Date of Award

8-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Thompson, Lonny L

Committee Member

Biggers , Sherrill B

Committee Member

Li , Gang

Abstract

ABSTRACT
With the goal of reduced weight, free-size finite element based optimization with constraints on stresses and displacements of a commercially available automotive seat backrest frame manufactured from several stamped and welded low carbon ductile steel sheets is performed using OptiStruct linear optimization package from Altair under the loading requirements of mandatory ECE R-17 backrest-moment test and headrest test for vehicles sold in Europe. In the free-size optimization, sheet metal thickness in a finite element shell model of the backrest frame are design variables with stress and displacement limits as the constraints, with an objective to minimize mass. Using the results from the free-size optimization and also by conforming to a minimum draw-able sheet metal thickness, a final design is derived which obtains a total mass reduction of 15.2%. To verify the functional performance of the final design, a non-linear finite element analysis including an elastic-plastic material model and geometric nonlinearity (large displacements) of the reference seat and the final optimized seat backrest frame is performed using the ABAQUS/Standard finite-element package. Results from the nonlinear analysis provide an accurate prediction of the material yielding and load path distribution on the backrest frame components during the ECE R-17 test loads for the backrest and headrest test and provide factor of safety estimates on yield and ultimate strength. Conservative load cases replicating the ECE R-17 backrest moment test and headrest test are applied as pressure loads on the upper support member of the backrest frame. The headrest load is applied as an equivalent force-couple on the supporting holes of the backrest frame. The ECE R17 loading for the headrest test is applied in two steps, first a moment is developed from the pressure load on the upper frame; in the second step, the force-couple is applied at the holes supporting the headrest. In an initial study, restraints at the bottom of the frame are applied at the connecting sleeve to the base-frame connector. In a second study, the connector part is included and tied to the frame model at the bearing interfaces and bolt connections.
Further investigation of the load path and application of forces for the ECE R17 load requirements is performed. In this analysis, ABAQUS/Explicit is used in a quasi-static simulation of the ECE R17 headrest test with the nonlinear finite model of the final optimized backrest frame, this time covered with a three-dimension solid PU foam material modeled with Hyperfoam properties in ABAQUS. The backrest moment load is applied in a more realistic load path by modeling a rigid body form making contact with the backrest in the upper region of the backrest foam draped across the frame whereas the headrest moment load is applied conservatively. The result of this simulation shows the acceptable performance of the final optimized seat under increased non-linearity in terms of contact and modeling of the foam material in the seat.
The significantly increased modeling and computer time required in simulations and analysis using solid finite elements with hyperfoam material for the accurate modeling of PU foam geometry lead to the question as to whether simplified shell foam models could be used with decreased computational time and cost, which approximates the behavior of the full three-dimensional, nonlinear crushing and expansion behavior of the seat back foam during the backrest loading with the body form. To answer this question, a study is performed to determine a suitable 3D shell surface replacement for the solid foam model. A series of non-linear quasi-static simulations are performed by varying the thickness of an equivalent shell surface, comparing both the PU foam material and an elastic material replacement, and also by varying the position of placement of the body form in front of the 3D shell surface representing the contact surface of the backrest foam. Backrest moment about the H-point and the deflection of the top-most point on the backrest frame is considered as an agreement criterion and a suitable replacement for the 3D solid foam model is suggested.

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