U.S. patent application number 14/943730 was filed with the patent office on 2016-03-24 for crush modelling.
The applicant listed for this patent is Engenuity Limited. Invention is credited to James Anderson, Graham Barnes, Ian Coles, Richard Roberts.
Application Number | 20160085892 14/943730 |
Document ID | / |
Family ID | 32843561 |
Filed Date | 2016-03-24 |
United States Patent
Application |
20160085892 |
Kind Code |
A1 |
Coles; Ian ; et al. |
March 24, 2016 |
CRUSH MODELLING
Abstract
A method of determining the impact resistance of a structure
including a crushable material comprises the steps of determining
for one or more layers of a finite element of the material during
an impact whether the element or layer thereof is to be treated as
failing by crushing. If the element or layer is determined to fail
by crushing, a load-bearing portion of the structure is defined and
the load-bearing portion is treated for the purpose of subsequent
calculations as exhibiting an ongoing resistance.
Inventors: |
Coles; Ian; (Sompting,
GB) ; Barnes; Graham; (Dormansland, GB) ;
Roberts; Richard; (Henfield, GB) ; Anderson;
James; (Brighton, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Engenuity Limited |
Cuckfield |
|
GB |
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|
Family ID: |
32843561 |
Appl. No.: |
14/943730 |
Filed: |
November 17, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12589776 |
Oct 28, 2009 |
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14943730 |
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10931273 |
Aug 31, 2004 |
7630871 |
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12589776 |
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Current U.S.
Class: |
703/8 |
Current CPC
Class: |
G06F 30/15 20200101;
G06F 30/23 20200101; G06F 2113/26 20200101 |
International
Class: |
G06F 17/50 20060101
G06F017/50 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 2, 2004 |
GB |
0414992.8 |
Claims
1. A data processing system which models the behaviour of a
structure during an impact, said structure incorporating a material
which can fail through a crush failure mode, whereby said material
is continuously consumed by disintegrating into debris, the system
comprising: a computer that: (i) determines for one or more layers
of a finite element of said material during an impact whether said
element or layer thereof is failing by said crush failure mode;
(ii) if said element or layer is determined to be failing by said
crush failure mode, said computer calculates a resistance force and
assigns said resistance force to said element or layer one or more
times such that a resistance force is assigned to said element or
layer for as long as the length of the element or layer is being
reduced by said crush failure mode; and (iii) outputs data from
which a predicted impact resistance of said structure can be
calculated.
2. The system as claimed in claim 1 wherein said resistance force
is assigned to a portion of said element or layer.
3. The system as claimed in claim 2, wherein said computer defines
a crush front or barrier and allows said element or layer to pass
through said crush front or barrier whilst being crushed.
4. The system as claimed in claim 2, wherein said computer applies
said resistance force to individual nodes of the element or layer
so that said portion comprises said nodes.
5. The system as claimed in claim 4, wherein said computer: defines
a crush front or barrier and allows said element or layer to pass
through said crush front or barrier whilst being crushed; and
divides said resistance force by allocating a first percentage of
said resistance force to a first set of nodes that have passed
through the crush front or barrier and a second percentage of said
resistance force to a second set of nodes that have not passed
through the crush front or barrier, wherein said first and second
percentages are calculated either as a function of the area of the
element or layer that has passed through the crush front or barrier
or as a function of the distance that said element or layer has
passed through the crush front or barrier.
6. The system as claimed in claim 5 wherein said first percentage
is the percentage of the area of the element or layer that has
passed through the crush front or barrier or the percentage of the
length of the element or layer normal to the crush front or barrier
that has passed through the crush front or barrier.
7. The system as claimed in claim 1, wherein said computer
determines whether the element or layer is failing by said crush
failure mode by determining whether an impactor barrier has
physically encroached into a space allocated to said element or
layer.
8. The system as claimed in claim 1, wherein said computer
determines whether the element or layer is failing by said crush
failure mode by calculating the stress or strain on the element or
layer and comparing said stress or strain with a threshold failure
value.
9. The system as claimed in claim 1, wherein said computer defines
a crush front or barrier and determines said resistance force as a
function of a thickness of the element or layer being crushed along
the crush front or barrier.
10. The system as claimed in claim 1, wherein said computer defines
a crush front or barrier and determines said resistance force as a
function of an area of contact at the crush front or barrier.
11. The system as claimed in claim 10, wherein for a given element
said resistance force has an actual value which is a constant
function of the area of contact.
12. The system as claimed in claim 11, wherein said computer
defines said resistance force as being directly proportional to the
area of contact.
13. The system as claimed in claim 1 wherein said crushable
material is a composite material having a plurality of layers,
wherein said computer determines said resistance force as a
function of the lay-up of said layers.
14. The system as claimed in claim 13, wherein said computer
determines said resistance force as a function of the order of said
layers in the composite.
15. The system as claimed in claim 1, wherein said computer
determines said resistance force as a function of one or more
dynamic parameters relating to the impact.
16. The system as claimed in claim 15, wherein said computer
determines said resistance force as a function of a velocity and/or
an angle at which said element or layer is struck.
17. The system as claimed in claim 15, wherein said computer
determines said resistance force as a function of an amount of
rotation imparted to the element or layer.
18. The system as claimed in claim 1, wherein said computer
designates a set of finite elements of the.
19. The system as claimed in claim 18 wherein said set is only a
subset of all available elements.
20. The system as claimed in claim 2, wherein said computer carries
out finite element calculations on said element or layer in
addition to assigning said resistance force to said portion and
uses the results calculated by said finite element calculations in
subsequent analysis instead of said resistance force if said
results indicate the element or layer is not failing by said crush
failure mode.
21. The system as claimed in claim 20, wherein said computer
allocates an element a degraded crush capability for future crush
analysis if the results calculated by said finite element analysis
are used.
22. The system as claimed in claim 1 wherein said finite elements
are shell elements.
23. The system as claimed in claim 1 wherein said finite elements
are solid elements.
24. The system as claimed in claim 1 wherein said finite elements
are beam elements.
25. The system as claimed in claim 1, wherein said computer defines
a crush front or barrier and adjusts a relative velocity between an
impactor and said element or layer during passage of the crush
front or barrier through the element.
26. The system as claimed in claim 25, wherein said computer
modifies the resistance force along a length of the element in
accordance with a predetermined function of the relative
velocity.
27. The system as claimed in claim 1, wherein said computer defines
a crush front or barrier and adjusts an angle of impact between an
impactor and said element or layer during passage of the crush
front or barrier through the element.
28. The system as claimed in claim 27, wherein said computer
modifies the resistance force along a length of the element in
accordance with a predetermined function of the angle of
impact.
29. The system as claimed in claim 1, wherein said computer defines
a crush front or barrier and specifies a friction of the element or
layer with the crush front or barrier.
30. The system as claimed in claim 1, wherein said computer
specifies material damping coefficients.
31. The system as claimed in claim 1 wherein said crushable
material comprises a composite material.
32. The system as claimed in claim 31 wherein said composite
material is a fiber-reinforced composite material.
33. The system as claimed in claim 31 wherein said composite
material is a carbon-fiber reinforced resin.
34. A data processing system which models the behaviour of a
structure during an impact comprising: a computer that: (i)
determines for one or more layers of a finite element of a material
during said impact whether said element or layer thereof is failing
by a crush failure mode whereby said material is continuously
consumed by disintegrating into debris; (ii) if said element is
determined to be failing by said crush failure mode pursuant to (i)
above, said computer assigns to a portion of said structure an
ongoing resistance throughout a consumed length of the element or
layer; and (iii) out puts data from which a predicted impact
resistance of said structure can be calculated.
35. A data processing system which models the behaviour of a
structure during an impact, said structure incorporating a material
which can fail through a crush failure mode whereby said material
is continuously consumed by disintegrating into debris, the system
comprising: a computer that: (i) determines for one or more layers
of a finite element of said material during said impact whether
said element or layer thereof is failing by said crush failure
mode; (ii) if said element or layer is determined to be failing by
said crush failure mode, said computer assigns to a portion of the
structure an ongoing resistance throughout a length of the element
or layer, and (iii) outputs data from which the predicted impact
resistance of said structure can be calculated.
Description
CROSS-REFERENCED APPLICATION
[0001] This application is a continuation application of U.S.
patent application, Ser. No. 12/589,776, filed on Oct. 28, 2009,
which is a continuation application of U.S. patent application,
Ser. No. 10/931,273, filed on Aug. 31, 2004, both of which are
incorporated herein by reference in their entireties.
BACKGROUND
[0002] 1. Field of the Disclosure
[0003] This invention relates to methods, apparatus and software
for modelling the behaviour of materials which are crushed
particularly, but not exclusively, in the context of composite
vehicle body parts under impact.
[0004] 2. Discussion of the Background Art
[0005] It has been recognised for a long time that fiber-reinforced
composite materials, particularly carbon fiber composites have
great potential for revolutionising the auto industry. It is well
known that composites are very light compared to their metal
equivalents, even aluminium, and can be formed into complex shapes
that can do the same job as many welded metal stampings.
[0006] Composites also have the ability to absorb high amounts of
energy during impacts which make them ideal for automotive, rail or
civil applications. For example, whereas steel can only absorb up
to 20 kilojoules per kilogram and aluminium approximately 30
kilojoules per kilogram, carbon composites can absorb up to 80
kilojoules per kilogram.
[0007] In addition, unlike metallic structures, the crushed
material has very little residual strength after it has absorbed
the energy. Instead, the composite material is essentially
transformed into small pieces of debris and loosely connected
fibres after it has been crushed which means that less space is
required than in an equivalent metal structure. This is because in
a metal structure space must be provided in designated crumple
zones to accommodate the buckled metal.
[0008] There is, therefore, a significant incentive to using
composite materials such as carbon fiber composites in mass
production vehicles. However, to date they have only been used in
very limited applications such as top-end sports cars, motor sport
and small, non-critical parts of mass produced cars.
[0009] Two significant current disadvantages of composites is that
they are relatively costly and have long manufacturing cycle times.
However, a significant barrier which still remains to their
widespread use in the automotive industry is the ability to be able
to model their performance in an impact. This is of course
essential to be able to do in order to design vehicles which are as
safe as possible and which will behave in a predictable way in the
event of a crash. Although crash performance testing can be carried
out by building prototypes, this is extremely expensive and is only
practically feasible in the latter stages of design to prove the
basic design and calibrate restraint systems. During the earlier
stages of design of vehicles made from metal, finite element
analysis is used to model the behaviour and interaction of the
various metal parts and to predict their performance in the event
of an impact. This means that designs can be proposed, tested and
modified using computer modelling with much less reliance on
producing and testing expensive prototypes.
[0010] However, this approach does not currently work for crushable
materials such as composites. The reason for this is that
composites absorb energy by a very different mechanism to metallic
structures. Metallic structures absorb energy by plastic folding of
the metal, initiated by local buckling of the material, which can
be characterised by a stress vs. strain curve to good effect. At
limit, final failure, which may be tearing or brittle fracture,
results in the element being unable to transfer load, although its
initial volume is essential unchanged.
[0011] On the microscopic scale however some materials such as
composites absorb energy by local crushing of the material, by
matrix cracking, fiber buckling and fracture, frictional heating
etc. Viewed on a macro scale, the material is essentially crushed
or consumed by the impact on a continuous basis, and the volume of
the material is reduced as the structural material is turned to
debris.
[0012] It is widely recognised in the art that no satisfactory way
of modelling the crush performance of composite materials exists.
Existing finite element analysis techniques tend to treat elements
of composite by treating the whole element or separate layers
thereof as maintaining their integrity until the appropriate
failure stress value is reached, whereafter the element or layer is
simply deleted from the analysis or the element or layer is deleted
from the analysis in a predefined period. In a typical example,
this might result in the element being deleted with only 5% of its
original edge length compressed. The conventional finite element
calculations essentially cannot deal with very large changes in
volume and therefore catastrophically fail the element where in
reality the unimpinged volume of material still had a significant
capacity to absorb energy. This has the effect that the results of
analysis based on such techniques do not correlate satisfactorily
with actual experimental results such that they cannot be relied
upon to predict the performance of structures, e.g., automotives in
the event of an impact.
[0013] This is clearly a serious drawback of conventional
techniques and in practice means that composite materials are not
used or in the few cases where they are used, either the structure
must be sufficiently over-engineered to ensure the required minimum
level of performance, or extensive prototyping and testing is
needed in order to assess performance, which is of course unduly
time consuming and expensive.
[0014] There exists a need, therefore, to be able to predict
reliably the performance of composite materials during an
impact.
SUMMARY
[0015] When viewed from a first aspect the present invention
provides a method of determining the impact resistance of a
structure including a crushable material comprising the steps of
determining for one or more layers of a finite element of said
material during an impact whether said element or layer thereof is
to be treated as failing by crushing; and if said element or layer
is determined so to fail, defining a load-bearing portion of the
structure and treating said load-bearing portion for the purpose of
subsequent calculations as exhibiting an ongoing resistance.
[0016] When viewed from a second aspect the invention provides
computer software which, when executed on suitable data processing
means, determines the impact resistance of a structure including a
crushable material by determining for one or more layers of a
finite element of said material during an impact whether said
element or layer thereof is to be treated as failing by crushing
and if said element or layer is determined so to fail, defining a
load-bearing portion of the structure and treating said
load-bearing portion for the purpose of subsequent calculations as
exhibiting an ongoing resistance.
[0017] When viewed from a further aspect the invention provides a
data processing apparatus programmed to determine the impact
resistance of a structure including a crushable material, by
determining for one or more layers of a finite element of said
material during an impact whether said element or layer thereof is
to be treated as failing by crushing and if said element or layer
is determined so to fail, defining a load-bearing portion of the
structure and treating said load-bearing portion for the purpose of
subsequent calculations as exhibiting an ongoing resistance.
[0018] The inventors have recognised that the actual failure mode
of crushable materials during crush can be approximated as giving
an ongoing resistance throughout the continuous consumption of the
element or layer at the crush front rather than letting the element
or layer as a whole suffer a single rapid failure.
[0019] The inventors have realised that the approach in accordance
with the invention gives much more reliable and accurate results in
circumstances where a material undergoes crush.
[0020] It should be appreciated that in general the resistive force
returned for the element or layer is not the peak failure stress
but is a somewhat lower value which may be calculated from
materials theory or determined empirically. To give one specific
example, for a typical high strength carbon composite such as T300
in a toughened resin system the compressive failure stress is of
the order of 600 Newtons per square millimeter (N/mm2). However, if
the material is crushed continually, the resistance to the impactor
is of the order of 100 N/mm.sup.2, i.e. approximately 1/6 of the
peak compression strength value.
[0021] The invention therefore effectively adds a new failure mode
for elements which are determined to be those which in reality will
undergo crush--i.e. return a resistance force throughout the
consumed length of the element. The crush front may simply be the
forward face of the barrier impacting the structure although this
is not essential and the crush front could instead be defined
elsewhere--e.g. in a fixed relationship relative to the
barrier.
[0022] The element or layer which is determined to be failing by
crushing could be deleted, the ongoing resistance being applied to
one or more elements or layers adjacent the deleted element or
layer, and/or another load bearing portion of the structure.
Preferably the load bearing portion is a portion of the element or
layer being crushed itself. For example the element or layer could
be resized or redefined (e.g. by splitting), the ongoing resistance
being distributed across the or each new element or layer. In both
of the foregoing alternatives the barrier is effectively treated as
being impenetratable (save possibly for an allowance for minimal
penetration to avoid computation difficulties at the boundary). The
nodes of the element or layer adjacent to the barrier are therefore
prevented from passing through. However both possibilities are to
be contrasted with conventional finite element in which analysis
rigid barriers are effectively treated as impenetratable and
analysis elements or layers are simply compressed against the
barrier until the failure stress is reached and the element or
layer is deleted with no residual effect.
[0023] In presently preferred embodiments of the invention the
crush front is allowed to progress across the element or layer so
that the space occupied by the element or layer "passes through"
the crush front.
[0024] The resistance will not in general be a fixed value but
rather may be a function of one or more parameters relating to the
element or layer. In a preferred example the resistance is a
function of the thickness of the element or layer being crushed
along the crush front. Additionally or alternatively the resistance
is preferably dependent upon the contact area at the crush front.
Preferably for a given element the actual value of the resistance
force is a constant function of the contact area. In the simplest
case the resistance force could be directly proportional to the
contact area although this is not essential. Additionally or
alternatively where the crushable material is a composite material,
the resistance may be determined as a function of the lay-up of the
layers of the composite, e.g. the order of the layers.
[0025] Furthermore in presently preferred embodiments of the
invention the crush resistance is also a function of one or more
dynamic parameters relating to the impact such as the velocity
and/or angle with which the impactor strikes the element or layer
in question or the amount of rotation imparted to it.
[0026] The variations with element/layer and/or dynamic parameters
may be determined by theory, empirically or both. Even if these
variations are determined theoretically, this does not imply that
the corresponding base value is so determined and vice versa. In
practice it is expected that at least the variation of crush
resistance with angle will be empirically determined since this is
very dependent upon the weave of a layer or on each of the layers
of a composite material.
[0027] Preferably a set of finite elements of the structure is
designated as being susceptible to crush. The set could comprise
all of the elements in the structure. However the Applicant has
realised from empirical experience that only a relatively small
zone of a composites structure in the immediate vicinity of an
impactor will undergo crash. In preferred embodiments therefore
only a subset of elements is designated as being susceptible to
crush, thereby defining a crush zone. These elements are thus
allowed to fail through the novel crushing mode of the present
invention and will therefore require data allowing their resistance
in this failure mode to be calculated. Elements outside the crush
zone will not have the option of failing by crush. However this
means that it is not necessary to establish data allowing their
failure resistance to be determined. Clearly this is beneficial
where empirical data is used to measure the resistance exhibited
during crush since it obviates the need to establish data for areas
outside the crush zone.
[0028] When it is determined in accordance with the invention that
a particular finite element is in the crush regime, the
conventional finite element analysis could simply be suspended in
favour of the novel crush failure mode set out herein--in other
words the conventional finite element analysis calculations would
simply not be carried out for the particular element or layer. In
at least some preferred embodiments however the conventional finite
element calculations are also carried out in parallel so that
analysis reverts to these in the event that at any the element is
calculated to have failed due to another, conventional failure mode
such as shear, tensile or inter-laminar failure at any point whilst
the element is being crushed. To give one example if the crush
resistance force gives rise to very large bending forces an element
might then fail as a result of tensile stress rather than being
crushed.
[0029] If the force pushing an element through the crush front is
not sufficient to overcome the resistive force calculated in
accordance with this invention the element can effectively can move
back into conventional finite element analysis. It should be
appreciated however that the element could again pass through the
crush front at a later stage as dictated by the finite element
analysis calculations.
[0030] Where analysis reverts to the conventional finite element
calculations the element or layer in question may be deemed
thereafter not to be capable of being crushed or to have a degraded
crush capability. For example the resistance force of the element
or layer in question might be reduced, for the purposes of any
future crush, in proportion to the amount of it which had
previously been consumed during the previous crush phase.
[0031] Where, as is preferred, the load bearing portion is a
portion of the element or layer being crushed itself, the load
bearing portion could be the whole element or layer, i.e. the
resistance force could conceivably be applied as a distributed
force across the element or layer. However for consistency with
normal finite element analysis it is preferred to apply the force
to the individual nodes of the element so that the nodes comprise
the load bearing portion. In some embodiments the force is divided
equally between the nodes. In other embodiments the force may be
biased towards one or more of the nodes. The force is preferably
divided between nodes that have passed through the crush front and
nodes that have not in proportions according to the amount of the
element by area or penetration distance that has passed through the
crush front. To give an example, if 70% of the element had passed
through the crush front, 70% of the calculated force would be
applied to the nodes that had not yet passed through.
[0032] The crush resistance which the element or layer will be
treated as offering may, as mentioned above, be determined using
materials theory. However, the internal mechanisms at work during
crush are often highly complex. For example in fiber composite
materials they depend on inter alia fiber type and sizing, the
resin properties, the cure cycle and the weave style. This
complexity is one reason why attempts to model crush in the past
have failed. However, one of the strengths of the present invention
is that it is not necessary to calculate or even understand the
internal mechanisms responsible since it has been appreciated that
for a given set of macroscopic conditions (area of contact with
impactor, velocity, angle of impact etc.) the crush resistance may
be approximated to a single macroscopic value. This value may
therefore be obtained empirically by performing tests on small
samples (known in the art as "coupons") of the material in question
which thereafter allows it to be modelled in large, complex
structures.
[0033] In accordance with the invention an element comprising the
entire material thickness could be modelled together or, where the
material comprises layers each layer or sub-group of layers could
be modelled separately.
[0034] In accordance with the invention, a determination is made
for analysis elements or layers as to whether or not they are to be
treated as undergoing crush. In embodiments preferred for
simplicity the determination is made by deciding whether the
impactor barrier has physically encroached into the space allocated
to a given element or vice versa. In terms of the model this
amounts to deciding whether any of the element's nodes have "passed
through" the barrier or in other embodiments a crush front defined
in another region of the model space. If failure of the element
through a conventional failure mode has not already taken place,
and the supporting structure has not collapsed, it may then be
deduced that the element will undergo crush. In alternative
embodiments an explicit calculation is made of the stress or strain
on the element which is compared with a threshold failure value.
The element is therefore denoted as being crushed if this threshold
value is exceeded. However the determination is made if an element
is determined to be undergoing crush, the treatment in accordance
with the invention is applied.
[0035] It will be appreciated that the ability in accordance with
the invention to model the behaviour of materials being crushed
does not, as has been previously attempted, require drastically
reducing the size of the finite elements used in the model which
would in any event lead to an inordinately large time or computing
power requirement. Rather a practical advantage of using an
essentially continuous model of the crush force, as the methods of
the present invention may be seen, is to allow element sizes which
are the same order of size as would be employed for an equivalent
analysis of a metal structure. This is because when an element has
been forced into the crush regime, as determined in accordance with
the invention, and providing the structure supporting the element
in question is capable of withstanding the forces involved, its
edge length is no longer compressed against the wall of the
impactor or other crush front but is effectively permitted to pass
through, subject, of course, to the resistive force on the wall
that the projected edge length, thickness and crush resistance
stress etc. dictate.
[0036] Although in many cases where the principles of the invention
are applied the impactor will be a rigid solid object striking the
structure, this is not essential and the impactor could comprise
another part or body of the structure with sufficient strength and
rigidity.
[0037] In presently preferred embodiments shell elements are
employed although alternatively solid or beam or other elements
could be used.
[0038] In some embodiments it may be preferred, e.g. for reasons of
computational efficiency, that the relative velocity of the
impactor wall or crush front and the element in question is taken
to be constant during consumption of the element. However, this is
not essential and preferably the relative velocity is adjusted
during the passage of the crush front through the element.
Preferably the resistive force is modified along the length of the
element in accordance with a predetermined function of the relative
velocity.
[0039] The same considerations apply to angle dependence to allow
for rotation during consumption of the element. Indeed in general
any parameter on which the crush resistance depends may be updated
during consumption of the element, another example being the
thickness, vibration, temperature etc.
[0040] In some preferred embodiments the friction of the crush
interface with the barrier or other crush front may be specified.
This is advantageous as it can influence whether a given element is
stable enough to undergo crush or whether it fails by another
mechanism.
[0041] Modelling of the effect of an impact of a structure
including a crushable materials in accordance with the invention
may be carried out without taking damping into account. In some
preferred embodiments however damping coefficients are specified
which could be internal, external or specified globally by the
overall finite element analysis model.
[0042] The invention may be applied to any material which can be
crushed, i.e. one which disintegrates with little or no residual
strength under certain conditions. Some possible and non-limiting
examples include concrete, wood, glasses, ceramics, honeycombs and
foams. In preferred embodiments of the invention the crushable
material comprises a composite material, more preferably a
reinforced-reinforced composite material and most preferably a
carbon-fiber reinforced resin.
[0043] Although the principles of the invention may be widely
applied, e.g. as part of an original analysis model, preferably
software implementing the invention is incorporated into an
existing finite element modelling package. The type of finite
element modelling is preferably non-linear and could be implicit,
explicit or another type of analysis mathematics, although explicit
non-linear analysis is preferred. In the currently preferred
embodiment for example, the software is incorporated into
MSC.Dytran (trade mark) explicit non-linear finite element analysis
software.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] A preferred embodiment of the present invention will now be
described, by way of example only, with reference to the
accompanying drawings in which:
[0045] FIG. 1 is a schematic flowchart showing the operation of
software embodying the present invention;
[0046] FIG. 2 is a graph showing resistive force against deflection
for a test coupon of a composite material;
[0047] FIG. 3 is a graph of deceleration against time for a test
cone which underwent an impact under controlled conditions,
[0048] FIG. 4 shows the sled velocity versus displacement for the
experiment of FIG. 3;
[0049] FIG. 5 shows the predicted deceleration profile is shown in
FIG. 5; and
[0050] FIG. 6 shows the predicted sled velocity.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0051] In a preferred embodiment of the invention, software
operating in accordance with the principles of the invention is
incorporated into MSC.Dytran (trade mark) 2004 finite element
analysis package which is available from MSC. Software Inc. This
known software can be programmed with failure stress values for
composite materials and thus for a given finite element of the
material can attempt to model the forces on that element until the
stress it experiences exceeds the failure stress whereupon the
element is deleted. However, in the embodiment of the invention now
being described, this part of the functionality of the software is
supplemented. Instead, the process shown in FIG. 1 is followed.
[0052] In this process, it is first determined, at 2, when there is
impact between the defined impactor and an element selected as
being capable of crush of the structure. If there is contact, it is
determined, at 4, whether any of the nodes of the element have
penetrated the impactor. If none of the nodes has penetrated the
impactor, the software moves to the next main step at 6 in which
the element stress is updated. However, if penetration is detected,
the software moves, at 7, to assess whether the element connected
to the node is already tagged as undergoing crush. If it is not the
software adds this tag to the node at 8 and then moves on to update
the element stress at 6. If the element connected to the node had
already been tagged as undergoing crush though, a further series of
subroutines is carried out first at 9. Firstly the contact force is
set to zero. Secondly the direction of crush is stored and lastly
the relative velocity is stored.
[0053] The next main step at 6 is to update the stress on the
element. To do this it is determined, at 10, how many of the nodes
of the element have been tagged as undergoing crush. If all of the
nodes of the element have been tagged, the element is taken to have
failed and is therefore removed from further calculations at 12. If
one or more, but not all of the nodes is tagged, the software, at
14, projects the crushing direction in the element co-ordinate
system to allow determination of the correct direction for material
properties to be calculated. It then determines the resistance
stress of the element from input data (explained in greater detail
below with reference to FIG. 2) and the whole element is tagged as
undergoing crush.
[0054] Alternatively, if at the assessment step 10 none of the
nodes is tagged as undergoing crush, the system simply does
nothing, at 16. Whichever of the possibilities 12, 14, 16 is
encountered, the software then moves to 23 where the conventional
finite element stress update is undertaken prior to moving on to
the third main step of the process in which crushing contact is
calculated, at 18.
[0055] In this stage, a determination is made, at 20, as to whether
the element has been tagged as undergoing crush. If the element has
not been tagged, processing continues within the previous
conventional analysis mode before returning to the beginning of the
process shown in FIG. 1.
[0056] However, if the element has been tagged, four actions are
taken. Firstly, the intersection between the element and the
impactor is calculated. The intersection is calculated to determine
the amount of material being crushed. If a triangle is crushed from
a vertex, the material being crushed will increase and, as a
result, the resistive force will increase as the element is
consumed through the barrier. Secondly, the crush direction is
obtained, thirdly the crush stress is obtained and finally the
crush forces are calculated. Thereafter, processing continues
within the previous conventional analysis mode before returning to
the beginning of the process shown in FIG. 1.
[0057] In order to calculate the predetermined resistance to be fed
into the model described above, a small coupon of the relevant
composite material is subjected to a crush test. In one example,
material sections of 60.times.30 mm are cut from flat plates and
bonded to a 50 mm thick honeycomb sandwich in order to promote
stabilized crush. The outer edges of each skin presented to the
impactor are chamfered at approximately 60.degree. to present a
sharp edge to minimize the spike in crush resistance exhibited at
the start of crushing and thereby minimize the risk of deamination
from the honeycomb at the start of crushing where the initial
failure corresponds to the compressive failure performance of the
element. The honeycomb cells are oriented perpendicular to the
direction of coupon crush and therefore do not absorb significant
energy but ensure that the skins do not buckle. A typical plot of
resistance force exhibited by a coupon versus deflection (i.e. the
amount of the coupon which has been crushed) is shown in FIG. 2.
From this it will be seen that throughout most of the range of
deflection the force is relatively constant. By taking a suitable
average value for this, the resistance force to be used in the
analysis model for a particular material may be determined. Since
the coupon has a constant cross-sectional area, there is no
variation of the resistance force with contact area. However in the
model the actual value of the resistance force is calculated as
directly proportional to the contact length.
[0058] It will be appreciated that this method of coupon testing
provides a low cost way of determining the stabilized crush
properties for a wide variety of lay-ups configurations and angles.
Thus typically such tests would be conducted for each of the
material constructions used in the structure to be modelled as
crush capable, and optionally each at a range of angles.
[0059] In an exemplary application of the embodiment described, a
rectangular-section cone structure of a T300 carbon fibre composite
material approximately 85.times.115 mm in section and approximately
455 mm long was mounted on a rigid barrier and a rigid sled is
propelled at a controlled velocity into the cone. FIG. 3 shows the
measured deceleration of the trolley versus displacement filtered
using a Butterworth Order4 low pass filter with upper cut-off
frequency of 300 Hz in this experiment (impact occurring at
Displacement=0). From this the actual resistance force encountered
may be calculated simply from the deceleration of the trolley and
its mass. FIG. 4 shows the sled velocity versus displacement for
the same experiment.
[0060] The cone was modelled using Dytran 2004 software modified as
described above with reference to FIG. 1. The predicted
deceleration profile is shown in FIG. 5 filtered in the same manner
as the test results, using a Butterworth Order4 low pass filter
with upper cut-off frequency of 300 Hz. From this it will be seen
that the profiles and absolute values of the deceleration are
similar. FIG. 6 shows the predicted sled velocity and here a
remarkable similarity exists between the tested and predicted
results. For example, the prediction of the distance taken to bring
the trolley to a rest was predicted at 327 mm and was measured at
328 mm meaning that the prediction was accurate to within 1%
percent. This is much more accurate than could be achieved with the
prior art methods.
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