# Airbag Fabric Porosity

Porosity of airbag material is modeled by the addition of a porosity card /LEAK/MAT to /MAT/LAW58 card.

```
#---1----|----2----|----3----|----4----|----5----|----6----|----7----|----8----|----9----|---10----|
/MAT/LAW58/1
Altair test fabric law58
# Init. dens. Ref. dens.
8E-7 0
# E1 B1 E2 B2 FLEX
0.380 0 0.380 0 1.0
# G0 GT AlphaT Sensor
0.0035 0.0055 7.175 1
# Df Ds Gfrot ZeroStress
0 0 0 1
# N1 N2 S1 S2
1 1 0 0
# Dir 1
500 1.00
# Dir 2
501 1.00
# Dir12
502 1.00
# FCT_ID4 FCT_ID5 Fscale4 Fscale5 FCT_ID6 Fscale6
600 600 1 1 502 1
/LEAK/MAT/1
Fabric
# Ileak AscaleT AscaleP
1
# AC' fct_IDAC’
# LC AC
1.0e-03 1.0e-03
#---1----|----2----|----3----|----4----|----5----|----6----|----7----|----8----|----9----|---10----|
```

The /LEAK/MAT specifies how the effective porous area of a
component having this material will vary as a function time or pressure. It is
determined by the value of `Ileakage`.

`Ileakage`=1:

`Ileakage`=2:

`Ileakage`=3:

`Ileakage`=4:

`Ileakage`=5:

$\text{\Delta}P=P/{P}_{ext}-1$

Where, ${\lambda}_{1}$ and ${\lambda}_{2}$ are the stretches in warp and weft directions and ${\alpha}_{12}$ is the angle between warp and weft directions.

`Ileakage`=6:

with ${r}_{s}=Are{a}_{n}/Are{a}_{0}$ and ${r}_{p}={P}_{ext}/P$ .

Parts with this material are considered porous, when referenced via a surface ID
using the /MONVOL/FVMBAG1
`surf_ID`_{ps} option.

```
#---1----|----2----|----3----|----4----|----5----|----6----|----7----|----8----|----9----|---10----|
/MONVOL/FVMBAG1/1
DAB
# Isur
4
# Scal_T Scal_P Scal_S Scal_A Scal_D
0 0 0 0 0
# Mat_ID Mu Pext Tphi Iequi Ittf
4 0.1 0.00010135 0 0 3
# Njet
1
# Inj_ID Isensor surf_IDinj
10 1 100
# Nvent Nps
0 1
#Sur_id_ps Iform_ps
4 1
# Tstart Tstop dPdef DtPdef ldtpdef
….
#---1----|----2----|----3----|----4----|----5----|----6----|----7----|----8----|----9----|---10----|
```

`Iform`

_{ps}and has the following formulations:

`Iform`_{ps}= 1 ${\dot{m}}_{\mathit{out}}={A}_{\mathit{eff}}\sqrt{2P\rho}{Q}^{\frac{1}{\gamma}}\sqrt{\frac{\gamma}{\gamma -1}\left[1-{Q}^{\frac{\gamma -1}{\gamma}}\right]}$ (Isentropic - Wang Nefske)`Iform`_{ps}= 2 ${\dot{m}}_{\mathit{out}}={A}_{\mathit{eff}}\rho v(P-{P}_{\mathit{ext}})$Where,

`v`is the outflow gas velocity (Chemkin)`Iform`_{ps}= 3 ${\dot{m}}_{\mathit{out}}={A}_{\mathit{eff}}\sqrt{2\rho (P-{P}_{\mathit{ext}})}$ (Graefe)

`Iform`

_{ps}=2 option is normally used to represent fabric porosity. The outflow gas velocity as function of airbag overpressure comes from test data.