Land vehicles: bodies and tops – Bodies – Structural detail
Reexamination Certificate
2001-04-10
2003-07-08
Dayoan, D. Glenn (Department: 3612)
Land vehicles: bodies and tops
Bodies
Structural detail
Reexamination Certificate
active
06588831
ABSTRACT:
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 imperfectio
Browning & Bushman P.C.
Helmreich Loren G.
Ochoa Carlos M.
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