The to the blast and so gives more

The
purpose of special kind of vehicle design is to increase the vehicle and crew
survivability by deflecting an upward blast from a landmine (or IED) away from
the vehicle. To reduce the vulnerability of the vehicle to anti-vehicular mine
blast is relying heavily on the numerical simulation to help design and
optimize design of armour systems. This paper deals with the simulation and analysis carried out on
the V-hull of a wheeled combat  vehicle.
Numerical analysis revealed that there is significant energy absorption because
of the use of deformable V-plate for vehicle hull. One of the great challenge
faced during the evaluation of the vulnerability of the vehicle to mines blast
is not only assessing the structural response of the vehicle but also
evaluating the damage caused to the vehicle due to the acceleration induced due
to blast. This paper presents the results of a simulation performed with
LS-DYNA using 8kg TNT under the belly of a wheeled combat vehicle. Local
deformation, acceleration, stress and peak pressure values were obtained and
analysed as results of analysis.

 

 

1.0     
Introduction

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The
design of the hull plays a vital role in the blast propagation. There are
multiple shapes like V, W, U are being used for designing the hull. They have
their merits and demerits based on the vehicle configuration. concluded that deflection
for the deep V-hull is the most effective and it was minimum deflection and
also the manufacturing difficulties are less compared to W-hull . The efficiency of the impact of the explosion on hull
bottom depends on the magnitude of shockwave resulting from the detonation of
the mine explosive. The shockwave pressure impulse effecting the vehicle
underside, particularly under a flat bottom late is much higher . The flat hull
gives more face area to the blast and so gives more space to propagate the
blast. Keeping the shape of the hull in such way that to cause minimum blast
propagation and minimum damage to the occupants. Simulations have been carried
out using different shapes of the vehicle hull, to find out the effectiveness
of the plate angle Under flat bottom plate, the blast wave gets reflected
pressure pulse, hence the resultant overpressure is higher compare to V-plate
where the pressure gets deflected due to V-shape. Manfred (2009) studied the
effects of mine blast on the vehicle structure.

                The effect of blast against the
hull of vehicle can be reduced considerably by incorporating steel plates at an
angle to direction of blast, because highest pressure are generated only when
the blast direction is at a 900 angle to the plate. This approach
has led to the introduction of V-hulls, which have been successfully used in
the protection of light-and medium sized vehicles against mines.

      A charge of   8 kg TNT was used to load the vehicle model. The
effect of blast was simulated and the results were analysed. A finite element
model of the wheeled combat vehicle was prepared using CAD software. Simulation
of penetration events requires a numerical technique that allows one body
(Penetrator) to pass another.

 

2.0     
Mine blast propagation

When a high order
explosion is initiated, a very rapid exothermic chemical reaction occurs. As
the reaction progresses, the solid or liquid explosive material is converted to
very hot, dense, high pressure gas. The explosion products initially expand at
very high velocities in an attempt to reach equilibrium with the surrounding
air, causing a shockwave. A shockwave consist of highly compressed air
travelling radial
outward from the source at supersonic velocities. Only 1/3rd of the
chemical energy available in most high explosive is released in detonation
process. The remaining 2/3rd is released more slowly as the
detonation products mix with air and burn. This after burning process has a
little effect on the initial blast wave because it occurs much slower than the
original detonation. As the shockwave expands, pressure decreases rapidly (with
the cube of distance) because of geometric divergence and dissipation of energy
in heating the air. The detonation is the process of a pressure wave blast wave
propagating chemical reaction to initiate behind it.

When the shock
wave impinges on a surface that is perpendicular to the direction it is
travelling, the point of impact will experience the maximum reflected pressure.
When the reflecting surface is parallel to the blast wave, the minimum
reflected pressure or incident pressure will be experienced. The magnitude of
the peak reflected pressure is dependent on angle of incidence, peak incident
pressure, which is a function of the net explosive weight and distance from the
detonation.

Mine blast
propagates in the actual manner. The actions of the blast above the ground and
below the ground are clearly mentioned in this figure. The explosive contains metal
and plastic pieces, which is to be placed below the ground to react on the
application of pressure. The explosion produces flame, fragments and blast wave
with inclusion of some debris.   In the present study soil ejecta is not considered
for simulation in order to simplify the analysis. 

 

3.0     
Simulation / FE Analysis Approach

3.1     
Model Geometry

The
model used in the present analysis is improved  version of Wheeled APC. The hull geometry is
designed in such a way to reduce the effect of the blast. The shape of the hull
bottom considered as shallow kept as V-shape. The turret, hatches, and interior
bulkheads have been removed as they have a negligible effect on the response of
the vehicle hull in the first few microseconds of the event, which is the
duration of the interest here. The wheels and the drive shafts have also been
removed to simplify the model. These components play a much more vital role in
the response of the vehicle and in the local deformation of the hull, e.g. the
wheels can absorb and deflect a significant amount of the blast, but the
loading model used in the analysis is incapable of explicitly model their
effect on the blast. The engine and transmission block along with their
attachment points are also hidden. For the purpose of assessing the blast
effect, only the bottom V-hull portion with a simplified axles and gear box
have been considered for further loading and analysis. The dimensions of the
vehicle model are 7150mm X 1775mm X 1995 mm (L X B X H). The complete vehicle
model is shown in figure 2 and 3. The stand-off is 450 mm. The location of the
blast is exactly at the centre of hull plate.

        The high hardness steel plate having
thickness of 8mm is used for the V-hull of the model and it has hardness in the
range of 300-350 BHN and the hull is made of 5mm high hardness steel. The
stiffeners are of 8mm thickness provided in the V-hull to restrict the upward
movement of the plate. The deformable V-plate reduces the risk of damage due to
blast pressure and fragmentation intrusion into the crew compartment. The
V-hull portion is directly faces the blast fragmentation, so to reduce the
process time and to achieve quick results, the focus is on V-hull portion only.

 

3.2     
Finite Element Model

The CAD/CAE
software package was used as a pre-processor to build the solid model and
finite element mesh of the vehicle. The vehicle hull is meshed using shell elements
(Quad elements). The model mesh consists of 2,42,694 nodes and 2,43,895 shell
elements. The vehicle bottom V-hull is shown in figure 4. The hull is
restricted in all DOF as the prime interest is to observe the maximum
deformation of the V-plate due to blast load. It is assumed that the explosion
is taking place above the ground and the detonator is not buried in the soil.  

The structure
of the hull is high strength steel having hardness in the range of 300-350 BHN.
The values for card format are as under.

 

Table  1 : Material properties of V-Hull

Property

Value

Mass density (RHO)

7850 kg/m3

Young’s modulus (E)

210 kN/mm2

Yield strength (SIGY)

1100 N/mm2

Poisson’s ratio (NU)

0.3

 

3.3     
Charge

The explosive ignited at
the centre and a ‘Programme burn’ model is commonly used with a burn fraction
enhancement. This approach is mainly used with hex-elements. The evolution of
the explosive after ignition is described by the Jones-Wilkins-Lee (JWL)
equation of state 10, which defines pressure as;

Where,

V –   Relative volume

E
–   Internal energy and   w,
R1, R2  are
constants

            The structural response of the
vehicle mainly depends on the blast pressure and the stand-off. The analysis
mainly depends on the acceleration generated inside the hull and finally
transmitted to the occupants. The CONWEP function was used in order to generate
blast equivalent pressure distribution on the hull. CONWEP assumes the
exponential decay of pressure with time.

            For shell elements, *LOAD_BLAST is
used to define air blast function for the application of the pressure loads
from the detonation of conventional explosives. *LOAD_BLAST function reproduces
a field of vectors on the target’s nodes that changes with time.

 

3.4     
Loading Model

The
vehicle hull model is developed using HyperMesh and the simulation of blast
loading is done by LS-DYNA. The model is used to predict the maximum damage
area due to the blast of 8 kg TNT. The data gathered from the blast simulation
is used to develop an empirical model, which accounts the effects such as size
of charge of TNT, location of charge, properties of material etc.

By using
LS-DYNA pre-processor, the input data like element thickness, properties of
material, boundary conditions and loading was assigned to the vehicle model.

 

3.5     
Result interpretation through
LS-PREPOST

The
blast is a microsecond phenomenon. The von-Mises stress values of  V-hull are shown in figure 5 followed by displacement
values of  V-hull in figure 6. As the detonation starts, the explosive emits the
energy in the form of heat which finally becomes the blast wave and acts as a
pressure wave on the vehicle structure.   The maximum displacement is found 59mm which
is within the permitted limits. The max displacement of 150 mm was permitted.
The max stress is found 1100 MPa, which is within the yield limit of plate. The
Max values for Von-Mises stress are coming close the contours of the vehicle
due to stress concentration in that region.

            As shown in figure 7, the nodal
displacement on the V-hull is found in the range of 35-38 mm. The graph shows
only few selected nodes on the V-plate where blast wave strikes in the
beginning.

Figure
8 shows peak acceleration values are in the range of 1520 mm/ms2 for
approximately 0.005 second. As the duration is very small, the fatal effect
will be less.. The above acceleration values are on the V-hull. By using
secondary energy absorption mechanism, the acceleration values can be brought
down further. It is observed that due to compression of spines and snaps necks,
the soldiers inside get killed due to the instant acceleration caused by blast.

           

4.0               
Conclusion

The
impulse is a function of total stand-off distance and plate angle. The V-hull
deflects the blast up and away from the crew cabin, the use of other secondary
mechanisms will help to absorb the blast energy. The V-shaped design of hull
could neutralise the overall blast effect compared to flat shaped hull design.
But to achieve higher protection against mines, more study and analysis is
required.