WO2002083466A1

WO2002083466A1 - Stiffeners for automotive sheet body structure

Info

Publication number: WO2002083466A1
Application number: PCT/US2002/010602
Authority: WO
Inventors: Carlos M. Ochoa
Original assignee: Icom Engineering Corporation
Priority date: 2001-04-10
Filing date: 2002-04-05
Publication date: 2002-10-24
Also published as: US6588831B1; GB0323843D0; CA2444229A1

Abstract

A reinforcing support structure for an automotive vehicle body having a sheet structure (10) including a pair of side sheet structures (11, 12). Upper stiffener (17) and lower stiffener (18) are secured to the interior of side sheet structures (11, 12). Stiffeners (17 and 18) may have a cross-sectional shape as illustrated by stiffener (39) in Figure 6. Stiffener (39) includes a mounting flange (41) and an opposed flange (42) which have free edge portions. Beads or curls (44, 46) are formed on the free edge portions for reinforcement.

Description

STIFFENERS FOR AUTOMOTIVE SHEET BODY STRUCTURE

Reference to Related Applications

This application is a continuation-in-part of application serial no.09/707,657 filed

November 7, 2000; which is a continuation-in-part of application serial no. 09/389,163

filed September 2, 1999; which is a continuation-in-part of application serial no.

09/263,684 filed March 5, 1999, now Patent No. 6,082,429 dated July 4, 2000; which

is a continuation-in-part of application serial no. 09/116,689 filed July 16, 1998, now Patent No. 5,954,111 dated September 21 , 1999. Field of the Invention

This invention relates generally to a reinforcing support structure for an

automotive vehicle body, and more particularly to stiffeners integrated with the sheet body structure. Background of the Invention

Stiffened sheet structures play a vital role in today's automotive body construction. To a significant degree these stiffened sheet structures serve to define the aesthetic shape and style as well as important safety elements of the vehicle. These structures have long provided an effective medium for addressing some of the

unique and demanding aesthetic and structural requirements of this industry. While

many technical advances have been made, improved stiffeners for automotive body

sheet structures are still needed.

Over the past several years government-mandated vehicle crash test

requirements as well as government and industry fuel economy and safety goals have

driven the automotive industry toward lighter, stronger, more energy absorbing, and

tailorable stiffeners for integration with automotive sheet material bodies, including those used in cars, trucks, minivans, and sport utility vehicles, in order to maximize

structural efficiencies related to these goals. Viable candidate stiffener concepts must be capable of addressing not only performance goals, but also economies related to processing, forming, structural integration/configuration, tooling, and assembly. The automotive industry has thus embarked on an exhaustive search for simple, enabling stiffener technologies that might fulfill these requirements.

Generally speaking, even the most favored conventional stiffener technologies have satisfied only some of the combined goals, in spite of the significant advantages

made possible by using the latest and most advanced technologies now available in structural simulation and stress analysis as well as in optimization computer software. These efforts have sometimes made use of very high and ultrahigh strength materials including specially developed steel alloys and extrudable aluminum alloys, as well as

ceramic fiber reinforced plastics.

The most commonly resulting trend has been for weight saving and strength

goals to be heroically accomplished in tandem with significant penalties in the areas

of tooling, material processing, fabrication, and assembly costs. At times, form and

style have yielded to functional goals. The total net cost savings has sometimes been

marginal. This is because of the complex cross-sections and interfaces that are sometimes created as mass is redistributed using an increasingly intricate and localized level of control over the cross section of each structural component. As a result, fabrication, integration, complexity and space claim issues have typically risen to join a growing list of challenges.

Some of the favored cross-sections have included modified tubes, hat-shaped cross-sections, and C-channels of various types. Each of these shapes offers specific challenges in the area of interfaces and joints. Some of these shapes have been

formed under very high fluid pressures that may themselves have presented new safety and training challenges in their implementation in the workplace. One common scenario in these cases has been that as design ratios of cross-sectional dimensions

such as outer diameter-to-thickness or depth-to-thickness ratios exceed the range of about 50, both closed and open sections may have entered a range of relatively high

sensitivity to local wall thinning during fabrication, as well as sectional buckling and

reduced bending rupture resistance in service.

Furthermore, the use of thinner material in traditional open-section stiffener configurations makes these stiffened sections more susceptible to edge stress

concentrations that are characteristic of open sections-especially under bending and

compression loads. This is because conventional thin open sections commonly have a "blade edge". This edge is very susceptible to imperfections in the sheet material along this edge as well as to damage during manufacture, shipping/handling and

installation. These imperfections along the blade edge become stress concentration

points or focal points at which failure of the stiffener can initiate. A more detailed

description of this failure initiation follows.

Even the most perfect, smooth edge of the conventional stiffener may

experience a very localized point of high stress gradient due to the characteristic edge

stress concentration associated with open sections under bending loads. Thus, initiation of an edge "bulge" or "crimp" on a perfect smooth edge is nothing more than the creation of an edge imperfection that is large enough to grow or "propagate" easily. It is significant that this stress concentration may be made worse by the presence of any relatively small local edge imperfections, even those on the order of size of the thickness of the stiffener material itself.

These imperfections near the edge can be in the form of edge notches, waviness (in-plane or out-of-plane), local thickness variations, local residual stress variations, or

variations in material yield strength. Where multiple imperfections occur together, they may all compound together to further increase the stress concentration effect, and thus lower the load level at which failure is initiated. Thus, the existence of any edge

imperfections in a conventional open section stiffener has the effect of enhancing an already established process of failure initiation.

Local complexity in the structural cross sectional shape of thin conventional

stiffeners can further degrade structural stiffness and buckling resistance. Buckling is an instability in a part of the stiffener associated with local compressive or shear

stresses. Buckling can precipitate section failure of the stiffener. This in turn can

cause a stress concentration in adjacent structures that can lead to failure of a larger

section. This effect is of particular concern in the evaluation of crash worthiness of

automotive bodies, because such failures may be less uniform or predictable, thus

making them less desirable from an occupant safety standpoint. Such unstable structures typically do not absorb sufficient crash energy or resist crash forces effectively enough to consistently meet safety performance goals without adding significant additional mass and further complexity to the overall design.

Moreover, some thinner conventional stiffeners can experience "rolling" when

placed under load. Rolling is when the shear stresses within the stiffener result in a net

torque about the centroid of the thin walled cross-section, thus causing the cross- section to twist, possibly making the stiffener unstable. Another cause of rolling is related to the curvature of the stiffener itself, after it has been formed to the local

contour of the vehicle. Some designers have increased the cross sectional length of the open-section member flanges having free edges while attempting to solve the rolling problem but were met with only marginal improvement. This is because the increased flange length has the simultaneous effect of increasing the distance from the centroid to the shear center of the stiffener. Also, increasing the cross-sectional flange

length sometimes causes difficulty in accessing the interior of the section during assembly or other operations and has typically raised additional space claim issues.

Yet another problem facing thinner conventional structural stiffeners is that of fastening or joining relatively thick sections to sections that are relatively less thick, or

relatively stiff sections to sections that are relatively less stiff near the joint or interface. This can result in a local stress concentration in the region of joining. These stress

concentrations may significantly weaken the joints or interfaces associated with

conventional stiffeners.

Computer optimization codes often help to improve the design of conventional

stiffeners. But they may not accurately represent the degradation in practical

performance and increased sensitivity to geometric and material imperfections that has

brought largely empirical guidelines into widespread acceptance over the years. Moreover, as conventional stiffening cross-sections are made thinner, damage

tolerance may rapidly become an even greater concern. Load paths and local fastener stresses become more difficult to evaluate. Even minor repair is sometimes out of the

question due to special welding or joining techniques that do not lend themselves to body shop environments. Because of ever increasing safety and performance standards, there is clearly an

established need for a new and innovative automotive body structural stiffening system. The new system should combine traditional versatility and simplicity with design flexibility and tailorable structural efficiency to achieve both weight saving and crash worthiness objectives, while not casting a shadow on form and style. Such a system should specifically address and overcome the significant limitations of conventional

stiffener shapes, without resorting to extreme measures, for example in fabrication or

joining approaches. Such a system should largely follow existing intuitive conventions

for joining and forming, handling and processing, without presenting additional

obstacles to workers or to the environment.

Summary of the Invention

The present invention alleviates and overcomes the above-mentioned problems

and shortcomings of the present state of the art through a novel stiffener for automotive

body sheet structures. The novelty and uniqueness of this invention is that it: 1 ) is

made of thinner material to reduce the in-plane interface stresses found in the sheet

interface areas, 2) resists deflection adequately to meet new higher structural stiffness requirements, 3) is resistant to buckling and rolling, 4) effectively addresses edge stress concentrations by modifying the blade edge to an area of relatively low stress,

5) provides a special interfacing flange capability that results in stronger and more

crash resistant joints, 6) is synergistic with the use of thinner and stronger materials, and, 7) can be manufactured cost-effectively by using conventional forming methods such as roll forming and stretch forming.

This novel invention may be described as a substantially reconfigured or stabilized J-stiffener having a specially configured interface and mounting flange capability. It should be noted here that due to their extreme susceptibility to rolling, conventional J-stiffeners are seldom used in automotive body applications. The unexpectedly strong synergisms of the unique characteristics found in the stabilized J- stiffener not only address the above problems, but simultaneously obtain significant material savings. More particularly the synergisms may be described as follows.

The instant invention has substantially redistributed material at critical locations

as compared with conventional stiffener or structural member configurations. This

material redistribution has the effect of altering considerably the behavior of the

stiffener as compared with conventional J-stiffeners and other stiffener configurations.

The material redistribution required to accomplish these collaborative effects is

accomplished by having specifically placed free edge portions, which are turned as folds to define tubular beads or curls along the free edges. Moreover it is not just the

presence of the tubular bead or curl that enables the substantial level of synergism, but

the discovery of specific ratios of curl diameter to other stiffener dimensions that

maximize these synergisms even to the extent of obtaining significant weight savings

Three main sets of synergisms combine to make the present invention

successful. The first set of synergisms is directly related to the ratio of the diameter of the curl to the stiffener section flange length and web length. Each tubular bead has a cross-sectional dimension which when combined in specific ratios with other stiffener dimensions substantially maximizes the moment of inertia of the overall section about the horizontal and vertical axes with a minimal use of material. Moreover, the tubular

bead size specified by these same ratios has the effect of altering the characteristic

failure mode normally associated with the free edge stress concentration of conventional stiffeners as described above. Not only does this substantially enhance in-service performance, but it also increases the formability of the stiffener by enabling

the section to respond more uniformly during three-dimensional fabrication operations such as roll forming, pressing, or stretch-forming. Finally, the cross-sectional dimensions of the tubular beads of the stabilized J-stiffener make this novel stiffener less sensitive to edge imperfections and damage because the blade edge has now

been placed in a position of relatively benign stress levels so that imperfections or

damage to the tube or edge region have to be on the order of size of the diameter of

the curl in order to have significant detrimental effect to the stiffener section.

Having established the above ratios, a second set of synergisms was discovered

by directly combining the above with specific ratios of the stiffener's cross-sectional

web dimension to cross-sectional flange dimension. The compounding effect of the first set of synergisms with this additional set of ratios makes the stabilized J-stiffener more resistant to rolling and buckling and thus avoids the problems that plague deeper conventional automotive body stiffeners using thinner gauge material. Additionally,

these compounding synergisms make this stiffener unique in that stresses are now more evenly distributed in the flanges, thus making the stiffener more stable and less

sensitive to dimensional imperfections. Because of these cooperative effects, the stabilized J-stiffener demonstrates its uniqueness and efficiency in using thinner gauge material to reduce in-plane stresses found in the fastener, interface, or joint areas, thus

allowing the other automotive body components and the stiffener to work together as a cohesive system instead of as individual components.

The third set of synergisms relates to intermediate interface flanges of the

stabilized J-stiffener. These intermediate flanges extend the capability of the J-stiffener

in the following way. They permit the stabilized J-stiffener to be integrated more easily and successfully with complex arrangements of structural members, while substantially maintaining the benefits of the present invention. These intermediate interface flanges are formed according to precise and specific ranges of the angle defined between adjacent flanges, and according to specific ranges of radius-to-thickness ratios of the bend region between these flanges. The result is that the adjacent flanges to an interface flange work together to provide a significantly stabilized and strengthened

interface region that lends itself to a variety of fastening or joining approaches, without

substantially compromising the performance of the stiffener. In addition, when

fasteners or local joints are used, an innovative barrier to crack propagation and

fracture in the joint region is created. Because this innovation allows the stabilized J-

stiffener to effectively address the problems of fracture and in-plane stresses at interfaces, increased performance is made possible at a larger level without

significantly adding complexity and tooling costs.

Some of the mechanisms and technical advantages of better stabilization

against crack growth that may originate near a bolt hole, a weld, or at other interface

or joint locations along the length of the stiffener may be further described as follows. Enhanced fracture resistance is enabled by the combined effect of radius and angle

between two adjacent flanges, and is still further enhanced in a compounding manner by the appropriate choice of radius to thickness ratio. This ratio serves to accomplish

the dual role, first of maximizing the amount of strain hardening in the radius region which itself serves as a barrier to crack growth, and second, of emphasizing the stiffening and constraining role of one flange with respect to the other which serves as

an additional crack barrier. Thus, within prescribed ranges, these variables may be

changed along the length of the stiffener along with flange widths to accomplish various design goals, while substantially retaining the benefits of the teachings of the present

invention. As previously stated, the dual stabilization that is achieved has a compounding effect against the growth of cracks that may originate near interfaces,

joints, or welds or at other locations along the length of the stiffener. It results in a stronger, more stable stiffener that is more predictable and better able to resist damage resulting from impact for example by another vehicle. The combined effect is so

significant that for some applications the strength of this unique interface flange joint

may be increased by as much as a factor of about three (3).

When compared with conventional stiffeners on the market today, the stabilized J-stiffener of the present invention uses substantially thinner material while obtaining

better structural performance. Thus, even though additional slit width (width of the sheet of material from which the stiffener is made) is required to reposition needed

material, the use of thinner gauge material more than offsets the additional slit width, bringing overall material savings as high as 20 percent in some instances. This innovation in system configuration also represents a substantial cost savings for the manufacturer, since material cost is a significant portion of total manufacturing costs

for automotive body hardware. Thus, this unique and novel stiffener is very cost effective.

For manufacturing process cost efficiency, the tubular bead is preferably an

open-section bead, meaning that the sheet material is formed in an almost complete

bend or curl, but the curl need not be closed at its outer edge, such as by welding. A

closed section tubular bead would work equally well, at a slightly higher manufacturing

cost. This edge feature is discussed in more detail in the following paragraph. The edge- flange section curl and the trough curl are folded tubular features, preferably open- sections, that are made by shaping the free edges or edge marginal portions of the stiffener cross-sections into a generally elliptical, preferably circular, cross-sectional

shape. As used herein, a circular cross-section is an embodiment of an elliptical cross-

section and is covered by the term "elliptical cross-section" or "elliptical cross-sectional

shape". The term "characteristic diameter" refers to a constant diameter in the case of

a circle, while other elliptical shapes will have major and minor axes or diameters, with

the minor axis or diameter being the "characteristic diameter".

Even though some configurations of a slightly non-circular elliptical shape may

be more desirable in some applications, the circular cross section embodiment is generally preferable, because it is simpler to manufacture, while still achieving the desired benefits to a significant degree.

It is important to contrast the edge curl approach against other possible edge

treatment approaches by noting that the dimensional order of size effect related to

imperfections or damages described above for the curl can not be achieved by simply folding the edge over, either once or multiple times, because in this case the characteristic dimension will be defined by the fold edge diameter and not by the length

of overlap of the fold. This is because the overlap direction is transverse to the edge and quickly moves out of the peak stress region, and because the edge fold diameter defines the maximum distance over which the edge stresses may be effectively spread.

The elliptical or circular open-section tubular shape or "edge curl" is contrasted

to tubular sections of rectangular cross-sectional shapes, including folded edges, and

to open-section tubular shapes of softened corner polygon cross-sectional shapes in that the characteristic diameter will be defined in each of these other cases by the fold diameter or by the softened corner diameter nearest to the stiffener edge, as opposed to the overall diameter of the edge curl section. It may be noted that in this context a polygon cross-section with very softened corners is in effect an imperfect ellipse or

circle.

In some instances, quasi-elliptical or quasi-circular cross-sections, teardrops,

imperfect ellipses, and imperfect circles, in the form of polygons or rectangular cross-

sections with some rounded regions may function adequately, but may also be more difficult to manufacture and may be less effective than a generally circular curl.

Including local offsets or adding material locally such as by bonding or welding strips

of material or high strength fibers or wires are additional examples. Yet other examples include local modifications to the material such as by heat, electromagnetic, chemical, or deformation treatment of the tubular bead cross-section or of adjacent regions. In spite of the potential for additional fabrication costs, some of the above variations may at times be desirable for example in local regions where the designer desires local

regions of modified cross-sectional shape for space claim, interfacing, or joining

reasons, or in special cases where such features may be combined with other features such as notches, folds, or hole patterns near the edge region in order to induce a

prescribed response of the stiffener, such as in addressing crash energy absorption

design-related goals. In some applications the curl may be formed by turning the

edges through an arc of up to 360 degrees, 720 degrees, or even more, so that the

edge loops over one or more times on itself, in order to concentrate mass locally or to

address other design objectives. In these cases manufacturing economy and

complexity are also considerations. Some of the substantial advantages made possible through the teachings of the present invention may be summarized as follows. They include the synergistic effect of the stabilized J-stiffener's material efficiency in obtaining the desired bending rigidity or moment of inertia, the alteration of the characteristic failure mode, the reduction in sensitivity to edge imperfections and damage, higher fracture resistance and more stable stresses in the regions of joints and interfaces, resistance to buckling and rolling,

as well as the ability to spread stresses more uniformly. These features offer the same

degree of compounding advantage as the conventional stiffener's compounding

disadvantage of low resistance to buckling and rolling combined with sensitivity to

relatively small edge or dimensional imperfections. Accordingly, it can now be

appreciated by those versed in this art, that the novel stabilized J-stiffener of the instant

invention provides a solution to some important problems that the stiffened sheet material automotive body art has sought to overcome.

In summary, the stabilized J-stiffeners of the present invention, having specially

configured interface capabilities including mounting flanges that are uniquely designed

to be compatible with substantially all types of standard sheet material automotive body

structures, are thereby significantly capable of lowering costs and reducing the number of stiffener types that designers must consider to achieve their objectives. These novel

stiffeners thus permit more stringent structural and safety requirements to be addressed. Since they are quite adaptable, they often permit this to be done without major modification of other hardware.

The description of the present invention may incorporate dimensions that are

representative of the dimensions which will be appropriate for some common

automotive applications. Recitation of these dimensions is not intended to be limiting, except to the extent that the dimensions reflect relative ratios between the sizes of various elements of the invention, as will be explained where appropriate.

It is an object of the invention to provide stiffeners for an automotive sheet

structure and particularly to a provide and effective stiffener for an automotive side

sheet.

It is a further object to provide an economical stiffener that is capable of

extending in a longitudinal direction for substantially the entire length of an automotive

vehicle.

Another object is the provision of a stiffener for an automotive side sheet structure in which the stiffener has a pair of flanges with free edges and a tubular bead extending along the free edge of each flange for reinforcing the stiffener. Brief Description of The Drawings

Figure 1 is a perspective of an automotive sheet structure body including side

sheets having stiffeners comprising the present invention mounted thereon;

Figure 2 is a side elevation of the upper and lower stiffeners shown in Figure 1 extending horizontally for substantially the entire length of the automotive body;

Figure 3 is a perspective of an automotive sheet material body interior showing

a roof portion and integral rear end portion of the upper stiffener shown in Figures 1

and 2 mounted on an automotive side sheet;

Figure 4 is a perspective of the stiffener shown in Figure 3 removed from the side sheet structure;

Figure 5 is a perspective of the front end portion of the upper and lower

stiffeners;

Figure 6 is an enlarged section of a stiffener removed from a body section; Figure 7 is an enlarged sectional view of a bead on a free end of the stiffener;

Figures 7A and 7B are enlarged sectional views of modified beads on a free end of a stiffener;

Figure 8 is an enlarged section of a modified stiffener in which the mounting

flange extends in an opposite direction from the mounting flange for the embodiment

of Figure 6; and

Figure 9 is an enlarged section of a modified stiffener in which an intermediate

interface flange extends from the mounting flange at an angle and interfaces the next adjacent flange at an angle.

Description of the Invention

Referring now to the drawings for a better understanding of the present

invention, and more particularly to Figures 1 and 2, an automotive body sheet structure is shown generally at 10 and includes side sheet structures generally indicated 11 and

12 connected by a roof panel 14. Door openings are shown at 16. For reinforcing side

sheet stiffeners 11 and 12, elongate upper and lower sheet reinforcing members or structures generally indicated at 17 and 18 respectively extend continuously for

substantially the entire length of body sheet structure 10 and are mounted on the inner surface of side sheet structures 11 and 12. Sheet reinforcing members 17 and 18 comprise stiffeners of the present invention. Upper reinforcing structure 17 includes

a front end portion 17a, a roof portion or rail 17b, a front curved connecting portion 17c, a rear end portion 17d, and a rear curved connecting portion 17e between roof portion

17b and rear end portion 17d. Lower sheet reinforcing structure 18 includes a front end

portion 18a, an intermediate base portion 18b, a front connecting portion 18c, a rear

end portion 18d, and a rear curved connecting portion 18e. A vertical post or pillar 19 extends between door openings 16 and upper and lower sheet reinforcing structures

17 and 18. Sheet enforcing structures 17 and 18 are normally in spaced relation to each other except for rear end portions 17d and 18d which may be bolted or welded to each other. Upper and lower sheet reinforcing structures 17 and 18 are of a generally

similar cross-section although a relatively small variation in dimensions or shape may occur along the length of sheet reinforcing structures 17 and 18.

As an example and referring to Figures 3 and 4, a suitable stiffener portion is

illustrated generally at 20 for roof portion 17b and rear curved connecting portion 17e. Stiffener portion 20 is effective to withstand bending loads near the mid-span of roof portion 17b and is also stable against axial loads introduced near the lower end of rear connecting portion 17e. Figure 4 shows stiffener portion 20 removed from the

automotive body sheet structure. It may be noted that the cross-section of stiffener

portion 20 may vary slightly since the stiffener portion may be required to conform to the curvatures of the vehicle body. This may be accomplished through relatively simple stretch-forming operations that may be performed subsequent to roll-forming the cross-

sectional shape.

One shape that has recently gained interest in the industry because of the

shortcomings of conventional open section stiffeners has been hydroformed tubes.

Comparison of the present invention to hydroformed tubes is used in order to highlight the novel capabilities introduced by the present invention. Hydroformed tubes are

typically made from thin, high strength steel. These tubes are susceptible to local

damage during fabrication and handling that may precipitate buckling of the tube cross- section. Furthermore, their attachment to the vehicle body along the length of the tube is not easily accomplished because access is a problem. Finally, in some cases it may be possible that damage is present in an installed tube, but that the damage cannot be

observed by inspectors subsequent to installation, because some of the tube is hidden from view once it is installed. In contrast, the stiffeners of the present invention are

open sections, and thus provide greater ease in inspecting the integrity of installed

parts.

The actual process of fabricating hydroformed tubes can involve sensitive steps

of precise three-dimensional stretch-forming during which it may be difficult to uniformly

grip and load the part in order to maintain a uniform cross-section and prevent local

thinning of the tube wall. This may limit the length of tube that may realistically be considered. Longer tubes may quickly rise in cost because these complexities increase

greatly with total tube length. Moreover, high pressure forming is also used, during

which additional local wall-thinning may occur. Wall thinning is an important concern

for hydroformed tubes because it may not be detectible through simple visual

inspection. In contrast, the stiffeners of the present invention typically deform in a more uniform fashion because the tubular edges promote uniformity in stresses that aids in

achieving consistent and uniform parts without requiring special inspection techniques. The stiffeners of the present invention lend themselves to "section-by-section" forming

operations. This enables long stiffeners of complex shapes to be formed relatively

simply, and at lower cost.

During installation, the hydroformed tubes may require special care in order to

secure them and join them to the sheet structure at either end so that they are able to

maintain their structural integrity under axial loads. This can be a formidable task

because access between the tube and the sheet is restricted. In contrast, the flanges of the present invention lend themselves to a variety of joining techniques because of easy access and reduced sensitivity to nonuniformity at the joints due to the special

tubular edge features and the special interface flange geometries that stabilize the joints.

Sheet structure 10 is commonly formed from one or several thin sheets of high strength and modulus materials such as steel or aluminum, although fiber reinforced

plastics or metals may also be used. When multiple sub-sheets are used, they may be

formed independently and then joined together such as by bonding or welding, to

extend sheet structure 10. The overall industry trend for automotive body structures is to form sheet structure 10 from as few sub-sheets as possible in order to maximize

the rigidity, strength and structural integrity of sheet structure 10. This is important because of the vital role that sheet structure 10 plays in maintaining the vehicle body shape and stiffness thereby reducing vibration, as well as in maintaining body strength and integrity during a vehicle crash or rollover. Sheet structure 10 must also absorb

energy and protect the vehicle occupants from intrusion into the vehicle interior

compartment during a crash. Because sheet structures typically have little inherent

bending stiffness of their own, they largely depend upon stiffening members to integrate

with them for support. These stiffeners are thus vital to each of the various roles of

sheet structure 10. The functions of the reinforcing structures or stiffeners 17 and 18

that interface with sheet structure 10 include reinforcing sheet structure 10, and also

providing load paths in areas where the body is attached to the vehicle frame (not shown) for transferring loads between the vehicle frame and the sheet structure 10 of

the vehicle body.

In order to further highlight the novelty and significance of the present invention, specific stiffener embodiments are chosen related to stiffeners that are interfaced with sheet structure 10. The following highlights the stiffeners of the present invention in

terms of their ability to address and substantially overcome many of the difficulties related to stiffeners presently in use. It also highlights the unique combination of local flexibility in the use of thin material, along with the substantial rigidity that is accomplished in the overall sheet structure. In this discussion, some unique interface features will be discussed that further maximize the structural and cost efficiency that

is achievable in implementing the present invention. Those versed in the art will readily recognize the simplicity and versatility of the modified J-stiffeners of the present

invention in reducing fabrication costs and in enabling the automotive designer to simultaneously and synergistically achieve a variety of design objectives related to

fabrication, performance, safety, interfaces, and aesthetics.

To illustrate a typical cross-section for stiffeners 17, 18, a stiffener such as stiffener 39 which includes the features shown in Figure 6 has been found to be satisfactory for the entire length of reinforcing structures 17,18 generally with only changes in dimensions being required for different length portions. Stiffener 39 is

commonly formed of a sheet material such as a steel alloy or a fiber reinforced material,

and comprises a web or body 40, an integral mounting flange 41 at right angles to body

40, and an integral outer bowed flange 42. The opposed free edge portions of

mounting flange 41 and bowed flange 42 are turned inwardly to form open-section tubular beads or edge curls 44,46. An open gap 48 as shown also in Figure 7 is

formed adjacent each tubular bead 44,46. Tubular beads 44, 46 are shown as being

of circular configurations or shapes in cross section and have outer diameters indicated

at d and d1. Tubular beads 44,46 are turned inwardly an angular amount A of about 270 degrees from the flange 41 and bowed flange 42 as shown in Figures 6 and 7 particularly. Thus, gap 48 is of an angular amount about 90 degrees. If desired, tubular beads 44, 46 could be closed although 270 degrees has been found to be optimum. An angular or circular shape for beads 44,46 as small as about 210 degrees would function in a satisfactory manner in most instances.

While a circular shape for tubular beads 44,46 is preferred, a generally elliptical non-circular shape would function adequately in most instances. A tubular bead or curl

of a non-circular elliptical shape has a major axis and a minor axis. Diameter or

dimension d or d1 for an elliptical shape is interpreted herein for all purposes as the

average dimension between the major axis and the minor axis. The major and minor

axes are at right angles to each other and are defined as the major and minor

dimensions of the open or closed tubular section. To provide an effective elliptical

shape for tubular beads 44 and 46, the length of the minor axis should be at least about

40 percent of the length of the major axis. The terms "elliptical" shape and "elliptical"

cross section are to be interpreted herein for all purposes as including circular

embodiments. For a circular embodiment diameters d and d1 may be equal in order

to simplify manufacturing operations. Preferably, diameter d1 for bead 46 is larger than

diameter d for bead 44 in a non-circular elliptical shape. Bowed flange 42 is generally bowl shaped and has a sloping wall portion 50 extending from body 40 to an arcuate

apex 52. An integral sloping wall portion 54 extends from arcuate apex 52 to bead 46.

In order for tubular beads 44, 46 to provide maximum strength with a minimal cross sectional area of stiffener 39, the diameter d1 of tubular bead 46 is selected according to the width W1 of bowed flange 42 as shown in Figure 6.

A ratio of about 5 to 1 between W1 and d1 has been found to provide optimum results. A ratio of W1 to d1 of between about 3 to 1 and 8 to 1 would provide satisfactory results. A similar ratio between W2 and d for tubular bead 44 is utilized.

As an example of a suitable stiffener 39, W1 is 1 inch, W2 is 1 inch, and W3 is 3¹/₂ inches. The diameter d for bead 44 is 3/16 inch and diameter d1 for bead 46 is inch.

In order to obtain the desired minimal weight stiffener, tubular curls or beads 44, 46 must be shaped and formed within precise ranges and sizes in order to provide maximum strength. Using various design formulae to determine the outer diameters

of tubular curls 44, 46, an optimum outer diameter of % inch was found to be satisfactory. However, it is preferred that diameter d1 for curl 46 be slightly larger than

diameter d for curl 44. W1 and W2 are at least two (2) times and preferably between about three (3) and five (5) times the outer diameter of tubular curls 44, 46 for best

results. Width W3 is between about two (2) and seven (7) times widths W1 and W2 for best results. By providing such a relationship between tubular curls 44,46 and

widths W1 and W2 the moment of inertia is maximized and edge stress concentrations

are minimized for stiffener 39 thereby permitting the light weight construction for

stiffener 39 of the present invention. Tubular curls 44, 46 are illustrated as turned

inwardly which is the most desirable. In some instances it may be desirable to have a tubular curl turned outwardly.

For mounting stiffeners 17,18 with body sheet structure 10 as shown particularly

in Figures 1-3, mounting flange 41 may be modified locally in order to accommodate attachment with the adjacent sheet structure. This may include removing a segment of the tubular edge, or simply modifying its shape such as by flattening it locally to accommodate joining in order to obtain a more rigid and integrated structural support of the sheet structure body. The following example will further highlight the novelty and utility of stiffener 39. When the typical cross-section of stiffener 39 is utilized for front end portions 17a and 18a, the continuous stiffeners 17,18 provide a load path and substantial integrated stiffening and strengthening of large sections of body side outer sheet structure 10.

This is accomplished with maximum simplicity and a minimum number of critical joints. In some locations continuous welding may not be required to adequately mount a

stiffener on body side outer sheet 10, and may be replaced by skip or spot welding.

This is enabled by the stabilized edges of the stiffener that cause the loads to distribute more uniformly into the interior of the stiffener cross section. Additional stiffeners such

as shown at 39 or 39A may be mounted at various angles on body side outer sheet

structure 10 in order to enhance the overall structural performance of the vehicle body,

or to achieve other design objectives. These may be interfaced with each other, and may even be extended such as to form large closed loops over the body side structure

10, for example, in order to maximize structural efficiency in achieving stability and

resistance of the automotive body to intrusion during a crash or vehicle rollover.

As a specific example of a typical stiffener 39 being utilized for stiffener 17 such

as roof portion 17b secured to body outer sheet structure 10, roof portion 17b may have a thickness of 19 gauge (.047 inch) with W1 and W2 being 1.25 inch and W3 being 2

3/4 inches. Diameter d is 3/8 inch and diameter d1 is 3/8 inch. When stiffener 39 is

utilized for front end portion 17a of sheet reinforcing member 17, front end portion 17a

may have a thickness of 16 gauge (.063 inch) with W1 and W2 each being about 2% inches. Diameter d may be chosen as 3/8 inch and diameter d1 may be chosen as 3/8 inch. High strength steel is normally used to enhance the strength and crash worthiness of stiffeners 17,18. However, certain fiber reinforced composite materials have been found to be satisfactory.

Figure 8 shows another embodiment of an automotive body stiffener in which stiffener 39A has a mounting flange 41 A extending from body 40A generally in the opposite direction as outer bowed flange 42A. Tubular curls or beads 44A and 46A together with the dimensions shown at W1 , W2, W3, d, and d1 are similar to the

embodiment of Figures 1-7. The only change in the embodiment of Figure 8 from the embodiment of Figures 1-7 is the direction in which mounting flange 41 A extends.

Figures 7A and 7B show two embodiments of alternative tubular edge features to that of Figure 7. Each of these two embodiments is directed to a bead or curl in

which the outer diameter to thickness ratio of the tubular edge is larger than about 15.

In these cases, additional stiffening of the tubular edge may be desired. As shown in

embodiment 110 a smaller elliptical curl 107 inside the tubular edge feature or large

diameter curl 54A is shown in Figure 7A, and includes a "curl flange" 116 with

associated thickness t, radius R11 and angle Θ11 in order to further stabilize tubular

edge 54A. For ratios of radius R11 to thickness t of 3.5 or less, and for angle Θ11 at

least about 25 degrees in the positive or negative direction, a special strengthening

effect is obtained which causes the curl flange 116 to act together with tubular edge

54A so that the entire edge is substantially strengthened against deformation such as may occur during a vehicle crash. This added stabilization is thus a very useful extension of the capabilities of the present invention.

As an alternative to the curl within a curl of Figure 7A, Figure 7B shows

embodiment 113 having a lip 114 of length L11 that may be applied to the free edge of the curl 54B such as is shown in Figure 7B. In this case the angle Θ12 is between 30 and 120 degrees for best results, and the length L11 of lip 114 is between 1/5 and 4/5 of the average of edge tube dimensions L12 and L13 for best results. Longer lips may be used, but usually with limited additional benefits over the benefits of the range given.

Referring now to Figure 9, a separate embodiment of a stiffener 39D is illustrated which is similar to stiffener 39 shown in Figure 6 except for an intermediate

connecting flange portion 45D extending between mounting flange 41 D and body or

web 40D. Connecting flange portion 45D extends at an angle A6 and radius R6 with

respect to mounting flange 41 D and at an angle A7 and radius R7 with respect to body 40D. Connecting flange portion 45D is of a width W4. Radius R6 and radius R7 are

each less than 3.5 times the thickness of stiffener 39D for best results. Angle A6 and

angle A7 are each preferably at least greater than about 25 degrees in a positive or negative direction. Dimensions W1 , W2, and W3 are similar to dimensions W1 , W2, and W3 shown in Figure 6. The length of W4 is generally smaller than the length of W3 for best results.

While stiffeners or sheet reinforcing members 17 and 18 have been illustrated as extending continuously for substantially the entire length of the automotive body, it

may be desirable in some instances to utilize only a portion of the length of stiffeners 17,18. For example, front end portions 17a and 18a may in some instances be separated from the remainder of stiffeners 17 and 18. The specific longitudinal shape of stiffeners 17 and 18 may also vary depending primarily on the shape of the automotive body and side sheet construction.

As a result of providing the intumed tubular beads or curls along the marginal

edge portions of the stiffener, an unexpected significantly thinner gauge material generally about twenty percent lighter has been utilized for the stiffener as compared with conventional prior art body stiffeners as utilized heretofore. By utilizing precise tubular beads as set forth herein on the selected members where it is most needed for strength, a manufacturer may utilize an unexpected substantially thinner gauge material while eliminating or minimizing problems encountered heretofore by prior art designs

of stiffeners for the sheet structures of automotive bodies.

While the particular invention as herein shown and disclosed in detail is fully

capable of obtaining the objects and providing the advantages hereinbefore stated, it is understood that this disclosure is merely illustrative of the presently preferred

embodiments of the invention and that no limitations are intended other than as

described in the appended claims.

Claims

What Is Claimed Is:

1. An automotive sheet body structure comprising: a side sheet member having an inner surface and an outer surface;

an elongate stiffener secured to said inner surface of said side sheet

member, said stiffener defining in cross-section a web and a flange extending outwardly from each end of said web, each of the flanges having a free edge; and

a tubular bead extending along the free edge of each flange for

reinforcing said stiffener.

2. The automotive sheet body structure as defined in Claim 1 , wherein the

tubular beads are inturned and of an elliptical cross section wherein the minor axis is at least about 40 percent of the major axis.

3. The automotive sheet body structure as defined in Claim 1 , wherein one of said flanges extends at right angles to said web to define a mounting flange, and the

other of said flanges is a bowed flange.

4. The automotive sheet body structure as defined in Claim 3, wherein the

width of said bowed flange and said mounting flange is at least about two times the

outer diameter of said beads.

5. The automotive sheet body structure as defined in Claim 4, wherein said

inturned tubular beads are of a circular cross-section and extend in a circular path of

at least about 210 degrees.

6. The automotive sheet body structure as defined in Claim 1 , wherein said web has a width between two times and seven times the width of said bowed flange and the width of said mounting flange.

7. The automotive sheet body structure as defined in Claim 2, wherein each bead has a closed end to define an enclosed area; and a curled end portion of said

bead extends within said enclosed area from said closed end to provide additional

reinforcing.

8. The automotive sheet body structure as defined in Claim 7, wherein said

curled end portion is of an elliptical cross section.

9. The automotive sheet body structure as defined in Claim 7, wherein said

curled end portion includes a planar flange extending within said enclosed area.

10. The automotive sheet body structure as defined in Claim 2, further

comprising; an intermediate connecting section between said web and one of said flanges, said connecting section extending from said web at an angle of at least about

25 degrees.

11. An automotive sheet body structure as defined in Claim 1 , wherein said elongate stiffener comprises an upper stiffener extending continuously for substantially the entire length of said sheet body construction.

12. An automotive sheet body structure as defined in Claim 11 , wherein said

elongate stiffener includes a front end portion, a roof portion, a rear end portion, and connecting portions between said roof portion and said end portions.

13. An automotive sheet body structure comprising: a side sheet member having an inner surface and an outer surface;

an upper stiffener secured to said inner surface of said side sheet

member; a lower stiffener secured to said inner surface of said side sheet member; each stiffener defining in cross-section a web and a flange extending outwardly from each end of said web, each of the flanges having a free edge; and a tubular bead extending along the free edge of each flange for reinforcing said stiffener.

14. The automotive sheet body structure as defined in Claim 13, wherein said upper and lower stiffeners include front end portions.

15. The automotive sheet body structure as defined in Claim 13, further

comprising: a door opening between said upper and lower stiffeners; each stiffener

extending for substantially the entire length of the body structure and including a front end portion, an intermediate portion, a rear end portion, and connecting portions

between said intermediate portion and said end portions.

16. The automotive sheet body structure as defined in Claim 13, wherein the tubular beads are inturned and of an elliptical cross-section wherein the minor axis is at least about 40 percent of the major axis.

17. The automotive sheet body structure as defined in Claim 16, wherein one of said flanges extends at right angles to said web to define a mounting flange, and the

other of said flanges is a bowed flange.

18. The automotive sheet body structure as defined in Claim 17, wherein the width of said bowed flange and said mounting flange is at least about two times the outer diameter of said tubular beads.

19. The automotive sheet body structure as defined in Claim 17, wherein said

web has a width between two times and seven times the width of said bowed flange and the width of said mounting flange.

20. An automotive sheet body structure comprising:

a front side sheet member having an inner surface and an outer surface; an upper front end stiffener secured to said inner surface of said front

side sheet member;

a lower front end stiffener secured to said inner surface of said front side

sheet member and aligned vertically with said upper front end stiffener;

each stiffener defining in cross-section a web and a flange extending

outwardly from each end of said web, each of the flanges having a free edge; and a tubular bead extending along the free edge of each flange for

reinforcing said stiffener, the tubular beads being inturned and of an elliptical cross- section wherein the minor axis is at least about 40 percent of the major axis.

21. The automotive sheet body structure as defined in Claim 20, wherein one of said flanges extends at right angles to said web to define a mounting flange, and the

other of said flanges is a bowed flange.

22. The automotive sheet body structure as defined in Claim 21 , wherein said web has a width between two times and seven times the width of said bowed flange

and the width of said mounting flange.