Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

Oasys LS-DYNA Environment: User Guide (Version 8.1)
Note:
! The parts affected by jetting are listed in a part set. Only the include the front face of the
airbag—otherwise the rear face will be pushed out instead.
! The jet focal point needs to be some distance away from the airbag, to avoid high forces being
concentrated on a single element.
! The direction from the focal point to the jet head defines the direction of the jetting forces.
! The angle is the cone semi-angle in radians.
CALIBRATION OF JETTING AIRBAGS
The jetting force is calculated automatically from thermodynamic equations. These do not take
account of the form of the orifice and many other factors. The total force applied by the model
jet must be calibrated to allow for this. Typical drivers-side airbag inflators have peak reactions
around 0.5 to 2kN.
The jetting force is not output directly by LS-DYNA it must be found using a separate model.
Use the same mass flow curve and thermodynamic data but double the size of the bag to avoid
developing significant overpressure. Apply the jet to a rigid back face, supported on stiff springs.
Sum the forces in the springs using T/HIS. This is the jetting force. The “efficiency factor” in the
model should then be scaled so that the peak force is correct.
Jetting can alter the way in which an airbag deploys, especially during the initial stages of
inflation However, it has a minimal effect on the airbag pressure time-history.
Spring 1E4 N/mm
F
B
A
2 x actual diameter
JET
Airbag = Parts 1 + 2 Jetting onto Part 2
t
fabric airbag
(Part 1)
These elements made
rigid (part 2)
A = Sum of spring forces
B = Actual reaction force peak
=> Factor = B/A
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