Blind fastener for composite materials

The fastener assembly includes a generally solid pin, positioned within a tubular fastener body and a buckle sleeve. The fastener body has a generally cylindrical configuration and includes an enlarged head adjacent one end thereof, an intermediate shank portion, and a nose portion adjacent the other end thereof. The nose portion engage the buckle sleeve, and during installation causes the buckle of the buckle sleeve to form prior to workpiece contact by the interaction between the nose portion of the fastener and the leading edge of the buckle sleeve. The sleeve buckles the intersection between a trailing section and the largest inner diameter at the end of a tapered interior section.

BACKGROUND OF THE INVENTION 
The present invention relates to blind fasteners employed to join composite 
materials. 
A particular type of such a fastener is a blind rivet, well known in the 
prior art, used to fasten components when only one side of the workpiece 
is accessible. These fasteners generally comprise a stem or mandrel having 
a buckle-forming head at one end and serrations at the other end for 
engagement by a pulling tool; a tubular body surrounding the mandrel and 
having a flanged head; and, a locking collar encircling the mandrel near 
the body head. In use, the head portion of the mandrel and the surrounding 
body together are inserted through a hole in the workpiece. A riveting 
tool including a pulling head is used to translate the mandrel axially 
away from the workpiece. During such translation, the buckle-forming 
elements of the mandrel head expand the rivet body laterally to form a 
buckle on the blind side of the workpiece. The pulling head then forces 
the locking collar into a groove in the mandrel to lock the headed, 
accessible end of the body to the stem. Finally, the stem portion of the 
mandrel extending from the workpiece is broken off to complete the 
installation. 
Another type of blind fastener has a threaded body to receive a setting 
screw or core bolt, with a separate buckle sleeve as the expandable 
element. The sleeve is expanded on the blind side of the workpiece by 
tightening the threaded core bolt and fastener body. The fastener body has 
a manufactured head for bearing on the setting side of the workpiece. This 
head may have wrenching flats for use of a wrenching tool to prevent 
fastener body rotation during setting. The manufactured head may protrude 
or be flush. The fastener body has a shank with a diameter for 
substantially complete occupancy of the aligned holes in the workpieces. A 
blind side end of the fastener body, the end opposite the manufactured 
head, is a nose which externally tapers to provide an expansion surface 
over which the sleeve expands and forms against the workpiece. 
The sleeve is cylindrical and has an external diameter no greater than the 
diameter of the fastener body so that the sleeve passes through the 
aligned holes in the workpieces, with an internal diameter for receipt of 
the core bolt. The core bolt head has a diameter no greater than the 
diameter of the sleeve for passage to the blind side and a radial shoulder 
for bearing on the end of the sleeve. The core bolt has wrenching flats or 
other means on the setting side for tightening in the female threads of 
the fastener body. As the core bolt is rotated in the thread and moved 
axially relative to the fastener body, the bolt head bears on the sleeve 
and the sleeve is forced over the nose of the fastener body and expanded 
against the blind side of a joint. Load determining means such as a 
breakneck in the bolt can break to stop tightening and determine the 
amount of clamp-up force. 
In joining materials such as aluminum, damage to the hidden, or "blind" 
side of the workpiece by these conventional fastener assemblies had to be 
considered, but was not of major concern due to the inherent strength and 
reduced frangibility of aluminum and other materials. Such damage 
generally occurred during the contact of the buckle sleeve with the blind 
side of the workpiece prior to buckle formation. Without the formed buckle 
to dissipate the force of the pulling mandrel or bolt along a large area 
of the workpiece, the relatively smaller area buckle sleeve contacted the 
workpiece during buckle formation, thereby transmitting the pulling force 
of the mandrel or bolt onto a relatively small area of the workpiece. 
However, the use and development of composite materials such as graphite or 
fiber/epoxy matrix has increased. Therefore, the problem of damaging the 
more sensitive composites from the blind side during buckle formation is 
of concern, as these composites are more prone to damage when such 
concentrated force is transmitted. While conventional fastener assembly 
configurations may prove useful in connection with certain materials and 
in certain workpiece stress situations, there exists a need for a fastener 
assembly which provides substantially reduced stress on a workpiece, 
specifically workpieces made of composite material, during fastener 
setting and buckle formation. Reduction of workpiece stress is especially 
important in aerospace applications which presently use a variety of newly 
developed composite materials. 
A number of blind fastener assemblies for composite materials have been 
developed, with the primary focus on formation of the buckle in the buckle 
sleeve substantially prior to contact with the workpiece. Forming the 
buckle prior to workpiece contact dissipates the force of the pulling 
mandrel along a larger area of the blind side of the workpiece, thereby 
reducing the chance that the composite workpiece will fail during 
compression. One such fastener is U.S. Pat. No. 4,312,613 to Binns, 
disclosing a multiple shoulder type rivet assembly. 
In the multiple shoulder configuration, the sleeve and rivet body are 
specifically configured such that, as the mandrel is pulled through the 
workpiece, the rivet sleeve begins to slide and expand laterally over the 
dual-domed tail portion of the body member, substantially forming two 
buckles at the shoulder areas prior to sleeve contact with the workpiece. 
However, a problem with this assembly is that the action on which the dual 
buckle formation and propagation depend requires multiple intermediate 
configurations of the buckle sleeve during buckle formation, thereby 
increasing the likelihood of incomplete or discontinuous buckle formation 
prior to workpiece contact, thus increasing the likelihood of workpiece 
damage. 
Furthermore, to the above shortcomings, the dual shoulder rivet requires a 
complex tail geometry on the rivet body and buckle sleeve, requiring 
expensive secondary manufacturing operations on the dual-domed tail end of 
the body member and the dual-shoulder buckle sleeve. 
SUMMARY OF THE INVENTION 
The present invention overcomes the problems associated with the prior art 
and provides a means for joining pieces together such that improvements in 
strength and integrity are achieved with a reduced likelihood of workpiece 
damage. 
The present invention pre-forms the large bearing area of the buckle prior 
to contact with the workpiece, thereby avoiding exceeding a predetermined 
unit load on a composite workpiece. The buckle is formed by compression 
between a tubular fastener body and pin head by advancing the pin and 
forming the buckle prior to its contact with the blind side of the 
workpiece, avoiding application of a high unit load on the workpiece which 
would occur if the workpiece reacted all the compression load. The buckle 
forms substantially prior to its contact against the backside workpiece 
progressively and in a controlled fashion. Assurance of buckle formation 
prior to contact on the backside of the blind side workpiece results from 
the interaction between a weakened internal shoulder in the buckle sleeve, 
and buckle sleeve interaction with the contact and slide sections of the 
nose of a fastener body. 
The present invention provides a blind fastener characterized by an 
expandable sleeve that upon expansion, produces a buckle having a large 
bearing area for transfer of load from the sleeve to a backside workpiece 
surface of a joint formed of at least two workpieces and the fastener. The 
buckle is substantially completely formed prior to contact with the 
workpiece. The sleeve buckles in response to compressive loading of the 
sleeve between the nose of a fastener body and a head of the fastener pin. 
Compressive loading is produced by tightening the threads of the core bolt 
within threads of the fastener body, or alternatively, by pulling the 
mandrel of a rivet-type fastener. 
A specific embodiment of the present invention comprises a fastener body 
having female threads for receipt of a male threaded core bolt or pin. The 
bolt in turn has a head for bearing on the buckle sleeve on the blind side 
of the workpiece. The buckle sleeve is received on the bolt between the 
bolt head and a contact section of the nose of the fastener body. The 
buckle sleeve has a relieved shoulder located at the juncture of the 
tapered and trailing sections of the buckle sleeve. A leading edge of the 
sleeve contacts the contact section of the nose of the fastener body for 
the formation of a blind side buckle. The tapered section of the sleeve is 
tapered internally with the taper extending from a smaller diameter at the 
leading edge of the sleeve to a larger diameter towards the trailing 
section. The wall thickness of the tapered section decreases the greater 
the distance away from the leading edge. The tapered section is backed 
axially at the end of the sleeve by the trailing section of the sleeve. 
The trailing section has an inside diameter that receives with a close fit 
the shank of the bolt. The trailing section of the sleeve prevents 
tuck-out of the sleeve over the head of the bolt and determines the zone 
of buckle formation to be at the shoulder. 
The fastener body has a manufactured head for bearing on a setting side of 
the workpiece. The fastener body has a nose end with a contact section and 
an adjoining slide section. The outer diameter of the fastener body 
increases from the end of the contact section, through the slide section 
to the shank of the fastener body. The contact and slide sections of the 
nose of the fastener body in conjunction with the shoulder of the buckle 
sleeve assure initiation of buckling on the sleeve and buckle formation 
prior to contact with the exposed surface of the workpiece. The buckle 
contact section of the fastener causes the sleeve to initially buckle or 
fold upon being loaded axially during contact with the contact section. 
The leading edge of the sleeve slides along the nose radius and onto the 
slide section. The buckle continues to form during the slide of the 
leading edge along the slide section. The sleeve leading edge next slides 
from the slide section along the cylindrical shank of the fastener body to 
present a large bearing area to a backside workpiece after the buckle has 
substantially completely formed. The presentation of a formed buckle to 
contact the workpiece keeps unit loading on the workpiece below the 
failure load of the workpiece material. However, the required clamp-up 
load on the workpieces along the axis of the fastener can still be met. 
The pin or core bolt preferably has a breakneck portion so that a tail 
section of the pin separates from the permanent section of the pin at a 
predetermined load corresponding to a predetermined clamp-up force. 
A method for forming the buckle sleeve of the instant invention is provided 
which comprises forming a semi-finished buckle sleeve comprising a 
trailing section, a tapered section and a leading edge. The semi-finished 
buckle sleeve has a constant inner diameter throughout the trailing 
section with the inner diameter increasing through a radius at an inner 
shoulder located between the trailing section and the tapered section, the 
inner diameter then remaining constant throughout the tapered section to 
the leading edge. 
The semi-finished sleeve has a constant outer diameter through the trailing 
section to the tapered section wherein the outer diameter and wall 
thickness gradually increases to the leading edge. The semi-finished 
buckle sleeve is then shaped to form the finished buckle sleeve by 
deforming the tapered section to produce a constant outer diameter along 
the entire length of the buckle sleeve. This is preferably performed by 
camming the tapered section inwardly through application of force at the 
outer portion of the leading edge. This action changes the inner diameter 
of the forward portion of the semi-finished buckle sleeve while, the inner 
diameter remains constant along the trailing section to the radius at the 
internal shoulder. The inner diameter then gradually decreases throughout 
the tapered section to the leading edge. In the finished buckle sleeve, 
the inner diameter of the leading edge and the inner diameter throughout 
the trailing section is substantially equal. 
The deformation of the semi-finished buckle sleeve is preferably 
accomplished through the insertion of the sleeve into a forming die. The 
die has a tapered receiving portion with cam surfaces which cam the 
tapered section of the semi-finished sleeve inwardly by application of 
force to the outer surface of the leading edge of the semi-finished 
sleeve. The sleeve is advanced axially into the forming die until the 
entire outer diameter of the sleeve is constant. 
These and other features, aspects and advantages of the present invention 
will become more apparent from the following description, appended claims 
and drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 and 2, the fastener of the invention comprises a 
tubular fastener body 10, a pin in the form of a core bolt 12 and a buckle 
sleeve 14. The tubular fastener body has a manufactured head 16 for 
bearing on a setting side 18 of a workpiece 20, a shank 22 of a diameter 
to fit into aligned holes 23 in workpiece 20 and 21 and a threaded axial 
bore 29. The blind end of the body 10 has a nose 26 with a contact section 
27 and an adjacent slide section 28. 
Core bolt 12 has male threads 30 for complementary receipt in the bore 29. 
The core bolt 12 has a tail section 32 joined to the permanent section 33 
of the bolt at break groove 34. The tail has wrenching means in the form 
of opposed parallel flats 36. The tail extends axially beyond the head of 
the tubular fastener body for driving by a wrenching tool. Core bolt 12 
has a head 38 at its blind end opposite tail 32. This head bears axially 
during compression on sleeve 14. 
The tubular buckle sleeve 14 has a generally cylindrical constant diameter 
outer surface 41 and an axial bore 42 with a diameter of a size to accept 
the major thread diameter of core bolt 12. Sleeve 14 interposes between 
head 38 of core bolt 12 and nose section 26 of the tubular fastener body 
10. 
The buckle sleeve 14 includes a leading edge 45, a tapered section 46 and a 
trailing section 48, with the trailing section of the buckle sleeve 
adjacent the head 38 of the pin and the leading edge abutting the nose 
portion 26 of the fastener body 10. The leading edge 45 of the buckle 
sleeve 14 abuts the section 27 of the nose 26 of the body 10. 
At the junction of the trailing section 48 and the tapered section 46 of 
the inner bore 42 of the buckle sleeve 14, an internal radially extending 
shoulder 44 is formed. The shoulder 44 has a larger outer diameter than 
the trailing section 48. The interior wall 47 of the leading edge 45 
tapers axially to the shoulder 44, with the tapered surface diverging away 
from the central axis of the fastener. This creates a thick wall leading 
edge tapering through a thinner walled tapered section to the shoulder 44. 
The inner and outer diameter of both the leading edge 45 of the buckle 
sleeve 14 and trailing section 48 are preferably substantially the same, 
but the inner diameter is less than the inner diameter of the sleeve along 
the shoulder of the tapered section 46. 
Trailing section 48 axially backs-up the tapered section 46 and bears 
against head 38 of core bolt 12. The tapered section 46 has a 
cross-sectional area for resisting axial column loading that is 
considerably smaller than the cross-sectional area of the trailing section 
48 that resists such loading. The tapered section fails at the shoulder 44 
upon sufficient axial loading. 
Shank 22 of the fastener body 10 extends axially from head 16 through the 
hole 23 in the workpieces to be fastened by the fastener. Nose 26 is 
axially spaced from head 16 relative to the thickness of the workpieces. 
This condition assures that the buckle of the sleeve will form prior to 
its contact against the backside workpiece. It also provides a band of 
material of the sleeve in tight, radial compression with the material of 
the workpieces so as to prevent rotation of the sleeve with respect to the 
workpieces. 
In greater detail, flush mounted head 16 of fastener body 10 includes 
internal, 90.degree. spaced slots 59 to provide the wrenching means for 
holding the fastener body stationary. In the alternative embodiment (not 
shown), a protruding head and a flush head nut may be used with external 
wrenching means. In either case, a flange or shoulder 17 provides bearing 
against the setting side 18 of workpiece 20. The area for such bearing is 
large enough so that the unit loading will not exceed the compression 
strength of the workpiece material. 
To install the blind fastener assembly of the present invention in a pair 
of composite workpieces, a hole 23 is initially drilled through the 
workpieces 20, 21 large enough to receive the bolt head 38 the sleeve 14 
and the shank 22 of the fastener body 10. The workpiece 20 is countersunk 
in an area around the hole so that when the assembly is inserted the 
fastener body is retained by its enlarged head 16 engaging the countersunk 
area in the workpiece. 
An axial force is exerted on the head 38 of the core bolt 12 by rotating 
the bolt with a wrench. Head 38 and nose 26 compressively bear on sleeve 
14 as bolt 12 is tightened in the fastener body 10. As the bolt advances 
axially, the head of the bolt applies axial force to the trailing section 
48 of the sleeve 14, producing axial movement of the sleeve. This causes 
the sleeve leading edge 45 to press against the nose contact section 27. 
Referring to FIG. 3, a partial buckle 60 initially forms in the vicinity of 
the shoulder 44, while the forward end of the sleeve is still abutting the 
contact section 27. Buckle formation is aided slightly by a tendency of 
the forward end to compress into the contact section 27 as buckle 
formation first begins. The trailing section 48 maintains its original 
shape during initial loading because of its thickness. Thus, the slide of 
the sleeve is retarded while buckling continues during its abutment with 
the contact section until the shoulder 44 of the buckle sleeve is deformed 
to a considerable extent. 
Continued axial advancement of the bolt 12 results in the leading edge 45 
of the sleeve 14 sliding on the contact section 27, over the nose radius 
and onto slide section 28, as shown in FIG. 3. The sliding of the leading 
edge of the sleeve 14 along the slide section provides additional 
resistance along the buckle sleeve 14 so that as the bolt 12 is advanced, 
buckle 60 formation continues so as to form a generally flat leading 
surface. 
The essentially completely formed buckle 60 slides along the slide section 
28 onto the shank 22 of the fastener body 10 until the broad face or 
surface of the buckle 60 makes contact with the workpiece 20. As the core 
bolt becomes tighter in the fastener body 10 during the setting of the 
fastener, the stress in breakneck groove 34 will increase to the ultimate 
strength of the core bolt at that point. The tail 32 will separate from 
the balance of the core bolt with the resulting failure and can be 
discarded. The separation occurs at a clamp-up force less than that which 
is calculated to provide damage to the workpiece 20. 
A very stable, final configuration results which produces a symmetrical and 
very strong blind buckle with consistent pressure on the composite 
material of the workpieces 20, 21. This is achieved by consistent pressure 
during the formation of the buckle by creating the slide of the buckle 
sleeve 14 against the rivet body in relation to the buckle formation, with 
differential rates of slide of the end of the leading edge 45 of the 
buckle sleeve providing the proper buckle formation. 
The generally flat surface contacting the workpiece, distributes the force 
transmitted by further translation of the mandrel throughout a maximized 
area of the blind side of the workpiece, thus substantially eliminating 
the problems of point pressure encountered in prior art fastener 
assemblies. Distribution of the force of contact over a broad area of the 
workpiece surface produces a low pressure in pounds per square inch on the 
workpiece 20. 
The sleeve 14 buckles in a controlled manner. If the angle of the slide 
section 28 is too great, the buckle 60 may form too quickly. This results 
in buckles that extend forwardly towards the workpieces, not flush, 
thereby causing excessive unit loading on the workpieces. With 
insufficient angle, incomplete buckle formation occurs away from the 
workpieces. This also increases unit load on the workpieces when contacted 
by the buckle by causing the buckle to finish forming while contacting the 
workpieces. The configuration and interaction of the fastener body and 
buckle sleeve substantially eliminates non-uniformity or skewing of the 
buckle being formed. That is, the buckle radius at any particular time 
during buckle 60 formation is uniform around the entire periphery of the 
buckle sleeve 14, and is also uniformly spaced from the leading edge 45 of 
the buckle sleeve 14. 
Referring to FIGS. 5-7, representative dimensions of the buckle sleeve are 
as follows: 
______________________________________ 
Length 81: .450 (in.) 
O.D. 84 of trailing section 48: 
.1955 
Length 84 of trailing section 48: 
.308 
I.D. 86 of trailing section 48: 
.1500 
O.D. 107 of leading edge 45: 
.197 
I.D. 105 of leading edge 45: 
.142 
Radius 111 of tapered section at shoulder 44: 
.013/.017 
Length 136 of leading edge 45: 
.0096 
Angle 103 of taper along tapered section 46: 
8.degree. 30' 
______________________________________ 
Referring to FIG. 1, representative dimensions of the fastener body are as 
follows: 
______________________________________ 
Nose radius: .005/.010 (in.) 
I.D. of contact section 27: 
.140 
O.D. 125 of contact section 27: 
.166 
Angle 11 of slide section 28: 
10.degree. 
O.D. 123 of shank 22: .197 
______________________________________ 
FIGS. 5, 6 and 7 illustrate a method of manufacturing the buckle sleeve. In 
FIG. 5 the buckle sleeve 14 is initially formed with a semi-finished 
shape, the sleeve having a trailing section 48, a tapered section 46 and a 
leading edge 45. The semi-finished sleeve may be made by any of a number 
of commercially available methods; for instance, machining or by forming 
or heading. If made by heading, the forming die could be incorporated into 
the last station of a multiple die header for convenience purposes. 
As can be seen, the semi-finished sleeve basically differs from the 
finished sleeve concerning its forward portion. That is, the trailing 
section 48 has constant inner and outer diameters. However, after the 
shoulder, the section 46 has a constant inner diameter. The outer diameter 
of the semi-finished sleeve gradually increases, starting in the vicinity 
of the shoulder 44, to the leading edge 45. The forward portion of that 
outwardly flared surface in an initial design of the product is a curve, 
about a radius of about 0.250 inch. 
The forming die 73 depicted in FIG. 5 has an interior bore 76 comprising a 
flared female receiving portion 75 and a cylindrical portion 78. The 
receiving portion preferably has an inner diameter at its opening 79 
approximately equal to or slightly larger than the outer diameter of the 
semi-finished buckle sleeve 70 at its leading edge 45. The receiving 
portion 75 is adapted to initially receive the leading portion 45 of the 
semi-finished sleeve 70. The receiving portion has its inner diameter 
gradually decreasing to the cylindrical portion 78 of the die, the 
cylindrical portion 78 having an inner diameter approximately equal to or 
greater than that of the outer diameter of the finished sleeve 14. The 
interaction between the leading edge of the semi-finished sleeve 70 and 
the opening 79 of the receiving portion 75 of the forming die prior to 
deformation of the semi-finished sleeve is depicted in FIG. 6. 
As axial force is applied to the semi-finished sleeve 70 to place it into 
the forming die 73, the leading edge 45 of the semi-finished sleeve is 
gradually cammed inwardly by its interaction with the camming surfaces 80 
of the receiving portion 75 of the die. By deforming the sleeve in this 
fashion, the outer diameter of the sleeve becomes constant along its 
length as it is fully contained within the cylindrical portion of the die, 
thus producing an inner diameter configuration as described above. FIG. 7 
depicts the final forming of the semi-finished sleeve 70 into a finished 
sleeve 14. 
Forming the finished sleeve in this method permits ready inspection of its 
inner and outer dimensions, specifically those dimensions relating to the 
shoulder 44 and taper 42 of the forward portion of the finished sleeve. 
The method also provides a less costly alternative in forming the finished 
sleeve 14 compared to producing it by machining an undercut into the 
tubular shell of the sleeve, thereby requiring complex machining and 
forming. 
Having thus described the present invention, it should therefore be 
understood that other changes and modifications can be made without 
departing from the true scope and spirit of this invention as recited in 
the appended claims.