This blog is an attempt to help people understand and improve upon their knowledge of FEA - Finite Element Analysis and other Mechanical Engineering disciplines
FMVSS 202A Introduction
Note: “H/R” here should be read as “head restraint.”
This procedure is based on the FMVSS 202A. FMVSS 202A contains two compliance options: Static and Dynamic.
Here we would specifically addresses Section S4.2.7 of the static compliance option: ‘Backset retention, displacement,and strength.’
Energy absorption (S4.2.5), height retention (S4.2.6), and initial displacement at 37 N-m (S4.2.7(a)(1)) are not covered in this short procedure.
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Some requirements of 202A
a) Seat back shall be rigidly held during the backset retention (37 N-m) and displacement (373 N-m) portions of the test.
b) For adjustable seat backs, the seat back shall be adjusted to a position that results in a 25° torso angle.
c) Back set retention requirement is 13 mm, as compared to 10 mm in the proposed 202A.
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Short flow chart depicting procedure
1) Position Head Restraint
2) Position Seat Cushion
3) Install Back Form and Position Seat Back
4) Establish Displaced Torso Line(373 N-m moment on back form)
5) Create Seat Back Bracing
6) Install Head Form and Measure Non-Structural Displacements
7) Head Restraint Displacement Requirement (373 N-m moment on head form)
8) Head Restraint Set Requirement (Backset retention at 37 N-m)
9) Remove Seat Back Bracing
10) Head Restraint Strength Requirement (890 N head form load, minimum)
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Input Definition
Set-up information (seat position for 202A testing, 202A torso angle, seat back bracing)
Requirements (acceptance criteria)
Proper CAD.
Lower Structure modelling
Previous experience points to the fact that the lower structure is not the weak link during ultimate loading of the H/R.
Hence, it may be possible to remove some or all of the seat lower structure from the FE model without significantly affecting the results. This can improve convergence and reduce run time.
There is a video I found on youtube which explains this partly
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1. Position Head Restraint
FMVSS 202A states that if the H/R is adjustable, it shall be adjusted to a height position closest to and not less than 800 mm above H-point for front seats, and 750 mm above H-point for rear seats (measured in the direction of the design torso line). The full-up H/R position is usually tested.
Mark the location on the H/R STO where the head form load will later be
applied. Measure 65 mm below the top of the H/R STO, measuring along the 202A torso angle.
This is a change from the prior FMVSS 202 regulation, in which the 65 mm
measurement was made along the displaced torso angle established during the back form loading.
2. Position Seat Cushion
If the seat cushion adjusts independently of the seat back, it shall be adjusted to the highest Hpoint position. Even if the cushion structure is not be modeled in the FEA, the H-point from this full-up position is still needed.
NOTE: There is no designated track position for 202A testing as per FMVSS. There may not be a clear “worst case” track position for a seat, because what’s worst for the H-point prediction portion of 202A might not be worst for the loading portion of 202A (both portions of the test must be done in the same position).
Full-rear is commonly assumed. (this need not be the case always)
3. Install Back Form and Position Seat Back
For adjustable seat backs, this 202A seat back position is intended to result in a 25° torso angle. This does not apply to non-adjustable seat backs.
FMVSS 202A requires that an adjustable seat back be adjusted to a position that produces a 25° torso angle.
Rotate the back form fixture (including torso line) until the torso line is at the specified 202A torso angle minus one degree (i.e. – one degree more upright). The back form is placed at an angle slightly more upright than the design torso angle.
Translate the back form to the 202A H-point.
Rotate the back form fixture (including torso line) until the torso line is at the specified 202A torso angle minus one degree.
4. Establish Displaced Torso Line
Constrain the back form rigid body at H-point in all DOF’s except rotation about the cross-car axis.
Create a duplicate copy of back form elements and rotate them rearward about H-Point until they first contact the seat frame.
Calculate 50% of the angle. The result is referred to as ΦTorsoFoam, and represents the contribution of foam crush to the displaced torso angle.
Record the angle ΦTorsoFoam as the ‘Rotation due to seat back foam crush, 373 N-m’ component of the displaced torso angle in the results summary.
Rotate all elements in the back form assembly except the duplicate elements (i.e. – rotate the original back form elements and the torso line) rearward about H-point by an angle equal to ΦTorsoFoam.
The resulting location of the torso line relative to the H/R is critical to 202A, because it determines where measurement of the head form displacement begins during head form loading. The more forward the torso line is relative to the H/R (assuming a forward facing seat), the higher the head form displacement will be.
Apply a 373 N-m moment about H-point to the back form rigid body.
Per the FMVSS 202A regulation, create the moment by applying a follower force (*CLOAD, FOLLOWER) along a vector that is perpendicular to the displaced torso line and is at a height of 290 mm above H-point.
The angle of rotation of the back form assembly (torso line) under the 373 N-m moment is referred to as ΦTorsoStructural.
Record the angle ΦTorsoStructural as the ‘Rotation due to structural deformation, 373 N-m’ component of the displaced torso angle in the results summary.
Add ΦTorsoFoam and ΦTorsoStructural to the initial torso procedure, ΦTorsoInitial. The result is the displaced torso angle, referred to as ΦTorsoDisp.
Record this angle as the ‘DISPLACED TORSO ANGLE’ in the results summary.
5. Create Seat Back Bracing
6. Install Head Form and Measure Non-Structural Displacements
Re-position the H/R STO CAD geometry so that it is correctly aligned with the displaced H/R frame mesh. Note that the displaced torso line may or may not penetrate the displaced H/R STO. The head form loading point on the STO geometry, should move along with the H/R STO.
Translate the head form shell mesh, beam element, and local coordinate system until they are roughly in front of the H/R. Align them in the cross-car direction with the H/R centerline.
Rotate the head form beam element and local coordinate system about the head form center until they are perpendicular to the displaced torso line.
Translate the head form shell mesh, beam element, and local coordinate system until the head form surface is just tangent to the head form loading point on the STO geometry. The head form beam element in this position represents the head form loading axis.
Translate the head form shell mesh along the head form loading axis (i.e. – along the 1-D beam element) until the head form sphere is just in contact with the displaced torso line. The displaced torso line may be inside or outside of the displaced H/R STO.
Translate the head form shell elements along the head form loading axis until they are just in contact with the displaced H/R STO.
This translation is referred to as D2TorsoToSTO. The term D2 to represent the head form displacement under 373 N-m, relative to the displaced torso line). As defined in this procedure, D2TorsoToSTO represents the gap (or
penetration) between the displaced torso line and the H/R STO after the 373 N-m back form moment has been applied to the seat and prior to applying any load to the head form.
Record the value D2TorsoToSTO as the ‘Air gap (between displaced torso line and STO)’ component of the head form displacement in the results summary.
Enter a positive value for D2TorsoToSTO if there is a gap between the displaced torso line and the H/R STO after the back form loading. In this case, D2TorsoToSTO will increase the total head form displacement.
Confirm that the head form shell elements are just in contact with the displaced H/R STO. Translate the elements along the head form loading axis until they first contact the H/R frame. This translation
represents the H/R foam thickness, referred to as TFoam in this procedure.
The contribution of H/R foam (and insert) crush to head form displacement at 373 Nm is referred to as D2Foam in this procedure. This foam displacement prediction must be provided by the project engineer. With the variety of H/R foam and inserts being used to meet 202A, it is more practical to predict foam displacement (and set) based on test data than by FEA.
Record the value of D2Foam in the results summary.
7. Head Restraint Displacement Requirement
Import the positioned head form.Make sure the head form reference node (at the center of the head form) is assigned to the head form local coordinate system. Constrain the reference node in all six D.O.F., including translation along the head form loading axis.
The translation of the head form from zero load to 37 N-m is referred to as D1Structural in this procedure.
This procedure does not propose an assumption for the amount of foam crush
during loading from 0 to 37 N-m, since it likely would account for the majority of the D1 displacement at this low load level.
Head form translation from zero load to 373 N-m is referred to as D2Structural.
Record the value D2Structural as the ‘Structure (H/R rod, H/R bracket, back frame)’component of the head form displacement in the results summary.
Add D2TorsoToSTO, D2Foam, D2sleeves, and D2Structural to arrive at the total head form displacement prediction at 373 N-m, D2. Record the value of D2 as ‘TOTAL HEAD FORM DISPLACEMENT’ in the results summary.
8. Head Restraint Set Requirement (Backset Retention)
The contribution of H/R foam (and insert) to permanent set at 37 N-m is referred to as DTfoam in this procedure.
Record the value of DTfoam as ‘H/R foam (and insert, if applicable)’ in the results summary.
10. Head Restraint Strength Requirement
Here seat back is no longer braced; back form remains locked.
Load the head form to failure by applying a force or enforcing a displacement to the head form reference node.
RBE2 and RBE3 are multi point constraint elements, but Rigid and RBE2 elements add stiffness to original structure while RBE3 does not.
RBE2 elements also distribute the force and moment equally among all connected nodes, irrespective of position of force or moment. RBE3 element is constraint equation to distribute force and moment as per the distance (least aquare weighted function).
Elform 2 is a fully integrated model, so there is a tendency for negative volume in soft materials due to crushing. Elform 1 is a single integration model. As the integration point is in the middle of the element, even if the crossing over of nodes of one edge over the other may cause a physical negative volume but for all calculations it is possible that the centre node of the element is still on the +ve side and hence negative volume will be avoided. In case of excessive compression or warping, this may also become a negative volume.
