Fundamentals of Automobile Body Structure Design 9780768047684, 9780768021691, 2010041059

Citation preview

Fundamentals of

Automobile Body Structure Design

by Donald E. Malen

Fundamentals of

Automobile Body Structure Design

Other SAE books of interest

The Passenger Car Body: Design, Deformation Behavior, Accident Repairs by Dieter Anselm (Product Code: R-307) Advanced Vehicle Technology, Second Edition by Heinz Heisler (Product Code: R-337) Handbook of Automotive Engineering Edited by Herman Braess and Ulrich Seiffert (Product Code: R-312)

For more information or to order a book, contact SAE International at 400 Commonwealth Drive, Warrendale, PA 15096-0001 USA; phone 877-606-7323 (U.S. and Canada only) or 724-776-4970 (outside U.S. and Canada); fax 724-776-0790; email [email protected]; website: http://books.sae.org

Fundamentals of

Automobile Body Structure Design By Donald E. Malen

Warrendale, Pennsylvania USA

Copyright © 2011 SAE International

eISBN: 978-0-7680-4768-4

400 Commonwealth Drive Warrendale, PA 15096-0001 USA E-mail: [email protected] Phone: 877-606-7323 (inside USA and Canada) 724-776-4970 (outside USA) Fax: 724-776-1615 Copyright © 2011 SAE International. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, distributed, or transmitted, in any form or by any means without the prior written permission of SAE. For permission and licensing requests, contact SAE Permissions, 400 Commonwealth Drive, Warrendale, PA 15096-0001 USA; e-mail: [email protected]; phone: 724-772-4028; fax: 724-772-9765. ISBN 978-0-7680-2169-1 SAE Order No. R-394 Library of Congress Cataloging-in-Publication Data Malen, Donald E. Fundamentals of automobile body structure design / by Donald E. Malen.     p. cm. “SAE order no. R-394.” ISBN 978–0–7680–2169–1 1.  Automobiles—Bodies—Design and construction.  I. Title. TL255.M255 2011 629.2´6—dc22 2010041059 Information contained in this work has been obtained by SAE International from sources believed to be reliable. However, neither SAE International nor its authors guarantee the accuracy or completeness of any information published herein and neither SAE International nor its authors shall be responsible for any errors, omissions, or damages arising out of use of this information. This work is published with the understanding that SAE International and its authors are supplying information, but are not attempting to render engineering or other professional services. If such services are required, the assistance of an appropriate professional should be sought. To purchase bulk quantities, please contact: SAE Customer Service E-mail: [email protected] Phone: 877-606-7323 (inside USA and Canada) 724-776-4970 (outside USA) Fax: 724-776-1615 Visit the SAE Bookstore at http://store.sae.org

Table of Contents Chapter 1 - The Automobile Body........................................................... 1 1.1 1.2 1.3 1.4 1.5

Description of the Automobile Body Types................................................. 3 Body Nomenclature........................................................................................ 5 Body Mass Benchmarking............................................................................ 10 The Body Structure as a System.................................................................. 11 Note on Design Philosophy......................................................................... 12

Chapter 2 - Body Structural Requirements.............................................13 2.1 2.2 2.3 2.4

Categories of Structural Requirements....................................................... 14 The Locate and Retain Function.................................................................. 18 Locate and Retain for Front Suspension Attachment Structure............. 21 Flow Down of Requirements from Vehicle-Level Functions.................. 31

Chapter 3 - Automotive Body Structural Elements..................................37 3.1 3.2 3.3 3.4 3.5 3.6 3.7

Overview of Classical Beam Behavior........................................................ 38 Design of Automotive Beam Sections......................................................... 43 Torsion of Thin-Wall Members.................................................................... 52 Thin-Wall Beam Section Design in Automobiles...................................... 72 Buckling of Thin-Walled Members............................................................. 79 Automobile Body Panels: Plates and Membranes.................................. 102 Summary: Automotive Structural Elements............................................ 118

Chapter 4 - Design for Body Bending...................................................121 4.1 4.2 4.3 4.4 4.5

Body Bending Strength Requirement....................................................... 122 Body Bending Stiffness Requirement....................................................... 126 Internal Loads During Global Bending: Load Path Analysis................ 131 Analysis of Body Bending Stiffness.......................................................... 138 Principles of Good Joint Design................................................................ 150

Chapter 5 - Design for Body Torsion.....................................................163 5.1 5.2 5.3 5.4 5.5

Body Torsion Strength Requirement......................................................... 164 Body Torsion Stiffness Requirement......................................................... 165 Internal Loads During Global Torsion: Load Path Analysis................. 169 Analysis of Body Torsional Stiffness......................................................... 181 Torsional Stiffness of Convertibles and Framed Vehicles...................... 196

Chapter 6 - Design for Crashworthiness...............................................215 6.1

Standardized Safety Test Conditions and Requirements....................... 216

v

Table of Contents cont. 6.2 6.3 6.4 6.5

Front Barrier................................................................................................. 217 Side Impact................................................................................................... 242 Note on Rear Impact................................................................................... 252 Note on Roof Crush..................................................................................... 254

Chapter 7 - Design for Vibration..........................................................257 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

First-Order Vibration Modeling................................................................ 258 Source-Path-Receiver Model of Vibration Systems................................ 262 Frequency Response of a Single-Degree-of-Freedom System............... 266 SDOF Models of Vehicle Vibration Systems........................................... 272 Strategies for Design for Vibration............................................................ 287 Body Structure Vibration Testing.............................................................. 289 Modeling the Body Structure Resonant Behavior..................................290 Vibration at Frequencies Above the Primary Structure Modes............ 296 Note on Use of Rotating Phasors to Solve Damped Vibration Problems...................................................................................... 312 7.10 Note on Mechanical Impedance Technique............................................. 314

Chapter 8 - Design for Vehicle and Styling Integration..........................319 8.1 8.2 8.3 8.4

Designing the Best Body Structure...........................................................320 Vehicle Layout.............................................................................................. 320 Exterior Body Surface................................................................................. 332 Constraints on Body Structure from Vehicle Layout and Exterior Surface............................................................................................ 337 8.5 Body Structure Topology............................................................................ 340 8.6 Load Path Design at Suspension Attachments........................................ 345 8.7 Summary....................................................................................................... 354

Chapter 9 - Material Selection and Mass Estimation in Preliminary Design............................................................................357 9.1 9.2

Materials for the Body-in-White................................................................ 358 Preliminary Mass Analysis......................................................................... 373

Appendices.......................................................................................387 A. Exercises........................................................................................................ 387 B. Nomenclature............................................................................................... 437 C. English & Metric Units & Typical Values for Key Parameters.............. 441

vi

Dedication This book is dedicated to my wife Char, who shared in the long hours required to bring this book to fruition.

vii

Acknowledgments The author is indebted to several individuals who significantly influenced the trajectory of his professional life. First, a very special thanks to Professor Noboru Kikuchi of the University of Michigan, Department of Mechanical Engineering, for his guidance on my new career as an adjunct faculty member, and his collaboration in developing the course ME513-Fundamentals of Auto Body Structure, on which this book is based. His contributions and suggestions were invaluable. Mr. William Elliott encouraged me to pursue the discipline of automobile body engineering as a new engineer at General Motors. His wise and steady mentoring at the onset of my career was greatly appreciated. Mr. Klaus Winkelmann of GM took a great risk by sending an engineer at mid-career back to school, and I am grateful for that opportunity. Finally, deep gratitude is extended to my colleagues in the Advance Product Engineering Structures Group at General Motors. It was an honor to work with this competent and creative group. Thanks to Professor Raghu Echempati of Kettering University, who offered many valuable comments on early drafts of this manuscript, and to Martha Swiss of SAE International, who patiently guided me through the editing process. Appreciation is also extended to several organizations who have given permission to use their materials in this text. These include the American Iron and Steel Institute, General Motors Corporation, the Motor Industry Research Association, and the SAE International. Portions of materials contained herein have been reprinted with the permission of General Motors Corporation, Service and Parts Operations, License Agreement #0910987.

ix

Preface The objective of this book is to give the reader an understanding of the behavior of auto body structural elements, and to provide insight into how changing design parameters will influence this behavior. We seek to answer questions such as: What is the best way to shape a structural element to achieve a desired function? What is the behavior of the structure relative to required performance? Why does it behave in this way? How can the performance be improved? If we have the ability to answer these questions without resorting to complex mathematical models, we have structural intuition; this is particularly relevant today when bringing new products with unique configurations to market quickly is so important. This book emphasizes simple models of a structure’s physical behavior—firstorder models. Students are often surprised that the first-order models they have encountered earlier in courses in Mechanics, Strength of Materials, and Vibration may be applied so successfully to a complex structure like the auto body. The benefit of these first-order models lays in the insight the student develops. In addition to physical testing, finite element analysis (FEA) is today a reliable and precise means to determine structural performance. Both these tools are important during design, but each has limitations. Both testing and FEA require a complete definition of structure shape. Often this complete geometrical definition does not exist during preliminary design. Both testing and FEA require considerable time for construction of the model and for evaluation. Often decisions on design alternatives must be made very quickly, and testing or FEA just takes too long. Finally, both testing and FEA can tell us very precisely how a structure will behave, but not why it behaves as it does or how we may improve it. We need to understand the whys in order to create the structure in the first place. The intent of this book is to supplement these more precise structural design tools with first-order models and structural intuition. First-order models enable instant analysis for the what-if questions so important during early design. Firstorder models can evaluate structural concepts with incomplete geometrical definition. First-order models provide a sense check on the results of a physical test or FEA. First-order models help the designer identify potential problems in a design, which then requires a more precise analysis. In this, we are following the advice of Robert Cook and Warren Young given in their book, Advanced Mechanics of Materials: Structural design seldom requires sophisticated mathematics or a powerful computer. Rather, it depends on the ability of the designer to gain clear insight into the phenomena, to identify simplifying assumptions, and to apply straightforward methods to the simplified problem.

xi

In this book we intend to develop a sense of the relevant: What are the most relevant requirements; the most relevant structural behavior; the most relevant first-order models to describe this behavior; and the most relevant interactions with other vehicle subsystems. In doing so, we hope to contribute to the development of skilled structural designers. Disclaimer: The first-order models in this book are intended for preliminary layout and sizing of automotive structure. Actual structural performance should always be validated by more detailed analysis and testing.

xii

Chapter 1

The Automobile Body

Chapter 1 | The Automobile Body

The automobile body, Figure 1.1, is an important vehicle subsystem that performs many functions [1]. These range from the very basic function of being the armature holding the parts of the vehicle together, to the function of noise and vibration refinement which differentiates a luxury vehicle from an economy vehicle. In this chapter, we briefly describe the contemporary body structure and terminology.

Figure 1.1  Typical body-in-white. (Courtesy of General Motors Corporation) A typical body is an assembly of metal stampings, Figure 1.2, usually of steel but also of aluminum. Several material grades or alloys are used to meet the structural requirements at the formability needed to achieve the part shape, Figure 1.3 [2]. The stampings are assembled to form thin-walled structural elements, Figure 1.4. The general arrangement of these structural elements leads to several different body types.

Figure 1.2  Body stampings. (Courtesy of General Motors Corporation)

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Figure 1.4  Typical section at rocker. (Courtesy of General Motors Corporation)

1.1  Description of the Automobile Body Types There are several types of body structure configurations today. The predominant types are space frame, central frame, body-on-frame, and monocoque (integral body-frame), Figure 1.5. The space frame configuration, Figure 1.5a, is characterized by a three-dimensional framework of beams connected at nodes. The framework provides the structural integrity with the exterior panels being unstressed. The space frame type can be fabricated using lower-cost tooling such as roll forming or hydroforming, and is usually targeted for lower-volume vehicles. The central frame configuration, Figure 1.5b, is characterized by a large, closed structural member down the center of the vehicle. This member provides the structural integrity for this type. Because of the intrusion into the cabin, this arrangement is limited to two- or four-seat interior configurations.

3

Chapter 1 | The Automobile Body

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The body-on-frame configuration, Figure 1.5c, was the predominant passenger car type until the 1980s, and remains the predominant type for light trucks. This type is characterized by a ladder frame to which the suspensions and powertrain are attached, and a body shell which is connected to the frame by flexible body mounts. The monocoque configuration, Figure 1.5d, is characterized by an integral structure which forms a shell, including exterior panels. The integral structure reacts to all major loads. This is currently the predominant type for passenger cars, and is considered the most mass-efficient configuration. Because of its predominance, this book will focus on the monocoque configuration; however, the principles we will develop are also applicable to the other configurations as well. Within the monocoque configuration, there are topology variants. Topology is the arrangement of structural elements—beams and panels—to meet requirements in the most efficient manner. Besides structural concerns, an effective body topology also satisfies the additional constraints which the package, styling, and manufacturing place on the positioning and size of structural elements. Figure 1.6 shows three of the most common monocoque topologies in use today [3]. In later chapters, we will discuss the fundamentals of determining a body topology.

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1.2  Body Nomenclature While a uniform naming convention for body structure elements does not exist, there are some more common terms in use. Figures 1.7 through 1.10 show common names for the major structural elements with respect to the overall body. In Figures 1.11 through 1.16, part names are shown in a useful hierarchical format. This hierarchy follows a typical manufacturing partition of the body structure.

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5

Chapter 1 | The Automobile Body

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Figure 1.11  Body in white. (Courtesy of General Motors Corporation)

7

Chapter 1 | The Automobile Body

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9

Chapter 1 | The Automobile Body

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1.3  Body Mass Benchmarking

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The body structure mass is a significant portion of the vehicle mass, and because of this, the body influences mass-sensitive vehicle functions such as fuel economy, acceleration performance, and handling. Looking at the body shell mass (no trim, glass, closures, or bolt-on panels) for several sedans, Figure 1.17, the average mass is approximately 325 kg (715 lb) [4]. A more useful way to look at body mass is in comparison with the other vehicle subsystems. Figure 1.18 shows this breakdown for a typical mid-size vehicle having integral body and front wheel drive.

 

   

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