Date of Award

8-2007

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Thompson, Lonny L

Committee Member

Vahidi , Ardalan

Committee Member

Biggers , Sherill B

Abstract

Computer aided engineering and finite element simulation are essential in order to predict accurately the safety performance of automotive structures in an event of crash. In this work, finite element simulation is used to evaluate the strength and deflection characteristics of a reference automotive front seat in an event of vehicle rear impact. Understanding the strength-deflection characteristics of front automotive seats during vehicle rear impact is important to ensure the safety function of the seat. The safety function is measured based on a moment test in accordance with government (ECE R17) regulations. Accurate finite element modeling of a reference seat which has passed ECE R17 moment test requirements is important to provide a benchmark reference to compare new concepts and designs which reduce weight while maintaining minimum test requirements.
In this work, simulation of the moment test of the reference seat is done beyond the component level by using a complete seat model formed by integrating the major structural components including the base frame, slider rails and backrest. The stamped sheet metal structural frames are represented in the finite element using shell elements. Contact within the seat structure is defined in order to simulate joints between side flanges and cross tube members of the base frame. Contact modeling is also used to simulate the interaction of a SAE J826 rigid body form with the backrest. Height adjustment, front tilt adjustment and the backrest tilt angle adjustment locking mechanisms are represented in the finite element model using rigid connections. The bolt connections between different parts in the model are represented using multi-point constraints. An elastic-plastic material is used to model the ductile steel structures. Different grades of steel with low, medium, and high ultimate strength are considered for the different components. In order to confirm the strength requirements, moment deflection characteristics of the seat are studied in accordance with ECE R17. The strength and deflection characteristics of the seat are obtained by simulating the quasi-static moment test in ABAQUS/Explicit using two complementary loading cases, constant horizontal force and constant angular velocity. For the moment test, simulated using constant horizontal force, the results show that the seat satisfies a maximum moment requirement and, at the end of the deformation travel, the developed moment is maintained above a minimum requirement. For the moment test simulated using the constant angular velocity, the maximum moment is not reached, yet after the end of deformation travel, the developed moment stays above the minimum.
Since the ECE R17 regulations do not provide precise specifications for the height of the applied force and test setup for the body form pivot mechanism, a study is conducted in order to understand the influence of body form rotation and height of the body form above the H-point, representing the physical pivot of the occupant hip area. The influence of plastic material properties of different grades of steel used for the seat model, front mesh contour on the backrest and connection between the backrest and connector are also modified to analyze the influence of load path on moment deflection characteristics of a seat. The moment test setup with increased distance of 430 mm between H-point and reference point of the body form shows higher strength for initial deflection and lower strength towards the end of the deformation path when compared to original length of 360mm. The moment test setup with body rotation about the reference point axis constrained shows higher strength for initial deflection and similar strength towards the end of the deformation path when compared to free rotation of the body form reference point axis. The front mesh without contour on the backrest decreases the strength of the seat below the requirement. Changing the ultimate strength of steel used on the major load bearing components does change the component stress, but shows only a small change in the moment deflection characteristics of the model. Using a high yield strength steel material for the connector increases the maximum moment that the seat can support

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