NASA CASE NO. LAEL15397-1

PRINT FIG. J

NOTICE

The invention disclosed in this document resulted from research in aeronautical

and space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the patent licensing regulations applicable to U.S.

Government-owned inventions (37 CFR 404.1 e.;t seq.).

To encourage commercial utilization of NASA-owned inventions, it is NASA

policy to grant licenses to commercial concerns.

nonexclusive licensing to promote competition and

Although NASA encourages

achieve the widest possible

utilization, NASA will provide the necessary incentive to the licensee to achieve early

practical application of the invention.

Address informational requests for this invention to the Technology

Applications Team, NASA Langley Research Center, Code 200, Hampton, Virginia

23681-0001.

Address applications for license to the Patent Counsel Office, Mail Stop 212,

3 Langley Boulevard, Hampton, Virginia 23681-0001.

Serial No.: 08/624,843

Filed: 03/20/96 La RC
NASA Case No. LAR 

CRASH-ENERGY ABSORBING 

15397-1 

COMPOSITE STRUCTURE 

AND METHOD OF FABRICATION 

A stand-alone, crash-energy absorbing structure and fabrication method are provided. 

A plurality of adjoining rigid cells are each constructed of resin-cured fiber 

reinforcement and are arranged in a geometric configuration. The fiber reinforcement 

can be in the form of a fabric or braided fibers wrapped about a core that is either left 

in place or removed from the ultimate cured structure. The geometric configuration 

of cells is held together with more fiber reinforcement (in the form of fabric or braided 

fibers) in order to integrate the cells in the geometric configuration. The additional 

fiber reinforcement is resin-cured to the cells. Curing of the cells and ultimate 

structure can occur in a single step. In applications where post-crash integrity is 

necessary, ductile fibers can be used to integrate the cells in the geometric 

configuration. 

The novelty of the present invention is that simple fabrication techniques are used to 

create structures that can be formed in a variety of net stable shapes without 

additional reinforcement and can withstand combined loading while crushing in a 

desired direction. 

Inventors: Sotiris Kellas Huey D. Carden 

Address: 51 2 Birkdale Court 126 Tanglewood Drive 

Yorktown, VA 23693 Hampton, VA 23666 

ss: - ss: 

Employer: LESC NASA LaRC 

Initial Evaluator: James H. Starnes, Jr. 

Serial No.: 081624,843 

Filed: 03120196 
NASA Case No. LAR 15397-1 -1- PATENT APPLICATION

CRASH-ENERGY ABSORBING COMPOSITE STRUCTURE

AND METHOD OF FABRICATION

5

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Origin_lnvendon

The invention was jointly made by an employee of the United States

Government and a contract employee during the performance of work under

NASA Contract NASI-19000 and is subject to the provisions of Section 305

of the National Aeronautics and Space Act of 1958, as amended, Public

Law 85-568 (72 Star. 435; 42 USC 2457.

Background of 4heLInvention

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1. EieldJ_{the lnven_on

This invention relates to energy absorbing composite structures.

More specifically, the invention is a crushable structure for use in moving

vehicles such as aircraft, automobiles, etc., as well as stationary structures

such as highway barriers, shipping containers, etc., and a method of

fabricating such a structure.

2. Description of the Belated Art

25

The effective management of crash energy is of interest in a wide

variety of applications. For example, the management of crash energy in

moving vehicles such as aircraft and automobiles is important from a safety

perspective in the event of an accident. The management of crash energy

in stationary structures such as shipping containers and highway barriers is

important from the perspective of damage control as well as safety. While
NASA Case No. LAR 15397-1

5

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-2- PATENT APPLICATION

there are several methods for managing crash energy, there are very few

cost effective solutions that can be accomplished without adding unwanted

weight. Examples of such solutions that have been used include thin-wall

steel tubes in automobiles for managing front and side impacts, and sine

wave composite beam webs in military helicopters for managing vertical

impacts.

The optimization of steel structures is generally limited by the uniform

thickness of the sheet material used in the construction of the energy

absorber. Any attempt to introduce thickness variation and/or "tee" or

"cross" intersections in the cross-section of a column structure (in order to

maximize the specific energy absorption) in a steel or aluminum structure

will result in prohibitive fabrication costs and/or weight penalties.

Therefore, optimization or structural tailoring of metal energy absorbers is

limited by the fabrication processes as well as the isotropic nature of metal

materials.

The use of composite sine wave beam web concepts for crash energy

management haS been demonstrated successfully in helicopter subfloor

structures. However, the fabrication of such structures requires the use of

an expensive molding tool. The optimization of the sine wave beam web is

therefore limited by the molding tool which has a fixed wave length and

wave amplitude. Another disadvantage is the sensitivity of this design to

fabrication irregularities. Yet, a reduction (or elimination) of fabrication

flaws often translates to higher production costs through the fabrication of

better quality molding tools and/or rejection of finished parts.

25

Summary of the 1_

Accordingly, it is an object of the present invention to provide a

structure that is effective in managing crash energy and a method for
NASA Case No. LAR 15397-1 -3- PATENT APPLICATION

fabricating the structure.

Another object of the present invention is to provide a crash-energy

absorbing structure that is lightweight.

Still another object of the present invention is to provide a crash5 energy absorbing structure that makes use of simple, cost-effective

fabrication techniques.

Yet another object of the present invention is to provide a crashenergy absorbing structure and fabrication method that can be adapted for

a variety of applications.

10 Other objects and advantages of the present invention will become

more obvious hereinafter in the specification and drawings.

In accordance with the present invention, a stand-alone, crash-energy

absorbing structure and fabrication method are provided. A plurality of

adjoining rigid cells are each constructed of resin-cured fiber reinforcement

15 and are arranged in a geometric configuration. The fiber reinforcement can

be in the form of a fabric or braided fibers wrapped about a core that is

either left in place or removed from the ultimate cured structure. The

geometric configuration of cells is wrapped with more fiber reinforcement

(in the form of fabric or braided fibers) in order to integrate the cells in the

20 geometric configuration. In applications where post-crash integrity is

necessary, ductile fibers can be used to integrate the cells in the geometric

configuration.

25

Brief Description of_the 13rawJngs

FIGs. 1A-1C depict a typical fabrication sequence for an embodiment

of the crash-energy absorbing structure according to the present invention

where:

FIG. 1A is a perspective view of a core being wrapped with
NASA Case No. LAR 15397-1 -4- PATENT APPLICATION

continuous fiber reinforcement in the form of a fabric in order to form a cell;

FIG. 1B is a perspective view of several cells arranged in a onedimensional configuration and further showing the containment skin prior

to being applied about the arranged cells;

5 FIG. 1C is a perspective view of the resulting crash-energy absorbing

structure showing the variety of forces it can experience when crushed;

FIG. 2 is a perspective view of a column structure according to the

present invention in which the cells are arranged in a two-dimensional

configuration;

10 FIG. 3 is an exploded schematic view of an alternative fabrication

approach in which a plurality of cores are wrapped in a square wave fashion

with a single piece of reinforcing fiber fabric;

FIG. 4 is a perspective view of a shaped crash-energy absorbing

beam according to the present invention; and

15 FIG. 5 is a perspective view of a shaped crash-energy absorbing

column according to the present invention.

Detailed Description of the _o.n

20 Referring now to the drawings, and more particularly to the sequence

of FIGs. 1A-1C, a typical process of fabricating an embodiment of the

crash-energy absorbing structure of the present invention will be explained.

However, as will be apparent to one skilled in the art of composites

fabrication, details of the method of fabrication can vary without departing

25 from the scope of the present invention. Furthermore, structures other than

the one shown could be fabricated as will be discussed later below.

In FIG. 1A, the fabrication process begins with the construction of a

single cell 10 that is based on a block or core 12 of a specified shape. Cell

10 has a longitudinal axis represented by dashed line 11. Core 12 is
NASA Case No. LAR 15397-1 -5- PATENT APPLICATION

typically made from a lightweight material such as a closed-cell, lowdensity, rigid structural foam if it is to remain as part of the fabricated

structure. Examples of such structural foam include polyvinyl chloride

foam, polyurethane foam, polystyrene foam and other syntactic foams.

5 However, if core 12 is to be removed from the ultimate structure, a denser

material that is easily shaped, e.g., wood, wax, or any other material that

is (or can be treated to be) easily separated from cured composite materials,

can be used for core 12.

As shown in FIG. 1A, core 12 is wrapped around the perimeter

10 thereof with a continuous fiber reinforcement in the form of, for example,

a woven fabric 14. By way of example, fabric 14 is an inexpensive fabric

such as a woven glass fabric that is generally wrapped substantially or

wholly about the perimeter of core 12 circumscribing longitudinal axis 11.

However, the present invention is not so limited. As will be explained

15 further below, continuous fiber reinforcement of core 12 could take the

form of fibers that are woven or braided directly about core 12 using

braiding methods well known in the art.

Whether using fabric 14 or braiding fibers directly on core 12,

eventually the fibers must be impregnated with a resin material (such as a

20 polymer resin) before curing. It is to be understood that a variety of matrix

impregnation methods are well known in the art and that such methods are

not part of the present invention. For example, one method involves the

wet lay-up of fabric 14 about core 12 and curing thereof to form cell 10.

Alternatively, fabric 14 could be tacked in place on core 12 and a resin

25 infusion process, e.g., a standard resin transfer method (RTM), utilized at

this point in the fabrication process or once a number of cells 10 are

arranged together as will be described below. The use of a closed-cell foam

for the core material prevents the absorption of the resin material during the

assembly process. Pre-impregnated fabrics and tapes can also be used for
NASA Case No. LAR 15397-1 -6- PATENT APPLICATION

the construction of the cells.

A plurality of cells 10, in either their cured or un-cured state, are

arranged in a geometric configuration suitable for a particular application

such that their respective longitudinal axes are parallel to one another. By

5 way of example, FIG. 1B shows four cells 10 arranged side-by-side in a line,

i.e., one dimensional, as would be the case in beam fabrication. Each cell

typically adjoins the next cell to form an area of contact that serves as

transverse reinforcement in the ultimate structure as will be explained

further below.

10 Once arranged in their desired geometric configuration, cells 10 are

assembled into a unitary structure by means of containment skin(s) 16.

Similar to fabric 14, skins 16 are wrapped around the perimeter of the

arranged cells 10 with a continuous fiber reinforcement in the form of, for

example, a fabric represented as skins 16. Skins 16 are generally wrapped

15 substantially or wholly about the perimeter of the arranged cells 10.

However, the present invention is not so limited. The continuous fiber

reinforcement provided by skins 16 could also take the form of fibers that

are woven or braided directly about the arranged cells 10 using braiding

methods well known in the art.

20 In applications where post-crash integrity of the unitary structure is

a requirement, skin 16 is constructed from ductile fibers such as aramid

fibers (e.g., Kevlar® manufactured by DuPont), polyethylene fibers (e.g.,

Spectra®manufactured by Allied Fibers), or combinations thereof. Skin 16

is wrapped about the so arranged cells 10 in sufficient quantity to both hold

25 the arrangement of cells together and to provide the desired degree of postcrash integrity for the structure as will be described further below. By way

of non-limiting example, two skins 16 are shown in FIG. lB. Naturally, a

single containment skin could also be used to enclose the arranged cells 10.

Note that when post-crash integrity is unimportant, a glass or other
NASA Case No. LAR 15397-1 -7- PATENT APPLICATION

inexpensive fabric could be used for skin 16.

Similar to fabric 14, the fibers of containment skin 16 must be

impregnated with resin and cured in place over the arranged cells 10. As

skin 16 is cured to cells 10, the resin links fabric 14 and skins 16. Note

5 that a single curing step can be used to cure the resin-impregnated fabrics

14 and skins 16. The resulting crash-energy absorbing structure is formed

as shown in FIG. 1C where the structure is referenced generally by numeral

100. Adjoining areas between cells 10 cure together to form transverse

reinforcements 18 between skins 16. Once again, it is to be understood

10 that a variety of resin impregnation and curing methods are well known in

the art and that the choice of a particular method is not critical to the

present invention. For example, skins 16 could be impregnated separately

from or simultaneously with fabric 14 (or the wrapped fiber reinforcement

of the cores 12 if braiding techniques are used). Thus, skins 16 could be

15 impregnated with resin by means of a wet lay-up, the use of preimpregnated fabrics and tapes, or a resin infusion process. The resulting

cured structure 100 can be used as is or is easily cut/shaped slightly to fit

a desired application.

The method of fabrication in the present invention provides great

20 flexibility in terms of customizing shape, size, stiffness and strength. With

respect to stiffness and strength, note that individual cells can be

customized by adjusting the fiber orientation or by providing additional

wraps of the continuous reinforcing fabric 14 (or fibers if braiding

techniques are used) about the core. Note also that each transverse

25 reinforcement 18 consists of at least two layers of reinforcing material, i.e.,

where cells adjoin one another, thereby increasing the transverse strength

of structure 100. This feature can be further enhanced simply by

constructing cells such that reinforcing fabric 14 (or fibers if braiding

techniques are used) overlaps at the portion of each core 12 that is to form
NASA Case No. LAR 15397-1 -8- PATENT APPLICATION

transverse reinforcements 18 in structure 100. Thus, the present invention

has great utility where combined loading can be expected, e.g., any crash

situation.

For purpose of illustration, it is assumed that cores 12 are foam cores

5 that remain a part of structure 100. However, cores 12 could also be

removed from the structure as noted above. In either case, structure 100

(e.g., a beam as shown in FIG. 1C) can be engineered to be stable in a

variety of directions yet crush in a single (desired) direction, e.g., along the

axial direction. The various directions are represented by force arrows and

10 are defined as follows. Structure 100 is configured with the assumption

that the primary crush loads will occur along the longitudinal axis of each

of cells 10. This axial or primary force is represented by arrow 201. The

transverse load direction is defined by arrow 202, and a force experienced

longitudinally with respect to structure 100 is represented by arrow 203.

15 Various shear forces include the in-plane shear force 204 acting in the plane

of structure 100 and transverse shear force 205.

In constructing structure 100, the longitudinal axes of the cells are

aligned with primary crush force direction, e.g., the direction indicated by

arrow 201. The fibers of fabric 12 and the fibers of skin 16 are thus

20 generally oriented relative to the primary crush load direction to ensure out

of plane stability, shear strength, bending stiffness, etc., to suit a particular

application. For example, assuming axial force 201 represents the direction

of the expected primary crush force, the fibers of fabric 12 could be

oriented with respect to the axial direction in order to improve the shear

25 stability of the structure. The fiber orientation of skins 16 can be oriented

at angles with respect to the axial direction to, for example, regulate the

crushing load.

As mentioned above, the fabrication method and resulting structure

are not limited to one-dimensional (beam) configurations. As shown in FIG.
NASA Case No. LAR 15397-1 -9- PATENT APPLICATION

2, cells 10 can be arranged in a two-dimensional geometric configuration to

produce, for example, a column structure 101. Column structure 101 could

be fabricated in the same fashion as described above.

Mass production of the present invention can be facilitated by using

5 braiding techniques. In such an instance, a long length of the core material

could have the core-reinforcing fibers braided therearound in continuous

fashion. Then, the so wrapped core material could be cut into smaller

pieces thereby forming a plurality of cells which can then be

arranged/assembled into a structure as described above. To simplify the

10 fabrication process even further, the containment skin could be braided over

the assembled cells.

Multiple celled structures of the present invention could also be

fabricated using a single piece of glass fabric. For example, as shown in

FIG. 3, a single piece of glass fabric 24 could be wrapped about a plurality

15 of cores 12 in a square wave fashion. The resulting arrangement of cells

would then be wrapped with a containment skin as described above.

The advantages of the present invention are numerous. The

composite structure is effective at managing crush energy while the

fabrication method is a simple, affordable approach for assembling the

20 structure. The cured structure is stable, can function on its own with no

additional reinforcement and can withstand combined loading while crushing

in the desired direction. Design and fabrication flexibility allows the present

invention to be used for practically any crash-energy management situation

that may occur in either moving or stationary structures. The use of

25 composites also means that the present invention is ideal for use in

environments that would be corrosive for metals.

Although the invention has been described relative to specific

embodiments thereof, there are numerous other variations and modifications

that will be readily apparent to those skilled in the art in light of the above
NASA Case No. LAR 15397-1 -10- PATENT APPLICATION

teachings. For example, while the cores, cells and resulting structures

described herein have been regularly shaped rectangles, beams and/or

columns, this need not be the case. The cores could be any geometric

shape such that, when arranged as cells to form a structure according to

5 the present invention, adjoining cells will be in sufficient contact with one

another to form the transverse reinforcements spanning substantially or

completely between the containment skin(s). In addition, cells of various

sizes and shapes could be combined to make an irregularly shaped

structure.

10 Two such irregularly shaped structures are shown in FIGs. 4 and 5.

In FIG. 4, a resulting beam structure 101 is shaped for a particular

application. Beam structure 101 is constructed from a combination of

rectangular cells 30 and triangular cells 32. The cores for each cell could

be cut from a single piece of core material that was shaped commensurate

15 with the ultimate shape of structure 101. Each core could be prepared to

form a cell (as described above) and the cells repositioned to once again

form the shape of structure 101. Containment skin(s) 16 could then be

applied as described above.

In FIG. 5, an irregularly shaped column structure 102 is shown.

20 Structure 102 is constructed from cells 40, 41, 42, 43 that are irregularly

shaped along the respective longitudinal axes. Once again, the cores for

each cell could be cut from a single piece of core material. Each core could

be prepared to form a cell and the cells repositioned to form the shape of

structure 102. Containment skin(s) 16 could then be applied as described

25 above. Note that structure 102 is formed such that transverse

reinforcements 18 span between skin(s) 16.

It is therefore to be that, within the scope of the appended claims,

the invention may be practiced other than as specifically described.

What is claimed as new and desired to be secured by Letters Patent
NASA Case No. LAR 15397-1

of the United States is:

-11- PATENT APPLICATION
"Pages missing from available version"
NASA Case No. LAR 15397-1 -17- PATENT APPLICATION

CRASH-ENERGYABSORBINGCOMPOSITESTRUCTURE

AND METHOD OF FABRICATION

Abstract

A stand-alone, crash-energy absorbing structure and fabrication

method are provided. A plurality of adjoining rigid cells are each

5 constructed of resin-cured fiber reinforcement and are arranged in a

geometric configuration. The geometric configuration of cells is

integrated by means of continuous fibers wrapped thereabout in order to

maintain the cells in the geometric configuration. The cured part results

in a net shape, stable structure that can function on its own with no

10 additional reinforcement and can withstand combined loading while

crushing in a desired direction.
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