Study windshields. Hence, there is a considerable interest in

Study of
Crack Propagation in Automobile Windshields subject to Light-weight Impacts

Abstract

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During
automobile accidents, it is commonly observed that the pedestrians and
passengers sustain head injuries due to the impact on the windshields. Hence, there
is a considerable interest in understanding the light-weight impacts on
automobile windshields and the crack pattern formed in the windshields as a
result. The lamination of the glass with a polyvinyl butyral (PVB) interlayer
plays an important role in reducing the injuries caused to the human head. In
this study, an experimental research to replicate the light-weight impact to
understand the crack patterns in a PVB laminated glass is analyzed. The results
of the study give a basic understanding of the crack morphology and propagation
through the material. Various factors influencing the velocity of crack growth like
impact velocity, weight of the object and PVB interlayer thickness is also
discussed.

 

1.    
Introduction

Laminated glass has been used in
various applications since it was invented in the early 1900s. Laminated glass
is a type of safety glass, which consists of at least two layers of glass or
plastic glazing sheets (brittle materials) bonded together by interlayers.i An interlayer, typically of
polyvinyl butyral (PVB), holds the glass in place when it is broken between its
two or more layers of glass. The toughening of the interlayer prevents the
glass from breaking up into large sharp pieces.ii This produces a characteristic “spider web”
cracking pattern (see Fig. 1) when the impact is not enough to completely
pierce the glass.iii  

Fig. 1 Automobile windshield displaying
“spider-web” cracking typical of laminated safety glass

Polyvinyl
Butyral (PVB) laminated glass has extensive applications in architectureiv, automobile industryv as well as the structural
parts during the past few decades where they usually play an important role in
human protection and structural integrity. In automobiles, the windshields need
to be brittle to absorb the impact energy of the pedestrian head by undergoing
plastic deformation. Also, the glass should not break down into large sharp
pieces to avoid injury. PVB laminated glass satisfies both criteria. For this
reason, most of the automobile manufacturers use PVB laminated glass for their
windshields to provide safety and protection to the pedestrians and passengers.
In case of automobile windshields, stones and pedestrian heads are the two
major potential impactors.vi Despite being one of the most
important safety features in a vehicle, the mechanical behavior of PVB
laminated glass has not been fully explored yet due to the complicity caused by
the composite material, i.e. polymer material sandwiched by two pieces of
brittle material. Cracks are the most obvious traces left on the windshield
plate after impactvii
and hence, it is useful to study the macroscopic cracking behaviors which helps
to understand the material properties of the composite.

 

2.     Windshield Structure

An automobile windshield is made of a viscoelastic
layer of polyvinyl butyral (PVB) sandwiched between two layers of curved glass.
The process of bonding the glass layers with the PVB layer is called
Laminating. The lamination is done
in an autoclave. The autoclave is a special oven that uses both heat (120?C)
and pressure (10 bar) to form a single unit of
laminated glass.viii
Unlike traditional safety glass, laminated glass can be further processed—cut,
drilled, and edge-worked, as necessary. A typical laminated windshield for
common passenger cars is very thin. Each glass layer is approximately .03 inch
(.76 millimeter) thick, while the plastic interlayer is approximately .098 inch
(2.5 millimeters) thick. However, the dimensions may vary based on the
requirement. In the United States, windshields are required by law (FMVSS 205) to
be made of laminated glass. Fig. 2 shows an example of a typical automobile
windshield. The PVB interlayer is often tinted to act as an ultraviolet filter. 

Fig. 2 Typical Automobile Windshield

3.     Material Properties

PVB
is used in automobile windshields mainly due to its properties like strong
binding, optical clarity, adhesion to many surfaces, toughness and flexibility.
Some of the material properties of PVB interlayer are listed in table 1.ix

Table 1 PVB Material Property

Property

Young’s Modulus,
Ep (GPa)

Short time Shear Modulus, G0
(GPa)

Long time Shear Modulus, G?
(MPa)

Bulk Modulus, K (GPa)

Poison’s Ratio, vp

Density, ?p (kg/m2)

Decay factor, ? (s-1)

PVB

0.1

0.33

0.69

20

0.49

1100

12.6

 

For
glass, Young’s Modulus Eg = 70 GPa, Poison’s ratio vg =
0.22 and density ?g = 2500 kg/m2.9 Glass is a typical linear elastic brittle material where
its facture strain is about 0.1%x while the PVB is a
rubber-like material which may sustain a much larger deformationxi during tension. The unique
design enables the possible protection against impact with small amount of
energy and dissipates larger impact energy through the large deformation of
interlayer.

4.     Light-weight Impact

Failure of an automobile windshield can
occur by various means. Impact of light-weight objects like stones or a
pedestrian head are the major causes of windshield failure for passenger cars.6 This light-weight impact can be simulated in a controlled
environment by performing a simple drop test. A typical drop test performed by
Xu et al8 is shown in Fig 3.

Fig. 3 Schematic of the drop-weight tower
experiment setup. The testing sample is illustrated along with the force
direction convertor.8

The impact
system consists of a weight block which can slide freely along the two standing
poles. A force direction converter is used to change the direction of the
impact from vertical to horizontal. The minimum mass of the drop-weight is 0.5
kg and the maximum height of the drop-weight is 1.84 m (corresponds to the maximum
impact velocity of 6 m/s). Both mass and the drop height are varied during the
experiment to simulate different impact conditions. The drop-weight is
suspended and held in place through an electromagnet before the experiment.

Upon impact of the drop-weight on the laminated
glass test sample, a high-speed camera is employed to capture the crack
geometry by shining spotlight on the sample at regular time intervals. The time
intervals are adjustable from 1?s to 9999?s by means of highly precise
controllers.

 

5.    
Results
and Discussion

5.1.  Crack Initiation

It
is observed during the experiment that the crack does not form immediately
after the impact in all the cases of impact velocity. The material undergoes a
plastic deformation before cracks appear. The time taken for crack initiation is
also found to be related to the thickness of the PVB layer used. The larger the
thickness of the PVB layer the longer it takes for the crack initiation9 which is shown in Fig. 4.

 

Fig. 4 Initiation
time of the radial and circular crack at various impact velocities from 3.13
m/s to 4.2 m/s with the PVB thickness of 0.76 mm.

5.2.  Crack Morphology

Analysis of the results of the
experiments shows that two kinds of crack patterns, radial and circular crack
patterns are observed in the laminated glass sample. Also, all the glass pieces
are found to remain bonded to the PVB layer after the impact of the
drop-weight. Radial cracks are found to form before the appearance of circular
cracks in the sample. Fig. 5 shows a typical crack propagation process
depicting the radial and circular cracks at different times after impact.

 

Fig. 5 Selected sequence of images depicting the circular crack growth at the
impact velocity of 3.13 m/s, with PVB thickness of 0.76 mm.9

5.3.  Crack Propagation

The velocity of the cracks in PVB
laminated glass is found to be dependent on various factors like impact
velocity of the object, weight of the impact object, shape of the glass and
thickness of the PVB layer. The crack velocity directly increases with increase
in the impact velocity and weight of the object.8 The PVB interlayer thickness is found to have a great
effect on delaying the passage of the elastic wave to the backing glass. The
amplitude and wavelength of the elastic wave rehang the backing glass layer is
also reduced due to the internal friction in the PVB interlayer.xii

 

6.    
Conclusion

Crack
patterns formed in automobile windshield created by light-weight impact has
been an important yet unsolved problem for a long time. It is essential to
understand the crack pattern as it is a critical factor in pedestrian and
passenger protection. A study of crack initiation and propagation in PVB
laminated glass through an experimental setup is investigated. The crack
initiation begins once a critical energy is reached. Radial cracks appear
before circular ones and found to travel faster. Cracks propagate faster for
higher impact velocities and weights. Crack initiation and propagation delayed
with increasing thickness of the PVB interlayer.

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Glass Structures: Design
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The Architecture Reference
& Specification Book: Everything Architects Need to Know Every Day.
Rockport Publishers. ISBN 9781592538485, (2013-08).

 

iii P. Metcalfe, R. Metcalfe

Excel Senior High School:
Engineering Studies. Pascal Press. ISBN 9781741250510,
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