WO1994012806A1

WO1994012806A1 - Energy absorber and method of absorbing energy

Info

Publication number: WO1994012806A1
Application number: PCT/GB1993/002460
Authority: WO
Inventors: David Gordon Boulter; James Gerard Mulholland; Amir Ziv-Av
Original assignee: Aircraft Furnishing Limited
Priority date: 1992-11-30
Filing date: 1993-11-30
Publication date: 1994-06-09
Also published as: GB9225001D0

Abstract

An energy absorber has first and second anchorages (18) and is adapted to absorb energy when and only when an applied load along a line of action extending generally through said anchorages exceeds a predetermined limit. The energy absorber comprises at least one leaf (10) extending between the anchorages, the or each leaf having corrugations (16) transverse to said line of action and being arranged to undergo permanent deformation through straightening or deepening of said corrugations as the applied load exceeds the predetermined limit. A structure including the energy absorber, a vehicle including said structure, a method of manufacturing an energy absorber and a method of absorbing energy are also disclosed.

Description

]

_.

ENERGY ABSORBER AND METHOD OF ABSORBING ENERGY

This invention relates to an energy absorber and in the most important example to an energy absorber for limiting 0 the effects of impact or other damage in aircraft, vehicles and other fixed or movable structures. It also relates to a structure including the energy absorber, to a vehicle including said structure, to a method of manufacturing an energy absorber, and to a method of absorbing energy. 5 A variety of forms of energy absorber have been proposed. In many cases, these absorb energy by undergoing catastrophic structural failure or radical deformation. For passenger-carrying and indeed many other applications, this is likely to be unacceptable. In such applications, 0 a form of energy absorber is required which can absorb energy in an impact without undergoing catastrophic failure and with limited deformation.

Mechanisms have been suggested which are capable of absorbing energy without catastrophic failure or deformation, using - for example - hydraulic rams. Such mechanisms are, however, generally more expensive and may require regular maintenance to guarantee operability. For a particular level of energy absorption, a hydraulic mechanism is likely to be significantly heavier than an element which absorbs energy through structural deformation. This increase in weight will be significant in aircraft, for example.

The present invention seeks to provide an improved energy absorber which is capable of significant energy absorption without catastrophic failure and with limited and controlled deformation.

According to the present invention there is provided in one aspect in an energy absorber having first and second anchorages and being adapted to absorb energy when and only when an applied load along a line of action extending generally through said anchorages exceeds a pre-determined limit, the energy absorber comprising at least one leaf extending between the anchorages, the or each leaf having corrugations transverse to said line of action and being arranged to undergo permanent deformation through straightening or deepening of said corrugations as the applied load exceeds the pre-determined limit.

As used herein, the term "transverse" connotes any direction not aligned with the line of action, it being understood that the corrugations will be effective provided that they are not actually aligned with the line of action. Preferably, for greatest effectiveness, the corrugations would usually be perpendicular to the line of action.

By the provision of such corrugations, within applied load limits a structure of which the energy absorber forms part need not undergo catastrophic failure. Further, the energy absorber can continue to serve a load bearing function even after deformation has taken place. Furthermore, the energy absorber of the present invention can be simple to manufacture and operate. Also, for a particular energy absorption characteristic, the energy absorber according to this invention can be relatively lightweight and thus ideally suited to use in applications such as aircraft where performance to weight ratios are critical.

The design of energy absorber according to this invention can enable the energy absorbing characteristics to be readily tailored for any particular application. For any given section and length of leaf material, the number and size of the corrugations may be changed to vary the load extension characteristic. The number of corrugated leaves can similarly be varied to provide an absorber having the desired overall characteristics.

In one particularly preferred embodiment, said permanent deformation is in straightening of said corrugations. Whilst an energy absorber according to the preferred form of this invention undergoes permanent deformation as energy is absorbed, if this deformation is restricted to straightening of the corrugations it can be controlled. Since the deformation is likely to take the form of only partial straightening of the corrugations, the physical appearance of the energy absorber need not change dramatically, even after substantial energy has been absorbed. There would usually of course be some minor change in physical appearance.

In an alternative preferred embodiment, said permanent deformation is in deepening of said corrugations.

If the deformation is caused by a compressive load (such as might deepen said corrugations), it has been found that there is a possibility that the energy absorber may suffer from out of plane buckling rather than that the corrugations might deepen. To guard against this, the absorber preferably includes means for preventing out of plane buckling of the or each leaf. Such a means may actually be important even if the deformation is in straightening of the corrugations, since, at least when the absorber is used on aircraft seats, it has been found that, on impact of the aircraft, the absorber might first elongate up to its full stretched length then actually shorten somewhat, so to speak "on the rebound". This shortening would involve a compressive load which might cause undesirable out of plane buckling unless prevented by the preventing means.

The preventing means may include a cover for the body of the absorber, said cover forming a close fit with the leaf or leaves. The energy absorber may in any event include a cover for the body of the absorber, to prevent the ingress of dirt and to prevent users coming into contact with the corrugations.

Preferably, for extra strength and to enable the desired characteristics of the energy absorber to be accurately pre-set, the absorber comprises a plurality of such leaves arranged in parallel. For compactness, preferably the corrugations of each leaf are similar, the corrugations of each leaf being arranged so as to mesh with the corrugations of the or each adjoining leaf. Preferably, each leaf has at least three, more preferably at least five, yet more preferably at least ten corrugations, since the greater the number of corrugations, the greater the extensibility (or compressibility) of the absorber and also the greater the amount of energy which can be absorbed.

However, as explained later, it may be desirable to limit the extensibility of the absorber. Hence, the full stretched length of the absorber, with the corrugations straightened, is preferably less than three times, more preferably less than twice, its unstretched length.

Unlike many existing energy absorbing mechanisms, the energy absorber according to the present invention can be manufactured using conventional machinery relatively inexpensively. In one form of the invention, a number of parallel leaves are preferably joined together through welding at opposite ends. Again, for manufacturing ease, the or each leaf preferably terminates in flattened end portions, and the anchorages take the form of simple apertures. The invention extends to a structure, for instance a seat structure, including an energy absorber as aforesaid.

The energy absorber may be incorporated into the remainder of the structure in such a way that at least one of the anchorages is capable of pivoting with respect to the remainder of the structure. This can prevent distortion of the structure in the event of impact. For the same reason, preferably other load bearing members of the structure are also pivotally mounted.

If the structure is a seat structure, the energy absorber is preferably anchored to the seat base, adjacent the seat back. Hence the absorber can operate essentially in tension if the impact is frontal. The invention further extends to a vehicle including said seat structure, the energy absorber being arranged to absorb energy associated with impact of the vehicle, such as might cause a sudden deceleration of the vehicle. A particular application for the present invention arises in passenger seating for civil aircraft. Requirements laid down by certification or regulatory authorities dictate the static or dynamic load which an aircraft seat may be capable of withstanding. For example, under recently introduced certification requirements, an aircraft seat should be capable of withstanding a 16g longitudinal dynamic load. This loading is likely to result in interface loads which exceed the levels which can safely be accommodated by the airframe. For this reason, it is necessary to incorporate within the seat design a form of energy absorber which can attenuate the peak interface loads but neither fail in a catastrophic mode nor result in a permanent deformation of the seat, beyond a specified amount. The energy absorber according to the present invention is well suited to this application since it is capable of absorbing high peak interface loads with a deformation which is limited and controlled.

The invention extends to a method of manufacturing an energy absorber as aforesaid, wherein the leaves are welded together adjacent their ends.

In a related method of manufacture, the corrugations are formed by a hot stamping process.

The invention further extends to a method of absorbing energy comprising providing at least one energy absorbing leaf, and anchoring the leaf adjacent its ends to respective first and second anchorage points, the or each leaf being adapted to absorb energy when and only when an applied load along a line of action extending generally through said anchorage points exceeds a pre-determined limit, the or each leaf having corrugations transverse to said line of action and being arranged to undergo permanent deformation through straightening or deepening of said corrugations as the applied load exceeds the pre-determined limit.

The permanent deformation may be in straightening of said corrugations. Preferred features of the present invention are now described by way of example with reference to the accompanying drawings in which: -

Figure 1 is a section through an energy absorber according to the present invention; Figure 2 is a plan view of the energy absorber shown in Figure 1; and

Figure 3 is a side elevational view of a seat structure incorporating the energy absorber of the present invention. The energy absorber shown in Figure 1 consists of four metal leaves 10 of approximately 25 cm in width and 2.5 mm in thickness. Each leaf has generally flattened portions

12 and 14 and a corrugated mid-portion 16. The corrugations in the individual leaves are formed so that the leaves can be brought together in an assembly with the portions 12 and 14 abutting. Through bores 18 are provided near the ends of the leaves to serve as anchorages and the leaves are held together through welding as shown generally at 20. The corrugations are formed, for example, in a hot stamping process. The constituent metal and the treatment processes are selected in accordance with known principles to provide the required tensile strength.

The number of leaves in the energy absorber can, as will be explained, be varied to suit a particular application. In certain cases, one leaf, rather than four as shown, may suffice.

As shown in Figure 1, the shape of the junctions between the corrugated mid-portion 16 of each leaf 10 and the flattened portions 12 and 14 varies according to the position of the leaf in the array of leaves. The further away from the centre of the array is the leaf, the more cranked is the junction in order that the flattened portions of all the leaves can be contiguous.

The choice of material for the leaves is important. In the preferred embodiment, a mild steel such as SAE 1010 or

S 510 was used. It was found that, for example, Aluminium was not a successful material since it did not permit satisfactory deformation of the leaves.

Referring now to Figure 3, an aircraft seat structure incorporating the energy absorber of the present invention comprises a seat body 100, and, supporting this above the floor of the aircraft, a support structure 102 including a front leg member 104, a rear leg member 106, a diagonal strut 108 adapted to bear a compressive load, and a track cover plate 110. Also shown in this figure is a baggage bar 112 for restraining baggage. Attached respectively to the front and rear leg members 104 and 106 are front and rear feet 114 and 116 which are bolted to the floor track (not shown) of the aircraft. The track cover plate serves the function not only of preventing the ingress of debris into the floor track, but it also serves to maintain a constant spacing between the front and rear feet 114 and 116. It is adapted to bear a substantial tensile load.

The rear leg member incorporates the energy absorber 118 as shown in Figure 3, the absorber being enclosed in a stainless steel cover 120 to prevent the ingress of dirt and to prevent passengers coming into direct contact with the corrugations of the absorber. As shown in this figure, the plane of the corrugations is aligned with the fore and aft direction, so that some side to side movement of the seat structure is possible without the corrugations becoming distorted. The absorber is anchored to the seat base 122 adjacent the seat back 124, in other words, near the rear of the seat, and extends downwardly and rearwardly from this anchorage point. The front and rear leg members 104 and 106 and the diagonal strut 108 are bolted at each of their ends to the other parts of the seat structure in such a way that they can pivot to a limited extent - and against the friction exerted by the bolts - relative to the other parts of the structure. The one exception to this is the top of the front leg member 104, which is attached relatively rigidly to the seat base 122.

In use, the energy absorber forms part of a passenger seat or other structure so designed that the energy absorber is subjected to peak loading. As that loading exceeds a pre-determined limit (that is, if the aircraft crashes) , along a line extending between the two anchorages, the corrugations will begin to straighten. The load at which this straightening commences can be optimised in any particular design ^'and the continuing energy absorbing characteristics can be tuned through straightforward design changes in the size and number of corrugations and the number of leaves within the assembly. This form of design change can be carried out much more simply and with more predictable results than a design change requiring, for example, the use of alternative material or materials of different size. However, different materials or sizes of materials may be used.

Since the main structural members of the seat structure are mounted for slight pivotal movement with respect to each other, if the aircraft is involved in a crash the energy absorber can extend without the other members becoming distorted, without the feet coming adrift from the floor track, and without also the floor becoming distorted.

It is a further advantage of the energy absorber according to this invention that there is, in the preferred form, an inherent degree of flexibility perpendicular to the line of action and to the plane of the corrugations. For aircraft seats and many other applications, this may eliminate the need for any formal universal joint in mounting the energy absorber. As part of certification testing, an aircraft seat may be distorted prior to dynamic testing. These distortions result in internal stressing of the seat structure. The lateral flexibility of the energy absorber according to the preferred form of this invention reduces the static forces and stresses within the seat.

Another important feature of the energy absorber is that it has what is to all intents and purposes a maximum extension, which is dictated by the point at which the corrugations have straightened fully. It will be understood that, for the safety of the passenger and for other reasons, it is important that the movement of the seat structure during a crash is limited in some way. In the preferred embodiment, the energy absorber, which includes about eight to ten corrugations per leaf, is limited to a roughly fifty per cent extension.

In an alternative embodiment, an energy absorber according to the present invention may be operated not in tension but in compression. In this variant, it may be particularly desirable to provide a cover or other guide to ensure that the leaves deform through an increase in the depth of the corrugations rather than a buckling action. The cover needs to form a close fit with the leaves of the absorber. It is to be noted that the cover 120 of the preferred embodiment serves a similar purpose, even though the energy absorber of this latter embodiment is operated basically in tension. This is because this absorber may also undergo a compressive load on the rebound following a full extension of the absorber, as explained previously.

It should be understood that this invention has been described purely by way of example and modifications can be made within the scope of the invention. For example, the energy absorber may be formed with anchorages in forms other than simple apertures. The individual leaves in an assembly can be held together through means other than welding and may indeed in certain cases extend freely and separately between the anchorages.

Claims

1. An energy absorber having first and second anchorages and being adapted to absorb energy when and only when an applied load along a line of action extending generally through said anchorages exceeds a pre-determined limit, the energy absorber comprising at least one leaf extending between the anchorages, the or each leaf having corrugations transverse to said line of action and being arranged to undergo permanent deformation through straightening or deepening of said corrugations as the applied load exceeds the pre-determined limit.

2. An energy absorber according to Claim 1 wherein the corrugations are perpendicular to the line of action.

3. An energy absorber according to Claim 1 or 2 wherein said permanent deformation is in straightening of said corrugations.

4. An energy absorber according to Claim 1 or 2 wherein said permanent deformation is in deepening of said corrugations.

5. An energy absorber according to any of the preceding claims including means for preventing out of plane buckling of the or each leaf.

6. An energy absorber according to Claim 5 wherein said preventing means includes a cover for the body of the absorber, said cover forming a close fit with the leaf or leaves.

7. An energy absorber according to any of Claims 1 to 5 including a cover for the body of the absorber.

8. An energy absorber according to any of the preceding claims comprising a plurality of leaves arranged in parallel.

9. An energy absorber according to Claim 8 wherein the corrugations of each leaf are similar, the corrugations of each leaf being arranged so as to mesh with the corrugations of the or each adjoining leaf.

10. An energy absorber according to any of the preceding claims wherein each leaf has at least three, preferably at least five, more preferably at least ten corrugations.

11. An energy absorber according to any of the preceding claims wherein the full stretched length of the absorber, with the corrugations straightened, is less than three times, preferably less than twice, its unstretched length.

12. An energy absorber according to Claim 8 or 9 wherein the leaves are welded together adjacent their ends.

13. An energy absorber according to any of the preceding claims wherein the or each leaf terminates in flattened end portions.

14. An energy absorber according to any of the preceding claims wherein the anchorages take the form of apertures in the or each leaf.

15. A structure including an energy absorber according to any of the preceding claims.

16. A structure according to Claim 15 wherein the energy absorber is incorporated into the remainder of the structure in such a way that at least one of the anchorages is capable of pivoting with respect to the remainder of the structure.

17. A structure according to Claim 15 or 16, the structure being a seat structure.

18. A seat structure according to Claim 17 wherein the energy absorber is anchored to the seat base, adjacent the seat back.

19. A vehicle including a seat structure according to Claim 17 or 18, the energy absorber being arranged to absorb energy associated with impact of the vehicle.

20. A method of manufacturing an energy absorber according to Claim 12 wherein the leaves are welded together adjacent their ends.

21. A method of manufacturing an energy absorber according to any of Claims 1 to 14 wherein the corrugations are formed by a hot stamping process.

22. A method of absorbing energy comprising providing at least one energy absorbing leaf, and anchoring the leaf adjacent its ends to respective first and second anchorage points, the or each leaf being adapted to absorb energy when and only when an applied load along a line of action extending generally through said anchorage points exceeds a pre-determined limit, the or each leaf having corrugations transverse to said line of action and being arranged to undergo permanent deformation through straightening or deepening of said corrugations as the applied load exceeds the pre-determined limit.

23. A method according to Claim 22 wherein said permanent deformation is in straightening of said corrugations.

24. An energy absorber substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

25. A seat structure substantially as hereinbefore described with reference to and as illustrated in Figure 3.

26. A method of absorbing energy substantially as herein described.