Mitigation Of Lower Leg Tibia Forces During A Mine Blast On
A Light Armoured Vehicle: Simulations And Experiments
Richard Smith1, Senior Mechanical Engineer; Dinesh Shanmugam2, Senior Materials Engineer;
Ashley Marr3, Senior Mechanical Engineer; Campbell James4, Engineering Manager
1,2,3,4
Protected Mobility Systems, Land & Joint Systems Division, Thales Australia
[email protected]; [email protected];
3
[email protected]; [email protected]
1
Abstract: Safety of an occupant has a high priority in peacekeeping military operations. During a mine detonation
under a light armoured vehicle, the impulse loads threaten occupant safety. These impulse loads are directly
transferred to any part of the body that is supported by the seat or floor including the occupant lower leg, potentially
causing serious injuries. A combined numerical and experimental approach was used in this study to obtain greater
confidence in the injury assessment method. Blast tests were performed using the standard Hybrid III 50th% dummy
and data such as acceleration/time history from the vehicle and force/time history from the Hybrid III dummies were
collected. The acceleration/time history measured from the floor plate under the occupant feet was used as inputs for
the dynamic numerical analyses. A simplified static modelling technique was also developed to study relative hull
distortion based on displacements at the time of approximate peak acceleration. This simple model enabled more
rapid experimentation in the simulation space using ANSYS© than would be possible with dynamic non-linear FEA.
The explicit code within the finite element software LS-DYNA© was used to perform the non-linear dynamic
numerical simulations which eliminated any inherent convergence issues often associated with implicit codes such
as ANSYS©. A standard Hybrid III 50th% dummy incorporated within LS-DYNA© was used to predict the forces
generated on the lower leg tibia due to the input accelerations that could then be compared to the measurements
taken from the actual Hybrid III’s. The simulations showed good agreement with the experimental data and
therefore can be used as a means to interpolate mine blasts. Further, these simulations were used to revise the design
to minimise the forces on the occupant lower leg tibia within the threshold conditions of fracture, subsequently
verified by experiments.
1. INTRODUCTION
During the detonation of buried explosives in the
form of mines under the wheels of an armoured
vehicle, it is understood that at this relative close
proximity to the vehicle the effect of soil ejecta is the
major component of loading, that is, the initial
loading comes in the form of a high-speed “plug” of
soil. The soil plug can have an initial velocity of up to
1500 m/s for a full-size charge [1], and the initial
impact of these relatively dense products against a
vehicle have the capacity to induce large loads in a
very short period of time. It is suggested that changes
in the flow field resulting from target geometry can
create localized pressure spikes, particularly if the
field stagnates inside re-entrant corners [2].
In general, the severity of the threat, and therefore,
the severity of the loads on the occupant, depends on
the distance between the occupant and the detonation
point as well as on the inertia and rigidity of the
vehicle hull structure and the interior structure such
as the seat and seat mountings and the floor plate
configurations [3]. When a seat or a footrest is
mounted on or close to the deforming floor or hull
plate large loads are transferred to the feet, ankles,
legs and lumbar spine respectively. Additionally, the
chance of injury depends on initial body posture, and
the presence and use of personal protection
equipment and restraint systems. Also age, gender,
health, training, etc., may influence the injury
probability [4].
It is expected that once the vehicle’s structural
integrity is secured, effects of fragments,
overpressure, gases and heat will have minor physical
effects on the occupant inside the vehicle [5].
However, the impact of the vehicle structure with the
occupant’s feet through the floor and other contact
points such as pedals and foot rests, through
extremely high acceleration pulses can result in
fracture and disintegration of the leg bones such as
the tibia. If these loads are not attenuated to
survivable levels, it may lead to severe leg injury of
the occupants [6].
In developing mine protection characteristics, the
design and tuning of the dynamic behaviour of the
vehicle structure, along with occupant protection
systems, are demanding tasks which can be solved
only in the context of the complete vehicle [6]. The
required test series and qualification trials, some of
which are conducted with fully equipped vehicles, are
very time-consuming and costly. Apart from the high
cost, the typically short procurement times sought by
the customer for new vehicle systems call for a
significant reduction in development times.
Research shows that the use of numerical simulations
in the design process is an important tool for the
design layout as well as to substantially reduce the
time requirements. One approach is to utilise finite
volume explicit codes to model loading events, such
as explosions, to generate the input loads on the
structures under investigation [7], [8]. However,
where the required experimental inputs are available
the finite volume approach is not required to generate
the input loads and the finite element method is then
used only to analyse the associated structural
responses [9]. In this paper the finite element
approach only has been used to predict the responses
of the vehicle and occupants of proposed solutions
based on past actual test measurements.
2. PROBLEM DEFINITION
The design of mine-protected vehicles places high
demands on the developmental process. In this case, a
study has been conducted to find the effects of a mine
detonated underneath one of the wheels of a light
armoured vehicle on the occupant. It is understood
that the floor structure must have a high level of
stiffness in order to resist deflection as much as
possible ensuring a minimal level of deformation of
the floor into the crew compartment. On the other
hand, the stiffness of the structure must not be so
high that the floor structure breaks up and collapses
due to material failure.
rigid seat with its feet resting on the EA mats under
investigation. The bottom face of the mat is fixed in
the same reference space as that of the fixed torso.
The input acceleration can then be applied to this
reference space simulating the acceleration of the
vehicle vertically upwards. Under the acceleration
input the inherent inertia of the legs generate the
destructive forces within the leg members that are
targeted for reduction to acceptable levels via the EA
mats under investigation.
The main focus of this paper is to study the blast
loading effects on the lower tibia of the occupant
seated at the driver/co-driver positions by 1) using
Energy Absorbing (EA) mats on the floor and 2)
increasing the structural integrity of the floor-well.
These were both performed via simulation and
experiment.
1) To predict the performance of the EA mats the LSsimulation
package
was
used.
DYNA©
Acceleration/time measurements recorded on the
floor (Test 1 mine blast) along with true stress-strain
curves of the mats were used as inputs. A digital
Hybrid III 50th% dummy inbuilt within the LSDYNA© code was then utilised to predict the impact
forces on the relevant parts of the body. In order to
initially validate the LS-DYNA© model, drop tests
were performed on a simplified test rig, which
mimicked a floor structure, to
develop
acceleration/time history data that was then input into
the simulation. The predicted force/time outputs from
the digital Hybrid III were then compared with the
actual drop test Hybrid III force/time results.
2) To give an indication of the relative improvement
in structural rigidity of the floor well, due to the
addition of stiffening plates, a simplified static
structural modelling technique was developed based
on simplified loading assumptions using ANSYS©.
This was done to enable a more rapid
experimentation process in the simulation space than
would be possible using a full non-linear dynamic
FEA approach of the complete hull.
For validations, the simulation Hybrid III force/time
history was compared with the measured force/time
history from the actual Test 2 mine blast.
3. SIMULATION AND EXPERIMENTAL
SETUP
3.1. Dynamic Matting LS-DYNA© Simulation
The actual seating position and region around the
occupant in the vehicle formed the basis for the
numerical simulations and the drop test
configuration. Figure 1 shows the numerical
simulation as set up within LS-DYNA© that was used
to predict the occupant responses to test acceleration
inputs. The torso is rigidly fixed in 3-D space as this
is assumed to have little or no influence on the
motion of the legs as under real conditions it is
secured to the seat using a belt. The occupant model
is a 50th% Hybrid III dummy as supplied with LSDYNA©. The dummy is assumed to be seated on this
Figure 1: LS-DYNA© Simulation Setup
The positions of the arms are left at their default
orientations as they play no role during the
simulation. Similarly an occupant restraint
mechanism such as a seatbelt has not been modelled
as the torso is fixed. The main region of activity lies
between the foot and hip of the dummy. The motion
of the fixed reference frame, i.e. the vehicle, is
controlled via the prescribed acceleration curves.
This same setup serves to simulate the conditions of
a) the mine blast, via the input of the typical
acceleration curve as seen in Figure 2, which was
measured at the approximate position on the floor
that the right foot would rest, and b) the drop test, via
the curve seen in Figure 3.
While modelling, care was taken to position the feet
as close as possible to the mat, as even the smallest
gap between the feet and mat can significantly alter
the lower tibia axial compressive force. During the
initial sequence of the simulation the feet are allowed
to settle onto the top mat under gravity therefore
eliminating any contact gap. For the EA mats within
LS-DYNA© the *MAT57 Low Density Foam
material algorithm was used with a characterised
stress-strain compression curve mapped to it from a
standard compression test.
Figure 1: Typical acceleration/time curve for floor response from Test 1 of the mine blast
Figure 1: Drop test acceleration impulse
©
3.2. Static Structural ANSYS Simulation
To obtain an indication of the relative improvement
in structural floor well rigidity a simplified static
ANSYS© model was developed. Within this
simplified model a section of the floor-well region
was extracted from a complete vehicle CAD model.
This section model included a portion of the inclined
hull outer wall as well as the appropriate internal
floor plate sections and relevant support members as
seen in Figure 4.
B
A
C
Figure 2: Static ANSYS© Simulation Setup
Reference A in Figure 4 indicates the position of the
rested feet. Reference B in Figure 4 indicates the
pressure applied to the outer hull plate. Reference C
in Figure 4 indicates the prescribed motion edge.
This simplified model was developed purely to
determine what the influence the addition of rigid
plates would be to the deflection of the floor. It was
determined during an earlier study that the deflection
of the floor region could be reduced via stiffening.
The static model was set up with an arbitrary pressure
applied to a region of the outer hull plate in and
around the region where a mine blast would load the
vehicle during a real event. The section was fixed in
space along a number of plate and member edges
while a single lower hull plate edge was allowed to
move a prescribed amount relative to the fixed
reference frame. This prescribed motion mimicked
the lower section of the hull moving under the
extreme loading of the blast relative to the rest of the
hull.
With this basic setup and assumptions a study could
be undertaken to determine the relative effects of the
additional rigid plates. Figure 4 shows the floor plate
section before the addition of the rigid plates which
reflects the results seen during Test 1 mine blast.
Figure 5 highlights the initial deflection levels for
comparative analysis before the additional rigid
plates were added. It is important to note that due to
the simplified nature of this analysis the actual
deflection levels indicated are arbitrary and only
useful for comparative purposes. In this fashion a
determination of the increase in rigidity could be
made due to the additional plates.
Figure 3: Static ANSYS© deformation showing
internal wall buckling and relative deflection of
floor plate
Figures 6 shows the additional rigid plates added to
the floor-well area. The positions of the plates were
chosen to minimise the floor vertical movement
which can adversely increase the impulse into the
occupant’s lower legs. The plates mounted on the
internal vertical cover wall were intended to prevent
the buckling of this wall plate (as indicated in Figure
5) and hence reduce allowable vertical movement of
the attached floor.
Figure 4: Static ANSYS© simulation setup with
additional plates
From earlier full scale mine blast tests it was found
that the force impulse exerted on the lower tibias of
the Hybrid III dummy were not of the same
magnitude between the left leg and the right even
though the position of the feet on the floor were only
a short distance apart. In reality the magnitude of the
right leg was noticeably greater than that of the left
leg even though the left leg was closer to the outer
hull and hence closer the blast. From this simple
static model with fixed boundary conditions and
prescribed motion it is believed that the underlying
cause and structural mechanism for this difference in
impulses was successfully modelled. By measuring
the relative deflection of points at the approximate
positions that the feet would lie on the floor from the
"before rigid plate addition" model and the "after
rigid plate addition" models a relative structural
stiffness improvement could be extrapolated.
Figure 7 highlights the relative influence of the
additional plates on the rigidity of the floor-well
region. The internal wall is predicted to be prevented
from buckling and the relative vertical deflection of
the feet positions is shown to significantly reduce.
During the subsequent Test 2 mine blast with rigid
plates added the actual results were observed to agree
with this simplified model very closely.
Figure 5: Static ANSYS© deformation with
additional plates
3.3. Experimental Setup
Experiments using 50th% Hybrid III dummies under
vertical drop tests and mine blast tests were carried
out to validate the simulations performed by LSDYNA©. Hybrid III dummies are able to withstand
crash and vehicle mine detonation tests while
simultaneously (and reproducibly) measuring the
accelerations, forces and moments in different body
parts. As seen in Figure 8, the dummy in the drop test
was buckled onto the seat of the test sled in the
vertical position with the legs placed on the floor
according to the driver/co-driver’s normal resting
position.
Figure 6: Drop test setup
The dummy’s arms were strapped around its lower
thighs as shown in Figure 9 so as not to cause
damage during the rapid upward movement of legs
during the drop test. In addition to the measurement
taken from instrumented dummy, accelerations were
recorded on the floor which provided input for the
simulation as shown earlier in Figure 3.
Figure 7: Drop test leg setup
For the mine blast, full scale tests were conducted
with test conditions followed according to Allied
Engineering Publication (AEP-55), Volume 2 [10].
The loads on the occupants were measured using
instrumented Hybrid III dummies together with other
vehicle measurement equipment, and in particular
accelerometers in this case.
The forces measured from these Hybrid III dummies
serve as the injury assessment criteria as described in
AEP-55, Volume 2 [10]. Also, these force
measurements are used for validating the numerical
analysis. The data from the numerical simulations are
compared against experimental data.
placed on the vehicle floor at the approximate
position of the right foot during Test 1 mine blast
before any of the additional rigid plates were in place.
The ANSYS© simulations predicted that the
additional plates should have a significant impact on
the acceleration response of the floor. It was
recognised therefore that any improvement in tibia
force/time results measured during Test 2 mine blast
would be due to the combined effect of both the EA
mats and the change in structural rigidity of the floor
region. For this reason, the results from the LSDYNA© simulation would not be strictly comparable
to the actual measured results and should by
inference over predict the resultant tibia forces.
The drop tests were therefore performed to gain a
level of confidence in the simulation setup that could
be transferred to the blast simulation. The drop tests
have a far more reproducible and predictable
acceleration curve (Figure 3). As described earlier,
this impulse curve was also used as an input into the
same LS-DYNA© simulation which could then be
directly compared with the Hybrid III tibia results
from the drop test. Figure 10 illustrates this direct
comparison showing a quite good correlation
between the drop test and simulation space. The
simulation is seen to over predict the tibia force
which is likely to be due to an overly stiff material
model or slight differences in the Hybrid III leg
placements.
Figure 11 illustrates the comparison between Test 2
mine blast and the LS-DYNA© simulation. As
expected, the simulation over predicted the force
levels mainly believed to be due to the increased
floor rigidity. Considering this, when comparing the
trace from the actual mine blast with that from the
LS-DYNA© simulation this is considered a good
correlation. Both the drop test simulation and the
blast input simulation have a high level of correlation
to the actual measurements while both being
conservative. This simulation methodology can
therefore be deemed to be valid for future design
studies.
4. VALIDATION
As described previously the LS-DYNA© simulation
developed for the EA mats was conducted using the
input acceleration as measured from an accelerometer
Figure 7: Drop test Hybrid III lower tibia Force/Time comparison
Figure 7: Mine blast Hybrid III lower tibia Force/Time comparison
5. CONCLUSIONS
Numerical and experimental studies were performed
to study the injury assessment on a fully instrumented
standard Hybrid III 50th% dummy. The
acceleration/time history measured from the floor
plate under the occupants feet were used as inputs for
the dynamic numerical analyses on the EA mats.
Both the drop test simulation and the blast input
simulation have a high level of correlation to the
actual measurements while both being conservative.
This simulation methodology can therefore be
deemed to be valid as a part of future design studies
in the effects of mine blasts on occupants
6. REFERENCES
1.
Bergeron, D.M. & Gonzalez, R. (2004)
“Towards a Better Understanding of Land
Mine Blast Loading”, The Technical
Cooperation Program, Weapons Group,
Conventional Weapons Technology, WPN TP1 Terminal Effects, Key Technical Activity 134, Published June.
2.
2. Tremblay, J.E. Bergeron, D.M. & Gonzalez,
R. (1998) “Protection of Soft-Skinned Vehicle
Occupants from Landmine Effects”, The
Technical
Cooperation
Program,
Subcommittee on Conventional Weapons
Technology, Technical Panel W-1, Key
Technical Activity 1-29, Published August.
3.
Radonić, V. Giunio, L. Biočić, M. Tripković,
A. Lukšić, B. & Primorac, D. (2004) “Injuries
from Antitank Mines in Southern Croatia”,
Military Medicine, Vol. 169, April, pp. 320324.
4.
Yoganandan, N. Pintar, F.A. Kumaresan, S. &
Boynton, M.D. (1997) “Axial Impact
Biomechanics of the Human Foot-Ankle
Complex”, Journal of Biomechanical
Engineering, Vol. 119, November, pp. 433437.
5.
RTO-TR-HFM-090 (NATO) (2007) “Test
Methodology for Protection of Vehicle
Occupants against Anti-Vehicular Landmine
Effects”, Final Report of HFM-090 Task
Group 25, Published April.
6.
Tabiei, A. & Nilakantan, G. (2007)
“Reduction of Acceleration Induced Injuries
from Mine Blasts under Infantry Vehicles”,
6th European LS-DYNA Users Conference,
Gothenburg, Sweden.
7.
Showichen, A. (2008) “Numerical Analysis of
Vehicle Bottom Structures Subjected to Antitank Mine Explosions”, PhD Thesis, Cranfield
University.
8.
Genson, K. (2006) “Vehicle Shaping for Mine
Blast Damage Reduction”, Master’s of
Science Thesis, University of Maryland.
9.
Honlinger, M. Glauch, U. & Steger. G (2002)
“Modelling and simulation in the design
process of armoured vehicles”. Reduction of
military vehicle acquisition time and cost
through advanced modelling and virtual
simulation. Paris, France, 22-25 April 2002.
10.
AEP-55 (2006), Procedures for Evaluating the
Protection Level of Logistic and Light
Armoured Vehicles - Mine Threat, Vol. 2, Ed.
1, Published September.