This describes seat-belt function and modelling its components like retractor, pretensioner, seatbelt webbing, sliprings, their material and contact.

1. EVERYTHING ABOUT
SEATBELTS
FUNCTION & MODELLING – LS DYNA
AKSHAY MISTRI
WAYNE STATE UNIVERSITY

2. CONTENTS Slide #
Chapter – 1 General Information 3 – 8
Chapter – 2 Modelling
• Getting the model ready (10 - 15)
9 - 35
2.1 Belt Modelling 16 – 21
2.2 Retractor/ Pretensioner Modelling 22 – 28
2.3 Slipring Modelling 29 – 31
2.4 Belt Material Modelling 32 – 35
Miscellaneous
• Modelling zero length elements in retractor
• Seatbelt contact definition
36 - 38

3. 1. GENERAL INFORMATION

4. WHY YOU NEED THEM?
• They spread crash forces across stronger parts of the
body to minimize injury.
• Force is not concentrated in a small area, so injury is
reduced.
• Belt mechanism locks up during a collision event thus
restraining the passenger.
This Photo by Unknown Author is licensed under CC
BY-NC

5. SEATBELT SYSTEM
Image Credits
Creator: Chang Hyun Baek
Information extracted from IPTC Photo Metadata
• Consists of:
• Seatbelt webbing
• Retractor mechanism
• Pretensioner
• Buckle
• Load limiters

6. RETRACTOR
• Keeps belt taut always.
• Locks the webbing spool out in two ways:
• Triggered by car’s sudden acceleration / deceleration:
1. Swinging pendulum locks the gear attached to the spool.
• Triggered by belt’s quick movement:
2. Like a centrifugal clutch. Fast spool rotation drives the lever attached on
it which locks the spool.
Image Credits
https://auto.howstuffworks.com/car-driving-safety/safety-
regulatory-devices/seatbelt3.htm
1
2

7. PRETENSIONER
• Tightens up slack in the belt webbing in the event of a
crash.
• This helps secure the occupant in the seat.
• Mechanism:
• When a collision is detected, an electrical signal generates a
spark. This spark ignites a combustible gas inside the
pretensioner which drives a piston upward as shown, which
rotates the spool.
• Some pretensioners pull the whole retractor mechanism
while some rotate the retractor spool to remove slack in the
webbing.
Image Credits
https://auto.howstuffworks.com/car-driving-safety/safety-
regulatory-devices/seatbelt3.htm

8. LOAD LIMITERS
• Load limiters reduce belt inflicted injuries by limiting the force belt webbing can induce on the
occupant.
• Earlier, load limiters had stitching in the webbing which used to break when the peak load is
reached.
• Advanced load limiters rely on torsion bars which twists and allows the belt to extend a bit
further.

9. 2. MODELLING

10. SEATBELT SYSTEM - SETUP
Slip ring
1
Slip ring
1
Slip ring
2
Slip ring
2
Anchor
Anchor
Anchor –
Lap Belt
Anchor –
Lap Belt
Retractor,
Pretensioner
– Pulls
shoulder belt
Retractor,
Pretensioner
– Pulls
shoulder belt
Actual setup Model setup
Image Credits
http://howcarpartswork.blogspot.com/2016/05/seat-belt-working.html

11. BEFORE WE BEGIN MODELLING
• I have tried this belt setup on a sled model developed by Livermore Software Technology (LSTC).
• Due to sled environment, belt will be designed with the help of 4 beams.
• These 4 beams represent vehicle body points about which the actual belt would be constrained (slip
rings) / fixed (anchor points).
• If you are modelling the belt in an actual vehicle model, these 4 beams might not be needed. You
can use nodes on the vehicle.

12. SEATBELT SYSTEM – MODEL SETUP
Slip ring
2
Lap Belt
anchored on
Beam 2
Retractor,
Pretensioner
on Beam 1Beam 4
Slip ring
1
Beam 3
Beams 1,
2
Vehicle Part
Seatbelt System
Part
All the 4 beams are constrained to vehicle via
*CONSTRAINED_RIGID_BODIES
Zone 1
Zone 2
Zone 3

13. GETTING THE MODEL READY: ZONE 1
• First, we need to setup node ID’s in the three zones as
shown in previous slide.
• These node ID’s we create will help us to create belt and
related components easily.
• For zone 1:
1. Create a node with ID 4, to be taken as the anchor node for lap
belt. This is where the belt ends and can belong to vehicle
component.
2. Create coincident nodes 1 and 11. ID 1 will be reserved for
defining retractor and will belong to the rigid beam while ID 11
(not belonging to any part) will be the starting node for the belt.
Node ID: 4
Node ID’s: 1, 11
Zone 1

14. • For zone 2:
1. Create coincident nodes 2 and 21.
2. Node ID 2 will be taken as the node for defining the slip ring and belongs to the beam.
3. ID 21 will be reserved as the common node between the belt elements 1 and 95 as shown.
Zone 2
Node ID’s: 2, 21
Beam
GETTING THE MODEL READY: ZONE 2

15. • For zone 3:
1. Create a node with ID 5, to be taken as the anchor node on the vehicle. Between nodes 5 and 3 is the belt part
which contains the buckle.
2. Create coincident nodes 3 and 31. ID 3 will be reserved for defining slip ring and will belong to seatbelt element
123 (part of the buckle) as shown. While ID 31 will be common to the seatbelt elements 16 and 96.
Node ID: 5
Node ID’s: 3, 31
Zone 3
Zone 3 – Zoomed view
Node ID’s: 3, 31
GETTING THE MODEL READY: ZONE 3

16. Modelling of the belt
Image Credits
http://howcarpartswork.blogspot.com/2016/05/seat-belt-working.html
2.1 BELT MODELLING

17. BELT CREATION
• Belt can be created from page 5, BeltFit option in LS-
PrePost.
• Choose “Simple” option.
• Check on Pick nodes and enter the two node ID’s as
shown.
• Click on “Fit” and “Accept”.
• This will create the 1D belt as shown.

18. BELT CREATION – SHOULDER BELT
• To create shoulder belt, click on “Mixed”.
• Also, do select the Set segment for shoulder belt which
helps to create the belt.
• Check on Pick nodes and enter the node ID 21 as
shown.
• After that, select some nodes on the dummy as shown.
• Again, enter the node ID 31 and press enter to select
the last node.
• Click on “Fit”, “Stretch” and “Accept” to create the belt
as shown below.
Node ID: 31
Node ID: 21

19. BELT CREATION – LAP BELT
• To create lap belt, keep the “Mixed” setting on.
• Also, do select the Set segment for lap belt.
• Check on Pick nodes and enter the node ID 31 as
shown.
• After that, select some nodes on the dummy as shown.
• Again, enter the node ID 4 and press enter to select the
last node.
• Click on “Fit”, “Stretch” and “Accept” to create the belt
as shown below.
Node ID: 31
Node ID: 4

20. BELT CREATION – ANCHOR/BUCKLE BELT
• To create this portion of the belt, keep the “Simple”
setting on.
• Check on Pick nodes and enter the node ID’s 3 and 5 as
shown.
• Click on “Fit” and “Accept” to create the belt as shown
below.
Node ID: 3
Node ID: 5

21. BELT ORGANIZATION
• Belt segments need to be organized into same components to model their behavior.
• This can be done on page 2, Movcpy.
• Now, you can click on belt segments as shown.
• Check mark on “Pick Target Part” and select the target belt component.
• Then hit Apply.
Segments
selected
Target part
Segments
selected
Target part

22. Modelling of retractor
and pretensioner.
Image Credits
http://howcarpartswork.blogspot.com/2016/05/seat-belt-working.html
2.2 RETRACTOR / PRETENSIONER
MODELLING

23. CREATING SENSORS
• Sensors need to be created to trigger pretensioner and retractor.
• This can be done on page 3, *Elem_Seatbelt_Sensor.
• Click on NewID with sensor type “SBSTYP” to be 3, triggered with time.
• I have created 1st sensor for retractor with trigger time of 1 ms, while the second one
with 13 ms for pretensioner.
Sensor type

24. CREATING RETRACTOR
• This can be done on page 3, *Elem_Seatbelt_Retractor.
• Click on NewID with retractor node ID to be 1.
• Belt element just outside the retractor and sensor created in the last slide also need
to be referenced.
Retractor Node ID Belt element just
outside retractor Sensor ID

25. RETRACTOR: LOADING &
UNLOADING
• Retractor will lock up at 1 ms according to our sensor trigger.
• That means, after 1 ms, belt spooling will not lock up but instead spooling out of the belt will be according to
Force-Deflection (payout; kN vs mm) curve.
• There is a steep rise initially in the curve which will stiffen up the belt payout quickly.
• As shown, the max value of 3.25 kN, retractor will payout the belt at a constant rate which will avoid injuries to
the occupant due to too stiff belt.
• Please note that unloading curve specified is same as loading curve.
Load curve

26. RETRACTOR: LOADING CURVE
MODIFICATION
• The loading curve shown in the last slide starts will 0, 0 and rises up constantly.
• That means belt payout is 0 mm for 0 kN and payout follows the initial rise of the curve.
• However, if the curve starts from a positive value, say 0.1 kN for 0 mm payout then retractor will lock up
and spool in the belt till 0.01 kN force is generated in the belt.
• This can help in situations where slack in the belt (outside retractor) is to be removed.
100 N

27. CREATING PRETENSIONER
• This can be done on page 3, *Elem_Seatbelt_Pretensioner.
• Click on NewID with pretensioner type to be 5.
• Sensor 2 created for pretensioner trigger along with retractor ID need to be referenced
here.
• Similar to retractor, we need to specify a pull in – time curve (PT LCID) to model the
pretensioning effect.
Pretensioner Type
Sensor ID
Retractor ID

28. PRETENSIONER VS RETRACTOR
• Actual scenario that occurs in a crash event:
• Pretensioner acts before the retractor takes over / locks up.
• Pretensioner pulls the belt and reels it back into the retractor following the pull in – time curve, thus
increasing forces and removing any slack in the belt.
• This pull in by the pretensioner is carried on by it until the limiting force is reached, where the
retractor comes into action.
• Retractor then locks up and belt pays out as per the load curve specified for the retractor.

29. Modelling of the sliprings
Image Credits
http://howcarpartswork.blogspot.com/2016/05/seat-belt-working.html
2.3 SLIPRING MODELLING
Modelling of the sliprings

30. CREATING SLIPRING
• This can be done on page 3, *Elem_Seatbelt_Slipring.
• Click on NewID and reference the two seatbelt elements on the either sides of the
slipring.
• Slip ring node ID given is 2. This should not belong to the seatbelt element.
• In our case, it belongs to a rigid beam.
Seatbelt elements
Node IDFriction
Beam

31. CREATING SLIPRING
• Similarly, we need to create a slipring on the buckle end.
• This time the slipring node ID is 3 which belongs to the seatbelt element 112 as shown.
• Seatbelt elements 95 and 96 are on the either side of the ring.
Seatbelt elements
Node IDFriction

32. Belt material modelling
Image Credits
http://howcarpartswork.blogspot.com/2016/05/seat-belt-working.html
2.4 BELT MATERIAL MODELLING

33. MATERIAL / SECTION ASSIGNMENT
• For 1D belt parts
• Section can be defined using *Section_Seatbelt.
• For material, *Mat_Seatbelt (B01) can be used.
• Load curves referenced model the force vs strain data for the belt elements.

34. MATERIAL / SECTION ASSIGNMENT
• For 2D belt parts
• Section can be defined using *Section_Shell.
• For material, *Mat_Fabric (034) can be used.
• Load curves referenced model the stress vs strain data for the belt elements in longitudinal and transverse
direction.

35. BELT FABRIC:
LONGITUDINAL VS LATERAL DIRECTION
• Go to page 2 > El edit > Direction (under Element). This will show
the seatbelt element direction which is also the longitudinal
direction of the belt fabric.
• Here, the geometrically lateral direction (arrow direction) of the
belt fabric will be considered as the longitudinal direction (EA,
LCA) of the belt during material assignment.
• So, EA and LCA in material assignment will correspond to the
lateral direction of the belt.
• Curves and properties obtained from physical tests on belts
for lateral direction should be applied for longitudinal
direction in this current condition.
• This confusion can be avoided if during section definition 90 degree
offset is given to the longitudinal axis of the belt.
• Provide ICOMP value of 1.
• Provide the value of 90 in Bi as shown below.
• Now, the longitudinal axis of the belt will coincide with “A”
direction values during material assignment.

36. • Modelling zero length seatbelt
elements inside retractor.
• Contact definitions for seatbelt.
Image Credits
http://howcarpartswork.blogspot.com/2016/05/seat-belt-working.html
2.5 MISCELLANEOUS

37. BELT CONTACT
• Contact definition between the belt and the dummy can be given using Nodes_To_Surface option.
• For shoulder belt (Slave: part), contact needs to be established with neck and torso (Master: part set) of the dummy.
• Similarly contact can be given between the lap belt and the pelvis of the dummy.

38. ELEMENTS INSIDE RETRACTOR
• Zero length elements need to be modelled inside the retractor, so it could pay out the belt during the analysis if needed.
• Here, element just outside the retractor has the ID 1.
• To create zero length elements we will create elements as shown below. These elements need to have referenced with
retractor ID.
• Please note that element ID 1 does not to have retractor ID referenced with it.
• Also node ID’s for these zero length elements will have the same coordinates.