WO1994027840A1

WO1994027840A1 - Vehicle bumper

Info

Publication number: WO1994027840A1
Application number: PCT/US1994/005784
Authority: WO
Inventors: Dinesh Bhutani; James Allen Crandall
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1993-05-21
Filing date: 1994-05-23
Publication date: 1994-12-08

Abstract

The present invention relates to a lightweight, high performance, impact absorbing vehicle bumper. The bumper comprises a slotted, reinforced I-section beam (11) formed from fiber reinforced thermoplastic or thermoset materials. The bumpers of the invention are capable of absorbing the force of a low speed impact without the use of attenuators or other shock absorbing systems.

Description

VEHICLE BUMPER

This is a Continuation-in-Part of United States Patent Application No. 981,906, filed November 23, 1992, which is a Continuation of United States Patent Application No. 834,016 filed February 10, 1992, now abandoned.

The present invention relates to a light weight, high performance, impact absorbing vehicle bumper. The bumper comprises a slotted, reinforced I-section beam formed from fiber reinforced thermoplastic or thermoset materials. The bumpers of the invention are capable of absorbing the force of a low speed impact without the use of attenuators or other shock absorbing systems.

BACKGROUND OF THE INVENTION

Current automotive bumper systems generally consist of a C-shaped beam which spans the vehicle front rails. Loads experienced by the beam in a low speed collision (typically 2.5 to 5.0 mph) are passed to the front rails of the vehicle. In order to prevent damage to the components protected by the bumper system, the kinetic energy is absorbed by the bumper system. Other components such as hydraulic attenuators, absorptive foams or egg-crating type parts are usually added to help dissipate system energy. Multi-step processing is. required because the present bumper systems are made up of several different types of materials.

United States Patent 4,762,352, for example, discloses a bumper assembly with a front made of fiber reinforced thermoplastic resin, a back-up beam and a layer of foam between the face and the back-up member. Each of these parts are made of different materials and requires a separate processing step to form the bumper.

United States Patents 4,635,984, 4,586,739 and 4,616,866 teach a bumper construction having a semi-rigid resilient face and a rear mounting member. An impact absorbing foam is layered between the two pieces. During a collision, the outer face deforms and compresses the foam dissipating at least some of the energy of the impact.

United States Patent 4,482,180 appears to disclose a bumper system which may comprise an I-section beam having a resilient outer covering and one or more layers of an energy absorbing foam between the I-section beam and the outer covering.

United States Patent 4,925,224 discloses a C-shaped bumper having one or more reinforcing ribs between the front wall and a rear metal support plate. In this bumper the ribs and walls are prepared from an elastomer such as polyurethane. Upon impact the walls and the ribs flex, absorbing at least some of the impact energy. It is the flexing of the walls and the ribs which absorbs the energy of the impact. While the invention purports to withstand impact forces of up to 20,000 lbs., the bumper used in the tests is almost twice the size of a conventional automotive bumper.

Finally, United States Patent 4,762,352 discloses a bumper having an inner support beam with a resilient outer cover. An impact absorbing foam is placed between the beam and the cover with an air space between the cover and the foam. The back-up beam is a hollow tube made of fiber reinforced thermoplastic resin.

Each of these designs is made up of several different components each formed from different materials. This means that many different steps are required to produce a single unit of limited effectiveness. These multiple steps and different materials add significantly to the cost of producing each vehicle. In addition, they add to the weight of the vehicle reducing the fuel efficiency of the vehicle.

In addition, as seen in the patents discussed above, most of the currently produced composite bumper beams have cross sections which are generally "C" shaped or closed sections. The closed sections are generally square or rectangular in cross section. The use of a particular section is determined as much by production methods and material properties as is by the requirements of the design.

C-sections are particularly suited to materials that are relatively difficult to form due to such problems as material flow limitations which limit the complexity of the finished part. For example, in C-sections made from reinforced thermoplastic resins, the resin and reinforcement material flows smoothly and evenly because the section is continuous with few or no branches. If preforms are needed, they can be simple with the best possibility of accurate placement in the tool.

Hollow closed sections derive naturally from bumper sections made from welded channel or combined C-section shapes. See, for example. United States Patent 3,779,592. These shapes provided the best torsional rigidity but are not the simplest or most cost effective beams to produce using high performance materials. Most high performance materials that are used in high speed production lines (e.g. one part produced every 30 to 90 seconds) require high forming pressures, high strength tooling or complicated machinery. Hollow sections have added complexity in that they require some means of forming an internal shape. This is usually accomplished by the use of tooling slides, tooling or casting cores, gas assisted molding or similar processes. These methods significantly increase the complexity of the molding process and result in a further increase in investment cost, cycle time or both.

I-section beams are widely used in architecture because they can carry heavy loads per unit weight. These loads, however, must be central to the beam. I- section beams cannot carry an eccentric load to the same extent that they can carry a central load. As vehicle bumpers often experience eccentric loads, the I-section beams inability to handle these loads has prevented their being used in bumper applications. For example, the bumper disclosed in U.S. Patent No. 4,482,180 discloses the use of an I-section beam as one of the supports which can be used in the practice of the invention. If an ordinary I-section beam were used, however, one would expect the front wall to collapse if it encountered an eccentric load. This is because the patent does not teach or suggest the use of any reinforcement for the front wall.

Hollow beam sections, on the other hand, can carry eccentric and central loads since they also have sections that resist torsion. For central loads, I-section beams and hollow beams of equal area have equal load capacity.

C-section bumper failure often occurs with some buckling of the horizontal walls. This indicates that the ultimate load for a C-section bumper is lower than the ideal load and is also difficult to predict. This lack of predictability is due to the fact that no specific buckling area is designed into the bumper. As a result, the design safety factor for a C-section bumper must be larger than for an I-section beam section. The buckling of the C-section bumper can be reduced by the addition of material to the bumper. While this increases the stiffness of the bumper, it is not as cost effective as shifting to a hollow beam or I-section beam construction. A major disadvantage of both C and closed sections is that two walls are used to carry the span load. For bumper beam applications, this means that each of the horizontal walls of the beam must be capable of withstanding a substantial portion of an eccentric load (high and low hits) therefore increasing beam weight. Increased beam weight means increased construction costs in the form of additional materials used and also reduces the fuel efficiency of the vehicle by adding to the overall weight of the vehicle.

In addition, conventional bumper systems require the use of impact attenuators or absorbers to prevent damage to the vehicle. These shock absorbers add yet more weight to the bumper system and further reduce fuel economy and add to cost of the vehicle.

SUMMARY OF THE INVENTION

The bumper of the invention comprises an I-section beam constructed of fiber reinforced thermoplastic or thermoset materials. A slot varying width is provided in the web of the beam. In addition, reinforcing ribs are provided to enable the beam to withstand eccentric loads and prevent localized failure of the front wall of the bumper.

The bumper is designed such that the front portion or region of the I beam is flexible enough that upon impact, the front portion of the bumper deflects inward closing the gap created by the slot in the web. As the gap or slot fully closes, the front portion of the bumper engages the rear section and the two closed parts act as a single piece. When the impact pressure is released, the resiliency of the materials used allows the front portion to return to its original position. In this manner, the bumper itself is capable of absorbing a low speed impact without resorting to the use of shock absorbers or other energy managing devices (e.g. foams)..

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a top perspective view of a bumper of the invention.

Figure 2 is a top perspective view of a bumper of the invention showing an alternative slot design.

Figure 3 shows a male/female connector for the front and rear sections of the bumper of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1 the bumper of the invention comprises a curved I-section beam 11 with closed ends 12. The I-section beam consists of a front wall 13, a rear wall 14 and a web 15. Reinforced mounting points 16 are provided to ensure proper joining of the bumper to the vehicle. The center of the web 15 has been cut away to form a slot 17 which extends from one attachment region to the other. The slot is oriented in the general direction of the front and rear walls dividing the bumper into front and rear sections 18, 19. In the figure, the slot is shown to have a tapered configuration with the gap being widest at the center of the beam and narrowest at the mounting points. While this is one possible configuration of the slot, other possible designs can be used.

One feature which affects the slot design is the fact that the deflection of the front section to close the slot absorbs approximately 2/3 of the impact energy and the entire bumper, when closed, absorbs the remaining 1/3. In addition, the outer section must not fail before the slot closes and the rear section is able to reinforce the front section.

Given these constraints, several different slot designs are possible. One such design is shown in Figure 1. In this configuration, the width of the web on either side of the slot is essentially equal with the web section being narrowest at the center and widening until they meet at either end of the bumper.

With this configuration, the center of the slot closes first with the ends closing last. In this configuration, care must be taken that the slot is not so large that the front section fails before the front section contacts the rear section. Other limiting factors effecting the slot size include the energy to be absorbed and the overall size of the bumper.

An alternative slot design is the "U" design shown in Figure 2. In this design, the web of the front section is widest in the center 23 and tapers down as it reaches the mounting points 24. The web of the rear section complements this pattern. The slot 25 tapers along the U shape, being narrowest at the ends and widest in the center.

In this design, the slot first closes at the ends and, as more energy is absorbed, it then closes in the center. In this design the chances that the front section will fail before it energizes the rear section are reduced because, as the energy increases, the distance between the load points decreases.

Other slot designs will be readily apparent to those skilled in the art. As discussed above, the key design factor is that the slot should close and the two sections should engage each other before the front section reaches maximum capacity.

The bumper also contains a series of reinforcing ribs 20 extending from the front and rear walls inwardly towards the slot. The ribs 20 provide reinforcement to the walls in the case of eccentric loads and prevent localized failure of the front wall. The number and orientation of the ribs can vary but in the preferred design, the ribs should form a "V" pattern with the top of each "V" at the wall and the corner of the "V" at the slot. The ribs should be placed such that when the slot closes on impact, the ribs come together to form a cross shaped pattern providing maximum torsional stiffness. At a minimum, the ribs should extend from one mounting point to the other. Optionally, the ribs may extend into the bumper corners 21 to provide additional corner support.

The ribs extend both above and below the web providing support to the upper and lower portions of the walls. The ribs should be of sufficient height to support the wall during eccentric loading. Other factors, such as die tooling limitations may restrict the height of the ribs.

Spacing of the ribs is also important. The distance between the points at which the ribs meet the front wall should be close enough to support the wall but not so close that the overall weight is increased without meaningful gain in strength. The number and spacing of the ribs will also depend on the desired length and strength of the bumper. For example, for a 1300 millimeter length bumper designed for use with a full sized car, it has been found that nine sets of ribs spaced 100 millimeters apart provided sufficient support to allow the bumper to withstand up to 14,000 pounds of static force and to pass the industry standard dynamic load test. The ribs serve several functions. In the centrally loaded condition, they serve as stabilizers for the center of the web. This means that the web does not have to withstand buckling or local vertical bending. Therefore, load capacity of the beam has a much tighter tolerance and higher value. The ribs when used in a cross pattern help prevent vertical rotation of. the beam when an eccentric impact occurs. The cross pattern ribs also act as stabilizers for the front wall. For eccentric loading, the ribs pass the load to the rear wall as necessary thus preventing bending or rotation of the front wall. The ribs also prevent local failure of the front wall due to eccentric concentrated loads.

The I-section beam bumper of the invention is designed to absorb most, if not all of the energy from a low speed impact. This is accomplished in the following manner. Upon impact, the front section of the bumper flexes inwardly causing the slot to narrow. This flexing allows at least some of the energy to be absorbed similar to the action of a leaf type spring. As additional force is applied, the slot closes, the front section engages the rear section creating a unitary member capable of withstanding a significantly greater force than the front section alone. The bumper then acts similar to a solid web I-section beam bumper similar to that described in co-pending, commonly assigned United States Patent Application 981,906 the teachings of which are hereby incorporated by reference. When the impact force is released or is removed, the front section disengages from the rear section and returns to its original position.

In the preferred embodiment, it is desirable to provide a means for positively locking the front and rear sections together when they come into contact. This will prevent lateral movement of the sections and allow shear stress transfer which will help ensure that the two sections act in unison when the slot is closed. The locking means should be such that when the impact force is removed, the sections become unlocked and are allowed to return to their original positions.

In the practice of this invention it is necessary to ensure that when the slot closes on impact, the front section mechanically connects with the rear section to pass shear loads between the sections. One possible means for accomplishing this task is through the use of a vertical member 22 (shown in Figure 1) at the back of the front section running along the contour of the slot. In this embodiment the vertical member 22 extends above and below the web 15 to a height sufficient to withstand maximum shear forces. The vertical member 22 will ensure proper connection between the front 18 and rear 19 sections even if the front section is out of alignment with the rear section.

One possible locking system is shown in Figure 3. In this embodiment, a cylindrical male locking member 31 is provided at the juncture of the web 15 and reinforcing ribs 20 of the front section. A portion of the cylinder extends into the slot. Opposing the cylindrical member in the rear section is a concave female member 33 located at the juncture of the reinforcing ribs of the rear section and the slot. In this design, when the slot closes, the male member engages the female member in such a manner as to prevent lateral movement of the front and rear sections. It is preferred that one of these locking mechanisms be provided for each set of reinforcing ribs.

The above discussion is but one method for locking the two sections together. Other methods will be obvious to those skilled in the art.

One advantage of the present design is that it provides a bumper system which can perform as well if not better than conventional bumper designs at a fraction of the weight. This is true even when the bumper of the invention is compared to solid web I-section beam bumpers.

The performance of a slotted bumper of this invention is compared with that of a solid I-section beam bumper. The solid beam bumper has an overall length of 1300 millimeters and a maximum width of 102mm and a height of 127mm. The slotted beam bumper of the same dimension except that a tapered slot, having a maximum width of 50mm at the center of the beam is cut into the bumper. The beams are subjected to increasing force until the beam fails. Energy absorbed and deflection are calculated. For the solid beam, a deflection of 1.06 (36.2 mm) inches and a total force of 7934 inch/lbs are calculated. For the slotted beam a total deflection of 3.42 inches (87.55 mm) and a total energy absorbed of 15,143 inch/lbs are calculated.

From this data it can be seen than the slotted beam is capable of absorbing over twice the energy than the solid beam at an equivalent weight. Moreover, the overall weight savings will increase as the slotted beams allows the elimination of additional energy absorbers for the bumper beams.

In addition, weight savings can also be achieved by adjusting the thickness of the various parts of the invention. For example, in the preferred embodiment, the front wall is thickest at the center portion of the beam and gradually thins as the wall moves towards the ends. The web on the other hand is thinnest in the center and thickest at the ends near the mounting points. For example. , for a full sized passenger car, the thickness of the front wall may vary from as thick as 9.5 millimeters at the center to 5.0 millimeters at the ends. The web in the same bumper would have a thickness of about 5.0 millimeters in the center to about 10 millimeters at the ends.

The bumpers of the invention can be fabricated from any suitable reinforced thermoplastic or thermoset material. The material should be such that the reinforcing material is uniformly distributed through out the molded beam to provide uniform strength throughout the beam. Typical thermoplastic resins which can be used in the practice of the invention include polyolefins (polyethylene, polypropylene copolymers) , vinyl polychloride, polystyrene, polyamides, saturated polyesters, polyphenylene ether, polycarbonates and plastic alloys. Typical thermoset resins which can be used include: polyesters, vinyl esters, epoxides, sheet moulding compound and bulk moulding compound.

The reinforcing fibers used in the practice of the invention are such that their physical structures are unchanged by the forming process. They can be selected from the group comprising glass fibers, carbon fibers, ceramic fibers, boron fibers, glass wool, rock wool, metallic fibers, and synthetic organic fibers of a high melting point (e.g. aromatic polyamides, polyesters and others) . Several different types of fiber can be used together in the same starting material.

One particularly good and preferred material for fabricating bumper beams of the present invention is a glass fiber reinforced polypropylene manufactured by Exxon Chemical Company and sold under the trademark TAFFEN. Method for preparing this material and for shaping the material are discussed in U.S. Patent 4,913,774; Published European Patent Application 88 402414.2 and Published International Application PCT/EP91/00021.

Claims

WHAT IS CLAIMED IS:

1. A vehicle bumper comprising

a) an I-section beam having a front wall, a rear wall and a web connecting said front and rear wall;

b) a slot within said web; and

c) a plurality of reinforcing ribs.

2. The bumper defined in claim l wherein said slot runs in the direction of said front and rear walls.

3. The bumper defined in claim 1 wherein said bumper is fabricated from fiber reinforced thermoset or thermoplastic materials.

4. The bumper defined in claim 1 wherein said bumper is fabricated from glass fiber reinforced polypropylene.

5. The bumper defined in claim 1 further comprising a cover.