Four Noded 2D Seatbelt Shell Elements

Alternative Shells + 2D Seatbelt shells method.

It is also possible to mesh belts using the 2D Seatbelt Shell elements available in Ansys LS-DYNA 971.

This image shows the same example as above with the 1D seatbelt elements replaced with 2D Seatbelt shells, allowing existing shell element properties to be preserved but better geometry to be achieved at slipring and retractor locations.

• Shell elements across the dummy are still used, in exactly the same way as above.
• The seatbelt shell mesh is continuous through sliprings and into retractors.
• The belt mesh is realistic in shape and orientation where it passes through sliprings, especially at the shoulder, making sensible contact between belt, dummy and structure at these points achievable.
• Nodal rigid bodies between Shell and Seatbelt elements are no longer required since the mesh is continuous.
• Mesh ends, whether seatbelt or ordinary shells, which connect directly to structure are still given Nodal Rigid Bodies or, if the structure is rigid, made "extra" nodes on that part.

Note that PRIMER generates new Part, Section and Material properties for each isolated section of 2D Seatbelt shell mesh. (Here blue from retractor to shoulder, light blue at left hand side of pelvis through slipring.)

This is because Ansys LS-DYNA requires 2D Seatbelt shell meshes to be continuous and it does not "track across" any intervening stretches of ordinary shells, so two separate definitions are required in this example.

Alternative fully 2D Seatbelt Shell method.

PRIMER is also able to generate a mesh that is wholly made up of 2D Seatbelt shells.

Since 2D seatbelt shells are actually genuine shell elements the result is visually indistinguishable from the mixed shell / 2D seatbelt shell result above, except that only a single part / section / material definition is used because the stretch of elements is continuous.

Alternative fully conventional shell method.

PRIMER can also generate a belt that is wholly made up of conventional shell (ie not 2d belt) elements.

Prior to version 13 this has not been a practical proposition because there was no way of meshing sliprings with these elements, but now that Meshed (radiused) sliprings are available this meshing method may become useful. Retractors and pre-tensioners will have to be meshed by alternative means, perhaps by a short separate belt definition attached to the end of the shells by a nodal rigid body.

From version 18, with its introduction of Advanced fitting, it is practical to create a continuous mesh over the whole length of the belt using explicitly meshed sliprings as an alternative to the "Meshed" slipring type. This overcomes the limitations of "meshed" sliprings, notably that they are limited to semi-circular shapes. It also permits *MAT_FABRIC and *AIRBAG_REFERENCE_GEOMETRY to be used in order to eliminate the small belt path distortions that occur when meshing complicated geometrics.

The full choice of meshing methods available

The choice of meshing method is up to the user. We anticipate that users with existing 1D seatbelt element + shell belt definitions wishing to achieve better contact behaviour at slipring and retractor locations will convert the 1D belt sections to 2D seatbelt shells, leaving the intervening shell sections unchanged in order to retain consistent behaviour. Users creating new belt definitions may wish to make them entirely from 2D belt elements or conventional shells using a fabric material.

PRIMER V14 onwards permits an arbitrary mixture of element types.

From Version 14 onwards you can mesh each segment of the seatbelt with any permutation of 1d belt and/or 2d belt and/or shell elements.

Prior to V14 combinations of elements in a segment were limited

PRIMER permits existing *BELT definitions previously created in PRIMER to be refitted and/or remeshed in any of the following:

1. 1D seatbelt elements only
2. 2D seatbelt elements only
3. Conventional shells only
4. Mixed 1D seatbelt elements + shells
5. Mixed 2D seatbelt elements + shells

Options (1) is unlikely to be used in practice, but is provided for completeness.

Contact between belt and dummy.

In modern modelling practice it is likely that the belt will be included in a larger contact surface that includes dummy, vehicle, seat, steering wheel and - possibly - airbag.

However PRIMER can generate a separate contact between belt and dummy if required.

Two possible contact surfaces are offered

AUTOMATIC_SURFACE_TO_SURFACE For contact between belt shell elements, 2d seatbelt shells and the underlying structure.

AUTOMATIC_NODES_TO_SURFACE For contact between 1D seatbelt element nodes and the underlying structure.

Either or both may be omitted if unnecessary, and all parameters may be altered as required.

In PRIMER the various fields on the contact all have their default values, with the exception of the "soft" option which is set to 1. Experience has shown that the relative stiffnesses of belt and dummy may be very different, and the use of this "soft constraint" option can improve contact behaviour. 

Refitting a belt when the dummy moves.

If the dummy on which the belt has been fitted is subsequently moved, perhaps by Dummy or Mechanism positioning, then the belt can be automatically refitted to the new dummy position using the AUTO_REFIT function. In most cases no user intervention is required and the belt, in its current form, is remeshed to fit the new dummy position.

It can also be refitted manually using the conventional FIT process below.

In both cases any basic belt path points that are located at nodes will "know" about movement of those nodes, enabling the basic path itself to move in space so that it remains in much the same position relative to the repositioned dummy. Path points that are not at nodes will have their movement interpolated from adjacent points, so they too will move. However this only works if the number of path points not at nodes is not excessive, otherwise there is no information from which to interpolate.

In practice this means that belts can usually be refitted to adult dummies, where the "locate point at node and project it forwards" approach works well, but not on child-in-seat dummies where points usually have to be moved to locations in space in order to thread the path through obstacles.

Replacing one dummy with another

It is often the case that dummies need to be swapped around, for example replacing a 50th percentile dummy with a 95th percentile one.

To save having to delete the old belt definition and creating a new one from scratch PRIMER has a New Dummy capability. The process is not automatic, but it provides assistance with remapping the old belt path onto the new dummy, preserving as much information as possible.

The process works by looking for "nearest nodes" on the new dummy which match existing belt path points, so as long as the dummies are broadly similar in shape, and not wildly different in size, this process should be reasonably helpful. However if there are gross differences in size or shape it is likely that restarting the belt definition from scratch will be more efficient.

Saving belt definitions: the *BELT keyword (after *END)

PRIMER permits any number of belt definitions to be defined in a model, (cf: Dummies and Airbag Folding definitions), and these are written out as a *BELT block of keywords after the *END card of an Ansys LS-DYNA deck so that they are not lost between PRIMER runs. This also means that if a dummy is moved it is possible to go back to the seatbelt definition and to refit the belt to the new dummy position.

These data are added automatically to your model when you create a belt fitting definition. Details of the card formats used are given in Appendix E .