Land vehicles: bodies and tops – Bodies – Door or window with specified vehicle feature
Reexamination Certificate
2001-04-10
2002-08-06
Pape, Joseph D. (Department: 3612)
Land vehicles: bodies and tops
Bodies
Door or window with specified vehicle feature
C296S182100
Reexamination Certificate
active
06428080
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a reinforcing support structure for an automotive closure, and more particularly to stiffeners integrated with the sheet-structural closures.
BACKGROUND OF THE INVENTION
Automotive closures consist predominantly of stiffened sheet structures. Closures include such components as doors, hood, trunk lid, and hatches. They represent a special challenge for the automotive designer because they must be light-weight and manageable for persons operating them during installation and service. In addition, they must be able to resist typical sources of cosmetic and functional damage from external loads during service. At the same time they must substantially contribute to the overall safety, cost effectiveness, structural rigidity and crash worthiness of the vehicle. These important design mandates have accented the necessity for a general, economical, and structurally viable sheet material stiffener capability for reinforcing automotive closures.
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 structures for integration with automotive closures, including those used in cars, trucks, minivans, and sport utility vehicles, in order to maximize structural efficiencies related to these goals. Viable candidate concepts must be capable of addressing not only performance goals, but also economies related to processing, forming, structural integration/configuration, tooling, and assembly requirements. The automotive industry has thus embarked on an exhaustive search for simple, enabling technologies that might fulfill these requirements.
Generally speaking, even the most favored technologies have met only some of the goals, in spite of the significant advantages made possible by using the latest and most advanced technologies now available in computer-based structural simulation and stress analysis as well as in optimization software. These efforts have sometimes made use of very high and ultrahigh strength materials including specially developed steel and extrudable aluminum alloys, as well as ceramic fiber reinforced plastics and metals. Yet they have largely failed to produce a lasting new approach to addressing the needs of the designer.
The most commonly resulting trend has been for weight saving 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, 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 stiffener 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 cause a larger section to fail. This effect is of great concern in the evaluation of crash worthiness of automotive closures, because such failures may be less uniform or predictable, making them less desirable from an occupant safety standpoint. In addition, they may not absorb sufficient crash energy or resist intrusion effectively enough to consistently meet safety performance goals.
Finally, 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 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 had the simultaneous effect of increasing the distance from the centroid to the shear center of the channel. Additionally, increasing the cross sectional flange length sometimes caused difficulty in accessing the interior of the section during assembly or other operations while worsening 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.
Hydroformed tubes have gained some favor recently, but are cost intensive in terms of tooling
Browning & Bushman P.C.
Carpenter Scott
Pape Joseph D.
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