GB2186240A

GB2186240A - Two-stage energy management unit for mounting vehicle bumpers

Info

Publication number: GB2186240A
Application number: GB08701312A
Authority: GB
Inventors: James Michael Shamaly
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1986-01-24
Filing date: 1987-01-21
Publication date: 1987-08-12
Anticipated expiration: 2007-01-21
Also published as: GB2186240B; DE3700854A1; GB8701312D0; DE3700854C2

Abstract

The unit (20) includes a hard thermoplastic outer case (24) and a microcellular foam inner core (26) slidingly positioned within the case to provide two-stage energy absorption in use as an energy absorbing support for bumper 12. The outer case 24 provides the means (flanges 27,28) by which the bumper bar 18 is fixed to the vehicle body 14. The preferred material for the case 24 is a hard thermoplastics such as Surlyn (RTM), Hytrel (RTM) or Santoprene (RTM). The core 26 is preferably of a microcellular polyurethane foam. <IMAGE>

Description

SPECIFICATION Two stage energy management unit The present invention relates generally to energy management units and more specifically to energy management units configured as supports for automotive vehicle bumpers.

Since the advent of energy absorbing bumpers in automotive vehicles, the energy management function of the bumper systems has been divided between the structure of the bumper itself and certain support members particularly designed for their energy absorption function. Most widely used, and particularly effective, have been energy absorbing bumper supports configured as hydropneumatic shock absorber-like devices. Disadvantageously, these devices are relatively complicated and expensive to produce, and additionally are sometimes undesirably heavy for the function they perform. The weight and simplicity disadvantages of these energy absorbing units have been considered in the prior art, and attention has been given to taking advantage of modern plastics forming technology to devise a structure for performing the desired energy absorbing function in a lighter, less complicated manner.Exemplary prior art patents in which formed rubber and plastic foams have been used include U.S. 4,258,641 to Wakamiya, U.S. 3,857,596 to Nakamura et al, and U.S.

3,606,295 to Appleton. None of these prior art non-metallic energy absorbing unit structures, however, are known to have as efficiently handled the crash load energy absorbing function of the commercially prevalent hydropneumatic devices. Further, they do not provide two stages of energy management in a manner found by experience in automotive design to be desirable to provide sharply rising resistance to load over a relatively small displacement of the bumper energy absorbing structure and to subsequently provide a relatively constant resistance upon further displacement.

This desirable operating schedule has been accomplished in the energy management unit of the present invention in a light, simple and economic structure.

According to the invention there is provided an energy management unit for a motor vehicle having a body (14) and a bumper bar (18) positioned adjacent to the body (14), the energy management unit (20) comprising a case member (24) fixedly secured between the body (14) and the bumper bar (18) and defining a chamber (34) extending axially in a direction parallel to the longitudinal axis of the body (14), and a core member (26) slidingly received within the chamber (34) and extending axially to a length less than the chamber (34), the case member (24) and the core member (26) being fabricated of materials having substantially dissimilar energy absorbing characteristics responsive to imposition of compressive forces in the direction of the body longitudinal axis.

The energy management unit is also designed to take advantage of the dissimilarity in materials between its primary components in shell and core to enhance the capability of tailoring the energy absorbing capability of the unit over a wide range of temperatures.

The invention will now be described further by way of example with reference to the accompanying drawings in which: Figure 1 is a partial cross-sectional perspective view of the front bumper area of an automobile illustrating the installation environment of the invention energy management unit; Figure 2 is a diagrammatic cross-sectional view of the energy management unit in its rest position; Figure 3 is a diagrammatic cross-sectional view of the energy management unit showing the configuration assumed after the first stage of energy absorbing operation; Figure 4 is a diagrammatic cross-sectionai view of the energy management unit of the present invention showing the configuration that the energy management unit assumes during the second stage of energy absorbing operation; Figure 5 is a diagrammatic cross-sectional view of the energy management unit in its post-impact position;; Figure 6 is a partial diagrammatic cross-sectional view illustrating an alternative configuration of the foam block of the invention energy management unit; Figure 7 is a graphical representation of the energy absorption characteristic of the solid plastic outer shell; Figure 8 is a graphical representation of the energy absorption characteristic of the foam block; and Figure 9 is a graphical representation of the energy absorption characteristic of the complete energy management unit.

Turning now to the drawings and particularly to Fig. 1 thereof, an automobile 10 is illustrated as including a bumper assembly indicated generally at 12 mounted to a portion of its body 14. The bumper assembly 12 may include an outer decorative fascia 16 carried in a known manner on a relatively rigid bumper bar, indicated at 18. Connection of the bumper assembly 12 to the body 14 is effected in the preferred embodiment through one or more energy management units 20 (one shown here) which are interposed between a suitable surface such as 22 of the body 14 and the bumper bar 18.

The energy management unit 20 comprises a pair of plastic members having dissimilar energy absorption properties. These are an outer case member 24 formed of a hard thermoplastic, such as that sold by Dupont under the trademarks "Surlyn" or "Hytrel", or that sold by Monsanto under the trademark "Santoprene", and an inner core member 26 formed as a block of foam, as for instance a microcellular polyurethane foam. It is through the outer case 24 that mechanical fastening to the body panel 22 and the bumper bar 18 is effected. Front and rear flange portions 27, 28 form a part of the case 24 and include apertures (all indicated by the numeral 30) for receiving suitable fasteners for effecting the mechanical connection. Axially extending side walls 32 join the flanges 27 and 28 and define a rectangular chamber 34 for receiving the core member 26.It should be understood that while a rectangular chamber 34 is described in this preferred embodiment and has in fact been found to be effective in use, it is recognized that other shapes are possible within the scope of the teaching of the present invention.

The foam core 26 is preferably slidingly received within the chamber 34 as is shown in Figs. 1 and 2. Corrugation of the outer surface of the foam core 26 as shown in Fig. 6 has also been found to provide acceptable operating characteristics through enhancing the notion of freedom between case and core.

Operation of the energy management unit 20 can best be appreciated by reference to the diagrammatic representations of Figs. 2-5 and the graphical representation of Figs. 7-9.

Considering first the depictions of Figs. 2-5, the energy management unit as shown in Fig.

2 in its at-rest position between the the body 14 of the vehicle 10 and the bumper bar 18.

In this position it is to be noted that the axial length of block 26 is less than the length of the chamber 34 formed within the case 24.

Upon impact of the vehicle through its bumper assembly 12, the relatively stiff bumper bar 18 acts on the case 24 to buckle the side walls of the case 24 in columnar fashion, as indicated in Fig. 3, so that for the initial amount of axial travel of the bumper assembly rearwardly equal to the difference in length between the core 26 and the case 24, the resistance is provided solely by the case 24.

Further travel, as indicated in Fig. 4 continues bending of the walls of the case 24 and additionally compresses the foam core 26 so that both case and core resist this additional movement. After impact, below some predetermined level of force and extent of displacement, removal of load will permit return of the energy management unit to its original configuration, as shown in Fig. 2. Such reversible deflection may be enhanced by modifying surface configurations of the block, as indicated by the corrugations shown in the Fig. 6 embodiment, to reduce frictional effects between the block 26 and the case 24. Some compression of the core, as indicated in Fig. 5, is likely and acceptable.

The significance of this two-stage movement may be better appreciated by reference to the graphical representations of Figs. 7-9.

In Fig. 7, the energy absorbing characteristic of the outer case 24 is illustrated, indicating that the elastic resistance of the plastic case to columnar buckling produces an energy absorption characteristic providing for relatively high resistance for little displacement, followed by a rapid reduction in load resistance as buckling radically increases. In Fig. 8, the absorption characteristic of the foam block 26 is graphically represented, indicating that after the inital relative lost motion portion of the travel, the core block 26 rapidly takes on load to a moderate, maximum figure and maintains relatively constant load absorbing capacity through the remainder of a desired stroke, typically two inches in the application described.Overlaying these two dissimilar energy absorbing characteristics yields the characteristic graphically illustrated in Fig. 9 for the composite system in which substantial energy absorption capacity is indicated in the total energy management unit 20, since case 24 and core 26 are arranged in mechanical parallel fashion so that the rapid, initial absorption of the case 24 is utilized and absorption with further stroke is enhanced by the backup cushioning of the foam core 26.

By choosing two dissimilar materials for the two major portions of the energy management unit 20, it has been found to be possible to tailor the performance of the energy management unit 20 to provide desirable energy absorption over a wide range of temperatures.

At low temperatures, the shell member becomes relatively stiff and may be the primary means of absorbing energy. Thus, the wall thickness of the case and its flexural modulus can be chosen to attain the desired peak load and maximum effective displacement. In a high temperature environment, on the other hand, the outer case may become relatively flexible and contribute less to the overall energy management scheme. Consequently, the foam block inner core 26 can be chosen to have a percent fill and a material density to achieve a required peak load and maximum displacement for an optimum high temperature performance.

Thus, the energy absorbing characteristics shown graphically in Figs. 7-9 can be seen to be somewhat temperature dependent and defining operation as illustrated in these figures for ambient temperature permits choice of material specifications and sizing in the components to yield appropriate compromises for high and low temperature operation.

Claims

1. An energy management unit for a motor vehicle having a body (14) and a bumper bar (18) positioned adjacent to the body (14), the energy management unit (20) comprising a case member (24) fixedly secured between the body (14) and the bumper bar (18) and defining a chamber (34) extending axially in a direction parallel to the longitudinal axis of the body (14), and a core member (26) slidingly received within the chamber (34) and extending axially to a length less than the chamber (34), the case member (24) and the core member (26) being fabricated of materials having substantially dissimilar energy absorbing characteristics responsive to imposition of compressive forces in the direction of the body longitudinal axis.

2. An energy management unit as claimed in Claim 1, wherein the case member is fabricated from a hard thermoplastic material.

3. An energy management unit as claimed in Claims 1 or 2, wherein the core member is fabricated from a polyurethane foam.

4. An energy management unit as claimed in any one of Claims 1 to 3, wherein the case member defines a hollow column and elastically deflects in columnar buckling to absorb energy to provide sharply increasing resistance to small deflections and thereafter to provide sharply decreasing resistance to further deflections, and the core member defines a block shaped elastomeric spring providing a relatively linear increase in resistance to deflection after deflection of the case member by an amount equal to the difference between the length of the core member and the case member chamber.

5. An energy management unit as claimed in any one of the preceding claims, wherein outer surfaces of the core member comprise corrugated surfaces.

6. An energy management unit for a motor vehicle substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.