VAA

Loads
Attention: This command is in the beta stage and the format may change over time.
*VAA
"Optional title"
coid
entype${}_a$, enid${}_a$, entype${}_v$, enid${}_v$, tid${}_m$, $\delta_{ref}$, $\Delta_{hit}$
$\psi_0$, $\psi_1$, $N_{\psi}$, $\theta_0$, $\theta_1$, $N_{\theta}$
tid${}_f$
Parameter definition
VariableDescription
coid Command ID
entype${}_a$ Armour entity type
options: P, PS, ALL
enid${}_a$ Armour entity identification number
entype${}_v$ Protected volume entity type
options: P, PS, ALL
enid${}_v$ Protected volume entity identification number
tid${}_m$ ID of table with material properties
$\delta_{ref}$ Critical reference material thickness at normal impact
$\Delta_{hit}$ Hit spot grid size
default: $\Delta_{hit} = 0.5 \cdot \delta_{ref}$
$\psi_0$ Horizontal (azimuth) start angle
$\psi_1$ Horizontal (azimuth) end angle
$N_{\psi}$ Number of horizontal frames
$\theta_0$ Vertical start angle
$\theta_1$ Vertical end angle
$N_{\theta}$ Number of vertical frames
tid${}_f$ ID of table with edge effect properties by face set
Description

This command is a tool that can be used to evaluate the ballistic protection level of armoured vehicles. It is an enhanced version of VULNERABLE_AREA_ASSESSMENT. The main improvement is a more efficient and straightforward treatment of edge effects.

The methodology is not based on any Finite Element calculations, but uses simplified material and geometric assumptions to quickly estimate the ballistic protection at many thousands or millions of hit spots. It is assumed that the threat follows a straight path through the armour and that that the protective performance of the different materials can be expressed as critical thickness at normal impact.

$\delta_{ref}$ defines the critical thickness of a reference material at normal impact. It is used in the post-processor to quantify armour deficit or surplus at the hit spots. $\Delta_{hit}$ is the incremental distance between the hit spots.

The command references to a TABLE (tid$_{}m$) listing the critical thicknesses of the armour materials. Optional parameters specifying capacity reductions near material edges, at gaps and in heat affected zones can also be provided. It is also possible to specify curves with material specific impact angle dependencies.

Table with material specific properties
Table with material specific properties

Default assumed relationship between critical material thickness and impact angle is based on "line of fire".

Default critical thickness versus impact angle for material $mid_i$
Default critical thickness versus impact angle for material $mid_i$

This relationship can be replaced by user defined curves for the different materials. The curves specify critical thickness versus impact angle.

User defined critical thickness versus impact angle for material $mid_i$
User defined critical thickness versus impact angle for material $mid_i$

Materials can be assigned a reduced penetraion resistance near edges. For material $mid_i$ this is done through the parameters $\Delta_{edgei}$ and $sf_{edgei}$.

Reduced capacity near edges for material $mid_i$
Reduced capacity near edges for material $mid_i$

Gaps can have a negative effect on the protective capacity. By default, gaps are treated as edges. That is the same reduction will be applied to narrow gaps as to free edges. However, with $sf_{gapi}$ it is possible to control the magniude of the capacity reduction.

Controlling the effect of gaps between armour plates ($d_g \leq \Delta_{edgei}$).
Controlling the effect of gaps between armour plates ($d_g \leq \Delta_{edgei}$).

The optional TABLE tid${}_f$ is used define edge effect properties on face set level. Face set properties override the material specific edge effect properties in the material property table. An example is given below.

Reduced protective capacity in heat affected zones (typically after welding) can be accounted for by defining initial damage $D$ with the command INITIAL_STATE_HAZ. Having damage in an element using material $i$, the effective material thickness is reduced with a factor $(1 - D \cdot sf_{dmgi})$.

The vehicle is supposed to be oriented according to the figure below, with vertical direction along global $z$-axis.

Definition of horizontal (azimuth) angles
Definition of horizontal (azimuth) angles
Example
Edge effect properties by face set

This example shows how to override material specific edge effect properties on face set level. Note that edge effects are turned off completely by setting $\Delta_{edge}=-1$. In this example this is done for face set 33.

*UNIT_SYSTEM
MM/TON/S
#
# --- PARAMETERS ---
#
*PARAMETER
face_set_1 = 33
delta_edge_1 = -1
face_set_2 = 44
delta_edge_2 = 80
sf_edge_2 = 0.5
#
# --- MESH ---
#
*INCLUDE
mesh.k
#
# --- FACE SETS ---
#
*INCLUDE
faces33.k
*INCLUDE
faces44.k
#
# --- MATERIAL ---
#
*MAT_RIGID
"armour"
1, 7.8e-9
*MAT_RIGID
"protected volume"
2, 1.0e-9
#
# --- PART ---
#
*PART
"tile 1"
1, 1
"tile 2"
2, 1
"protected volume"
3, 2
#
# --- VAA ---
#
*VAA
"testing face sets"
1
PS, 12, P, 3, 100, 50, 10
0, 0, 1, 0, 0, 1
200
*SET_PART
"armour"
12
1, 2
*TABLE
"material properties"
100
1, 30, 40, 0
*TABLE
"face set properties"
200
[%face_set_1], [%delta_edge_1]
[%face_set_2], [%delta_edge_2], [%sf_edge_2]
*END
Overview and face sets
Overview and face sets
Results with and without face set specific properties
Results with and without face set specific properties