Apparatus and method for upsetting composite fasteners

A rotating anvil having a cavity in the shape of a desired upset head is urged against the tail of a composite fastener protruding through a workpiece and the tail is heated and deformed into an upset head sufficiently larger than the fastener shank to prevent the fastener from being withdrawn from the workpiece.

BACKGROUND OF THE INVENTION 
The use of composite materials in the construction of military and 
commercial aerospace vehicles is widespread and increasing. The most 
commonly used composites consist of a polymer matrix reinforced with 
fibers of such materials as carbon, glass and Kevlar. 
In many cases relatively thin composite panels are used. At the present 
time, panels 0.088 to 0.189 inches in thickness appear to be most common, 
although thicker and thinner parts are also used. 
To fasten such panels together, adhesive bonding is used wherever possible. 
When adhesive bonding alone is judged insufficient, fasteners are used in 
addition to, or in place of, adhesives. 
In the manufacture of conventional aluminum aircraft, solid aluminum rivets 
have been used successfully in very large quantities to fasten the sheet 
thicknesses mentioned, starting in about 1935 and continuing to the 
present time. Aluminum rivets are not suitable, however, for use in 
composite sheet material for several reasons. Some composite materials 
cause accelerated corrosion of aluminum rivets. The coefficients of 
expansion of aluminum and the composite materials may be too widely 
different. A lightning strike problem can arise when aluminum or other 
metal rivets are used in a composite material. For these reasons a need 
exists for a rivet which is itself made from composite material. 
Considerable work is now being done to enable rivets made from composite 
material to be used to fasten composite sheets, with or without prior 
adhesive bonding. Up to the present time, these composite solid rivets 
have been fabricated using the injection molding process to form a rivet 
having the required manufactured head shape at one end of the usual 
cylindrical shank portion. The shank portion is made sufficiently long to 
pass through the workpieces to be fastened and then protrude approximately 
two shank diameters beyond. Later, this two diameter protruding length is 
upset to form a "shop formed" head, which in conjunction with the 
manufactured head serves to hold the workpieces together. 
Examples of suitable materials for making composite solid rivets by 
injection molding are PEI (polyetherimide) reinforced with short glass 
fibers and PEEK (polyetheretherketone) reinforced with short carbon 
fibers. Both these materials belong to a composite materials category 
which is commonly known as "thermoplastic." Such materials become soft and 
formable at temperatures in the 600.degree. to 700.degree. range but when 
cooled to room temperature exhibit useful structural strength. 
Rivets made from these short fiber reinforced thermoplastic materials have 
typical average shear strengths of about 16 KSI when measured using the 
method of MIL-STD-1312 Test 20 and have typical average tension strength 
in the range 14 to 16 KSI. These rivets may be upset with any simple 
tooling which is capable of providing a heat input to the two diameters of 
protruding rivet tail, followed by a pressure to upset this tail when it 
becomes soft and formable. 
This riveting process seems to be gaining favor for fastening composite 
materials because the rivets themselves are easily made on readily 
available, high production injection molding machines and they are 
inexpensive compared to other suitable fasteners for composites, such as 
titanium threaded shear pins and collars. Also, experts in the composite 
aircraft field believe that the drilling of holes in a workpiece and the 
insertion and upsetting of these rivets can be easily automated. In this 
way, fastening may be accomplished with a simple inexpensive fastener 
using an inexpensive and reliable installation method. 
In some thick or high bearing strength thin workpieces the 16 KSI shear 
strength and the 12 to 14 KSI tension strength of common short fiber 
reinforced thermoplastic materials is not adequate. Thus the need exists 
for a rivet made from composite material and having higher shear and 
tension strengths. 
Long fiber reinforced threaded shear pin type fasteners having 40 to 60 KSI 
average ultimate shear strength have been described in U.S. patent 
application Ser. No. 397,659. High shear strength rivets can also be made 
from laid up panels as described in that application or from pultruded 
rod. However, it is difficult to upset rivets made in this way in a 
workpiece. Accordingly, there exists a need for a simple and practical 
method of upsetting rivets made from long fiber reinforced material. There 
also exists a need to provide a shop upset profile which is sufficiently 
strong in tension to optimize lap joint shear strength. 
SUMMARY OF THE INVENTION 
The method of this invention involves a shaped upsetting anvil which is 
rotated at high speed about its axis and then forced into contact with a 
rivet tail end that preferably extends approximately three shank diameters 
beyond the workpiece. Friction causes the rivet tail to heat up until it 
becomes formable at which time the pressure urging the shaped anvil 
forward is allowed to progressively heat and upset the rivet tail until a 
shop formed head having the desired shape and dimensions is formed against 
the workpiece. 
While it may be possible to use the method of the invention to upset 
composite rivets using a hand held tool, it is considered preferable that 
a machine be used on which the speed of the rotation of the anvil, the 
pressure applied to urge the anvil towards the workpiece and the speed of 
this advance are controllable. It is expected that such capabilities as 
this could most readily be built into such machines as are currently used 
extensively to install aluminum rivets and other types of aircraft 
fasteners automatically. 
The method of the invention was primarily developed to upset long fiber 
reinforced rivets made from thermoplastic materials, which are difficult 
to upset satisfactorily by other methods. However, the method can also be 
used for upsetting short fiber injection molded thermoplastic rivets. 
In forming the upset head, the anvil in the tool may be made in such a 
shape that the excess of melted material from the protruding rivet shank 
is spun radially outward in the form of a melted flash. This flash, when 
formed into a washer of appreciable thickness may serve as a useful 
provision for limiting the advance of the anvil and avoiding damage to the 
surface of the workpiece. This washer is also useful as a means of 
delaying the tilting of the rivet axis which occurs in a thin sheet lap 
shear joint under load, thereby increasing the shear strength of the rivet 
and joint combination.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a solid rivet 10 made from a composite of a polymer matrix 
reinforced with fibers. FIGS. 4-10 illustrate one method of making such a 
rivet. The composite material is initially formed in thin sheet-like 
tapes, called plies, containing fibers which add to the strength of the 
plies. A suitable tape has a thickness of about 0.005", but other 
thicknesses can be utilized. Each tape contains a plurality of individual 
carbon or other reinforcing fibers which are twisted or otherwise held 
together in a bundle referred to as a tow. A commonly used fiber is about 
7 microns in diameter. A commonly used tow contains 12,000 individual 
fibers. The tows are arranged parallel to each other and are bound 
together using a binder of polymeric resin. A common binder for binding 
the carbon fibers and tows together is PEEK, referred to above. As will be 
appreciated from FIG. 6, the schematically illustrated tows 16, each 
composed of individual filaments, are generally parallel to each other and 
greatly increase the shear strength of the tape in the direction 
perpendicular to the fibers. These tows also greatly increase the tension 
strength in the direction of the fiber flow axes. Since tows are often 
referred to as fibers, and a composite material in theory could have 
individual fibers rather than bundles or tows, the elements 16 will be for 
convenience referred to fibers, which is the most common term. 
The tapes can be made into a panel 12, illustrated in FIG. 4. To make a 
panel, a number of plies 14 of tape are stacked one on top of the other, 
as more clearly seen in FIGS. 5 and 9. The stack is then heated while 
applying a compressive load. This process causes the binder and adjacent 
plies 14 of tape to bind the plies together. After the panel 12 is formed, 
it is cooled. This creates a rigid composite panel having the desirable 
characteristics discussed above. 
To use the material to make a fastener, a section or bar 18 is cut from the 
edge of the panel, as indicated in FIG. 4. The bar 18 can then be machined 
either by turning it on a lathe or either by grinding it between centers 
and cutting it into shorter pieces to produce a cylindrical rod 20, as 
shown in FIG. 6. 
FIGS. 7a-7c show the manufacture of an individual rivet in stages. The 
square cross-section bar stock 22 is turned on a lathe or ground to 
produce a cylindrical blank, as shown in FIG. 7b. The blank 23 can then be 
heated and compressed to form a head 24, shown in FIG. 7c and in FIG. 8. 
With the fibers 16 in all of the layers or plies of tape in the same 
direction, the layup produces what is termed a unidirectional, reinforced 
long fiber rivet, as seen in FIGS. 8 and 10. Other orientations may be 
useful; for example, a +45/-45/0 layup could be used. The layup angles 
describe the direction of the fibers 16 in each single tape layer relative 
to a zero line. This zero line coincides with what later becomes the long 
axis of the rivet. Referring to FIG. 11, the top layer 65 represents a +45 
angle. The second layer 66 represents a -45 angle, which is at 90.degree. 
with respect to the layer 30, and the lower ply 66 represents a 0.degree. 
or no angle. FIG. 12 show how these fiber angles relate to the zero line 
68 and to a manufactured fastener. 
Although the layup and cut method is illustrated for producing blanks for 
forming the rivet of FIG. 8, it should be realized that suitable blanks 
may also be made from many thermoplastic materials and continuous fibers 
by the pultrusion method, which is a well-known process in the plastic 
industry. 
Referring again to FIG. 1, the rivet 10 is shown inserted in the workpiece 
15, and is of a length such that its tail 11 protrudes beyond the face 17 
of the workpiece, approximately 3 diameters of the workpiece shank 13. 
Also illustrated in FIGS. 1 and 2 is a suitable tool 38 for producing the 
desired upset in the rivet. 
The tool 38 includes an anvil 40, mounted within the lower end of an 
elongated spindle 42. The upper end of the spindle engages with a suitable 
mechanism 53 (schematically shown) for rotating the spindle at 
considerable speeds. Surrounding the lower end of the spindle is a sleeve 
46, connected by threads or other suitable means to a shroud 48, which 
surrounds the lower end of the sleeve and the spindle with the anvil in 
it, and extends beyond the lower end of the anvil in one position, as 
viewed in Figure A spacer 49 extends between the upper end of the shroud 
and a shoulder of the sleeve. A coil spring 50 surrounds a portion of the 
spindle and extends between the sleeve and the lower portion of a thrust 
bearing 52 in which the spindle is mounted. Thus, the spring continuously 
urges the sleeve and the shroud downwardly in a direction to urge the 
shroud to extend beyond the anvil. Note from FIG. 1 that the downward 
movement of the sleeve and shroud is limited by the lower end of the 
sleeve engaging the upper surface of the flared lower end of the anvil. In 
addition to the spindle being mounted for rotation, the entire tool is 
mounted for axial movement by suitable means 51 (schematically shown), and 
the shroud and spindle can move axially and rotationally relative to each 
other. 
The lower end of the anvil has an internal cavity or recess 54 having a 
desired internal surface of revolution in the shape of a desired upset 
head to be formed on the rivet. The cavity has a frusto conical shape with 
the lower larger diameter end being an open mouth leading to an upper 
smaller diameter end. 
To produce an upset, the axis 56 of the anvil of the tool is first aligned 
with the axis of the rivet to be upset. The spindle is then rotated at an 
appreciable speed; for example, 3,000 RPM, and the whole tool is moved at 
a preset rate of movement towards the rivet and the workpiece. 
Referring to FIG. 2, the lower end of the shroud 48 contacts the upper 
surface 17 of the workpiece 15 and ceases to rotate, while the anvil 40 
and the spindle 42 continue to rotate and approach the rivet tail 11. The 
spring 50 maintains the shroud 48 in contact with the workpiece 15. The 
thrust bearing 52 permits the spindle 42 to rotate while the shroud 48 
remains stationary. 
When the rotating anvil 40 contacts the end of the rivet tail 11 it is held 
in contact using a suitable pressure such that friction between the anvil 
and the rivet produces heat, without prematurely deforming the cold 
portion of the shank 13. When the melting temperature of the rivet is 
reached, (625.degree. F. approximately in the case of PEEK) the melted 
portion of the rivet end begins to flow. Under the continued influence of 
pressure and friction the rivet upset continues to form. Melted PEEK 
matrix materials and reinforcing fibers are forced downwards and outwards 
until further downward and outward movement of the material is limited by 
the space available within the shroud. As noted above, the tail end volume 
is larger than the cavity volume such that melted material is spun 
radially outward to form a melted flash. This flash becomes a washer that 
limits the advance of the anvil. The washer also strengthens the finished 
joint with the workpiece. 
Friction heating, melting and flowing continues until the available rivet 
tail volume has been deformed into a space of equal volume, defined by the 
anvil profile, the internal walls of the shroud and the top surface 17 of 
the workpiece 15. 
At this point rotation of the tool spindle 42 is halted; but the pressure 
is maintained on the anvil 40, until the temperature of the deformed 
material in the upset head 19 falls to appreciably below the melting point 
of the PEEK material (for example, 400.degree. F. or below) at which time 
the tool may be removed and a second rivet upset commenced. 
The set fastener and finished joint are seen in FIG. 3. As may be seen, the 
tail upset head 19 has a frusto-conical shape, with the smaller diameter 
upper end of the head tapering to a large diameter before flaring into an 
outwardly extending flange or washer 21. The fastener shank 13 in FIGS. 2 
and 3 is shown with schematically indicated fiber lines comparable to 
FIGS. 8, in that it remains essentially unchanged during the tail 
upsetting. However, the upset head 19 is shown in conventional cross 
hatching, since the fibers appear distorted and indistinct in an enlarged 
cross section of a prototype. 
The tool illustrated is an experimental prototype. It is expected that it 
will be further developed. For example, it is desirable that the tool be 
warm at the start of the operation so that heat transfer to the rivet is 
speeded up. Thus, a means for preheating the tool other than friction may 
be provided. Also, it is desirable that immediately after formation of the 
upset head is complete and rotation is halted, the cooling of the upset 
head and tool is as rapid as possible. Therefore cooling fins may be added 
to the shroud 48 and cooling may also be applied to the spindle 42 and the 
anvil 40.