Note: Descriptions are shown in the official language in which they were submitted.
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High strength high slenderness part having excellent
energy absorption
The present invention relates to a high strength structural part having
excellent energy absorption properties in the case of an impact. In
particular, the
present invention relates to a structural part for use in an automotive
vehicle.
High strength high slenderness structural parts play an important role in the
crash resistance of a vehicle. They are long and narrow assemblies comprising
a
hollow cavity. During an impact they act to absorb energy by buckling and thus
forming folds which absorb part of the crash energy. They also act as
important
relays on the load path of the vehicle architecture and contribute to
transmitting and
diffusing the crash energy from one end of the vehicle to the other, thus
ensuring
that a maximum amount of the vehicle's architecture is involved in absorbing
the
crash energy.
As such, high strength high slenderness structural parts play a fundamental
role in promoting the safety of the vehicle's occupants.
When a crash occurs, the high slenderness parts are subjected to a
compressive force, which is not necessarily strictly parallel to the length
direction of
the part. In order to absorb the maximum amount of energy it is important that
the
high slenderness part bottles onto itself as much as possible. When there is
an angle
between the compressive force and the length direction of the part, there is a
risk
that the part will bend before fully bottling. Once the part is bent it is no
more
available for bottling and therefore will not have absorbed the maximum amount
of
energy possible.
The purpose of the current invention is to address this issue by providing a
high slenderness part having a robust buckling behavior even in the case of an
angled compressive load. This is particularly critical in the case of current
vehicles
which are submitted to both stringent safety requirements and weight lightning
requirements for energy consumption.
The object of the present invention is achieved by providing a high
slenderness part according to claim 1, optionally comprising the features of
claims
2 to 5.
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The invention will now be described in detail and illustrated by examples
without introducing limitations, with reference to the appended figures:
-Figure 1 is a schematic of a high slenderness part according to an
embodiment of the invention, with Figure la being an insert detailing the
definition of the different angles defined in the description,
-Figure 2 is a top view of the compared simulated behavior during a crash
test of a high slenderness part according to the invention (Ii) and a
reference
example (R2) with an angle between the impactor and the longitudinal
direction of the part,
-Figure 3 is a side view of the compared simulated behavior during a crash
test of a high slenderness part according to the invention (Ii) and a
reference
example (R2) with an angle between the impactor and the length direction of
the part,
The slenderness ratio, commonly used in Leonhard Euler's buckling theory,
is defined by the following formula, where L is the length of the part, S is
the area of
its straight section, and Imin is the minimum quadratic moment of area in the
section
being considered.
L
Slenderness ratio = -
_\11min
S
In general, the minimum quadratic moment of area Imin over a cross section
A in a set of cartesian coordinates (x,y) is defined by the following formula:
Imin = min (fi y2dxdy ; x2 dxdy)
A A
For example, the minimum quadratic moment of area Imin for a hollow
rectangular section having outer dimension b and h and inner dimensions bl and
hl is calculated using the following formula:
Ch3 ¨ b1h13 . hb3 ¨ h1b13)
Imin = min _________________________________ , ________
12 12
For example, the minimum quadratic moment of area Imin for a hollow
annular section having outer radius R and inner radius R1 is calculated using
the
following formula:
TC
/min = ¨4 (R4 ¨ R14)
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A part can be considered to have a high slenderness when its slenderness
ratio is above 10.
The bending angle is measured according to the VDA-238-100 bending
standard. In the current invention, the bending angles are measured after
springback. For the same material, the bending angle depends on the thickness.
For the sake of simplicity, the bending angle values of the current invention
refer to
a thickness of 1.5mm. If the thickness is different than 1.5mm, the bending
angle
value needs to be normalized to 1.5mm by the following calculation where al .5
is
the bending angle normalized at 1.5mm, t is the thickness, and at is the
bending
angle for thickness t:
a1.5 = (at )( \It)/'I1.5
The bending angle of a part is representative of the ability of the part to
resist
deformation without the formation of cracks.
The ultimate tensile strength, the yield strength and the elongation are
measured according to ISO standard ISO 6892-1, published in October 2009. The
tensile test specimens are cut-out from flat areas. If necessary, small size
tensile
test samples are taken to accommodate for the total available flat area on the
part.
The term fracture strain refers to the fracture strain criterion defined by
Pascal
Dietsch et al. in "Methodology to assess fracture during crash simulation:
fracture
strain criteria and their calibration", in Metallurgical Research Technology
Volume
114, Number 6, 2017. The fracture strain is the equivalent strain within the
material
at the deformation point when the critical bending angle has been reached. The
critical bending angle defines the angle at which the first cracks are
detected on the
extrados of a sample which has been deformed according to the standardized VDA-
238-100 Standard.
The term "bottling" refers to the mode of deformation of a part subjected to a
compressive load, typically a high slenderness part, where the part
progressively
absorbs the mechanical energy of the compressive load by forming a series of
successive waves resulting from successive local buckling deformations of the
part.
As a result, the length of the part as measured in the direction of the
compressive
load is smaller after the deformation than the initial length of the part in
said direction.
In other words, when a part reacts to a compressive load by controlled
buckling, it
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folds onto itself in the same way as a plastic bottle on which a compressive
load is
applied between the top and the bottom of the bottle.
Hot stamping is a forming technology for steel which involves heating a blank
up to a temperature at which the microstructure of the steel has at least
partially
transformed to austenite, forming the blank at high temperature by stamping it
and
quenching the formed part to obtain a microstructure having a very high
strength,
possibly with an additional partitioning or tempering step in the heat
treatment. Hot
stamping allows to obtain very high strength parts with complex shapes and
presents many technical advantages. It should be understood that the thermal
treatment to which a part is submitted includes not only the above described
thermal
cycle of the hot stamping process itself, but also possibly other subsequent
heat
treatment cycles such as for example the paint baking step, performed after
the part
has been painted in order to bake the paint. The mechanical properties of hot
stamped parts below are those measured after the full thermal cycle, including
optionally for example a paint baking step, in case paint baking has indeed
been
performed.
A blank refers to a flat sheet, which has been cut to any shape suitable for
its
use. A blank has a top and bottom face, which are also referred to as a top
and
bottom side or as a top and bottom surface. The distance between said faces is
designated as the thickness of the blank. The thickness can be measured for
example using a micrometer, the spindle and anvil of which are placed on the
top
and bottom faces. In a similar way, the thickness can also be measured on a
formed
part.
Hardness is a measure of the resistance to localized plastic deformation
induced by mechanical indentation. It is well correlated to the mechanical
properties
of a material and is a useful local measurement method which does not require
to
cut out a sample for tensile testing. In the current invention, the hardness
measurements are made using a Vickers indenter according to standard ISO 6507-
1. The Vickers hardness is expressed using the unit Hv.
The heat affected zone is the area of material surrounding a weld which has
been heated up during the welding operation. In the case of high strength
materials,
for example high strength steels, it is well known that the heat affected zone
can
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have weaker mechanical properties. Indeed, the heat affected zone undergoes a
thermal treatment akin to tempering, which can lead to softening.
The hardness drop in the heat affected zone of a material is measured using
the following protocol:
5 1/ A
hardness profile of the weld is taken by measuring the Vickers hardness
under a 0,5kg load every 0,2mm along a staggered line crossing through the
heat
affected zones of both welded plates and through the spot weld itself,
2/ The weakest point of said hardness profile, usually present in the heat
affected zone, is determined and noted HVmin,
3/ The hardness of the base metal away from the weld is measured by taking
the average measurement of several points and is noted HVBM,
4/ The hardness drop is computed as the difference HVBM ¨ HVmin,
expressed in Hv.
Referring to figure 1, a high slenderness part 1 extends in a main
longitudinal
direction Ldir between two ends El and E2 and in a transverse direction Tdir.
It
comprises a hollow volume 4 encased between a top part 3 and a bottom part 2.
In a particular embodiment, as depicted on figure 1, the high slenderness part
1 is made by forming separately and then joining together the top part 3,
which is a
generally omega shaped part, and the bottom part 2, which is a flat closing
plate.
For example, the top part 3 and the bottom part 2 are joined together by
welding, for
example by spot welding on flanges 6, which produces spot welds 5.
In another embodiment, the high slenderness part is made in one piece
comprising the top part and the bottom part. For example, the high slenderness
part
is made by extrusion. For example, the high slenderness part is formed by roll
forming. For example, the high slenderness part is made from a formed metallic
tube.
High slenderness parts abound in vehicle architectures, some examples are
the front parts joining the front crash boxes to the rocker assembly, the rear
parts
joining the rear crash boxes to the rocker assembly, cross parts extending
transversally in the vehicle, the rocker panels themselves etc. The high
slenderness
part is generally attached to the rest of the vehicle structure at each of its
ends El
and E2.
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High slenderness parts are designed to absorb energy under the
compressive stress resulting from a crash. Referring to figure 1, the high
slenderness part is subjected to the joint effort of Fl exerted on El and the
opposite
reaction force exerted on E2. These forces are exerted by the other parts to
which
the high slenderness part is attached in El and E2. Referring to figure la, Fl
forms
an angle beta with the longitudinal direction Ldir. This angle results in a
bending
moment on the high slenderness part in addition to the compressive load
exerted
by Fl and its reaction.
As a result, the high slenderness part is submitted both to a compressive
stress, which it can accommodate through a bottling deformation (which keeps
the
part in the Ldir direction) or a bending deformation away from the Ldir
direction, and
a bending moment, which it can accommodate through a bending direction away
from the Ldir direction. The bottling deformation, which keeps the part in the
Ldir
direction, and the bending deformation, which bends the part away from the
Ldir
direction, compete with one another. Once bending starts to occur, the angle
between the normal vectors to El and E2 will increase, which will further
promote
the bending deformation mode and discourage the bottling deformation mode. At
this point, the high slenderness part will not anymore deform by bottling and
instead
will deform by bending onto itself only.
Bottling forms multiple folds in the material whereas bending only forms one
fold in the material. Therefore, bottling absorbs a much higher amount of
energy and
it is interesting to promote the bottling deformation mode over the bending
mode to
increase the energy absorption effectiveness of the part. Furthermore,
bottling
maintains the general direction of the part during the crash, whereas bending
will
make it deform in rather unpredictable directions and in a catastrophic way.
Bottling
therefore makes the behaviour of the part during crash much more predictable
then
bending and makes it available to collaborate predictably and correctly with
the rest
of the vehicle's structure as the crash scenario unfolds, which is a further
important
advantage of bottling over bending.
It is also interesting to manufacture the high slenderness part with high
strength materials in order to absorb as much energy as possible. For example,
the
material used to manufacture at least a portion or all of the high slenderness
part
has a tensile strength after forming above 1000MPa. For example, the material
used
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to manufacture at least a portion or all of the high slenderness part has a
tensile
strength after forming above 1300MPa. For example, the material used to
manufacture at least a portion or all of the high slenderness part has a
tensile
strength after forming above 1500MPa. For example, the material used to
manufacture at least a portion or all of the high slenderness part has a
tensile
strength after forming above 1800MPa.
Surprisingly, the inventors have found that in the case of an angle beta
strictly
greater than 0, the buckling deformation mode is promoted when using materials
having a high Yield Strength over Ultimate Tensile Strength (YS/UTS) ratio. In
particular, the inventors have found that high slenderness parts having a
YS/UTS
ratio above 0,85, even more preferably a YS/UTS ratio above 0,9, exhibit very
good
bottling behaviour and low bending reactions.
Furthermore, the inventors have found that the energy absorption amount is
increased when using material having a higher bending angle. Indeed, this
means
that the material can form folds without cracks occurring in the highest
deformation
areas of the folds. Such cracks lower the energy absorption because it takes
much
less energy to deform a cracked area. Cracks can also lead to crack
propagation
and catastrophic failure of the part, which is to be prevented to ensure
energy
absorption and energy transmission through the load path and to stick to a
predictable overall vehicle crash scenario.
Even though it is important to have sufficient bending ability of the material
in
order to avoid cracks occurring, the inventors found that it was not necessary
to
have a very high bending angle. For example, a bending angle normalized to
1,5mm
of 550 is sufficient for good energy absorption. More preferably, a material
having a
bending angle normalized to 1,5mm of 70 can be used for good energy
absorption.
The inventors have also found that a minimum level of fracture strain can be
beneficial for increased energy absorption. For example, it is interesting to
have a
minimum fracture strain of 0,5 to promote high energy absorption and to avoid
catastrophic failure of the part.
In the case of a high slenderness part made of at least two different parts
joined together by welding, such as the one illustrated on figure 1, the
mechanical
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resistance of the spot welds 5 and their surrounding area will have an impact
on the
total energy absorption.
In the case of high strength materials, for example high strength steels, it
is
well known that the heat affected zone can have weaker mechanical properties.
Indeed, the heat affected zone undergoes a thermal treatment akin to
tempering,
which can lead to softening.
The inventors have found that in this type of configuration, it is
advantageous
to use materials exhibiting a low hardness drop in the heat affected zone.
More
specifically, it is advantageous to use materials exhibiting less than 100Hv
hardness
drop compared to the base metal in the heat affected zone. Preferably, it is
advantageous to have a hardness drop below 80Hv, even more preferably below
50Hv.
In a particular embodiment, the material used to manufacture at least a
portion of the high slenderness part or the entire high slenderness part is
steel
comprising the following elements expressed in weight% :
C:0.1 - 0.25 %
Mn: 3.00 ¨ 5.00 %
Si: 0.80 ¨ 1.60 %
B: 0.0003 - 0.004 %
S 0.010 %
P 0.020 %
N 0.008 %
and comprising optionally one or more of the following elements, in weight
percentage:
Ti 0.04 %
Nb 0.05 %
Mo 0.3% 15
Al 0.90 %
Cr 0.80%
The remainder of the composition being iron and unavoidable impurities
resulting from the smelting
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This material is worked using hot stamping and the resulting hot stamped part
has for example a UTS above 1000MPa, an elongation above 10%, a YS / UTS ratio
above 0,9, a bending angle above 55 and a hardness drop in the heat affected
zone below 80Hv.
In a particular embodiment, the material used to manufacture at least a
portion of the high slenderness part or the entire high slenderness part is
steel
comprising the following elements expressed in weight% :
C: 0.03 - 0.18 %
Mn: 6.0 ¨ 11.0 %
Mo: 0.05 - 0.5 %
B: 0.0005 ¨ 0.005%
S 0.010%
P 0.020 %
N 0.008%
and comprising optionally one or more of the following elements, in weight
percentage:
A I < 3%
Si 1.20%
Ti 0.050%
Nb 0.050 %
Cr 0.5%
This material is worked using hot stamping and the resulting hot stamped part
has for example a UTS above 1000MPa, an elongation above 10%, a YS / UTS ratio
above 0,9, a bending angle above 55 and a hardness drop in the heat affected
zone below 80Hv.
In a particular embodiment, the material used to manufacture at least part of
the high slenderness part or the entire high slenderness part is steel
comprising the
following elements expressed in weight%:
C : 0.2 - 0.34 %
Mn: 0.50 ¨ 2.20 %
Si: 0.5 ¨ 2 %
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P 0.020 %
S 0.010 %
N 0.010 %
and comprising optionally one or more of the following elements, by weight
5 percent:
Al: ).2 %
Cr 0.8%
B 0.005%
Ti 0.06 %
10 Nb 0.06%
the remainder of the composition being iron and unavoidable impurities
resulting from the smelting.
For example, the portion of said high slenderness part being made of said
material has a microstructure comprising, in surface fraction:
95% or more of tempered martensite,
and less than 5 % of bainite.
For example, the portion of said high slenderness part being made of said
material has the following mechanical properties: an ultimate tensile strength
TS
higher than 1000 MPa, a fracture strain higher than 0.5, a bending angle
higher than
55 and a hardness drop in the heat affected zone below 80Hv.
In a particular embodiment, the material used to manufacture at least part of
the high slenderness part or the entire high slenderness part is steel
comprising the
following elements expressed in weight%:
0.20% C 0.25%
1.1% Mn 1.4%
0.15% Si 0.35%
Cr 0.30%
0.020% Ti 0.060%
0.020% Al 0.060%
S 0.005%
P 0.025%
0.002% B 0.004%
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the remainder being iron and unavoidable impurities resulting from the
elaboration.
The invention will now be illustrated by the following examples, which are by
no way !imitative.
The reaction to an impact having an angle on a high slenderness part made
with different materials was simulated using LS-DYNA R11.1Ø The mesh size
used
is 3mm.
Referring to figure 1, the simulated high slenderness part 1 is made by
forming separately and then joining together the top part 3, which is a
generally
omega shaped part, and the bottom part 2, which is a flat closing plate by
spot
welding on flanges 6, which produces spot welds 5. The joining is performed by
20
spot welds on each side every 30mm along each flange. Each spot weld 5 has a
6mm diameter nugget and the heat affected zone is simulated by a 3mm ring
around
each nugget.
The high slenderness part 1 has the following dimensions:
-sheet metal thickness of 1,5mm before forming,
-length L of 600mm
-closing plate 2 having a total width in the transverse direction of 130mm,
comprising two flanges 6 of 25mm each. Hence the width of the closing plate
enclosing the hollow volume 4 is 130 ¨ 2*25 = 80mm.
-height of the hollow volume 4 of 60mm
For simplicity sage, the slenderness factor below was calculated for a
perfectly rectangular part having the same hollow volume 4 and the same sheet
metal thickness. That is to say, the slenderness factor is calculated without
taking
into account the contribution of the flanges, which will be very minimal.
In the formula below, the factors b1 and b correspond respectively to the
inner
width (i.e. 60mm) and outer width (i.e. b=b1+2*thickness = b1+3mm) of the
rectangular part, the factors h1 and h correspond respectively to the inner
height
(i.e. 60mm) and outer height (i.e. h=h1+2*thickness = h1+3mm) of the
rectangular
part. The minimum quadratic moment is given by the formula:
Ch3 ¨ b1h13 . hb3 ¨ h1b13)
Imin = min ________________________________
12 , ___ 12
Which computes as:
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(83 * 633 - 80 * 603 63 * 833 ¨ 60 * 803)
Imin = min ________________________________
12 12
Imin = min(441881,75; 289491,75)
Imin = 289491,75
The slenderness ratio is given by the following formula:
Slenderness ratio = ¨
_\11min
In which the area of the straight section S = h*b ¨ hl*ID1
Which computes as
600
Slenderness ratio = _____________________________________
,\1289491,75
63 * 83 ¨ 60 * 80
Slenderness ratio = 23.1
The described shape therefore results in a slenderness ratio of 23.1.
The part 1 is fixed at one end E2 and impacted at its other end El by a flat
impactor 7 travelling at an angle beta of 10 with the longitudinal direction
Ldir and
an initial impact velocity of 16m/s and having a mass of 417kg.
The results are expressed in terms of energy absorption, as provided directly
by the software and in terms of deleted elements to represent the level of
fracture
resulting from the crash.
The behaviour of the spot welds under load was simulated applying the
method developed in the Fosta 806 project: "P 806 ¨ Characterization and
simplified
modeling of the fracture behavior of spot welds from ultra-high strength
steels for
crash simulation with consideration of the effects of the joints on component
behaviour" (Fosta stands for "Forschungsvereinigung Stahlanwendung", i.e. The
Research Association for Steel Application). In order to dissociate the effect
of the
spot weld behaviour from the rest of the material behaviour, the simulations
were
done with and without taking into account the presence of spot welds (in table
1 the
columns indicating no weld" for line "Hardness drop in Heat affected zone"
correspond to the simulation in which the weld behaviour was not taken into
account).
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The failure behavior and associated deleted elements calculation is simulated
using the material cards MAT123 and MAT_ADD_EROS1ON. Further explanation
on the methodology can be found for example in "Simulation of Spot Welds and
Weld Seams of Press-Hardened Steel (PHS) Assemblies", Stanislaw Klimek,
International Automotive Body Congress 2008.
The number of deleted elements is an evaluation of the amount of fracture
that occurs during the crash. Because the failure modelling does not take into
account the propagation of cracks, it can be said that the effect of fracture
on the
overall results is probably underestimated in the simulations and that in
actual
physical crash tests the energy absorption levels would probably be lower when
the
number of deleted elements are high because of failure propagation and
eventual
total failure of the part (such as for example the part being cut in two). It
should be
noted that such catastrophic failure is an issue for energy absorption but
also for the
overall behavior of the part in the predicted crash scenario of the vehicle.
Indeed, it
disrupts the anticipated load path and means that the different parts of the
vehicle
will travel in uncontrolled directions because they are not anymore joined
together.
This lack of control leads to unpredictable and catastrophic behavior of the
vehicle
during a crash.
Referring to table 1, the evolution of the amount of absorbed energy and the
number of deleted elements is represented throughout the crash scenario,
taking
sample points at 1/4 of the crash time, 1/2 of the crash time, 3/4 of the
crash time
and the end crash time. The inventive examples bear references 11 and 11w,
corresponding respectively to the case in which the spot weld behavior was not
taken into account and in which it was taken into account. The reference
examples,
corresponding to cases outside the invention, are termed R1, R1w, R2, R2w, R3
and R3w.
The inventive examples show the highest amount of energy absorption and
the lowest number of deleted elements, i.e. the most favorable response.
R1, R1w have lower YS/UTS ratio and lower bending angle than the
invention. This results in a combination of lower energy absorption and higher
amount of fracture (number of deleted elements).
R2, R2w have lower YS/UTS ratio than the invention. This results in a lower
energy absorption. Referring to figures 2 and 3, which are respectively top
and side
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views of the crash simulation at 1/4,1/2, 3/4 and the full crash time of 11
and R2, the
angle of the impactor 7 results in eventual bending of the part 1 starting at
3/4 crash
time. This bending translates in a lower amount of energy absorption for R2.
Actually, 11 and R2 have very similar energy absorption levels at 1/4 time and
1/2
time ¨ they start to deviate from one another at 3/4 time, which corresponds
to the
onset of bending in R2. On the other hand the high slenderness part of 11
continues
to deform by bottling until the end of the crash.
R3, R3w have lower bending angles than the invention, which results in a
significantly higher number of deleted elements.
When comparing the examples with and without the effect of the weld spots,
it is apparent that an important hardness drop in the heat affected zone has a
detrimental effect on the energy absorption. This is consistently the case for
RI vs
RI w, R2 vs R2w and R3 vs R3w, which all have an estimated hardness drop in
the
heat affected zone of 200Hv. The resulting decrease in energy absorption
ranges
from 0,4kJ for RI vs RI w to 2,4kJ for R2 vs R2w.
inventive example reference example 1 reference example 2 reference example 3
0
11 11w R1 R1w R2
R2w R3 R3w w
o
N
(44
Slenderness ratio 23 23 23 23 23
23 23 23 'a
.6.
w
Yield Strength (MPa) 1260 1260 1070 1070 1070
1070 1366 1366
(44
I..
(1)
.1) Ultimate Tensile Strength (MPa) 1337 1337 1500 1500
1500 1500 1500 1500
la-.)
0_ Hardness drop in Heat affected No weld 81 No weld 200
No weld 200 No weld 200
2
a_ zone (Hv)
Tri YS / UTS 0,94 0,94 0 71 0 71 0 71
0 71 0,91 0,91
.,_
a)
crj bending angle 89 89 47 47 80
80
Fracture Strain 0,6 0,6 0,36 0,36 0,6
0,6 0,37 0,37
Energy absorption Time 1/4 (kJ) 16.6 16.2 14,7 14,3
16,4 16,2 15,2 14,9 N
- v
Energy absorption Time 1/2 (kJ) 30.3 29.2 26 26,2 30,3
29,2 28,5 26,6
Energy absorption Time 3/4 (kJ) 40.2 38.9 35,1 34,6 37,2
34,7 36,7 36,1
0) Energy absorption End Time 45.3 45.9 41,2 40,8 40,9
38,5 43,2 42,2
TT) (kJ)
a) Deleted elements Time 1/4
cc 20 32 226 277 21
76 209 254
Deleted elements Time 1/2 65 90 420 458 36
128 396 443
Deleted elements Time 3/4 97 139 543 632 51
143 519 607
Deleted elements End Time 101 158 643 742 56
145 603 702 oo
n
1-i
w
Table 1: crash simulation results
=
w
w
-a
u,
oe
(44
01
