NASA CASE NO.

PRINT FIG.

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 NASA Patent Licensing Regulation (14 Code of

Federal Regulations 1245.2).

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

grant licenses to commercial concerns. Although NASA encourages nonexclusive

licensing to promote competition and achieve the widest possible utilization, NASA

will provide the necessary incentive to the licensee to achieve early practical

application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 143, Hampton, Virginia 23681-0001.

Serial No.: 08/265,361

06/24/94 LaRC

(NASA-Case-LAR-14892-1) IMPROVED

COMPOSITE MATERIAL AND METHOD FOR

PRODUCTION OF IMPROVED COMPOSITE

MATERIAL Patent Application (NASA.

Langley Research Center) 15 p

N95-13836

Unclas

G3/24 002986)
AWARDS ABSTRACT 

IMPROVED COMPOSITE MATERIAL AND METHOD 

FOR PRODUCTION OF IMPROVED COMPOSITE MATERIAL 

The present invention relates to composite materials, and specifically to 

laminated composite materials with improved interlaminar strength and high damage 

resistance. 

The interlaminar strength of a laminated composite material is improved by the 

inclusion of short rods distributed evenly throughout the laminate, with the rods 

perpendicular to the lamina. The rods are shorter than the thickness of the laminate, 

but several times longer than the thickness of each lamina. The rods function as 

known through-the thickness reinforcements, but the resulting material can be made 

at much lower cost than other through-the thickness reinforced materials. 

In a method for making the improved laminate according to the invention, short 

rods are inserted into prepreg material, e.g. by pushing continuous strands of rod 

material through the pliable prepreg material and cutting the rods to proper length 

after penetration. The prepreg layers with rods inserted are laid up, compressed and 

cured as usual in the art. The rods penetrate adjacent layers of prepreg during the 

lay-up and curing process, but are confined to the interior of the laminate by the press 

surfaces, so a smooth laminate is produced. 

-. Inventor: Gary L. Farley 

Home address: 205 Terry's Run 

Yorktown. VA 23693 

U.S. Army Aerostructure Directorate 

Langley Research Center, Hampton, VA 

Initial Evaluator: Charles J. Camarda 

Serial No.: 08/265,361 

Filed: 06/24/94 
LAR 14892-1 -1- PATENT APPLICATION

IMPROVED COMPOSITE MATERIAL AND METHOD

FOR PRODUCTION OF IMPROVED COMPOSITE MATERIAL

10

Origin of the Invention

The invention described herein was made in the performance of work

done by an employee of the U.S. Government and may be manufactured

and used by or for the government for governmental purposes without the

payment of any royalties thereon or therefor.

Background of the Invention

Field of the Invention

15 This invention relates in general to composite materials, and

specifically to laminated composite materials with improved interlaminar

strength.

20

25

Description of the Related Art

Laminated composite materials are widely used as construction

materials in applications covering the range from printed circuit boards to

aircraft skin. Typically, laminated composite materials are made by

arranging a number of thin sheets of reinforcing material impregnated with

an uncured binder in a stack, and then compressing the stack while the

binder is cured by heat or other means. Each lamina of reinforcing material

impregnated with an uncured binder is commonly called a "prepreg". The

cured binder in the finished material is commonly referred to as the

"matrix".
LAR 14892-1 -2-

Laminated composite materials

delamination induced failures due to

PATENT APPLICATION

are susceptible to damage and

low interlaminar and intralaminar

strength. Interlaminar and intralaminar damage can occur, for example,

when runway debris impacts the skin of an aircraft during landing, or a tool

5 is dropped on the aircraft during ground servicing. The mechanical

properties of both the matrix and the reinforcing fiber control the

interlaminar or intralaminar response mechanisms of a composite laminate,

but it is primarily the mechanical properties at the interface between

adjacent plies that control the interlaminar response.

10 Three methods are commonly used to improve the damage tolerance

of composite materials: 1) adding a compliant layer between lamina in the

laminate, 2) using a high failure strain matrix, and 3) inserting through-thethickness (TTT) reinforcement in a laminate. It has yet to be conclusively

proven which technique is best for improving the overall damage tolerance

15 of laminated composite materials. It is likely that no single technique is

universally optimal.

Compliant material between layers of composite material in a laminate

allows relative movement between adjacent layers of composite material,

and thereby reduces the interlaminar stresses along the interface of the

20 compliant layer and the composite material. There are several techniques

for adding a compliant layer between composite lamina. One technique is

to place a layer of compliant film between composite lamina. Another

technique is to add small compliant particles, such as small rubber spheres,

between the layers of composite materials. Usually, the compliant film or

25 compliant particles are applied to individual lamina during the fiber

impregnation ("prepregging") process. These techniques are equally

applicable to fabric, unidirectional tape and single yarn material forms. In

general, the compliant material does not change the processing or handling

qualities of the prepreg material. The compliant layer approach for
LAR 14892-1 -3- PATENT APPLICATION

improving damage tolerance is, however, not applicable to all composite

material systems. For example, most organic matrix materials for elevated

temperature applications are incompatible with the compliant layer

techniques for toughening composite materials. Matrix materials for

5 elevated temperature applications usually exhibit low failure strains which

requires the designer to use a conservative design approach. The compliant

layer also causes a reduction in the laminate fiber volume fraction, which

proportionately reduces the in-plane mechanical properties of the composite

laminate.

10 A high failure strain matrix, such as thermoplastic, also allows relative

movement of adjacent layers. In this case the relative motion is due to the

high strain capability of the matrix. High strain matrices and fibers also

allow greater straining of a layer. Composite materials with compliant layers

or composite materials of high failure strains are sometimes referred to as

15 toughened composite materials. High strain matrix materials that have high

stiffness are generally more expensive than high strain matrix materials that

have lower stiffness. However, the high strain and high stiffness matrix

materials are more difficult to process.

Through-the-thickness ("TTT") reinforcement increases the

20 interlaminar strength and, to a limited extent, increases the intralaminar

strength of the laminate therebyincreasing the damage tolerance of a

material. There are many types of through-the-thickness reinforcement and

insertion methods. For example, stitching and tufting have been

successfully applied to dry fiber preforms prior to resin infiltration and

25 processing into composite laminates as a method of improving damage

tolerance.

While

tolerance, it

densities are between 60 and 160 per square inch.

stitching or tufting of dry fiber preforms improve damage

has significant costs associated with it. Nominal stitch

Using a single needle
LAR 14892-1 -4- PATENT APPLICATION

stitching machine requires between 1 and 3 minutes to stitch a one square

inch region. When this process is applied to an entire wing skin or fuselage

of an aircraft, it is obvious that considerable time and hence cost are

involved in stitching. Furthermore, the preform can be damaged from the

5 stitching/tufting process and from preform handling when it is transferred

from the stitching/tufting tooling to the infiltration and cure tooling. The

automated stitching/tufting machines, the infiltration equipment, and the

development of resins compatible with the infiltration process are all costly.

Most of the resins that can be infiltrated into dry fiber preforms have

10 mechanical properties substantially inferior to those of the state-of-the-art

toughened resins ordinarily used with prepreg materials. These inferior

mechanical properties adversely affect the load carrying ability of the

composite.

Through-the-thickness (TTT) reinforcements have also been applied

15 to composite laminates composed of prepreg material. While stitching and

tufting techniques have met with little success, the insertion of metal and

composite rods as TTT reinforcement has been successfully applied to multilayer stacks of prepreg material.

One known method for insertion of rods into a stack of prepregs

20 requires the forming of small diameter holes using a hollow ultrasonic

needle, and then inserting a rod, either metallic or composite, through the

hollow needle into the stack of prepreg. This process is repeated across the

surface of the prepreg stack until the desired density of TTT reinforcement

is obtained. The rods typically extend from the upper to the lower surface

25 of the prepreg stack. Although this discontinuous reinforcement improves

the damage tolerance of the composite material, currently this technique is

labor intensive and economically impractical.

Another technique for inserting rods through the thickness of stacks

of composite prepreg is by a foam block transfer method. Discontinuous
LAR 14892-1 -5- PATENT APPLICATION

rods are first inserted through the thickness of a layer of foam. After

stacking the layers of prepreg, the rod filled foam block is placed on the

surface of the prepreg stack. The assembly of prepreg laminate and foam

block is bagged in the conventional manner and cured. As the temperature

5 increases in the cure cycle, the foam softens and the viscosity of the resin

decreases. When pressure is applied in the cure process the foam collapses

and the rods are forced into the composite laminate. A substantial cost is

incurred in making the rod filled foam layers, and the collapsed remains of

a foam block may be unacceptable as a residue on a composite material.

10 Further, the foam block transfer method is not readily applicable to cocured, built-up structures such as stiffened panels.

Summary nf the Invention

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

laminated composite material with improved interlaminar and intralaminar

strength that can be used in a wide range of applications.

It is a further object of the present invention to provide a laminated

composite material with through-the-thickness reinforcement that is less

20 costly than known materials.

It is a still further object of the present invention to provide an

inexpensive method for making a laminated composite material with

through-the-thickness reinforcement.

These and other objects are achieved by a laminated composite

25 material with a predetermined thickness comprising a number of rods

substantially shorter than the thickness of the composite material arranged

substantially parallel to the thickness of the composite material and

distributed substantially evenly throughout the volume of the composite

material.
LAR 14892-1 -6- PATENT APPLICATION

A method according to the present invention for making a composite

material comprises the steps of providing a sufficient number of uncured

laminae of reinforcing material impregnated by an uncured matrix material

to form a composite material of predetermined finished thickness, selecting

5 a number of the uncured laminae, providing a source of thin rods, inserting

a number of rods shorter than the finished thickness of the composite

material perpendicularly through each of the selected laminae, layering all

of the uncured laminae on top of each other to form a stack with the

laminae having rods inserted evenly distributed in the stack, compressing

10 the stack of layered uncured laminae, and curing the stack of layered

laminae to form a dense composite material.

Brief Description of the Drawings

15 The present invention and the objects achieved by it will be

understood from the description herein, with reference to the accompanying

drawings, in which:

FIG. 1 is an enlarged top view of a small piece of composite material

according to a preferred embodiment of the invention.

20 FIG. 2 is a sectional view taken along line 2-2 in FIG. 1.

FIG. 3 is an enlarged perspective view of a small piece of a single

prepreg for a composite material with short rods inserted.

FIG. 4 is an enlarged section of a stack of prepregs with

discontinuous through-the-thickness rods according to the invention before

25 the stack is compressed and cured.

Detailed Description of a Preferred Embodiment

FIG. 1 is an enlarged top view of a small piece of laminated
LAR 14892-1 -7- PATENT APPLICATION

composite material 10 according to a preferred embodiment of the present

invention. FIG. 2 is a sectional view along line 2-2 in FIG. 1. The laminated

composite material 10 consists of a number of laminae 12 of reinforcing

material, e.g. woven fibers of graphite, in a matrix of cured resin, e.g.

5 epoxy.

According to the preferred embodiment of the invention, a number of

short rods 16 are included in the laminate. These rods are arranged

perpendicular to the plane of each laminae 12. Each rod 16 is shorter than

the total thickness of the laminate 10, but several times as long as the

10 thickness of each lamina 12. The rods 16, accordingly, do not extend

through the entire thickness of the laminate 10, but rather form

discontinuous through-the-thickness reinforcements between laminae 12,

and thereby increase the interlaminar strength of the composite material 10.

The rods 16 may be made, for example, from metal or from composite

15 material.

The short rods 16 are preferably evenly distributed throughout the

volume of the finished composite material 10, such that some rods 16 are

in the middle of the laminate, remote from both the top and the bottom

surface of the laminate 10, as is clearly seen from FIG. 2. This

20 distinguishes the composite material according to the invention from known

materials with through-the-thickness rod reinforcement wherein the rods are

always inserted from one or both sides of a finished laminate, or a stack of

prepregs. The rods in these known materials accordingly extend from one

or both sides of the finished laminate, and usually through the full thickness

25 of the laminate.

A manufacturing method for a laminated composite material 10 with

discontinuous through-the-thickness rods 16 is illustrated in FIGS. 3 and 4.

Basically, the process involves the lay-up of a sufficient number of prepreg

sheets 13, 14 to form a laminate 10 of desired finished thickness. Short
LAR 14892-1 -8- PATENT APPLICATION

rods 16 are inserted in some or all of the individual prepreg sheets 13 during

the lay-up process.

A number of short rods 16 are inserted through the thickness of

individual sheets of prepreg material 13. The prepreg material 13 is soft

5 and pliable, so little force is required to insert the rods 16 through the

prepreg material 13. The ends of the rods stick out at each side of the

prepreg material 13 as shown in FIG. 3. The rods 16 may, for instance, be

inserted by pushing one end of a continuous strand of wire or composite

through the prepreg material 13, and then cutting off the strand at a

10 predetermined distance from the penetrating end such that a short rod 16

is formed.

Layers of prepreg 13, with rods 16 inserted, are assembled into

laminates using the same automated and hand lay-up techniques used for

conventional prepreg and towpreg materials. When layers of prepreg 13

15 with rods 16 inserted are stacked together, possibly interleaved with layers

of prepreg material 14 without rods as shown in FIG. 4, the rod 16

reinforcements penetrate into neighboring layers. Partial penetration of

neighboring layers occur during the prepreg lay-up process. Complete

penetration by the rod 16 reinforcements occurs during debulking

20 operations and when the laminate 10 is cured. When the prepreg stack is

heated during the cure process the viscosity of the resin decreases and the

rods 16 are forced into the adjacent layers of the laminate. The rods 16

can not penetrate the top and bottom surfaces of the curing press,

however, so the rods 16 are contained within the interior of the finished

25 laminate, which ensures a smooth top and bottom surfaces, as shown in

FIG. 2. Because the rods 16 are incorporated during the prepregging

process, this through-the-thickness reinforcement method can be used with

a wide variety of prepreg materials.

The length, the diameter, the insertion pattern, and the spacing of the
LAR 14892-1 -9- PATENT APPLICATION

rods 16 are functions of the requirements of the end application. The lateral

distance between the rods 16 will vary with the diameter of the rods 16 and

the interlaminar strength required. The area density of the rods 16, which

is defined as the total rod 16 cross section per unit area of laminate 10,

5 typically varies between 0.1% and 3%.

Each lamina 12, in a laminated composite material 10, typically has

a thickness between 0.05 mm and 0.25 ram. The diameter of the rods 16

will typically vary from a few _m to about 0.2 mm. The views shown in

FIGS. 1-4 are accordingly greatly enlarged views of the actual laminate and

10 lay-up process.

In the manufacturing process according to the invention the rods 16

penetrate into neighboring layers 12, but do not extend through the entire

thickness of the laminate, unlike reinforcements made by stitching or

tufting, or by previous methods for inserting discontinuous rods. The use

15 of the short rods 16 results in increased interlaminar strength and, to some

extent, increased intralaminar strength.

The invention offers several improvements over prior art methods for

improving the interlaminar strength and damage tolerance of laminated

composites. Compliant interlayers cannot be used in all composite

20 laminates. For example, the mechanical properties of some compliant layers

degrade with increasing temperature and are not readily applicable to high

temperature applications. A material according to the present invention is,

in contrast, useful for high temperature applications.

Through-the-thickness reinforcement is an effective method of

25 suppressing interlaminar damage. However, conventional TTT reinforcing

techniques are not practical for many resins because these resins are not

infiltrateable into dry fiber preforms due to their high viscosity. The

proposed invention is compatible with a broad range of resins because it is

applied during the prepregging operation. The TTT reinforced prepreg
LAR 14892-1 -10- PATENT APPLICATION

according to the invention also has an advantage over alternate methods of

improving damage tolerance in that the properties of the TTT reinforcement

can be readily tailored to achieve the desired level of damage tolerance.

Through-the-thickness reinforced prepreg according to the invention

5 further has the advantage over other TTT reinforcement techniques in that

no new technology needs to be developed in the application of this method

to conventional materials. The high cost associated with robotic

stitching/tufting machines, resin infiltration processes, higher performance

infiltrateable resins, or custom stitching/tufting tooling can be eliminated.

10 Through-the-thickness reinforced prepreg according to the invention can

also be used in existing prepreg and towpreg lay-up processes.

No substantial increase in prepreg cost will be incurred with the

method according to the invention, because the cost of the rods 16 is on

the order of the cost of compliant layer material. Also, the rod insertion

15 process can be performed in line with the existing prepregging operation,

so it will not significantly impede existing production rates.

Numerous modifications and adaptations of the present invention will

be apparent to those skilled in the art. Thus, the following claims are

intended to cover all such modifications and adaptations which fall within

20 the true spirit and scope of the present invention.

What is claimed is:
LAR 14892-1 PATENT APPLICATION

IMPROVED COMPOSITE MATERIAL AND METHOD

FORPRODUCTION OF IMPROVED COMPOSITE MATERIAL

Ab__traet ef the Disclosure

A laminated composite material with improved interlaminar strength

and damage tolerance having short rods distributed evenly throughout the

composite material perpendicular to the laminae. Each rod is shorter than

the thickness of the finished laminate, but several times as long as the

thickness of each lamina. The laminate is made by inserting short rods in

layers of prepreg material, and then stacking and curing prepreg material

with rods inserted therethrough.
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