Composite fastener

A fastener member and method of formation are provided wherein a bolt and nut are formed from a block of resin-impregnated fibers woven in a first, second and third plane each perpendicular to the other wherein the fibers in the first plane extend continuously and longitudinally through the fastener member and the fibers in the second and third plane extend to form the peaks and valleys of the helical thread.

FIELD OF THE INVENTION 
This invention relates primarily to fastener members and, more 
particularly, to reinforced fiber composite fasteners and to a production 
method therefor. 
BACKGROUND AND SUMMARY OF THE INVENTION 
In recent years, it has beoome a desideratum to provide a high strength 
non-metallic fastener. Such a fastener would provide significant 
advantages in that plastic components which are secured with metallic 
fasteners may tend to deteriorate due to galvanic corrosion and variations 
in coefficients of thermal expansion. Moreover, the use of metallic 
fasteners in aircraft and other such applications may produce a deviation 
in navigational equipment and problems with electronic equipment. 
However, non-metallic fasteners presently known have generally lacked the 
strength exhibited by metallic fasteners of comparative size and this low 
strength is very apparent both in thread or threadlike areas. When axial 
compressive forces are exerted on matingly threaded member, e.g., a nut 
and a bolt, the threads of the members are subjected to shearing forces 
which may distort or otherwise damage the thread. Similarly, when such 
forces are applied to the enlarged head portion of a bolt, the compressive 
force required to sufficiently secure the workpieces may weaken the 
fastener at the load bearing plange which extends laterally beyond the 
fastener shaft. 
To withstand such shearing forces, threaded plastic members have included 
resin impregnated fibers. For example, glass fibers have been helically 
wound about the longitudinal axis of the fastener, i.e., longitudinally of 
the threads. In this instance, the forces on the threads have been opposed 
primarily by the resin bonding between the fibers. As the shear strength 
of the resin in generally less than that of the resin-impregnated fiber 
the strength of the members has been less than adequate, due primarily to 
the two-dimensional fiber orientation which is inevitably subjected to 
delamination. 
Other methods which have been employed to increase the strength of threads 
include the use of mat reinforcement around pulltruded unidirectional 
fiber rod. The mat reinforcement has random fiber orientation with 
multi-directional properties, but as the fibers are not continuous the 
tendency toward delamination and relatively low strength persists. 
U.S. Pat. No. 2,510,693 describes fastening members having a longitudinal 
fibrous reinforcing medium extending along the stem portion and into the 
head of the fastener. U.S. Pat. No. 2,685,813 describes a glass-fiber 
rivet body, including a spirally wound longitudinal fibers and, in an 
alternative embodiment, the longitudinal fibers are surrounded by braided 
threads in an essentially helical form. 
U.S. Pat. No. 2,928,764 and No. 3,283,050 disclose methods and apparatus 
for the production of threaded fiber fasteners employing circumferential 
or helical fiber to form threads. U.S. Pat. Nos. 2,943,967 and 4,063,838 
also detail the formation of threaded members with a combination of 
longitudinal and helical fibers or filaments. 
U.S. Pat. No. 3,995,092 details the use of a plurality of laminated 
sections which are glued together to form the fastener, with the 
laminations substantially perpendicular to the surface of the threaded 
shaft. U.S. Pat. No. 3,495,494 again details the use of longitudinal but 
spirally wound fibers which are oriented along a generally serpentine path 
to conform substantially to the course of the threads. 
According to the present invention, fastener members are formed from 
multi-dimensional woven fiber preforms having uniform isotropic 
properties, e.g., three-dimensional orthogonal blocks, molded with organic 
resins. The fasteners and the threads thereon thus contain a plurality of 
fibers disposed continuously through the fastener. Specifically, the 
fibers are disposed in a plane which is essentially perpendicular to the 
longitudinal axis of the fastener, i.e., as chords of the cross section of 
the fastener, with the end portions of such chordal fibers extending into 
the threads and other angular load-bearing flanges of the fastener. For 
example, the fibers are disposed at right angles in the lateral 
cross-sectional plane of the fastener so that the end portions of the 
fibers form the peaks and valleys of the threads. Such fasteners have 
increased resistance to compressive and shearing forces, and resist 
delamination as a result of the continuous three-dimensional fiber 
placement. Moreover, the use of preformed, impregnated blocks allows the 
economical and advantageous production of fastener members without the 
need for complex winding or casting machinery or techniques.

DETAILED DESCRIPTION 
A detailed illustrative embodiment of the invention is disclosed herein. 
However, it is to be understood that this embodiment merely exemplifies 
the invention, which may take forms that are different from the 
illustration disclosed. Therefore, specific details are not to be 
interpretated as necessarily limiting, but rather as forming a basis for 
the claims which define the scope of the invention. 
Three-dimensional composite fastener members are formed by the use of 
multi-dimensional weave fiber preforms which are impregnated with organic 
thermosetting or thermoplastic resins. The fiber employed can be selected 
according to the properties desired and the manner in which the fastener 
is to be used, for example, fiberglass, Kevlar (TM) graphite or carbon 
fibers may be employed. It is preferred that the filaments be spun into a 
yarn-like fiber ranging from 12 K (coarse) to as fine as 1 K, with an 
intermediate yarn of about 6 K believed to provide superior properties for 
most general applications. The resin system may also be varied according 
to the desired strength, temperature and environmental requirements. 
Polyester, epoxy, phenolic, polyimide and bismaleimide resins have proven 
appropriate. Thermoplastic resins such as polyphenylene sulfide (PPS) or 
PEEK may be used if post-forming is desirable. In the preferred embodiment 
described herein, Thermid MC-600, a polyimide resin sold by the National 
Starch and Chemical Corporation, is employed due to its superior strength, 
low moisture absorption and excellent characteristics particularly in high 
temperature applications. 
The woven preforms hereinafter described are obtainable from Fiber 
Materials, Inc., Bedford, Massachusetts and Techniweave, Inc., East 
Rochester, N.H. In the preferred embodiment herein described, the preforms 
have a center-to-center spacing of 6 K fiber of about 0.050 inches, equal 
fiber volume in all axes (x,y and z) and 40 to 50% fiber volume in the 
preform. Generally, the fiber volume is the preeminent characteristic in 
the preform specification, with the fiber spacing being determined by the 
fiber volume and yarn type. At a fiber volume of about 35 percent or less 
the properties of the fastener, and particularly the strength, begin to 
subside due to the fact that the fiber is the load-carrying component of 
such a composite. As the fiber volume of greater than 50 percent, the 
preform becomes difficult to thoroughly impregnate with resin resulting in 
a "resin-dry" composite, i.e., a composite containing resin-free voids or 
bubbles dispersed therethrough. Accordingly, a fiber volume of 40 to 50 
percent provides superior strength with most resins. 
The woven preform is impregnated with the appropriate resin, and preferably 
pressure and temperature cured as required. Impregnation may be performed 
by a variety of methods dependent upon the viscosity of the resin 
employed, but most generally it is preferred to submerge the preform in a 
resin solution, i.e., a resin which has been thinned with an appropriate 
solvent to assist in the dispersion of the resin. If possible, it is 
desirable to heat the resin-solvent mixture, or the preform, to a 
temperature which is sufficient to assist in the resin-solvent dispersion. 
For example, with a thermosetting resin such as the polyimide resin 
hereinafter specifically described, the preform may be submerged in a 
resin solution heated to a temperature of about 350.degree. F., such a 
temperature being below the boiling point of the solvent and also below 
the curing temperature of the resin. 
After impregnation, any solvent which has been added to the resin should be 
removed to avoid the evolution of the solvent if higher temperatures are 
applied during the curing process. Thus, the impregnated preform is heated 
at a temperature which is below both the boiling point of the solvent and 
the curing temperature of the resin for a period of time which is 
sufficient to permit solvent vapors to escape. In order to minimize the 
formation of voids or bubbles within the composite, it is preferable to 
compress the impregnated preform as the resin begins to cure. Compression 
molding is known in the art and is generally accomplished by the use of a 
hydraulic press, wherein the preform is compressed between metal platens 
coated with a mold release compound. 
With any given resin, pressure is applied to the composite only as the 
resin begins to cure, to avoid deforming the composite. This gelling point 
may be ascertained by a number of methods. For example, the gelling time 
of the resin may be predetermined, or uncured resin which is cured under 
conditions similar to that in the molding press may be observed to detect 
the onset of the curing process. However, these methods are inherently 
estimates and thus subject to a certain amount of error. 
Accordingly, it is preferred that the gel time be determined by feedback 
through the press, either by the sensing of the travel of the platens with 
relation to an increase in pressure by the press operator, or through the 
use of a microprocessor-controlled, closed-loop system with an automated 
feedback circuit, such as are sold by Pasadena Hydraulics, Inc. of 
Pasadena, Calif. Such a system measures resistance by slight platen 
movements, and as soon as the resistance increases, indicating that the 
cure is starting, the pressure is increased to that required for 
compression molding. 
For example, with the polyimide resin hereinafter employed in the specific 
embodiment, a thermosetting resin, the impregnated preform is heated in 
the molding press to a temperature of 490.degree. F., the curing 
temperature of the resin. During this initial heating process, a "contact" 
pressure (10 to 50 psi) is employed. This pressure is applied during the 
preheating process and is then released or "bumped" momentarily to allow 
any remaining solvent to escape, and then the "contact" pressure is 
reapplied. At this point, a light pressure slightly in excess of 50 psi is 
employed until it is sensed, either by the operator or by the feedback 
circuit, that the resin is beginning to cure. The pressure is then 
increased to 500 to 1,000 psi, with a preferred pressure being from about 
600 to 700 psi. 
This pressure is maintained until the cure of the resin is completed. After 
cure, additional pressure is not required, although it is preferred to 
maintain the high temperature employed in the compression molding process 
to provide increased fastener strength where use in high temperature 
environments is anticipated. Such post-cure techniques are known in the 
art for particular resins, and advantageous techniques for the post-cure 
of Thermid MC-600 are hereinafter set forth wherein the temperature is 
incrementally increased from the curing temperature in incremental steps, 
and held at 700.degree. F. for about four hours. 
With regard to the specific fastener hereinafter described and tested, a 
woven preform comprising carbon fiber (Union Carbide T-300 6 K) having 
center-to-center fiber spacing of 0.050 inches and 50% total fiber volume 
was employed. A polyimide resin (Thermid MC-600) was prepared by mixing 2 
parts of NMP (N-methyl pyrrolidone; 1-methyl-2-pyrrolidinone) with one 
part of MC-600 by weight, and heating to 350.degree. F. The woven preform 
was submerged in the resin-solvent mixture and thus impregnated. After 
impregnation the NMP was driven off by heat (360 F.) in an air-circulating 
oven until volatile content of the impregnated preform was about 2 to 3% 
by weight. Generally, the time in the oven is approximately 1 hour. 
However, if the air circulating oven does not adequately release the 
solvent, a vacuum oven may be employed at a temperature of 300.degree. to 
325.degree. F. and a pressure of 25 to 30 inches of mercury to hasten the 
solvent release. 
After such evaporation of the solvent, the preform was molded between 
platens at 485.degree. F. In this regard, it should be noted that the gel 
time of Thermid MC-600 is approximately 90 seconds. The preform was then 
placed between the platens of a hydraulic press at a temperature of 
495.degree. F. and a pressure of about 20 psi was applied for 30 seconds. 
This pressure was then released for a few seconds, followed by a 
re-application of the 20 pound pressure for about one minute. Thereafter, 
a pressure of 650 psi was applied as the resin began to gel. The preform 
was cured for about 1 hour at 485.degree. F. with the 600-700 psi 
pressure. 
Thereafter, the cured preform is cooled under pressure to below 350.degree. 
F. before pressure release. The block was then post-cured in an oven with 
a temperature rise of approximately 1.degree. F. per minute to 600.degree. 
F., and left at this temperature for about 1 hour. Thereafter, the 
temperature is again increased at 1.degree. F. per minute to 700.degree. 
F., which temperature was maintained for 4 hours. 
Thereafter, cooling is begun at a rate of approximately 2.degree. F. per 
minute, and upon cooling to room temperature the block is ready to 
machine, as described. 
The block thus formed may be ground in any precision lathe generally used 
to grind fasteners. However, when the fibers and resins described herein 
are employed, a fine grinding wheel is desirable and a carbide or diamond 
grinding wheel with a grit of 100 to 120 is preferred. A typical grinding 
speed is about 3000 revolutions per minute with a 11/2 inch diameter 
wheel. Speed of rotation and travel of the lathe should be relatively slow 
to keep the grinding pressure low. It will be appreciated that the fine 
particles and dust created during such grinding should be collected with a 
vacuum system. 
FIG. 1 shows the three-dimensional arrangement of the fibers in the woven 
preform of the preferred embodiment, although it should be understood that 
other types of multi-dimensional weaves may be employed as required by 
specific applications. In that figure a woven preform 10 is shown as 
comprising a plurality of fibers 12 disposed along an x-axis, a second 
plurality of fibers 14 disposed along a y-axis and a third plurality of 
fibers 16 along a z-axis. Each of the pluralities, 12, 14 and 16 are seen 
to be disposed at angles of 90.degree.. The center-to-center spacing of 
the fibers of about 0.050 inches provides a 40 to 50% fiber volume in the 
preform 10. The preform 10 is impregnated with a curable resin, as 
described above, to form a block from which fastener members are machined. 
Turning now to FIG. 2, a bolt 20 machined from the impregnated preform 10 
will be described. The bolt 20 is seen to include a shank 22 including a 
threaded portion 24. At an end opposite the threaded portion 24, the bolt 
20 is seen to include an enlarged head portion 26, including a bearing 
face 28 and a slot 30 for the insertion of a driving tool such as a screw 
driver, which is not specifically shown. 
Turning now to FIGS. 3 and 4, cross sectional views of the bolt 20 will be 
described. In those figures, it should be noted that the bolt 20 includes 
a plurality of fibers indicated by the reference numeral 32, disposed at 
right angles to the fibers indicated by the numeral 34. Each of the 
plurality of fibers 32 and 34 are, in turn, disposed at 90.degree. to the 
plurality of fibers 36, which are oriented axially with respect to the 
shank 22. 
Turning now to FIG. 5, a nut 40 is shown. The nut 40 is formed from a 
resin-impregnated composite-fiber block identical to that described with 
respect to the bolt 20, and is seen to include fibers 42, 44 and 46. The 
nut 40 is seen to include an aperture 48 including internal threads 50 
adapted to matingly engage the threaded portion 24 of the bolt 20. 
As shown in FIG. 6, a two-dimensional composite panel 60 is seen to overlie 
a similar panel 62. Each of the panels 60 and 62 are seen to be formed, 
respectively, from fibers 64, 66 and 68,70. The panels 60 and 62 are seen 
to be bored to form an aperture 72 adapted to receive a bolt 74 therein. 
The bolt 4 is seen to cooperate with washers 76 and 78, and the nut 80, to 
secure the panels 60 and 62. Since the panels 60 and 62 lack a vertical 
fiber orientation which would provide resistence to vertical compressive 
force, the higher compression bearing stress provided by the use of a 
three-dimensional composite fastener could well damage the panels 60 and 
62. Accordingly, a three-dimensional composite bushing 82 is provided to 
enable a higher fastener compression bearing stress to be applied without 
damage to the composite panels 60 and 62 which are being secured. The 
bushing 82 also isolates the fastener 74 from the aperture 72 to ensure 
against delamination of the two-dimensional components during insertion of 
the fastener. 
As seen in FIG. 7, the bushing 82 is seen to have been machined from a 
resin-impregnated woven preform 10 in a manner similar to that described 
with regard to the fastener 20 according to grinding means which are known 
in the art, and includes axial fibers 84 and transverse fibers 86 and 88. 
The fastener members of the present invention provide significant 
advantages with regard to shear strength due to the fact that the 
three-dimensional reinforced fibers extend transversely and continuously 
along the three dimensions of the fastener member. In a specific example, 
a fastener having a 3/16 inch shank diameter with a 10-32 UNJF-3A thread 
was machined, as described above, to an existing metal fastener design as 
shown in FIG. 2. Test results indicated that the shear strength through 
the shank was greater than 31,000 pounds per square inch. The same 
fastener design, when tested in tension with the load being applied 
through the head and a nut screwed into the threads, failed by shearing 
the countersunk head from the shank without any damage to the threads. 
This sheer strength value is comparable to that provided by the metallic 
fastener of similar design. 
It should be understood that while the preferred embodiment described 
herein is directed to threaded fasteners, fasteners such as pins, washers, 
collars, studs, rivets and other cylindrical fasteners are within the 
ambit of the invention, the scope of which is limited by the following 
claims.