are coincident with the higher elastic modulus of SGP
at room temperature as indicated by the DMA results in Figure
5.
Fig. 4 Effect of interlayer structure on the impact properties
of laminated glass at 23°C (SGP – Case 1, TPU/SGP/TPU - Case 2, TPU
- Case 3, PVB - Case 4), for an impact energy of 15 J: (a) Force -
displacement over full 26 mm, (b) Force-displacement over first 6
mm,(c) Energy – time, (d) Velocity -time curves.
Fig. 5 The dynamic mechanical analysis results of: a) Storage
Modulus (E’) and b) Tan Delta, plotted against temperature for SGP,
TPU and PVB interlayers.
Table 2 Low velocity impact of laminated glass with different
interlayers (SGP – Case1, TPU/SGP/TPU - Case 2, TPU - Case 3, PVB -
Case 4) at 23°C.
Specimen
Ea(J)
Ee(J)
Ffl(kN)
Fsl(kN)
Dfl(mm)
Dsl(mm)
Dul(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±0.052
2.82±0.079
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
To illustrate the repeatability of the method, here, the energy
versus time curves of the repeated tests for each configuration at
room temperature are plotted together as shown in Fig. 6. It can be
seen that the scatter of the same three specimens is not large, the
method and analysis are considered to be reliable.
Fig. 6 The energy versus time curves of the repeated tests for
each configuration at room temperature. SGP – Case 1, TPU/SGP/TPU -
Case 2, TPU - Case 3, PVB - Case 4
3.2 Effect of temperature
The experiments were conducted on each of the four Cases at
temperatures of 50°C, 23°C, 0°C and -30°C with an impact energy of
15 J. The force versus deformation and energy versus time curves
for Cases 1-4 are presented in Fig. 7 and 8 respectively. For
clarity, the initial part of the force versus deformation responses
is plotted in Fig. 9.
Fig. 7 Force versus deformation responses for laminated glass
plates with different interlayers: Case 1 – SGP, Case 2 –
SGP/TPU/SGP, Case 3 – TPU, Case 4 – PVB tested at temperatures of
50°C, 23°C, 0°C and -30°C, with impact energy of 15 J.
Fig. 8 Energy versus time graphs for laminated glass plates with
different interlayers: Case 1 – SGP, Case 2 – SGP/TPU/SGP, Case 3 –
TPU, Case 4 – PVB tested at temperatures of 50°C, 23°C, 0°C and
-30°C,with impact energy of 15 J.
Fig. 9 The initial part of the force versus deformation
responses for laminated glass plates with different interlayers:
Case 1 – SGP, Case 2 – SGP/TPU/SGP, Case 3 – TPU, Case 4 – PVB
tested at temperatures of 50°C, 23°C, 0°C and -30°C, with impact
energy of 15 J.
3.2.1 Case 1 (SGP Interlayer)
It can be seen from Table 3 and Fig. 7a that although the Ee
value obtained at 0°C was the highest amongst the temperatures
tested, the loading values of Ffl and Fsl were slightly higher at
23°C compared to 0°C. The higher stiffness of the SGP interlayer at
lower temperatures provides a more rigid support against the
localised deformation of frontal glass layer when comes to the
contact with the impactor. As a result, the fracture in the glass
layers occurs earlier. On the other hand, when both glass layers
are broken the remaining load will be carried by the polymer
interlayer itself. The higher stiffness of the SGP interlayer at
low temperatures results in a greater elastic energy stored in the
structure.
From Fig. 8a, it can be seen that for Case 1 the absorbed
energy, Ea, value decreased as the temperature was lowered except
at -30°C where Ea was almost equal to the impact energy. As can be
seen in Fig. 5a, the stiffness of SGP interlayer increases as the
temperatures are lowered. The stiffer interlayer material
contributed to the higher strength and better elastic properties of
the whole laminated glass specimens. However, if the temperature
decreases to a certain level, as in the case of -30°C, the SGP
interlayer material became brittle and susceptible to fracture
instead of elastic deformation. As can be seen in Figs 7a and 9a,
the laminated glass plate with SGP interlayer fractures at much
lower loads and displacements. This results in much smaller elastic
deformation stored in the glass layers prior to fracture. In
contrast most of the absorbed energy was dissipated through
fracture in the glass layers as well as deformation and
subsequently fracture in the SGP interlayer. As the impactor
perforate the structure in this case, the transferred energy to the
structure does not reach 15 J as indicated in Fig 8a. The velocity
of the impactor after strike is measured to be 1.13±0.21m/s (as
shown in Fig. 10) and the corresponding residual energy of
1.34±0.47J. As mentioned earlier, the remaining energy which is
only small portion of the initial energy, 0.39±0.37J, might be
dissipated through friction or bending the remaining intact parts
of the plate. The ultimate deformation, Dul, of the specimen after
striking at -30°C could not be determined because there was no
rebound of the impactor as shown in Fig. 7a.
The above results show that the monolithic SGP loses its
ductility at low temperatures. This can lead to fracture in the
polymer interlayer itself which consequently affects the structural
integrity of the whole laminated glass. As post impact breakage of
laminated glass is an important safety aspect in the design, usage
of monolithic SGP is not recommended for low temperatures.
Fig. 10 Velocity versus time results of three repeat tests for
Case 1 impacted at temperature of -30°C.
Table 3. The low velocity impact properties of the laminated
glass at different temperatures with SGP interlayer.
T(°C)
Ea(J)
Ee(J)
Ffl(kN)
Fsl(kN)
Dfl(mm)
Dsl(mm)
Dul(mm)
50
14.28±0.39
0.72±0.36
1.53±0.25
1.94±0.18
0.88±0.12
2.37±0.13
18.25±0.26
23
11.57±0.12
3.31±0.12
4.98±0.46
4.46±0.51
1.13±0.053
2.34±0.23
10.04±0.15
0
10.14±0.23
4.86±0.23
3.83±0.37
3.14±0.25
1.26±0.048
2.82±0.088
10.21±1.33
-30
13.25±0.85
0.39±0.37
3.94±0.39
3.35±0.36
1.22±0.014
2.41±0.023
-
3.2.2 Case 2 (TPU/SGP/TPU Hybrid Interlayer)
From Fig. 7b and 9b it can be seen that as the temperature was
decreased, the initial gradient of the force versus deformation
curves was increased. The increase in stiffness of the laminated
glass is because of increase in shear modulus of TPU as well as SGP
interlayer at low temperatures. The higher the shear modulus the
higher shear stresses transfer between the two glass layers and the
higher the stiffness of the laminated glass.
The energy dissipation property of Case 2 at -30°C was better
that of Case 1 (pure SGP) which showed poor elastic properties at
-30°C. After the introduction of the TPU interlayer as the
transition layer in Case 2, the impact resistance property of the
whole structure was significantly improved because the TPU
interlayer is much more flexible than SGP at -30°C as can be seen
in the storage modulus (E’) versus temperature plots of Fig. 5a.
This helps to retain the structural integrity of the laminated
glass plates after both glass layers breakage and results in much
better post breakage performance. From the DMA data shown in Fig
5b, it could be seen that the glass transition temperature, Tg, of
TPU is -32°C, much lower than that of SGP (54°C) and close to the
test temperature of -30°C. As a result, TPU layer has not yet
reached its glassy state. No sign of fracture was observed in the
SGP layer for this case because the TPU transition layer can
dissipate some absorbed energy and the energy left to SGP become
less, and will not cause the fracture of the SGP interlayer.
Therefore, in this case SGP keep carrying the remaining load while
TPU interlayer ensure the structural integrity of the laminated
glass, which resulted in better impact property of the whole
laminated glass.
Table 4. The low velocity impact properties of the laminated
glass at different temperatures with TPU/SGP/TPU hybrid
interlayer.
T(°C)
Ea(J)
Ee(J)
Ffl(kN)
Fsl(kN)
Dfl(mm)
Dsl(mm)
Dul(mm)
50
14.86±0.062
0.14±0.062
1.37±0.16
2.93±0.19
1.23±0.08
3.45±0.12
24.47±2.36
23
12.06±0.047
2.82±0.078
2.56±0.16
4.54±0.29
1.16±0.23
3.55±0.12
10.88±0.21
0
10.97±0.27
4.03±0.27
4.30±0.22
3.44±0.26
1.30±0.13
2.92±0.33
11.45±1.19
-30
10.94±0.58
4.06±0.58
4.36±0.33
2.24±0.13
0.96±0.20
2.48±0.25
8.08±1.11
The impact energy is dissipated through three possible
mechanisms: i) fracturing in glass layer and in the polymer
interlayer (in certain conditions), ii) the deformation of polymer
interlayer and iii) the delamination between the polymer interlayer
and glass layers. The energy dissipated through the delamination is
believed to be smaller compared with the other two mechanisms. For
the impact tests conducted at 50°C, the deformation of the
laminated glass plate was much larger than the results obtained at
the three lower temperatures, as indicated by the high value of Dul
in Table 4. Therefore, it seems that the concentration of radial
and circumferential cracks on both the glass layers tested at 50°C
was less than the glass layers tested at 23°C and 0°C, shown in
Fig. 14b. At low temperatures, the higher stiffness of the polymer
interlayer provided a greater support against the local deformation
of the frontal glass layer when comes to the contact with impactor.
This resulted in a highly localised fracture pattern, as can be
seen in Fig 14b. In contrast, at higher temperatures the polymer
interlayer became softer and more complaint as a result. Therefore,
the deformation is distributed over the larger area and
consequently the fracture occurs over a larger area.
3.2.3 Case 3 (TPU Interlayer)
From the results shown in Fig. 8c and Table 5 for Case 3, it can
be seen that the Ea value decreased while Ee value increased as the
temperature was lowered except at -30°C. This is consistent with
mechanisms leading to enhanced elasticity of TPU material when the
temperature is lowered from 50°C to 0°C, however, -30°C is nearly
as low as the Tg value of the TPU material, at which temperature
TPU became less ductile capable of undergoing large deformations,
inducing the relative lower Ee value of the specimen at -30°C
compare with that at 0 °C.
However, when struck at 50 °C, the Ee value is about
0.077±0.0028 as the absorbed energy is 14.85±0.065J and the final
kinetic energy of the impactor are 0.15 J, 0.08J, 0.00 J as the
velocity of the impactor for the three testing was 0.38 m/s,
0.28m/s and -0.02m/s, as indicated in Fig. 11. This is because the
TPU interlayer became very soft at 50 °C and its load carrying
capacity was significantly reduced after the fracture of the
glasses, and very little rebound happened for the impactor.
Consequently serious deformation was induced as shown in Fig.
12.
Fig. 11 Velocity versus time results of three repeat tests for
Case 3 impacted at temperatures of 50°C.
Fig. 12 Fracture plate of Case 3 impacted at 50°C with large
deformation
Compared to Case 1 and Case 2, with the stiffer SGP interlayer,
monolithic TPU provided inferior impact performance at high
temperatures. Conversely, at low temperatures, TPU can still
maintain good flexibility and thus the impact resistant properties
of the laminated glass with the monolithic TPU interlayer (Case 3)
were better than that with monolithic SGP interlayer (Case 1),
maintaining the structural integrity after impact. However, the
impact properties were still inferior to that of the laminated
glass with TPU/SGP/TPU hybrid interlayers (Case 2). This can be
seen by comparing Tables 4 and 5 for recoverable elastic energy, Ee
of 1.26 J for TPU compared with 4.06 J for hybrid interlayer and
for first peak force, Ffl, of 4.36 kN for TPU compared with 2.32 kN
for hybrid interlayer at -30°C. It can be concluded that the hybrid
interlayer structure of Case 2 has integrated the advantages of
both Case 1 and Case 3.
Table 5. The low velocity impact properties of the laminated
glass at different temperatures with TPU interlayer (Case 3)
T(°C)
Ea(J)
Ee(J)
Ffl(kN)
Fsl(kN)
Dfl(mm)
Dsl(mm)
Dul(mm)
50
14.85±0.065
0.077±0.0028
2.41±0.40
3.22±0.11
2.11±0.15
3.84±0.05
-
23
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
0
12.48±0.15
2.52±0.15
4.95±0.66
5.64±0.31
1.35±0.05
1.92±0.09
12.11±0.18
-30
13.74±0.10
1.26±0.10
2.80±0.18
2.64±0.54
1.06±0.02
2.46±0.12
13.00±1.64
3.2.4 Case 4 (PVB Interlayer)
Again, similar to Case 3, when struck at 50 °C, the energy-time
curve was terminated before reaching the impact energy of 15J as
shown in Fig 8d, for the similar reason as Case 3, as PVB
interlayer became very soft at 50 °C and the specimen was almost
penetrated during the impact, indicating that Case 4 also possessed
poor impact resistant property at high temperature. The Ea value is
approximately 14.05 J as seen in Table 6, indicating there still
left some energy on the impactor after the striking, and the
velocity of the impactor for the three experiments are 1.01m/s,
0.90m/s, 1.05m/s, then the Ee value is almost zero, as the same
reason as that for Case 3. The Dul value is very large indicating
that the rebound property of the PVB interlayer is very poor after
the fracturing of the glass layers.
However, at 23°C and 0°C, the Case 4 laminated glass plate
possessed similar impact resistant property to that of Case 3. The
Ee values at 23°C and 0°C are relatively lower than that of Case 1
and Case 2 due to the high mechanical strength of the SGP material.
However, the Dul value of Case 4 was the highest amongst all the
structures at 23°C and 0°C, indicating that the elastic property
and also the rebound capacity of the PVB interlayer was the lowest
compared to the other three types of interlayer materials.
Also, Case 4 showed poor impact resistant property at -30°C
which was fractured directly instead of being deformed upon being
struck. This is because PVB had become brittle at -30°C which was
lower than its Tg temperature of about 27°C, as indicated in Fig.
5b. Similar to the testing at -30°C of Case 1, the elastic energy
in the glass plate can be obtained through the subtraction of the
kinetic energy of the impactor from the difference between the
initial impact energy (15 J) and the absorbed energy. As the
velocity of the impactor after striking is still about 0.37±0.21m/s
and the corresponding kinetic energy is about 0.17±0.16J, then the
elastic energy in the glass plate is calculated to be about
0.86±0.67J.
Table 6 The low velocity impact properties of the laminated
glass at different temperatures with PVB interlayer (Case 4)
T(°C)
Ea(J)
Ee(J)
Ffl(kN)
Fsl(kN)
Dfl(mm)
Dsl(mm)
Dul(mm)
50
14.05±0.59
0.00±0.00
1.26±0.23
1.50±0.30
0.58±0.15
1.26±0.23
-
23
14.09±0.20
1.01±0.23
2.32±0.31
2.76±0.22
1.06±0.28
3.02±0.17
25.80±0.78
0
11.60±0.23
3.40±0.23
3.16±0.16
2.64±0.23
1.16±0.28
2.83±0.05
19.18±1.13
-30
13.96±0.81
0.86±0.67
4.54±0.94
-
0.94±0.051
-
-
Fig. 13 Absorbed energy (Ea) and elastic energy (Eb) versus
temperature curves for the four configurations tested at different
temperatures with impact energy of 15 J.
In order to directly compare the impact performance of the
different configurations, the absorbed energy (Ea) and elastic
energy (Eb) versus temperature curves have been plotted in Fig. 13a
and b. It can be seen that Case 2 (with TPU-SGP-TPU hybrid
interlayer) has the lowest Ea value but highest Ee value at -30°C.
Although Case 1 shows lower Ea value than Case 3 and Case 4, the
laminated glass specimens are apt to be fracturing instead of
deformation, leading to the lower Ee value. At 0°C, the sequence of
Ea values for the four configurations is Case 1< Case 2< Case
4< Case 3, and contrast to the Ee values. This is also true for
the results obtained at 23°C except the Ea value of Case 4 almost
being same to Case 3. Lower Ea value is considered to be beneficial
to ensuring the safety of vehicle occupants. For the high
temperature impact, the Ea and Ee values of the four configurations
are not very different from each other, but Case 3 and Case 4 are
almost penetrated after the impact. Case 1 and Case 2 with at least
one layer of SGP interlayer can keep the structural integrity of
the whole laminated glass after impact.
3.3 Crack patterns
Images of the crack patterns of each Case tested at temperatures
of 50°C, 23°C, 0°C and -30°C with an impact energy of 15 J are
shown in Figure 14.
31
Fig. 14 Crack patterns on the front (f) and back (b) face of the
laminated glass for a) Case 1 – SGP, b) Case 2 – SGP/TPU/SGP, c)
Case 3 – TPU, d) Case 4 – PVB tested at temperatures of 50°C, 23°C,
0°C and -30°C with impact energy of 15 J.
3.3.1 Case1-SGP interlayer
As the temperature of the test was lowered, the density of the
radial and circumferential cracks decreased except for the specimen
impacted at -30°C, which resulted in brittle fracture as shown in
Fig. 14a. Also, the damage becomes more localized because the rigid
SGP interlayer is bad for the energy transmittance between the two
glass layers. This was consistent with the lowered energy
absorption results shown in Fig. 8a, and fewer circumferential
cracks were visible on the back layer of the glass impacted at 0°C.
This was because the SGP became stiffer at lower temperatures and
consequently the laminate bending resistance increased, as
mentioned in Section 3.2.1. However, if the test temperature is
dropped below a certain level, e.g. tests at -30°C, ductility of
the SGP would be affected and resulted in fracture in the SGP layer
upon impact. This undermines the benefit from the increased bending
resistance of the structure at low temperatures and results in
lower absorbed energy values.
3.3.2 Case 2-TPU/SGP/TPU interlayer
For the impact test conducted at 50°C, the deformation of the
glass layers and the interlayer material was the largest of the
four temperatures tested for Case 2 according to Table 4. From Fig.
14b, it can be seen that the density of radial and circumferential
cracks on both the glass layers tested at 50°C was less than the
glass layers tested at 23°C, 0°C and -30°C. Again, the damage
became more localized as the temperature dropped also caused by the
bad energy transmittance capacity of the rigid TPU/SGP/TPU
interlayer at lower temperatures.
3.3.3 Case 3-TPU interlayer
From Fig. 12, 14c and Table 5, it can be seen that there was
large deformation and fragmentation of the specimen when impacted
at 50°C. The radial and circumferential cracks on both the glass
layers became sparse as the temperature was lowered to 0°C, this is
because the absorbed energy at 0°C is the lowest.
3.3.4 Case 4- PVB interlayer
The fractured specimen with the PVB interlayer is shown in Fig.
14d. It was very different from that of Case 3 with TPU as the
interlayer material. The specimen impacted at 0°C resulted in the
least deformation and fragment sizes, corresponding to a lower Ea
value. Similar to Case 3, high temperature caused larger
deformation and fragmentation, while the interlayer was almost
fully penetrated due to the poor load carrying capacity of the PVB
material after fracture of the glass layers at 50°C. However, at
the very low temperature of -30°C, the PVB interlayer became
brittle, leading to direct fracture of the specimen when impact
occurred, similar to what happened to Case 1 at -30°C.
4. Conclusions
In this paper, the low velocity hard impact properties and
damage mechanisms of laminated glass with four different types of
interlayer materials were evaluated. All the structures were tested
at four different temperatures whilst the impact energy was kept
fixed. Through a systematic study, the effect of the interlayer
material and temperature on the energy dissipation, loading
distribution and crack patterns were measured and analysed. The
following conclusions were drawn:
· The type of interlayer has a strong effect on impact
performance of laminated glass plates. The stiffest polymer
interlayer, SGP (Case 1), provided the highest elastic bending
strength to the laminated glass, resulting in the best load
carrying capacity after the fracturing of the glass layers at room
and high temperatures. However, the SGP became brittle when the
temperature was lowered to -30°C, leading to the direct fracturing
upon impact.
· The laminated glasses using soft polymer interlayer TPU (Case
3) offers better impact performance at lower temperatures compared
to laminates using SGP interlayer. This is because of the good
flexibility of the TPU at low temperatures. However, it is almost
penetrated at higher temperatures, e.g. 50°C as a result of
relatively low stiffness of TPU (i.e. 50°C is well above the glass
transition temperature of the material at which the stiffness is
significantly reduced), which is considered to be bad for the
occupants and instruments in the cabin.
· The laminated glass using PVB (Case 4) as an interlayer,
showed similar impact performance to TPU (Case 3) at 50°C, 23°C and
0°C and almost perforation when impacted at 50°C. Unlike the impact
performance of Case 3 at -30°C, the laminated glass using PVB
interlayer fractured directly after striking, which was similar to
the SGP interlayer specimen at -30°C.
· The hybrid interlayer (Case 2), consisting of an SGP layer
sandwiched between two layers of TPU, showed similar impact
performance to monolithic SGP (Case 1) at 50°C, 23°C and 0°C, but
at -30°C its impact resistance properties were considerably better
than the other three configurations because of the lowest absorbed
energy and less damage to the specimens, which is considered to be
safer for the occupants and instruments in the cabin. It can be
concluded that the hybrid interlayer (Case 2) possessed the most
attractive impact resistant properties among the four
configurations studied over the entire range of temperatures tests,
with the hybrid interlayer integrating the advantages of both SGP
and TPU.
Acknowledgement
This research was performed in collaboration with the Imperial
research teams at the BIAM Centre for Materials Characterisation,
Processing and Modelling at Imperial College London. This research
work is financially supported by the Innovation Foundation
Programme of Beijing.
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32
01234560246810Force [kN]Deformation
[mm]02468101214160246810121416Energy [J]Time [ms]abImpact
EnergyElastic EnergyAbsorbed EnergyFflFslDflDslDulStage IStage
IIStage III
-2-1012340246810121416Velocity [m/s]Time [ms]Case 1Case 2Case
3Case 4-101234502468101214161820222426Force [kN]Deformation
[mm]Case 1Case 2Case 3Case 4-10123450123456Force [kN]Deformation
[mm]Case 1Case 2Case 3Case 402468101214160246810121416Energy
[J]Time [ms]Case 1Case 2Case 3Case 4abcd
00.20.40.60.811.2-100-50050100Tan DeltaTemperature
[C]SGPTPUPVB1.E+001.E+011.E+021.E+031.E+04-100-50050100E'
[MPa]Temperature [C]SGPTPUPVBab
02468101214160246810121416Energy [J]Time
[ms]50230-3002468101214160246810121416Energy [J]Time
[ms]50230-3002468101214160246810121416Energy [J]Time
[ms]50230-3002468101214160246810121416Energy [J]Time
[ms]50230-30Case 1 -SGPCase 2 -HybridCase 3 -TPUCase 4 -PVBabcd
