Z-pin reinforced bonds for connecting composite structures

I improve the impact shock resistance of bonds between composite elements by including Z-pin reinforcement. I prepare stubbled composite structure by using peel plys over the appropriate surface of the composite during pin insertion using conventional processes. I then use the stubbled composite structure with padups, as necessary, to produce the Z-pin reinforced joint or bond between composite elements using any of adhesive bonding, cocuring, or thermoplastic welding.

TECHNICAL FIELD 
The present invention relates to a method for bonding or otherwise 
connecting two or more composite elements with bonds having Z-pin 
reinforcement and to products made using the method. 
BACKGROUND ART 
The use of composites in primary structure in aerospace applications is 
limited today because of the relatively high cost. A significant 
contribution to the total cost is the assembly cost where the precured 
composite elements are assembled, drilled, and fastened. The necessary 
design for mechanical fastening complicates the structure, especially in 
thin sections, because of the need for access to both sides of the bond 
line. 
While composites might be adhesively bonded, cocured, or welded, these 
connecting processes generally produce bonds that rely upon the resin 
matrix for strength. The bond line lacks any reinforcing material to help 
with load transfer. These bonds generally have modest strength, and are 
susceptible to disbanding with shock impact or other "out of plane" forces 
affecting the assembly. Such forces often arise in environments prone to 
vibration. The present invention is a method to introduce Z-pin 
reinforcement along the bond line especially in fastenerless composite 
assemblies. 
1. Composite Manufacturing 
Fiber-reinforced organic resin matrix composites have a high 
strength-to-weight ratio (specific strength) or a high stiffness-to-weight 
ratio (specific stiffness) and desirable fatigue characteristics that make 
them attractive as replacements for metal in aerospace applications where 
weight, strength, or fatigue is critical. Thermoplastic or thermoset 
organic resin composites, would be more economical with improved 
manufacturing processes that reduced touch labor and forming time. 
Prepregs combine continuous, woven, or chopped reinforcing fibers with an 
uncured matrix resin, and usually comprise fiber sheets with a thin film 
of the matrix. Sheets of prepreg generally are placed (laid-up) by hand or 
with fiber placement machines directly upon a tool or die having a forming 
surface contoured to the desired shape of the completed part or are 
laid-up in a flat sheet which is then draped and formed over the tool or 
die to the contour of the tool. Then the resin in the prepreg lay up is 
consolidated (i.e. pressed to remove any air, gas, or vapor) and cured 
(i.e., chemically converted to its final form usually through 
chain-extension or fused into a single piece) in a vacuum bag process in 
an autoclave (i.e., a pressure oven) to complete the part. 
The tools or dies for composite processing typically are formed to close 
dimensional tolerances. They are massive, must be heated along with the 
workpiece, and must be cooled prior to removing the completed part. The 
delay caused to heat and to cool the mass of the tools adds substantially 
to the overall time necessary to fabricate each part. These delays are 
especially significant when the manufacturing run is low rate where the 
dies need to be changed frequently, often after producing only a few parts 
of each kind. An autoclave has similar limitations; it is a batch 
operation. 
In hot press forming, the prepreg is laid-up to create a preform, which is 
bagged (if necessary) and placed between matched metal tools that include 
forming surfaces to define the internal, external, or both mold lines of 
the completed part. The tools and composite preform are placed within a 
press and then the tools, press, and preform are heated. 
The tooling in autoclave or hot press fabrication is a significant heat 
sink that consumes substantial energy. Furthermore, the tooling takes 
significant time to heat the composite material to its consolidation 
temperature and, after curing the composite, to cool to a temperature at 
which it is safe to remove the finished composite part. 
As described in U.S. Pat. No. 4,657,717 a flat composite prepreg panel was 
sandwiched between two metal sheets made from a superplastically formable 
alloy and was formed against a die having a surface precisely contoured to 
the final shape of the part. 
Attempts have been made to reduce composite fabrication times by actively 
cooling the tools after forming the composite part. These attempts have 
shortened the time necessary to produce a composite part, but actively 
cooled tools are expensive and the cycle time for heating and cooling 
remains so fabrication costs remain high. 
Boeing described a process for organic matrix forming and consolidation 
using induction heating in U.S. Pat. No. 5,530,227. There, prepregs were 
laid up in a flat sheet and were sandwiched between aluminum susceptor 
facesheets. The facesheets were susceptible to heating by induction and 
formed a retort to enclose the prepreg preform. To ensure an inert 
atmosphere around the composite during curing and to permit withdrawing 
volatiles and outgassing from around the composite during the 
consolidation, the facesheets are sealed around their periphery. Such 
welding unduly increased the preparation time and the cost for part 
fabrication. It also ruined the facesheets (i.e., prohibited their reuse 
which added a significant cost penalty to each part fabricated with this 
approach). So, Boeing described in U.S. Pat. No. 5,599,472 a technique 
that readily and reliably sealed the retort without the need for welding 
and permits reuse of the facesheets in certain circumstances. This 
"bag-and-seal" technique applies to both resin composite and metal 
processing. 
2. Processing in an Induction Press 
Boeing dies or tooling for induction processing are ceramic because a 
ceramic is not susceptible to induction heating and, preferably, is a 
thermal insulator (i.e., a relatively poor conductor of heat). Ceramic 
tooling is strengthened and reinforced internally with fiberglass rods or 
other appropriate reinforcements and externally with metal or other 
durable strongbacks to permit it to withstand the temperatures and 
pressures necessary to form, to consolidate, or otherwise to process the 
composite materials or metals. Ceramic tools cost less to fabricate than 
metal tools of comparable size and have less thermal mass than metal 
tooling because they are unaffected by the induction field. Because the 
ceramic tooling is not susceptible to induction heating, it is possible to 
embed induction heating elements in the ceramic tooling and to heat the 
composite or metal retort without significantly heating the tools. Thus, 
induction heating can reduce the time required and energy consumed to 
fabricate a part. 
While graphite or boron fibers can be heated directly by induction, most 
organic matrix composites require a susceptor in or adjacent to the 
composite material preform to achieve the necessary heating for 
consolidation or forming. The susceptor is heated inductively and 
transfers its heat principally through conduction to the preform or 
workpiece that usually is sealed within the susceptor retort. Enclosed in 
the metal retort, the workpiece does not experience the oscillating 
magnetic field which instead is absorbed in the retort sheets. Heating is 
by conduction from the retort to the workpiece. 
Induction focuses heating on the retort (and workpiece) and eliminates 
wasteful, inefficient heat sinks. Because the ceramic tools in the Boeing 
induction heating workcell do not heat to as high a temperature as the 
metal tooling of conventional, prior art presses, problems caused by 
different coefficients of thermal expansion between the tools and the 
workpiece are reduced. Furthermore, Boeing's induction heating press is 
energy efficient because significantly higher percentages of input energy 
go to heating the workpiece than occurs with conventional presses. The 
reduced thermal mass and ability to focus the heating energy permits us to 
change the operating temperature rapidly which improves the products we 
produce. Finally, the shop environment is not heated as significantly from 
the radiation of the large thermal mass of a conventional press, and is a 
safer and more pleasant environment for the press operators. 
In induction heating for consolidating or forming organic matrix composite 
materials, Boeing generally places a thermoplastic organic matrix 
composite preform of PEEK or ULTEM, for example, within the retort. These 
thermoplastics have a low concentration of residual volatile solvents and 
are easy to use. The susceptor facesheets of the retort are inductively 
heated to heat the preform. They apply consolidation and forming pressure 
to consolidate and, if applicable, to form the preform at its curing 
temperature. The sealed susceptor sheets form a pressure zone that 
functions like a conventional vacuum bag processes for resin 
consolidation. The retort is placed in an induction heating press on the 
forming surfaces of dies having the desired shape of the molded composite 
part. After the retort and preform are inductively heated to the desired 
elevated temperature, differential pressure (while maintaining the vacuum 
in the pressure zone around the preform) across the retort which functions 
as a diaphragm in the press forms the preform against the die into the 
desired shape of the completed composite panel. 
The retort often includes three susceptor sheets sealed around their 
periphery to define two pressure zones. The first pressure zone surrounds 
the composite panel/preform or metal workpiece and is evacuated and 
maintained under vacuum. The second pressure zone is pressurized (i.e., 
flooded with gas) at the appropriate time to help form the composite panel 
or workpiece. The shared wall of the three layer sandwich that defines the 
two pressure zones acts as the diaphragm in this situation. 
Boeing can perform a wide range of manufacturing operations in its 
induction heating press. These operations have optimum operating 
temperatures ranging from about 350.degree. F. (175.degree. C.) to at 
least about 1850.degree. F. (1010.degree. C.). For each operation, the 
temperature is held relatively constant for several minutes to several 
hours. Controlling the input power fed to the induction coil provides 
temperature control, but a better and simpler way capitalizes on the Curie 
temperature. By judicious selection of the metal or alloy in the retort's 
susceptor facesheets, we can avoid excessive heating irrespective of the 
input power. With improved control and improved temperature uniformity in 
the workpiece, we produce better products. The method capitalizes on the 
Curie temperature phenomenon to control the absolute temperature of the 
workpiece and to obtain substantial thermal uniformity in the workpiece by 
matching the Curie temperature of the susceptor to the desired temperature 
of the induction heating operation being performed. This temperature 
control method is explained in greater detail in U.S. Pat. No. 5,723,849. 
3. Thermoplastic Welding 
Three major joining technologies exist for aerospace composite structure: 
mechanical fastening; adhesive bonding; and welding. Both mechanical 
fastening and adhesive bonding are costly, time consuming assembly steps 
that introduce excess cost even if the parts that are assembled are 
fabricated from components produced by an emerging, cost efficient 
process. Mechanical fastening requires expensive hole locating, drilling, 
shimming, and fastener installation, while adhesive bonding often requires 
complicated surface pretreatments. 
In contrast, thermoplastic welding, which eliminates fasteners, features 
the ability to join thermoplastic composite components at high speeds with 
minimum touch labor and little, if any, pretreatments. In our experience, 
the welding interlayer (compromising the susceptor and surrounding 
thermoplastic resin either coating the susceptor or sandwiching it) also 
can simultaneously take the place of shims required in mechanical 
fastening. As such, composite welding holds promise to be an affordable 
joining process. For "welding" a combination of thermoplastic and 
thermoset composite parts together, the resin that the susceptor melts 
functions as a hot melt adhesive. If fully realized, 
thermoplastic-thermoset bonding will further reduce the cost of composite 
assembly. 
There is a large stake in developing a successful welding process. Its 
advantages versus traditional composite joining methods are: 
reduced parts count versus fasteners 
minimal surface preparation, in most cases a simple solvent wipe to remove 
surface contaminants 
indefinite shelf life at room temperature 
short process cycle time, typically measured in minutes 
enhanced joint performance, especially hot/wet and fatigue 
permits rapid field repair of composites or other structures. 
There is little or no loss of bond strength after prolonged exposure to 
environmental influences. 
U.S. Pat. No. 4,673,450 describes a method to spot weld graphite fiber 
reinforced PEEK composites using a pair of electrodes After roughening the 
surfaces of the prefabricated PEEK composites in the region of the bond, 
Burke placed a PEEK adhesive ply along the bond line, applied a pressure 
of about 50-100 psi through the electrodes, and heated the embedded 
graphite fibers by applying a voltage in the range of 20-40 volts at 30-40 
amps for approximately 5-10 seconds with the electrodes. Access to both 
sides of the assembly is required in this process which limits its 
application. 
Prior art disclosing thermoplastic welding with induction heating is 
illustrated by U.S. Pat. Nos. 3,966,402 and 4,120,712. In these patents, 
conventional metallic susceptors are used and have a regular pattern of 
openings of traditional manufacture. Achieving a uniform, controllable 
temperature in the bond line, which is crucial to preparing a 
thermoplastic weld of adequate integrity to permit use of welding in 
aerospace primary structure, is difficult with conventional susceptors. 
Thermoplastic welding is a process for forming a fusion bond between two 
faying thermoplastic faces of two parts. A fusion bond is created when the 
thermoplastic on the surface of the two thermoplastic composite parts is 
heated to the melting or softening point and the two surfaces are brought 
into contact, so that the molten thermoplastic mixes, and the surfaces are 
held in contact while the thermoplastic cools below the softening 
temperature. 
Simple as thermoplastic welding sounds, it is difficult to perform reliably 
and repeatably in a real factory on full-scale parts to build a large 
structure such as an airplane wing box. One difficulty is heating to the 
bond line properly without overheating the entire structure. It is 
difficult to achieve intimate contact of the faying surfaces of the two 
parts at the bond line during heating and cooling despite the normal 
imperfections in the flatness of composite parts, thermal expansion of the 
thermoplastic during heating to the softening or melting temperature, flow 
of the thermoplastic out of the bond line under pressure (i.e., squeeze 
out), and then contraction of the thermoplastic in the bond line during 
cooling. 
The exponential decay of the strength of magnetic fields dictates that, in 
induction welding processes, the susceptible structure closest to the 
induction coil will be the hottest, since it experiences the strongest 
field. Therefore, it is difficult to obtain adequate heating at the bond 
line between two graphite or carbon fiber reinforced resin matrix 
composites relying on the susceptibility of the fibers alone as the source 
of heating in the assembly. For the inner plies to be hot enough to melt 
the resin, the outer plies closer to the induction coil and in the 
stronger magnetic field are too hot. The matrix resin in the entire piece 
of composite melts. The overheating results in porosity in the product, 
delamination, and, in some cases, destruction or denaturing of the resin. 
To avoid overheating of the outer plies and to insure adequate heating of 
the inner plies, we use a susceptor of significantly higher conductivity 
than the fibers to peak the heating selectively at the bond line. An 
electromagnetic induction coil heats a susceptor to melt and cure a 
thermoplastic resin (also sometimes referred to as an adhesive) to bond 
the elements of the assembly together. 
The current density in the susceptor may be higher at the edges of the 
susceptor than in the center because of the nonlinearity of the coil, such 
as occurs when using a cup core induction coil like that described in U.S. 
Pat. No. 5,313,037. Overheating the edges of the assembly can result in 
underheating the center, either condition leading to inferior welds 
because of non-uniform curing. It is necessary to have an open or mesh 
pattern in the susceptor embedded at the bond line to allow the resin to 
create the fusion bond between the composite elements of the assembly when 
the resin heats and melts. 
a. Moving coil welding processes 
In U.S. Pat. No. 5,500,511, Boeing described a tailored susceptor for 
approaching the desired temperature uniformity. This susceptor, designed 
for use with the cup coil of U.S. Pat. No. 5,313,037, relied upon 
carefully controlling the geometry of openings in the susceptor (both 
their orientation and their spacing) to distribute the heat evenly. The 
susceptor used a regular array of anisotropic, diamond shaped openings 
with a ratio of the length (L) to the width (W) greater than 1 to provide 
a superior weld by producing a more uniform temperature than obtainable 
using a susceptor having a similar array, but one where the L/W ratio was 
one. By changing the length to width ratio (the aspect ratio) of the 
diamond-shaped openings in the susceptor, they achieved a large difference 
in the longitudinal and transverse conductivity in the susceptor, and, 
thereby, tailored the current density within the susceptor. A tailored 
susceptor having openings with a length (L) to width (W) ratio of 2:1 has 
a longitudinal conductivity about four times the transverse conductivity. 
In addition to tailoring the shape of the openings to tailor the 
susceptor, we altered the current density in regions near the edges by 
increasing the foil density (i.e., the absolute amount of metal). 
Increasing the foil density along the edge of the susceptor increased the 
conductivity along the edge and reduced the current density and the edge 
heating. Increased foil density was achieved by folding the susceptor to 
form edge strips of double thickness or by compressing openings near the 
edge of an otherwise uniform susceptor. These susceptors were difficult to 
reproduce reliably. Also, their use forced careful placement and alignment 
to achieve the desired effect. 
The tailored susceptor was designed to use with the cup coil of U.S. Pat. 
No. 5,313,037, where the magnetic field is strongest near the edges 
because the central pole creates a null at the center. Therefore, the 
tailored susceptor was designed to counter the higher field at the edges 
by accommodating the induced current near the edges. The high longitudinal 
conductivity encouraged induced currents to flow longitudinally. 
Boeing's selvaged susceptor (FIGS. 13 & 14) for thermoplastic welding which 
is described in U.S. Pat. No. 5,508,496 controls the current density 
pattern during eddy current heating by an induction coil to provide 
substantially uniform heating to a composite assembly and to insure the 
strength and integrity of the weld in the completed part. This susceptor 
is particularly desirable for welding ribs between prior welded spars 
using an asymmetric induction coil (described in U.S. Pat. No. 5,444,220, 
which I incorporate by reference). It provides a controllable area of 
intense, uniform heating, a trailing region with essentially no heating, 
and a leading region with minor preheating. 
Boeing achieved better performance (i.e., more uniform heating) in rib 
welding by using a selvaged susceptor having edge strips without openings. 
The resulting susceptor, then, has a center portion with a regular pattern 
of opening and solid foil edges, which are referred to as selvage edge 
strips. Embedded in a thermoplastic resin to make a susceptor/resin tape, 
the susceptor is easy to handle and to use in performing the composite 
pieces prior to welding. Also, with a selvaged susceptor, the impedance of 
the central portion should be anisotropic with a lower transverse 
impedance than the longitudinal impedance. Here, the L/W ratio of diamond 
shaped openings should be less than or equal to one. With the selvaged 
susceptor in the region immediately under the asymmetric induction work 
coil, current flows across the susceptor to the edges where the current 
density is lowest and the conductivity, highest. 
Generally, the selvaged susceptor is somewhat wider than normal so that the 
selvage edge strips are not in the bond line. Boeing sometimes removes the 
selvage edge strips after forming the weld, leaving only a perforated 
susceptor foil in the weld. This foil has a relatively high open area 
fraction. 
Significant effort has been expended in developing inductor and susceptor 
systems to optimize the heating of the bond line. A difficulty in 
producing large scale aerospace-quality structures in a production 
environment is the aspect of the process dealing with the control of the 
surface contact of the faying surfaces of the two parts to be welded 
together. This problem involves the timing, intensity, and schedule of 
heat application so the material at the faying surfaces is brought to and 
maintained within the proper temperature range for the requisite amount of 
time for an adequate bond to form. Intimate contact is maintained while 
the melted or softened material hardens in its bonded condition. 
Large scale parts such as wing spars and ribs, and the wing skins that are 
bonded to the spars and ribs, are typically on the order of 20-30 feet 
long at present, and potentially can be as much as 100 feet in length when 
the process is perfected for commercial transport aircraft. Parts of this 
magnitude are difficult to produce with perfect flatness. Instead, the 
typical part will have various combinations of surface deviations from 
perfect flatness, including large scale waviness in the direction of the 
major length dimension, twist about the longitudinal axis, dishing or 
sagging of "I" beam flanges, and small scale surface defects such as 
asperities and depressions. These irregularities interfere with full 
surface area contact between the faying surfaces of the two parts and 
actually result in surface contact only at a few "high points" across the 
intended bond line. Applying pressure to the parts to force the faying 
surfaces into contact achieves additional surface contact, but full 
intimate contact is difficult or impossible to achieve in this way. 
Applying heat to the interface by electrically heating the susceptor in 
connection with pressure on the parts tends to flatten the irregularities 
further. Still, the time needed to achieve full intimate contact with the 
use of heat and pressure is excessive, can result in deformation of the 
top part, and tends to raise the overall temperature of the "I" beam 
flanges to the softening point. When softened, they begin to yield or sag 
under the application of the pressure needed to achieve a good bond. 
Boeing's multipass thermoplastic welding process described in U.S. Pat. No. 
5,486,684 (which I also incorporate by reference) enables a moving coil 
welding process to produce continuous or nearly continuous fusion bonds 
over the full area of the bond line. The result is high strength welds 
that we produce reliably, repeatably, and with consistent quality. This 
multipass welding process produces improved low cost, high strength 
composite assemblies of large scale parts fusion bonded together with 
consistent quality. It uses a schedule of heat application that maintains 
the overall temperature of the structure within the limit in which it 
retains its high strength. Therefore, it does not require internal tooling 
to support the structure against sagging which otherwise could occur when 
the bond line is heated above the high strength temperature limit. The 
process also produces nearly complete bond line area fusion on standard 
production composite material parts having the usual surface imperfections 
and deviations from perfect flatness. The multipass welding process 
eliminates fasteners and the expense of drilling holes, inspecting the 
holes and the fasteners, inspecting the fasteners after installation, 
sealing between the parts and around the fastener and the holes; reduces 
mismatch of materials; and eliminates arcing from the fasteners. 
In the multipass process, an induction heating work coil is passed multiple 
times over a bond line while applying pressure in the region of the coil 
to the components to be welded, and maintaining the pressure until the 
resin hardens. The resin at the bond line is heated to the softening or 
melting temperature with each pass of the induction work coil. Pressure is 
exerted to flow the softened/melted resin in the bond line and to reduce 
the thickness of the bond line. The pressure improves the intimacy of the 
faying surface contact with each pass to improve continuity of the bond. 
The total time at the softened or melted condition of the thermoplastic in 
the faying surfaces is sufficient to attain deep interdiffusion of the 
polymer chains in the materials of the two faying surfaces throughout the 
entire length and area of the bond line. The process produces a bond line 
of improved strength and integrity in the completed part. The total time 
of the faying surfaces at the melting temperature is divided into separate 
time segments which allows time for the heat in the interface to dissipate 
without raising the temperature of the entire structure to the degree at 
which it loses its strength and begins to sag. The desired shape and size 
of the final assembly is maintained. 
A structural susceptor allows us to include fiber reinforcement within the 
weld resin to alleviate residual tensile strain otherwise present in an 
unreinforced weld. The susceptor includes alternating layers of thin film 
thermoplastic resin sheets and fiber reinforcement (usually woven 
fiberglass fiber) sandwiching the conventional metal susceptor that is 
embedded in the resin. While the number of total plies in this structural 
susceptor is usually not critical, we prefer to use at least two plies of 
fiber reinforcement on each side of the susceptor. This structural 
susceptor is described in greater detail in U.S. Pat. No. 5,717,191, which 
I incorporate by reference. 
The structural susceptor permits gap filling between the welded composite 
laminates which tailors the thickness (number of plies) in the structural 
susceptor to fill the gaps, thereby eliminating costly profilometry of the 
faying surfaces and the inherent associated problem of resin depletion at 
the faying surfaces caused by machining the surfaces to have complementary 
contours. Standard manufacturing tolerances produce gaps as large as 0.120 
inch, which are too wide to create a quality weld using the conventional 
susceptors. 
It is easy to tailor the thickness of the structural susceptor to match the 
measured gap by scoring through the appropriate number of plies of resin 
and fiber reinforcement and peeling them off. In doing so, a resin rich 
layer will be on both faying surfaces and this layer should insure better 
performance from the weld. 
b. Fixed coil induction welding 
Thermoplastic welding using Boeing's induction heating workcell differs 
from the moving coil processes because of the coil design and resulting 
magnetic field. The fixed coil workcell promises welding at faster cycle 
times than the moving coil processes because it can heat multiple 
susceptors simultaneously. The fixed coil can reduce operations to minutes 
where the moving coil takes hours. The keys to the process, however, are 
achieving controllable temperatures at the bond line in a reliable and 
reproducible process that assure quality welds of high bond strength. The 
fixed coil induces currents to flow in the susceptor differently from the 
moving coils and covers a larger area. Nevertheless, proper processing 
parameters permit welding with a fixed coil induction heating workcell 
using a susceptor at the bond line. 
Another advantage with the fixed coil process is that welding can occur 
using the same tooling and processing equipment that we use to consolidate 
the skin, thereby greatly reducing tooling costs. Finally, the fixed coil 
heats the entire bond line at one time to eliminate the need for shims 
that are currently used with the moving coil. We can control the 
temperature and protect against overheating by using "smart" susceptors as 
a retort or as the bond line susceptor material or both. 
The need for a susceptor in the bond line poses many obstacles to the 
preparation of quality parts. The metal which is used because of its high 
susceptibility differs markedly in physical properties from the resin or 
fiber reinforcement so dealing with it becomes a significant issue. The 
reinforced susceptor of U.S. Pat. No. 5,808,281 (which I also incorporate 
by reference) overcomes problems with conventional susceptors by including 
the delicate metal foils (0.10-0.20 inch wide.times.0.005-0.010 inch 
thick; preferably 0.10.times.0.007 inch) in tandem with the warp fibers of 
the woven reinforcement fabric. The foil is always on the remote side of 
the fabric because it is between the warp thread and the weave threads. 
This arrangement holds the foils in place longitudinally in the fabric in 
electrical isolation from each other yet substantially covering the entire 
width of the weld surface while still having adequate space for the flow 
and fusion of the thermoplastic resin. Furthermore, in the bond line, the 
resin can contact, wet, and bond with the reinforcing fiber rather than 
being presented with the resinphilic metal of the conventional systems. 
There will be a resin-fiber interface with only short runs of a 
resin-metal interface. The short runs are the length of the diameter of 
two weave fibers plus the spatial gap between the weave fibers, which is 
quite small. Thus, the metal is shielded within the fabric and a better 
bond results. In this woven arrangement to foil can assume readily the 
contour of the reinforcement. Finally, the arrangement permits efficient 
heat transfer from the foil to the resin in the spatial region where the 
bond will focus. 
I improve the strength and durability of adhesive bonds or thermoplastic 
welds connecting composite structure by adding Z-pin mechanical 
reinforcement to the bond line. 
4. Z-Pin Reinforcement 
First, some general discussion about the benefits of Z-pins in composite 
assemblies. 
Composite sandwich structures having resin matrix skins or facesheets 
adhered to a honeycomb or foam core are used in aerospace, automotive, and 
marine applications for primary and secondary structure. The facesheets 
typically are reinforced organic matrix resin composites, made from 
fiberglass, carbon, ceramic, or graphite fibers reinforcing a 
thermosetting or thermoplastic matrix resin. The facesheets carry the 
applied loads, and the core transfers the load from one face sheet to the 
other or absorbs a portion of the applied load. In either case, it is 
important that all layers of the structure remain rigidly connected to one 
another. Noise suppression sandwich structure or sandwich structures for 
other applications are described in U.S. Pat. No. 5,445,861, which I also 
incorporate by reference. 
Keeping the facesheets adhered to the foam is problematic. For simplicity, 
I refer to a foam core sandwich structure as an example. The most common 
source of delamination stems from a relatively weak adhesive bond that 
forms between the facesheets and the foam core. That is, pulloff strength 
of the facesheets in shear is low. Efforts to strengthen the bond have 
generally focused on improving the adhesive, but those efforts have had 
limited success. 
Delamination can arise from differences in the coefficient of thermal 
expansion (CTE) of the different material layers. As a result, as 
temperatures oscillate, the facesheet or foam may expand or contract more 
quickly than its adjoining layer. In addition to causing layer separation, 
CTE differences can significantly distort the shape of a structure, making 
it difficult to maintain overall dimensional stability. Conventional 
sandwich structure optimizes the thickness of a structure to meet the 
weight and/or space limitations of its proposed application. Sandwich 
structures are desirable because they are usually lighter than solid metal 
or composite counterparts, but they may be undesirable if they must be 
larger or thicker to achieve the same structural performance. Providing 
pass-throughs (i.e., holes), which is relatively easy in a solid metal 
structure by simply cutting holes in the desired locations, is more 
difficult in a composite sandwich structure because holes may 
significantly reduce the load carrying capability of the overall 
structure. 
Foster-Miller has been active in basic Z-pin research. U.S. Pat. No. 
5,186,776 describes a technique for placing Z-pins in composite laminates 
involves heating and softening the laminates with ultrasonic energy with a 
pin insertion tool which penetrates the laminate, moving fibers in the 
laminate aside. The pins are inserted either when the insertion tool is 
withdrawn or through a barrel in the tool prior to its being withdrawn. 
Cooling yields a pin-reinforced composite. U.S. Pat. No. 4,808,461 
describes a structure for localized reinforcement of composite structure 
including a body of thermally decomposable material that has substantially 
opposed surfaces, a plurality of Z-pin reinforcing elements captured in 
the body and arranged generally perpendicular to one body surface. A 
pressure plate (i.e., a caul plate) on the other opposed body surface 
drives the Z-pins into the composite structure at the same time the body 
is heated under pressure and decomposes. I incorporate U.S. Pat. Nos. 
4,808,461 and 5,186,776 by reference. 
A need exists for a method to form a sandwich structure that (1) resists 
distortion and separation between layers, in particular, separation of the 
facesheets from the core; (2) maintains high structural integrity; (3) 
resists crack propagation; and (4) easily accommodates the removal of 
portions of core, as required by specific applications. The method should 
allow the structure to be easily manufactured and formed into a variety of 
shapes. In U.S. patent application Ser. No. 08/582,297 (which I 
incorporate by reference), I described such a method of forming a 
pin-reinforced foam core sandwich structure. Facesheets of uncured 
fiber-reinforced resin (i.e., prepreg or B-stage thermoset) are placed on 
opposite sides of a foam core. The core has at least one compressible 
sublayer and contains a plurality of Z-pins spanning the foam between the 
facesheets. Z-pins are driven into the facesheets during autoclave curing 
of the face sheet resin when a compressible sublayer is crushed and the 
back pressure applied through the caul plate or other suitable means 
drives the Z-pins into one or both of the facesheets to form a 
pin-reinforced foam core sandwich structure. By removing some of the foam 
core by dissolving, eroding, melting, drilling, or the like to leave a gap 
between the facesheets, I produce my corresponding column core structure. 
The foam core generally is itself a sandwich that includes a high density 
foam sublayer, and at least one low density, compressible or crushable 
foam sublayer. The preferred arrangement includes a first and second low 
density foam sublayer sandwiching the high density sublayer. The Z-pins 
are placed throughout the foam core in a regular array normal to the 
surface or slightly off-normal at an areal density of about 0.375 to 1.50% 
or higher, as appropriate, extending from the outer surface of the first 
low density foam sublayer through to the outer surface of the second low 
density foam sublayer. Expressed another way with respect to the 
arrangement of the pins, I typically use 200 pin/in.sup.2 or more. 
Preferably, the foam sublayers are polyimide or polystyrene, the Z-pins 
are stainless steel or graphite, and the facesheets are fiber-reinforced, 
partially cured or precured thermosetting or thermoplastic resin 
composites. 
In U.S. Pat. No. 5,589,016, Hoopingarner et al., describe a honeycomb core 
composite sandwich panel having a surrounding border element (i.e., a 
"closeout") made of rigid foam board. The two planar faces of the rigid 
foam board are embossed or scored with a pattern of indentations usually 
in the form of interlinked equilateral triangles. The scoring is 
sufficiently deep and numerous to provide escape paths for volatiles 
generated inside the panel during curing and bonding of the resin in the 
facesheets to the honeycomb core and peripheral foam. The scoring prevents 
the development of excessive pressure between the facesheets in the 
honeycomb core that otherwise would interfere with the bonding. I 
incorporate this application by reference. 
Rorabaugh and Falcone discovered two ways to increase the pulloff strength 
in foam core sandwich structure. First, they form resin fillets around the 
fiber/resin interfaces at the contact faces of the foam core by dimpling 
the foam to create a fillet pocket prior to resin flow during curing. 
Second, they arrange the pins in an ordered fashion such as a tetrahedral 
configuration or a hat section configuration. In tetrahedral or hat 
section configurations, the pins not only provide a tie between the two 
skins but they also provide miniature structural support suited better for 
load transfer than normal or random off-normal (interlaced) or less 
ordered pin configurations. With ordering of the pins, they produce 
anisotropic properties. More details concerning their Z-pin improvements 
are available in their U.S. patent application Ser. No. 08/628,879 
entitled "Highly Ordered Z-Pin Structures," which I incorporate by 
reference. 
In U.S. patent application Ser. No. 08/660,060 entitled "Joining Composites 
Using Z-pinned Precured Strips," Pannell describes bond line reinforcement 
achieved by using precured strips to carry the Z-pins. Pannell position 
his strips between two detail parts and joins the three piece assembly 
with bonding, cocuring, or welding. I incorporate this Pannell application 
by reference. 
I have discovered how to add Z-pins to thermoplastic welds or to adhesive 
bonds in induction heating bonding processes to improve the bond or weld 
pulloff strength. 
SUMMARY OF THE INVENTION 
The present invention introduces Z-pin mechanical reinforcement to the bond 
line of two or more composite elements by prefabricating cured composite 
elements that include protruding Z-pins (or stubble) along the element 
face that will contact the bond line. The stubble is formed by including 
peel plys on this face during pin insertion using, for example, the 
process described in U.S. patent application Ser. No. 08/582,297, entitled 
"Pin-Reinforced Sandwich Structure." When connecting the element to other 
composite structure, I remove the peel plys to expose the stubble. Then, I 
assemble the several elements in the completed assembly to define the bond 
line. 
The stubble is typically about 1/16 inch high. So, if the element which the 
stubble contacts is a precured element, I use uncured resin padups or 
adhesive to fill the gaps. If the contacting element is uncured or 
partially cured, I generally do not use the padup. The padup might 
actually be a thermoplastic welding susceptor. 
Following assembly, I complete the bonding, cocuring, or welding using 
conventional techniques. If the stubble is backed by a foam core sandwich 
structure of the type I described in U.S. patent application Ser. No. 
08/582,297, during the connecting operation, I might compress or decompose 
a low density sublayer in the foam to drive the Z-pins deeper into the 
contacted element along the bond line. 
Assemblies having Z-pin mechanical reinforcement are better able to 
withstand impact shock without peel failure.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Z-pin bonding produces stronger bonds between composite detail parts than 
are achievable with adhesive bonds or fusion (i.e., welded) bonds. Z-pin 
bonding adds Z-direction reinforcement to the otherwise unreinforced joint 
in the organic matrix resin. The Z-pins mechanically reinforce the bond, 
especially in environments prone to vibration. I can use Z-pins at the 
joints between detail parts in adhesive bonding, cocuring, or welding 
processes to join two or more precured parts or a combination of precured 
and uncured detail parts. 
In my Z-pin bonding process, I prepare a precured composite that has Z-pins 
(or "stubble") protruding from the detail along the intended bond line. To 
insert the pins in their intended location, I use an insertion process 
like one of those described in U.S. patent application Ser. No. 
08/582,297) or U.S. Pat. No. 5,736,222 or any other suitable insertion 
process. My basic approach is shown in FIG. 8-10. I can also use Avila's 
pin insertion tool (FIG. 15). Before describing the pin insertion process 
in detail, I will first describe how I use the Z-pinned detail parts to 
prepare bonded assemblies. 
Throughout this discussion, I use "composite" to mean a fiber reinforced 
organic resin matrix material. The fibers should be of suitable strength 
to make aerospace structural parts, such as graphite, fiberglass, or 
carbon fibers. The organic resin can be a thermosetting resin, such as 
epoxy or bismaleimide, or a thermoplastic, such as ULTEM or KIIIB 
polyimide. Z-pinning is compatible with all fiber and resin systems. 
The function and properties of the Z-pins are described in my copending 
applications Ser. Nos. 08/618,650, now U.S. Pat. No. 5,736,222 and 
08/582,297 which I incorporate by reference. In Z-pin bonding, the resins 
should be compatible with the intended joint. The Z-pins might be the same 
material as the reinforcing fibers in the composite detail parts or can be 
different, as the situation dictates. 
Now turning to FIG. 1, the Z-pin bonding process uses a composite detail 
part 10 having a region 12 of Z-pin stubble along the intended bond line 
for connecting part 10 with other detail parts. Each Z-pin generally 
protrudes about 1/16 inch above the surface of part 10 (like the Indian 
"bed of nails") for ultimate insertion into the facing parts at the joint, 
but the height can change with the intended application. To protect the 
stubble during manufacture and inventory prior to laying up the assembly 
for bonding, I cover the stubble with Teflon peel plys 14 and the residue 
of the pin-carrier foam 16 which I use to hold the pins prior to their 
insertion into the detail part 10. 
In some applications, especially with a decomposable foam, it may be 
unnecessary to use a peel ply 14. The peel ply 14 functions to protect the 
Z-pin stubble during storage while leaving a clean surface in the stubble 
region when peeled away during the lay up process, and can be any suitable 
material. 
The pins in the stubble 12 may be normal to the surface of part 10 or 
interlaced or highly ordered, as described in Boeing's copending 
applications Ser. Nos. 08/618,650, now U.S. Pat. No. 5,736,222 and 
08/628,879. That is, the pins can assume any desired arrangement. The 
density of pins is also variable to suit the application. Differences in 
the orientation of pins, their length, their strength, their density, etc. 
can change in different regions of the bond line. That is, the areal 
density of pins might be 1.0% on the left side of the part in FIG. 1 while 
being 1.5% on the right side. Alternately, the pin density might be higher 
around fasteners or might be higher near the edges of the bond line as 
opposed to along the centerline. Also, of course, the orientation may 
change at different regions along the bond line and orientations might 
even be mixed together, if desired. 
By "orientation," I mean normal, interlaced, highly ordered, or the like as 
defined in Boeing's copending Z-pinning applications. For clarity in the 
drawings in the present application, I simply illustrate the "normal" 
orientation. 
The pin-carrying foam 16 is described in greater detail in my copending 
applications and prior art patents that I earlier incorporated by 
reference. 
The composite detail part 10 can be a laminate of plys of fiber reinforced 
organic resin matrix material, or might be a sandwich panel with a foam, 
honeycomb or other core, or might even be column core. Basically, part 10 
can be any material that has a resin interface for bonding to another 
detail part. I reinforce the joint between the resin and the other part at 
their interface. 
The height to which the stubble of pins extends can vary widely to suit the 
intended application. Of course, the strength, shape, size, and 
orientation of the pins effect their effectiveness when the protrusion 
height gets large. 
As shown in FIG. 2, the detail part 10a might be fabricated as an I-beam 
spar rather than as a panel. In fact, the detail part can assume any shape 
so long as the shape is susceptible of Z-pin insertion to create the 
reinforcing stubble along the bond line. 
FIG. 2 illustrates placing the spar detail part 10a on an uncured panel 18 
with the stubble 12 at the interface. FIG. 3 illustrates a typical 
cross-section of the spar-panel assembly. While the stubble 12 is shown on 
the spar in FIG. 2, the Z-pins could be on either the spar, the panel or 
both detail parts. 
As shown in FIG. 3, when the assembly of the spar 10a and panel 18 are 
bonded, the Z-pins in the stubble 12 penetrate into the uncured panel 18. 
In the circumstance where the panel 18a is precured, as shown in FIGS. 4 
and 5, I introduce a bond padup strip 20, typically of the same materials 
as the detail parts being joined. The padup strip 20 is uncured during 
assembly and functions to bond the precured, thermoset detail parts when 
the bonding process is complete. The padup strip can be an uncured 
thermosetting resin prepreg (in which cases bonding is a cocuring process) 
or might be any suitable adhesive bonding material. The padup strip might 
be a resin encased susceptor of the type shown in FIGS. 13 & 14 and as 
Boeing uses in its thermoplastic welding operations. In this case, the 
detail parts would generally be precured. 
As best shown in FIG. 5, the spar detail part 10a includes a stubble 
surface so that the padup strip 20 ends up having pins extending upwardly 
from the panel 18a as well as downwardly from the spar flange 10a. Shawn 
Pannell describes in his application "Joining Composites Using Z-Pinned 
Precured Strips," 08/660,060, that the pins might be carried in the padup 
strip with stubble on both faces with longer, integral pins if the detail 
parts are thermoplastic rather I insert the pins into the spar and panel 
prior to their curing. 
FIGS. 6 and 7 illustrate another embodiment of the present invention with 
reference to the bonding of a wing skin to a spar. FIG. 6 shows an 
exploded view of the wing skin 100, padup strip 20, and spar 200 while 
FIG. 7 shows a typical cross-section taken along the bond line. While 
FIGS. 6 & 7 illustrate a wing skin-spar joint, the process is applicable 
to any joint. This embodiment describes bonding using a sandwich core 
structure for the wing skin to produce the stubble region and subsequent 
bonding of the skin to the spar with an uncured padup strip in a cocure, 
adhesive bonding, or welding operation. 
As best shown in FIG. 7, the skin 100 comprises a sandwich core structure 
of the type described in U.S. patent application Ser. No. 08/582,297 
having outer facesheets 105 & 110, crushable foam layers 115 and 120, and 
a central foam core 125 with Z-pins 130 extending through all five layers. 
Stubble on the interface surface is achieved by crushing layers 115 and 
120 more than the combined thickness of facesheets 105 and 110 during the 
autoclave cycle when the pins are inserted into the facesheets. Of course, 
after curing, the central foam 115, 120 and 125 might be dissolved to make 
a column core skin structure. 
The facesheets 105 & 110 are positioned adjacent the foam core 115, 120 and 
125. I usually use a layer of adhesive to attach adjoining layers. I form 
the pin-reinforced foam core using known methods (e.g., stitching or 
needling) or purchase it from companies such as Foster-Miller, Inc., in 
Waltham, Mass. I can score the foam core according to the Hoopingarner 
method to provide channels for venting of volatiles during curing. 
The core generally is a closed cell foam that includes three sublayers: a 
high density central sublayer 125 and first and second low density, 
crushable foam sublayers 115 and 120 located on each side of the high 
density foam sublayer. While three layers are shown, the foam core may be 
composed of any number of sublayers depending on the particular 
application. For example, the foam core may be a single low density 
sublayer; or, it may be a stack of alternating low density and high 
density sublayers. The foam core should be crushable during autoclave 
curing to permit the pins to extend into the facesheets. Low density 
polyimide (e.g., Rohacel.TM.) or low density polystyrene (e.g., 
Styrofoam.TM.) foams are the presently preferred low density sublayer 
materials, since they are easy to form and do not require extremely high 
temperatures or pressures to crush during autoclave curing. The low 
density sublayer may decompose at the autoclave temperatures. 
If a high density sublayer 125 is included, it usually should be made of a 
material that will not crush during autoclave curing. Obviously, the 
precise temperatures and pressures to be used during autoclave curing will 
affect the selection of the material used to form the high density 
sublayer. Further considerations to be taken into account when selecting 
an appropriate high density sublayer material include whether the high 
density sublayer is to be removed after autoclave processing and the 
preferred method for removing it. Typically it is high density polystyrene 
or polyimide foam. It might be (i) syntactic foam having internal 
reinforcing spheres, (ii) a fiber-reinforced resin prepreg or composite, 
(iii) a fiberform or microform ceramic such as described in U.S. Pat. Nos. 
5,376,598; 5,441,682; and 5,041,321 or in copending U.S. patent 
applications Ser. Nos. 08/209,847 or 08/460,788, (iv) a metal foil, (v) a 
metal foil resin laminate of the type described in U.S. Pat. No. 4,489,123 
or U.S. patent application Ser. No. 08/585,304 entitled "Titanium-Polymer 
Hybrid Laminates," or (vi) a foam filled honeycomb core. The central 
sublayer 125 might also be a honeycomb core with the cells arranged normal 
to the plane of the facesheets. As Hoopingarner suggests, the core might 
be a combination of these alternatives, like a central honeycomb core 
bordered by a foam closeout frame. If the high density sublayer is a 
prepreg or a composite, the product itself is a laminated composite. In 
such case, generally the resin in the facesheets would be the same as the 
resin in the high density sublayer. 
The Z-pins 130 (here and in all the embodiments) may be any suitably rigid 
material, e.g., stainless steel, titanium, copper, graphite, epoxy, 
composite, glass, carbon, etc. The Young's modulus of elasticity for the 
Z-pins is generally greater than 10.sup.7. Additionally, the Z-pins may be 
barbed, where appropriate, to increase their holding strength in the 
facesheets. 
In the case of thermosets, the facesheets are preferably formed of a 
partially cured (B-stage) fiber-reinforced composite material. If 
composites are used as facesheets, the effect that the autoclave cure 
cycle will have on the facesheets needs to be considered to determine and, 
then, to follow the optimal temperature/pressure autoclave cure regime. 
Suitable reinforcing fibers include glass, graphite, arimide, ceramic, and 
the like. Suitable resins include epoxy, bismaleimide, polyimide, 
phenolic, or the like. (Virtually any thermoplastic or thermoset resin 
will suffice.) 
Various procedures are available for laying up the composite facesheets. 
Since such procedures are generally known to those skilled in the arts 
they are not described here. Although thick, metal sheets do not work well 
as facesheets, I can use metal foil or metal foil/resin laminated 
composites. The metal foil in such cases might be welded to metallic 
Z-pins in the fashion described in my copending U.S. patent application 
Ser. No. 08/619,957 entitled "Composite I Metal Structural Joint with 
Welded Z-Pins." 
The stubbled skin is bonded to a stubbled spar with a padup strip in the 
process previously described. 
FIGS. 8-10 illustrate a preferred process for inserting the Z-pins into a 
detail part to leave a stubble interface. The detail part 500 (here a 
laminated panel having several layers of prepreg) is mounted on a work 
surface or layup mandrel 550 with appropriate release films between the 
part and tool. Another release film 600 caps the detail part 500 and 
separates the part 500 from a Z-pin preform 650 (i.e., a foam loaded with 
Z-pins 130 in a predetermined orientation). A rigid caul plate or backing 
tool 700 completes the assembly. All the layers are then wrapped in a 
conventional vacuum bag film 750 which is sealed to permit drawing a 
suction within the closed volume surrounding the assembly. 
As shown in FIG. 9, in an autoclave under elevated temperature and 
pressure, the foam in the Z-pin preform 650 crushes and the Z-pins 130 are 
driven into the uncured detail part 500. After completing the cure cycle, 
the detail part 500 is cured and has the Z-pins 130 anchored within it. 
The crushed foam 650 and release ply 600 protect the stubble until 
assembly of the detail parts is desired. Thus, the process of FIGS. 8-10 
yields a cured detail part having a stubble field. Other processes can be 
used to achieve the same result, including ultrasonic insertion into 
precured thermoplastic laminates as described in the prior art or Boeing's 
other, copending Z-pin applications. 
As shown in FIG. 11, including Z-pin reinforcement in the joint improves 
compression after impact strength of the assembly. Boeing tests show about 
a 50% increase when using an areal density of 0.5% of 0.006 inch diameter 
pins in AS4-3501-6 test specimens following a 20 ft-lb impact. The joint 
nominally has the same compression and tensile strength prior to impact, 
but the inclusion of pins increase the compression ultimate strength when 
the assembly is subjected to low speed impact energy. In fact, the strain 
remains essentially constant over the range of impacts less than the 
impact needed to observe surface damage. 
The following examples further illustrate my Z-pin experiments. 
EXAMPLE 1 
I made 3/16 inch quasi-isotropic composite test specimens from AS4/3501-6 
having 0.5% areal density, 16 mil diameter T300/3501-6 Z-pins with 
sufficient surface peel plys to yield 0.080 inch stubble. As a control, on 
one-half of the specimens, I did not insert Z-pins. I assembled two of the 
stubbled parts around an AS4/3501-6 uncured scrim padup about 0.090 inch 
thick with the stubble from each part overlapping. I bonded the assembled 
parts using the conventional bonding cycle. Then, I cut the resulting 
bonded assembly into 1.times.10 inch coupons, thereby having some 
pin-reinforced, bonded coupons and some coupons lacking pin reinforcement. 
I tested the specimens in the G.sub.1c Mode 1 fatigue test cycle with pull 
tabs glued to the faces pulled in a standard G.sub.1c test fixture. I 
included a crack starter initiating peel in the bond area. I correlated 
the data with the standard load v. head extension (in-lbs/in). The results 
are summarized in Table 1: 
TABLE 1 
______________________________________ 
Specimen Load Comments 
______________________________________ 
Pinned: 
1 5.4 
2 4.8 
3 2.86 *Failed in the laminate above the bond line 
Unpinned: 4 2.75 
5 3.64 
6 3.49 
______________________________________ 
Ignoring specimen 3, the Z-pin reinforcement at this relatively low density 
improved the bond strength with this Mode 1 fatigue measure by about a 45% 
increase in the peel strength. Upon analysis of the pinned specimens, I 
discovered that some pins were bent, which I believe lowered the 
reinforcing value (reduced the load I measured). I also believe that I 
could prepare even better bonds (i.e., joints) using higher pressure 
during the bonding cycle. 
EXAMPLE 2 
I prepared additional specimens using AS4/3501-6 prepreg with 2% by area 
0.020 diameter titanium Z-pins inserted into a spar cap. This spar was 
then cured at 350 degrees F. with Z-pin stubble left exposed on the spar 
cap. The Z-pin stubble was 0.20 inches long. This cured spar was then 
placed on an uncured skin laminate 0.30 inches thick, with the Z-pin 
stubble placed against the uncured skin. The spar, associated spar 
tooling, and skin were then vacuum bagged and autoclave cured at 350 
degrees F., using a 100 psi autoclave pressure. The vacuum and autoclave 
pressure drove the spar down onto the uncured skin and inserted the Z-pin 
stubble into the skin. The cured final part was then trimmed for pull 
testing. 
Pull testing results showed the Z-Bonded parts had an 83 percent greater 
load carrying capability than the control parts. My results are summarized 
in FIG. 12. 
In a thermoplastic welding process, the padup strip 20 might include a 
susceptor for integrating with an oscillating magnetic field to generate 
eddy currents sufficient to melt and cure the bond line resins and to form 
a weld. Of course, I can use any other arrangement to get the appropriate 
heating at the bond line when completing the weld. If welding, I prefer to 
use pins in the detail parts that penetrate further into the parts than 
the region which softens during the formation of a fusion bond between the 
details. In this way, the pins stay firmly anchored in their desired 
orientation. A suitable padup strip is illustrated in FIG. 13. I can heat 
the bond line with induction or resistance heating produced in susceptor. 
Any of Boeing's susceptors might be used. Energy can be introduced for 
heating to the susceptor by induction, resistance, a combination of both, 
or any other suitable means. 
As shown in FIG. 13, the susceptor 1300 comprises a metal mesh 1305 encased 
in a resin 1310. The susceptor of FIG. 13 includes selvage edge strips 
1315. The mesh 1305 includes a repeating pattern of diamond shaped 
openings of length (L) and width (W) separated by fine-line elements. 
FIGS. 15-17 illustrate Avila's pin insertion tool that I can use to form 
detail parts having pin stubble. Avila's tool 1500 includes a housing 1505 
holding a sliding piston 1510 which is reciprocal between a loading 
position for receiving a pin-carrying foam 1550 in a cavity 1515 and an 
insertion position where the piston moves upwardly to crush the foam and 
to insert the pins 1545. Seals 1520 permit the piston 1510 to slide along 
the walls of housing 1505 when pneumatic pressure is applied through inlet 
1525 to chamber 1530 behind the piston. Motion of the piston 1510 toward 
removable cure tool 1535 is arrested with stop 1540 which also serves to 
control the depth of insertion of pins 1545 in the pin-carrying foam 1550 
into the detail part 1555. The stop 1540 contacts replaceable stop 1560 
that seats in the fixed support frame of the cure tool 1535 that is 
rigidly attached to the housing 1505 as the fixed wall defining cavity 
1515. The replaceable stop allows adjustment of the depth of penetration 
of the pins into the detail part 1555. The cure tool 1535 fits rigidly in 
a matching receiving surface in the frame and does not move when piston 
1510 moves upwardly. Yet, cure tool 1535 is replaceable to permit 
controlled insertion of different Z-pin orientations or different 
insertion depths into the detail part 1555. During pin insertion through 
movement of the piston 1510, the detail part 1555 is held rigidly on the 
surface of the cure tool 1535 so that the Z-pins 1545 are positioned 
correctly. 
All parts of the pin insertion tool 1500 are designed to withstand the 
temperatures and pressures associated with autoclave curing of the resin 
composite detail parts. Any necessary release films can be used between 
the pin-carrying foam 1550 or the detail part 1555 and the working parts 
of Avila's insertion tool. 
As the piston 1510 moves upwardly to compress the pin-carrying foam 1550 
against the cure tool 1535, the Z-pins 1545 in the foam register with an 
associated hole 1605 (FIGS. 16 or 17) in the cure tool 1535. To assure 
registration between the pin 1545 and hole 1605, each hole has a funnel 
nozzle 1705 to guide the pin into the hole and into its proper orientation 
in the detail part. 
The cure tool has the arrangement of holes that corresponds with the 
desired Z-pin orientation in the detail part. The tool helps placing the 
pins accurately. Because the foam decomposes at the autoclave curing 
temperature, without Avila's tool, the pins lose their lateral support and 
can move or buckle to disturb the desired pin orientation, especially when 
the stubble field in the detail part covers a large area. For further 
assurance of proper pin placement, the contact face of piston 1510 might 
be knurled to keep the pins from sliding. 
Avila's tool might include a shearing ram on the contact surface between 
the cure tool and the detail part or at the interface between the cure 
tool and the pin-carrying foam for cutting the pins after their insertion. 
In the alternative where the ram is at the cure tool-foam interface, the 
width of the cure tool becomes a reliable gauge for setting the height of 
the stubble, since this much of the pins will protrude when the detail 
part is removed from the tool. 
Avila's pin insertion tool is especially beneficial when making relatively 
large production runs of detail parts. The tool reduces part-to-part 
variation by inserting the Z-pins accurately and repeatedly where they are 
designed to be. Avila's tool is described in greater detail in U.S. Pat. 
No. 5,835,585 entitled "Tooling for Inserting Z-Pins," which I 
incorporated by reference. 
While I have described preferred embodiments, those skilled in the art will 
readily recognize alterations, variations, and modifications which might 
be made without departing from the inventive concept. Therefore, interpret 
the claims liberally with the support of the full range of equivalents 
known to those of ordinary skill based upon this description. The examples 
illustrate the invention and are not intended to limit it. Accordingly, 
define the invention with the claims and limit the claims only as 
necessary in view of the pertinent prior art.