The characterisation of hail and fraudulent impacts to vehicle body panels

The characterisation of hail and
fraudulent impacts to vehicle body
panels
Visvarajah Somasundaram
Abstract
On the 25th December 2011 there was a hail storm in the state of Victoria, Australia,
which caused approximately AU $712 million worth of damage. Some of this damage was
caused to passenger vehicles. Through the investigation of damaged vehicles its shows that
some of the damages are created intentionally by the vehicles owners with the use of
different tools and/or objects'. Hence, a robust method is required in conclusively
differentiating genuine hail damage from a fraudulent damage on vehicle body panels. The
aim of this project to characterise the hail and fraudulent impacts to vehicle body panels
with respect to deformation size and shape as well as investigate FEA modelling for hail
impact.
Due to the availability of resources and time constraints only hail impact
experiments to a passenger vehicle bonnet (from a late model Proton Satria) using 40mm
spherical hail were permitted. FEA simulations using Abaqus/Explicit assumed a smooth and
spherical shaped hail impacting a flat thin panel.
It was observed that 40mm hail impacting a bonnet at around terminal velocity gave
dent diameters ranging from 9mm to 28mm and dent depths ranging from 1.49 mm to 0.20
mm. The simulated results showed that a 40mm hail impact on mild steel plate of sizes
100mm2
, 150mm2
, 200mm2
, and 250mm2
yielded dent depths from 1.79 mm to 1.59 mm
and dent diameters from 25.6mm to 24.9 mm.
Keywords
Hail, Abaqus/Explicit, terminal velocity,
1 Introduction
The need to understand the full
impact of hail damage to vehicles has
been an ongoing topic of concern to both
owners and insurance industry since there
is a recent rise in fraudulent insurance
claims. According to historical disaster
statistics published by insurance council
of Australia, 11 hail storms were recorded
in the past decade. On Christmas day
2011, a large hail storm hit Melbourne
and country Victoria causing $712 million
worth property damage and also resulted
in a large number of fraudulent insurance
claims.
The Magistrate Court of Victoria
held a dispute case concerning a Ford
utility vehicle damaged in the Christmas
day hail storm. The insured, AP Carpentry
Pty Ltd had been denied indemnity by the
insurer, Insurance Manufacturers of
Australia Pry Ltd. Through forensic
investigation, the insurer found that
1
School of Aerospace, Mechanical, and Manufacturing Engineering, RMIT University, AUSTRALIA

approximately 90% of damages were
inconsistent with hail damage and had
been manufactured with tools (“The
inconsistent indentations exhibited
scratch marks and often appeared in
linear or clustered patterns”2
).
The insured’s expert witness
conceded that approximately 16% to
36.6% of damages were inconsistent with
hail damage. The court found that in
addition to some genuine hail damage,
the insured had intentionally
manufactured a significant amount of
damage over the vehicle in an attempt to
render the vehicle a total loss and make a
claim for the agreed value of the vehicle.
Consequently, the insurer was entitled to
deny indemnity.
Previous experiments conducted
at Delta-V Experts concluded on cosmetic
observations of damaged surfaces on
vehicle body panels. These observations
alone could not conclusively distinguish
between genuine and fraudulent hail
damages in all cases. In contribution to
these observations, additional
investigation into hail impacts were
required to provide numerical backing.
The objective of this paper is to
characterise hail and fraudulent damages
to vehicle body panels with respect to
deformation size and shape and
investigate mathematical simulations.
2 Hail
Hail is a naturally occurring and often
highly localised phenomenon where small
balls, or irregular lump clusters are
precipitated from an ice, water, and air
mixture. It is generally opaque and has a
layered structure. Common hail sizes can
range between 5-100 mm in diameter.
For sizes between 5-10 mm, they
generally appear spherical or conical in
shape. 10-20 mm sized hailstones tend to
be ellipsoidal or conical. Larger 10-50 mm
hailstones take on ellipsoidal shapes with
lobes, while still larger hailstones between
40-100 mm appear irregular (including
disk shapes) with protuberances5
.
The Australia Bureau of
Meteorology defines a large damaging hail
as requiring Ø20mm ($2 coin size) or
greater. This is also a defining parameter
for a severe thunderstorm.
Large Hail commonly occurs in mid
latitude between 30˚ and 50˚ in both
northern and southern hemispheres
(between the Arctic Circle and Tropic of
Cancer and between the Antarctic Circle
and Tropic of Capricorn) during late Spring
and early Summer. During this period, the
surface temperatures are sufficiently
warm to promote instability associated
with strong thunderstorms, while the
upper atmosphere is still cool enough to
support ice3
.
2.1 Hail Formation
Hail is formed in thunderstorm clouds
called cumulonimbus consisting of ice
crystals and liquid water droplets below
0°C. Collisions with supercooled water
droplets caused by powerful updrafts of
air within the storm promote hail growth.
The supercooled water droplets freeze
over the surface of an ice crystal, frozen
raindrop, dust or other nuclei. The cycle of
freezing supercooled water droplets forms
the layered structure of hail. The cycling
updrafts cause continual hail growth until
it can no longer support the weight of the
hail or when the hail is pushed out of the
draft and falls to Earth (illustrated in
Figure 1).
1

Figure 1. Hail Formation7
.
There are two forms of hail growth; dry
and wet growth. In dry hail growth, the air
temperature is well below freezing,
causing the water droplets to freeze upon
contact with the nucleus. This sudden
freezing process traps air bubbles in the
hail, which gives a cloudy layer of ice.
In wet growth, the air temperature
is below freezing but not enough to
promote sudden freezing upon contact.
The water droplets spreads and slowly
freezes over the surface of the hailstone
allowing air bubbles to escape leaving a
translucent layer of ice.
2.2 Density of Hail
The density of hail can vary significantly
and is dependent on numerous factors
such the formation process (e.g. wet or
dry growth), temperature and size. Field
et al. shows that hail with sizes smaller
than 20 mm can have its density
significantly varying between 50 to 890 kg
m-3
, while hail with larger sizes can have
its density varying between 810 to 915 kg
m-3
(Listed in Table 1).
Table 1. Experimentally determined densities of graupel and hail (>5mm) and size ranges5
.
Size Range [mm] Density Range [kg m-3
] Source
0.5 - 3.0 50 - 450 Locatelli & Hobbs (1974)
0.5 - 1.0 450 - 700 Zikamunda and Vali (1972)
1.0 - 2.0 250 - 450 Zikamunda and Vali (1972)
0.4 - 3.0 80 - 350 Bashkirova and Pershina (1964)
0.5 - 3.0 850 - 890 Braham (1963)
0.5 - 6.0 500 - 700 List (1985)
0.8 - 3.0 130 - 130 Magono (1953)
8.0 - 19.0 310 - 610 Knight and Heymsfield (1983)
1.0 - 7.0 200 - 700 Heymsfield (1978)
26.0 - 36.0 834 - 856 Prodi (1970)
11.0 - 31.0 810 - 900 Vittori and Di Caporiacco (1959)
9.0 - 39.0 870 - 915 Macklin et al. (1960)
2.3 Hail Fall Speed
The fall speed of hail can be highly
variable and can be affected by
environmental factors such as updrafts
and downdrafts. A simplification is made
by assuming the hail to be a smooth
spherical object and falling at terminal
velocity for worst case scenario. The
theoretical terminal velocity (VT) of hail is
expressed in equation (1).
1

(1)
Where VT is terminal velocity (m/s), g is
acceleration due to gravity (9.81 m/s2
),
ρHail is the density of hail (kg/m3
), DHail is
the diameter of hail (m), CdSphere is the
drag coefficient of a sphere, and ρAir is the
density of air (kg/m3
). The expression
shows that as the size and density of hail
increases, the terminal velocity increases.
However, as the drag coefficient and air
density increases, the terminal velocity
decreases.
The drag coefficient of hail is a
function of Reynold’s number, which
generally exceeds 102
. Figure 2 shows
drag characteristics with increasing
Reynold’s number for a smooth and rough
sphere. A Cd range from 0.5 and 1.0 can
be expected for hail impacts to vehicle
body panels.
Figure 2. Drag Coefficient of a sphere9
.
Figure 3 shows the fall speed range of
different hail sizes bounded by terminal
velocities for Cd of 0.5 (fastest fall speed)
and 1.0 (slowest fall speed) calculated
using hail density of 900 kg/m3
, 9.81 m/s
for acceleration due to gravity, and air
density of 1.225 kg/m3
at 15˚C, which is
the approximate ambient temperature
during a hail storm in Australia.
Figure 3. Fall speed range.
2.4 Economic Impact of Hail
Historical Disaster Statistics published by
the Insurance Council of Australia detail at
least 11 recorded hail related events
within the past decade. The figures are
only an approximation of the insured
losses (e.g. home, content, vehicle
damage) based upon reported data. Only
events with potential losses exceeding
AUD$10 million are recorded. Therefore,
actual hail related events may be greater
than recorded and the associated costs of
recorded events may be an underestimate
of actual figures.
From Table 2, the total 2011
normalised cost of all 11 events is
approximately AUD$3.2 billion, averaging
to about AUD$291 million per event.
Though actual figures concerning vehicle
damages are not specified, even if 20% is
attributed to vehicle damages in each
event, the normalised cost per event
concerning vehicle damages would
approximately be AUD$58 million.
1

Table 2. Historical disaster statistics from the Insurance Council of Australia for hail related events
over the past decade4
.
Event
Catastrophe
Number
Date
(dd/mm/yyyy)
State
Original Cost
(AUD$million)
2011
Normalised
Cost
(AUD$million)
VIC Christmas
Day Storms*
CAT 118 25/12/2011 VIC 712 N/A
Melbourne
Storms*
CAT 102 6/03/2010 VIC 1044 1160
Severe Hail
storms
CAT NSW 07/6 9/12/2007 NSW 415 486
Hail Storm CAT NSW 07/4 9/10/2007 NSW/QLD 97 109
Hail CAT NSW 06/1 31/10/2006 NSW 51 60
Hail CAT QLD 05/2 12/10/2005 QLD 61 89
Hail N/A 19/05/2005 QLD 17.6 28
Hail, Storm CAT VIC 05/1 16/05/2005 NSW/TAS/VIC 216.7 304
Hail, Storm CAT NSW 04/1 13/12/2004 NSW 32.3 46
Hail CAT QLD 04/1 24/01/2004 QLD 28.5 54
Hail N/A 31/12/2003 VIC 100 156
*denote events known to have contained hail, N/A - Not Available
3 Literature Review
Published literature regarding hail damage
to vehicle body panels are very limited.
However, hail impact studies are mainly
regarding aerospace applications (e.g.
aircraft skin) at high impact speeds (excess
of 80 m/s) with large deflections. This is
excessive compared to hail impacts to
vehicle body panels, which generally
involve relatively low impact speeds (less
than 50 m/s) and small deflections.
Though there may be some theoretical
differences between low and high impact
speeds, the same foundation can be used
to develop a method to analyse low hail
impact speeds. Through literature
reviews, the expectation is to find and
develop a mathematical model/simulation
that can be applied to hail impacts to
vehicle body panels.
Hail impacts occur within a
fraction of a second and are highly
dynamic. Therefore, a transient dynamic
finite element analysis approach is used.
Common commercially available finite
element packages used in literature are
LS-DYNA and Abaqus. Both offer different
integration methods in which hail impacts
can be analysed. These are Lagragian,
arbitrary Lagrangian Eulerian (ALE), and
smoothed particles hydrodynamics (SP).
Lagragian correlates the mesh with the
material properties that may result in
excessive distortions and numerical
instability with large material
deformations. ALE is a combination of
Lagragian and Eulerian integration
methods, allowing large deformations
because of the Eulerians method fixing
the mesh in space and allowing the
material to flow through it. SPH allows
large material deformation and is a
meshless integration method. The
material property is represented as
grouped particles rather than a mesh. The
material property is distributed
1

throughout the model based on the
distance between each particle.
Previous finite element models
include the use of an elastic-plastic model
with plastic strain and pressure failure
criterion, where the hail behaves like a
fluid (only carrying hydrostatic stresses)
after failure. Recent studies of hail impact
try to account for the strain rate
sensitivity of ice under compression,
where ice is ductile under low strain rates
and brittle under high strain rate. This
observation usually occurs at strain rates
greater than 10-2
s-1
.
4 Finite Element Analysis of
Hail Impacts using
Abaqus/Explicit
The impact of hail onto vehicle body
panels was simulated using Abaqus
explicit. The intent of this simulation to
validate the experimentally measured
depth and diameter of the dent, thus
demonstrate that a suitable model has
been developed. Though there are
numerous factors influencing hail impacts,
not all factors can be accounted for within
this project. A simplification was made
when developing the model where it is
assumed that the hail is smooth, sphere
shaped, and impacts normally to a flat
thin plate (vehicle body panel).The set of
units used in Abaqus are shown in Table 3.
Table 3. Units used in Abaqus
Quantity SI
Length m
Force N
Mass kg
Time s
Stress Pa (N/m2
)
Energy J
4.1 Symmetry
The hail and thin flat plate was originally
modelled as a ¼ scale using symmetry to
reduced computational cost. However,
the ¼ scale model encountered mesh
distortion and instability problems (shown
in Figure 4) and a decision was made to
develop a full scale model.
Figure 4. Mesh distortion.
4.2 Material Characteristic of Hail
The ice material model in Abaqus is
composed of a simple elastic - plastic
behaviour with failure criteria based on
tensile hydrostatic pressure. The elastic
material properties for hail are obtained
from literature are shown in table 4 .
Table 4. Abaqus material property input for
hail10
.
Material Characteristics Values
Density (kg/m3) 900
Young's Modulus (Pa) 9.38E+09
Poisson's Ratio 0.33
Yield stress at εp = 0 and 1
(MPa)
5.2E+06
Figure 5 shows three curves fitted to
experimental data published by Jones and
Kim and Kuene. These curves are defined
as stress ratios scaling 5.2MPa yield
strength over the range of strain rates.
1

Figure 5. Curve fits to compressive strength versus strain rate data10
.
Based on simulations, Tippmann, J.10
found that the lower bound curve showed
a better comparison to peak impact forces
of experimental data than the average
and higher bound curve. Therefore, the
lower bound curve was used as the strain
rate input for the hail material
characteristic. The curve value is then
scaled to stress ratios over the 5.2 MPa
yield strength for the range of strain
rates10
. The exact values for the lower
bound curve listed in Table 5.
Table 5. Lower Yield Strength Ratio Input10
.
Stress Ratio Strain Rate (S-1
)
1 0
1.01 0.1
1.267017189 0.5
1.382015232 1
1.649032421 5
1.764030465 10
2.031047654 50
2.146045697 100
2.413062886 500
2.52086093 1000
2.795078118 5000
2.910076162 10000
3.177093351 50000
3.292091395 100000
3.559108583 500000
3.674106627 1000000
The tensile failure pressure setting used in
the Abaqus model was to specify the
deviatory stress failure as brittle and the
pressure stress as brittle. The deviatory
stress of the failed material is set to zero
when the failure threshold pressure is
reached, while the hydrostatic stresses
(both compression and tension) in the
material remain up to the cut-off stress.
Tippmann, J.10
showed that a
tensile failure pressure of 517kPa gave
good correlation with experimental data.
The tensile failure pressure criterion was
used and inputted into the hail material
characteristic in Abaqus via keyword.
1

4.3 Hail Size and Velocity
Data obtained from the Bureau of
Meteorology Australia show damaging
hail sizes commonly found range between
Ø40 mm to Ø100 mm. Taking into account
the previous results obtained at Delta-V
Experts and limitations of the
experimental equipment, the hail sizes
originally chosen for FEA modelling were
Ø40mm, Ø60mm and Ø80mm. The hail
impacts were assumed to occur at
terminal velocity with a drag coefficient
(Cd) of 0.5 and with a density of 900kg/m3
.
Table 6 shows the terminal velocity
associated with each hail size used in
developing the FEA models.
Table 6. Hail terminal fall speed for hail
diameter sizes of 40 mm, 60 mm, and 80 mm.
Hail Size
(mm)
Fall Speed
(m/s)
Fall Speed
(km/h)
40 19.8 71.3
60 24.2 87.1
80 28.0 101
4.4 Material Characteristic of
Vehicle Panels
Tensile testing is one of the most
fundamental tests for engineering to
determine the nonlinear properties of a
material, therefore a tensile test was
conducted to the specimen collected from
vehicle body panel of Proton Satria.
Figure 6. Specimen collected from proton Satria.
the results obtained from tensile test are
engineering stress and engineering strain,
however Abaqus expects the stress strain
data to be entered as true plastic strain
and true stress. Hence an appropriate
conversion were made and imported to
Abaqus. Vehicle body panels can be very
complex in shape. For the finite element
model, the panel is assumed thin and flat.
Figure 7 True Stress Strain curve derived from
tensile test for Proton Satria.
The Proton Satria bonnet obtained
showed a general thickness of 1mm.
Based on this, the flat plate was modelled
with 1mm thickness using material
properties derived from tensile test. The
material characteristics of panel is listed in
Table 7 and Table 8.
Table 7. Material Characteristics of Mild Steel
Material Characteristics Values
Density (kg/m3) 7850
Young's Modulus (Pa) 180.56E+09
Poisson's Ratio 0.33
Table 8. Plastic Characteristic of Mild Steel
Yield Stress Plastic Strain
2.43E+08 0
2.50E+08 0.006816926
2.76E+08 0.018797865
3.00E+08 0.035155276
1

3.25E+08 0.056113752
3.50E+08 0.081851277
3.75E+08 0.115709227
4.00E+08 0.156856325
4.25E+08 0.202525738
4.50E+08 0.251981658
4.70E+08 0.300070023
Plate sizes 100mm2
, 150mm2
, 200mm2
,
and 250mm2
were modelled based on the
effective sheet metal exposed on the
bonnet with no bonnet frame reinforcing
underneath observed on both the Holden
Commodore VX sedan and Proton Satria
bonnets.
4.5 Elements
The solution procedure in Abaqus/Explicit
uses an explicit integration rule and
diagonal, lumped mass matrices,
therefore, the Abaqus/Explicit element
library is limited to linear elements with
reduced integration. 3D8R elements were
used to model the hail and flat plate.
4.6 Mesh
Abaqus/Explicit is conditionally stable
because the stability of the solution is
dependent on the time increment (time
increment must be less than the critical
time increment). Equation (2) below
expresses the stable time increment. This
is determined only by Abaqus/Explicit and
is influenced by the smallest element in
the model. Therefore, it is desired to have
uniform elements in the model for a faster
computational time.
(2)
Where,
Le = Length of smallest element
E = Young’s Modulus
ρ = density
A partition scheme recommended by
Tippman, J.10
was used to partition the
hail as shown in Figure 6.
Figure 8. Sketch of key sphere dimensions
with normalised unit diameter
10
.
Figure 9. Fully partitioned 40mm diameter hailmodel
A structured mesh seed size of 0.001 was
used to model the 40mm hail. Bias mesh
is used as shown in the figure 9 hence
finer elements near the impact face.
Figure 10. applied mesh on 40mm diameter hail model
1

4 elements where used across the
thickness (seed size of 0.00025 across
plate thickness) to accurately visualise
deflection of the plate using 3D8R
elements. Since the FEA models aimed at
visualising the deflection of the plate a
finer mesh was required at the impact
area. A seed bias was used on the plate
with a bias ratio 4 and created 40
elements around the impact area.
Figure 11. Full Scale Ø40mm Hail and
10000mm2
plate Model Mesh.
4.7 Constraints
Between hail and bonnet models a
general contact with a hard and
frictionless interaction property was
specified as shown in figure 8.
Figure 12. general Interaction between Outer
Surface of Hail and plate Surface.
Dynamic, Explicit step was created for the
impact between the hail and the bonnet
with a time period of 0.003 s, which was
the average impact duration from the
impact experiment. A Linear bulk viscosity
of 1.2 and quadratic bulk viscosity of 0
was used based on Park, H.13 sensitivity
studies.
Fixed boundaries with pinned joint
were added to all four edges of the flat
plate and a terminal velocity of 19.8m/s
was specified for the hail via predefined
fields as shown in Figure 9.
Figure 13. Hail Impact Model Constraints
5 Hail Impact Experiment
Setup and Procedure
Due to time constraints and availability of
resources only a small-scale hail impact
experiment was carried out on a late
model Proton Satria bonnet to validate
the FEA model using Ø40mm moulded
hail. The Ø60mm and Ø80mm hail moulds
made using a 3D printer encountered
sealing problems and were abandoned
due lack to time and resources.
The equipments used during the
hail impact experiment are detailed in
Table 9.
Table 9 - Experimental Equipment
Experimental Equipment
Ø19mm x 1.5m Powerband x 1
Clamp Hose Fit 13-25mm x 2
Table Tennis Balls x 24
Ø19mm x 2m Reinforced Pressure Hose
Ratchet
Crossbow trigger
Delta-V Experts' crossbow frame
Proton Satria Bonnet
High speed camera
1

Camera tripod
Nylon rope
Steel bolt x4
Steel nut x 4
Steel plate x 2
Laptop with VLC Player
LED lamp
Black marker
Digital depth micrometer
Ruler
Blu-tack
A red bonnet from a late model Proton
Satria was purchased for the impact
experiment. All pre-existing damages
were marked (marked in black in Figure
10) before the impact test.
Figure 14. A red late model Proton Satria
bonnet with pre-existing damages marked.
The bonnet was placed upright against a
wall facing the crossbow on plastic boxes
at a measured distance using a tape
measure (shown in Figure 11). A high
speed camera was placed on a tripod
perpendicular to the experimental setup
to record the hail impact.
Figure 15. Experimental setup of hail impact
test.
5.1 Modified Crossbow
To save time and resource, the crossbow
previously used at Delta-V Experts for
their impact experiments was reused.
However, existing problems with the
current design had to be addressed. It was
noted that the crossbow was not durable,
requiring constant replacing of rubber
cords and zip ties, which affected the
precision and accuracy and was not able
to launch larger hail sizes (e.g. Ø80mm) at
terminal speeds.
The previous crossbow was a T-
frame construction (a straight lathe and
stock) with two rubber cords affixed to
the lathe using nuts, bolts, and zip ties
(shown in Figure 12).
Figure 16. Rubber cord affixed to lathe on
previous crossbow using nuts, bolts and zip
ties.
1

The two rubber cords were joined to
single nylon rope using zip ties (shown in
Figure 13).
Figure 17. String component on previous
Delta-V Experts crossbow made from rubber
cords, zip ties, nylon cord, and a cardboard
pad.
The nylon rope had a cardboard pad
attached and was used to pull back the
rubber cords and mount on the trigger
(shown in Figure 14).
Figure 18. String component on previous
Delta-V Expert crossbow mounted on trigger
in safety lock position.
The new crossbow utilised the existing T-
frame. A single Ø19mm x 1.5m
spearfishing powerband was double
knotted at both ends and mounted to the
lathe using bolts, nuts, and steel plates as
shown in Figure 15.
Figure 19. Current crossbow with powerband
mounted on lathe using nuts, bolts, and steel
plate.
The introduction of the thicker
powerband eliminated the use of cable
ties and provided greater durability and
elastic potential energy. The cardboard
pad was replaced with a reinforced
pressure hose that was slotted on the
powerband and fixed using 2 clamps
shown in Figure 16. The reinforced hose
ensured a fixed spacing between the
clamps (approximately 80mm, which was
the size of the largest hail planned for
testing) by resisting the compression of
the powerband as it was drawn back.
Figure 20. Current crossbow string
component.
1

As a consequence of using the
powerband, greater effort was required to
draw back the powerband. Therefore, a
ratchet was required to draw back the
powerband, which was mounted to a bolt
on the T-frame. Two nylon ropes were
tied to the hose, one was used to draw
back the powerband using a ratchet and
the other was used as a trigger release
rope (shown in Figure
17).
Figure 21. String component on current
crossbow mounted on trigger in safety lock
position.
5.2 Ø40mm Hail Impact Speed
Validation Test
Before commencing the hail impact test
on the bonnet, a series of impacts against
a wall using table tennis balls filled with
water were conducted. This was done to
determine the appropriate release rope
length used to achieve an impact velocity
of 19.8 m/s (the calculated terminal
velocity for a Ø40mm hail). The launch
speed of the hail was found using a high
speed camera with a recording rate of
1000 fps (frames per second). The number
of elapsed frames between the hail
leaving the crossbow and impacting
against the wall were counted using a
laptop with VLC player. The velocity of the
hail was calculated using the formula
below.
(3)
Where smeasured is the measured distance
between the crossbow and the impacted
object, fcamera is the recording rate of the
camera, and felapsed is the number of
frames elapsed between the hail leaving
the crossbow and contact with the
impacted object.
The appropriate rope length was
found to be approximately 80cm
accounting for some length lost due to
tying. The rope gave a reach of 25cm
between hose and the trigger and gave an
impact velocity range between 17.8 and
21.8 m/s.
5.3 Ø40mm Hail Impact Testing and
Analysis Procedure
The procedure used during the impact
test once the equipment was setup are as
follows:
1. Measure and record distance
between crossbow and bonnet.
2. Ratchet the powerband back using
the nylon rope and mount release
rope on trigger.
3. Place hail in front of hose.
4. Press record button on high speed
camera to start recording.
5. Pull trigger to release hail.
6. Press record button on high speed
camera to stop recording.
7. Mark and number impact point on
bonnet.
A total of 20 impacts were done following
steps 1 to 7. The hail impact speeds were
1

calculated using the procedure described
in the previous section.
Measurements were taken on the
bonnet concerning the diameter and
depth of each hail dent. This was done in a
dark room using an LED lamp to scan
across the dented surface. This method
was previously used at Delta-V Experts
and provides better clarity of the dented
surface. The procedure used is detailed
below.
1. Find and mark dent center by using a
LED lamp to scan over dented surface.
2. Use a calibrated digital depth
micrometer to measure the depth of
the dent. Record measurement.
3. Repeat step 2 twice.
4. Attach a ruler below the dented area
on the bonnet using Blu-tack.
5. Align camera with dented area using
the tripod.
6. Use the LED lamp to highlight the
dented circumference and take a
photograph (shown in Figure 18).
7. Repeat step 5 twice at different light
angles.
The photographs taken were analysed on
a laptop to determine the diameter of
each hail dent.
Figure 22. Hail dent diameter measured using
a ruler.
6 Results
6.1 Experimental Results
Figure 19 shows the location of each hail
impact highlighted in yellow on the
bonnet.
Figure 23. Hail Impact Points highlighted in
yellow on Red Late Model Proton Satria
Bonnet.
Figure 20 is an image of the underside of
the bonnet rotated 180° about the vertical
axis with the hail impact points highlight
in yellow.
Figure 24. 180° rotated (about vertical axis)
image of Red Late Model Proton Satria bonnet
underside with hail impact points numbered in
yellow and sheet metal section number in
black.
1

Table 10 shows the dimensions and
surface area of exposed sheet metal on
the bonnet with respect to the numbered
sections in Figure 20 in black.
Table 10. Dimensions and Surface Area of
Sheet Metal Sections on Bonnet
Table 11 shows the taken measurements
(e.g. dent diameter, dent depth, frames,
etc.) and calculated results (velocity) from
the Ø40mm hail impact test on the Proton
Satria bonnet. Highlighted in green are the
impacts with little to no hail breakage
(such as the hail shown in Figure 21),
highlighted in yellow are impacts with no
measureable panel damage (e.g. no dent),
and highlight in grey are invalid results
due to improper launching of the hail.
Figure 25. Hail from impact test H40I15 after
bonnet impact.
Section Dimensions (m) Area (m2)
1 0.115 0.11 6.33E-03
2 0.31 0.155 4.81E-02
3 0.275 0.14 3.85E-02
4 0.19 0.17 3.23E-02
5 0.18 0.115 2.07E-02
6 0.17 0.16 2.72E-02
7 0.27 0.145 3.92E-02
8 0.17 0.13 2.21E-02
9 0.23 0.155 3.57E-02
10 0.34 0.18 6.12E-02
11 0.15 0.11 8.25E-03
12 0.3 0.14 4.20E-02
13 0.25 0.14 3.50E-02
14 0.19 0.15 2.85E-02
1

Table 11. Hail Impact Experimental Results
Hail
test
No.
Dent
Diameter
(mm)
Dent Depth
Measurement
1 (mm)
Dent Depth
Measurement
2 (mm)
Dent Depth
Measurement
3 (mm)
Average
Dent
Depth
(mm)
Camera
FPS
Frames
Time Taken
from
crossbow to
bonnet
Distance
between
crossbow
and bonnet
Velocity
(m/s)
H40I01 21 0.44 0.46 0.45 0.45 1000 46 0.046 1.03 22.39
H40I02 26 0.97 1.05 0.99 1.00 1000 46 0.046 1.03 22.39
H40I03 25 1.21 1.24 1.23 1.23 1000 48 0.048 1.03 21.46
H40I04 16 0.21 0.19 0.19 0.20 1000 49 0.049 1.03 21.02
H40I05 18 0.22 0.23 0.21 0.22 1000 50 0.05 1.03 20.6
H40I06 24 1.5 1.48 1.5 1.49 1000 54 0.054 1.03 19.07
H40I07 24 0.63 0.66 0.65 0.65 1000 53 0.053 1.03 19.43
H40I08 16 0.23 0.23 0.25 0.24 1000 54 0.054 1.03 19.07
H40I09 9 0.52 0.57 0.56 0.55 1000 54 0.054 1.03 19.07
H40I10 0 0.01 0.00 0.01 0.00 1000 59 0.059 1.03 17.46
H40I11 7 0.1 0.09 0.12 0.10 1000 99 0.099 1.3 13.13
H40I12 25 0.77 0.79 0.78 0.78 1000 63 0.063 1.3 20.63
H40I13 16 0.25 0.26 0.21 0.24 1000 61 0.061 1.3 21.31
H40I14 12 0.23 0.22 0.24 0.23 1000 59 0.059 1.3 22.03
H40I15 28 1.08 1.04 1.02 1.05 1000 60 0.06 1.3 21.67
H40I16 22 0.46 0.47 0.5 0.48 1000 63 0.063 1.3 20.63
H40I17 19 0.5 0.55 0.54 0.53 1000 64 0.064 1.3 20.31
H40I18 22 1.22 1.18 1.17 1.19 1000 65 0.065 1.3 20
H40I19 19 0.48 0.51 0.49 0.49 1000 67 0.067 1.3 19.40
H40I20 23 0.54 0.58 0.54 0.55 1000 65 0.065 1.3 20
Green indicates little to no hail breakage
Yellow indicates no measurable panel damage (e.g. no dent).
Grey indicates invalid result.
1

6.2 Finite Element Analysis Result
Proton Satria body panel
Figure 26. Proton Satria Plate Hail Impact Deflection
Figure 27 Simulated Hail Impact on 10000mm2
Proton Satria
Table 12 Impact Depth and Diameter for different Size Plates of Proton Satria.
Plate Size (mm x mm) Impact Depth (mm) Impact Diameter (mm)
100 x 100 1.79 25.6
`150 x 150 1.64 25.1
200 x 200 1.59 24.9
250 x 250 1.59 24.9
1

Hail impact simulations for Different hail sizes
Figure 28. Proton Satria Plate Hail Impact Deflection for different hail sizes
Table 13. Impact Depth and Diameter for different Size Plates of Proton Satria.
Hail Size (mm) Fall Speed(m/s) Impact Depth (mm) Impact Diameter (mm)
40 mm 19.8 1.59 24.9
60 mm 24.2 3.6 47.9
80 mm 28.0 5.24 72.3
Figure 29. Simulated Hail Impact on 10000mm2
Flat Plate.
1

7 Discussion
Hail impact experiment shows
that most of the hail impacts occurred at
or around the bonnet frame as shown in
figure 20. The experimental results
showed that a 40mm spherical hail
impacting a bonnet at around terminal
velocity caused dent diameters between
9mm to 28mm and dent depths between
0.20mm to 1.49mm.
The experimental results also
indicate that the diameter of the dents
and the depth of the dents are
independent of the presence of the
bonnet frame. This is due to a sufficient
gap between the bonnet sheet metal and
the bonnet frame at the impacted areas
preventing the bonnet frame from having
influence on the dent resistance of the
bonnet.
Figure 30: The bonnet Frame back of the Bonnet.
Based on the observation of the
dents following findings were revealed : 1.
hail impact will not scratch or mark the
paint but the paint may chip ; 2. dents
caused by hail will cause the light to move
smoothly and continuously across the
dent and the light will not "break" or
crease; 3. folds and curves on the panels
did not affect the shape of the (physical
appearance) dent caused to the panel;
and 4. for the same size hail the higher
impact speed hail caused more damage.
The body panel sizes between
(100mm)2
to (250mm)2
were simulated
for impact. Results showed that the dent
diameter varied between 25.6mm to
24.9mm between plate sizes and a depth
ranged of 1.79mm to 1.59 mm with
increasing plate size, which is up to 6.7%
larger than the largest dent depth
(1.49mm) observed during the
experiments. The numerical results have
good correlations with the experimental
results, as its deviate only 6 .7 %.
The hail simulations show very
little hail deformation similar to Figure 26.
It can be seen in Figure 27 that as the
plate becomes bigger, a greater deflection
peak is reached but a smaller final
deflection of the plate is observed. The
greater deflection peak may be attributed
to the increased distance between the
fixed plate boundary and the centre of
impact as the plate became larger, similar
to increasing the length of a beam and
observing a greater deflection under
bending. The lower final deflection may
be due to the spring back effect of the
plate. The larger the plate, the greater this
effect.
Factors which may be attributed to
the differences between the simulated
and experimental results include:
1. layered structure of the hail: The hail
model used for experiment is 40mm
moulded ice(monolithic ice). These
were created using a spherical split
mould having a filling hole. however
the Actual hail ice has a spherically
layered, or 'onion skin', construction.
these layered structure provide an
extra degree toughness to the hail.
which is not considered during the
experiment.
1

Figure 31. Comparison of moulded ice and Layered Hail
Stone
2. Inconsistency of projection: the
Projection of ice was achieved using a
crossbow hence inconsistency in
projecting speed and angle , which
would be better if we used a nitrogen
gas canon which can control the speed
from 10m/s - 200m/s.
Figure 32. Nitrogen gas cannon Setup.
3. Inconsistent hail temperature: The
mechanical properties of the ice are
strongly depend on the temperature,
however during the experiment ice was
not launched with consistent
temperature.
4. Density of the hail is not determined :
during the experiment the weight of
the hail is not recorded. hence it is not
possible to compare the density
between experimental and numerical
hail models.
5. The material characteristic of the hail:
It would be assumed that if hail
breakage can be included into the hail
model, the expected plate deflection
would be less due to the portion of the
impact energy that would be dissipated
through crack propagation and
breakage of the hail.
6. Curvature and edges on the bonnet:
These would aid in the dent resistance
of the bonnet. When an impact occurs
normally to an edge or curvature the
effective cross-sectional thickness of
the bonnet is greater than a flat plate,
therefore is more dent resistant.
7. Paint and clear coat: The paint and
clear coat of a bonnet and other body
panels may also influence in providing
some dent resistance.
8. Sources of error: These include the
way the depth of the dent was
measured, which can significantly vary
when measuring on a curvature or
edge to the crossbow not launching the
hail normal to the bonnet.
Data obtained from the Bureau of
Meteorology Australia show damaging
hail sizes commonly found range between
Ø40 mm to Ø100 mm. The Table 13 shows
the Depth and the Diameter of the dents
for different sizes of the hail. It is
observed that the damage is larger as the
hail size increases. this is due to the
momentum increases as the size of the
hail increases.
The developed numerical model
will be a great tool to investigate the hail
damage in the future. Finite element
analysis methods are frequently used in
forensic engineering. It gives more
detailed results than a physical
experiment (quantities can be measured).
Since experimental tests are expensive,
difficult to perform and time consuming it
is obvious to understand the importance
of developing numerical models.
1

8 Conclusion
The aim of this paper is to characterise
hail and fraudulent damages to vehicle
body panels with respect to deformation
size and shape. To achieve this aim two
different methods were implemented.
Initially hail impact experiment was
conducted to a Proton Satria bonnet using
40mm moulded hail. Then the
experimental results were compared with
numerical results. For the numerical
method a 40mm hail and bonnet models
were made, refined and compared with
the experimental data.
It was observed that 40mm hail
impacting a bonnet at around terminal
velocity gave a dent diameter range of
9mm to 28mm and a dent depth range of
0.20mm to 1.49mm. The simulated results
showed that a 40mm hail impact on mild
steel plate of sizes (100mm)2, (150mm)2,
(200mm)2, and (250mm)2 yielded dent
depths between 1.79mm to 1.59mm and
dent diameters between 25.6mm to
24.9mm.
As the good correlation between
experimental and numerical results , the
current numerical model could be used to
verify the real hail damages. Only by
changing the material properties
according to the vehicle model the
maximum depth and diameter of the dent
could be determined. Materials properties
of the panel is vary according to the
vehicle, hence the non linear material
properties of the panel needs to be
determined through the tensile test and
the imported to the model.
Acknowledgements
The author acknowledge the guidance and
feedback throughout the project from the
academic supervisor, Dr. Xu Wang, the
industrial supervisor from Delta-V Experts,
Dr. Shane Richardson and Mr. Andreas
Sandvik for their guidance and advices
throughout the project, and to Dr. Toh
Yen Pang for his consultation times and
guidance with Abaqus FEA modelling.
References
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1

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