Source: http://www.google.com/patents/US5876540?dq=6,418,462
Timestamp: 2017-10-21 18:17:37
Document Index: 152034737

Matched Legal Cases: ['art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 500', 'art 500', 'art 500']

Patent US5876540 - Joining composites using Z-pinned precured strips - Google Patents
Prefabricated composite detail parts use precured strips that include Z-pin reinforcement along the bond line. Each strip has Z-pin stubble protruding from opposed faces so that the pins are embedded into the detail parts when the joint forms....http://www.google.com/patents/US5876540?utm_source=gb-gplus-sharePatent US5876540 - Joining composites using Z-pinned precured strips
Publication number US5876540 A
Application number US 08/660,060
Also published as US5876832, US5935698, US6436507
Publication number 08660060, 660060, US 5876540 A, US 5876540A, US-A-5876540, US5876540 A, US5876540A
Inventors Shawn D. Pannell
Patent Citations (15), Referenced by (58), Classifications (45), Legal Events (5)
US 5876540 A
1. A method for forming a reinforced joint at a bond line in composite structure, comprising the steps of:
(a) positioning along the bond line between at least two composite elements a Z-pinned precured strip,
the strip having reinforcing pins generally extending in the Z-direction, embedded in a cured resin and projecting on opposed faces of the strip to define stubble, the strip being positioned so that the Z-direction pins extend substantially perpendicular to or at an angle with the plane of the bond line;
(b) optionally, applying an adhesive on the interfaces between the strip and the elements;
(c) heating the bond line to connect the elements about the strip to form a joint containing Z-pin reinforcement.
2. The method of claim 1 wherein the composite elements are thermoplastic and heating the strip forms a thermoplastic weld between the strip and the composite elements.
3. The method of claim 1 wherein the Z-direction pins have a Young's modulus of elasticity of generally greater than 107 and, optionally, include barbs to increase their holding strength.
4. The method of claim 3 wherein the strip is a plurality of plies of fiber-reinforced resin or a laminated resin-metal foil composite.
5. A method for forming a reinforced joint at a bond line in composite structure, comprising the steps of:
the strip having reinforcing pins general extending in the Z-direction, embedded in a cured resin, and projecting on opposed faces of the strip to define stubble,
the strip also having a susceptor heatable with an induction coil the positioning placing the Z-direction pins so that the Z-direction pins extend substantially perpendicular to or at an angle with the plane of the bond line;
6. The method of claim 5 wherein the step of heating includes magnetically inducing eddy currents in the susceptor.
7. The method of claim 5 wherein the susceptor includes a metal mesh.
8. The method of claim 5 wherein the resin is a polyimide.
9. The method of claim 5 wherein the stubble is about 1/16th inch high and, optionally, include barbs to improve their holding strength.
covering the stubble with uncured resin pad ups to fill at least one gap between the composite elements and the strip.
11. The method of claim 5 wherein the composite elements are thermoplastic and heating the strip forms a thermoplastic weld between the strip and the composite elements.
12. The method of claim 5 wherein the strip is a laminated resin-metal foil composite.
13. The method of claim 5 wherein the strip is thin enough to remain flexible so that the strip can assume the contour of the bond line.
14. A method for forming a reinforced joint at a bond line in composite structure, comprising the steps of:
(b) covering the stubble with uncured resin pad ups to fill a gap between one of the composite elements and the strip;
(c) optionally, applying an adhesive on the interfaces between the strip and the elements; and
(d) heating the bond line to connect the elements about the strip to form a joint containing Z-pin reinforcement.
Boeing can perform a wide range of manufacturing operations in its induction heating press. These operations have optimum operating temperatures ranging from about 350° F. (175° C.) to at least about 1850° F. (1010° C.). For each operation, the temperature is held relatively constant for several minutes to several hours. While controlling the input power fed to the induction coil controls the temperature, a better and simpler way capitalizes on the Curie temperature. Judicious selection of the metal or alloy in the retort's susceptor facesheets avoids excessive heating irrespective of the input power. With improved control and improved temperature uniformity in the workpiece, they produce better products. This 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. patent application Ser. No. 08/469,986.
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×0.005-0.010 inch thick; preferably 0.10×0.007 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.
In U.S. patent application Ser. No. 08/658,927, entitled "Z-Pin Reinforced Bonds for Connecting Composite Structures," Childress introduced Z-pin mechanical reinforcement to the bond line of two or more composite elements by prefabricating cured composite elements that included protruding Z-pins (or stubble) along the element face that will contacted the bond line. The stubble was formed by including peel plies 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, Childress removed the peel plies to expose the stubble. Then, he assembled the several elements in the completed assembly to define the bond line.
Childress's Z-pin bonding process (FIG. 1) 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 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, although the length of the stubble varies with the application. To protect the stubble during manufacture and inventory of the part 10 prior to laying up the assembly for bonding, Childress covers the stubble with Teflon peel plies 14 and the residue of the pin-carrier foam 16 which holds 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.
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, Childress introduces a bond padup strip 20, typically of the same material 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 with bonding becoming 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 be precured.
FIGS. 6 and 7 illustrate Childress's process for 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 FIG. 6 & 7 illustrate a wing skin-spar joint, the process is applicable to any structural aircraft joint. This embodiment shown uses a sandwich core structure for the wing skin to produce the stubble region and subsequent bonding of the skin to the precured spar with an uncured padup strip in a cocure, adhesive bonding, or welding operation. As I will describe later in this description, the wingskin and spar could also be joined with my precured strip carrying the Z-pins as shown in FIG. 15.
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 application 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 combine 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 107. Additionally, the Z-pins may be barbed, where appropriate, to increase their holding strength.
Various procedures are available for laying up the composite facesheets. Since such procedures are generally known to those skilled in the art 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 U.S. patent application Ser. No. 08/619,957 entitled "Composite/Metal Structural Joint with Welded Z-Pins."
FIGS. 8-10 illustrate Childress's 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.
Childress 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 plies to yield 0.080 inch stubble. As a control, one-half of the specimens did not include Z-pins. Childress assembled two of the stubbled parts around an AS4/3501-6 uncured scrim pad up about 0.090 inch thick with the stubble from each part overlapping, and bonded the assembled parts using the conventional bonding cycle. Then, the resulting bonded assembly was cut into 1×10 inch coupons, thereby having some pin-reinforced, bonded coupons and some coupons lacking pin reinforcement.
TABLE 1______________________________________Specimen   Load   Comments______________________________________Pinned:  1       5.4  2       4.8  3       2.86   *Failed in the laminate above the bond lineUnpinned:  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, Childress discovered that some pins were bent, which lowered the reinforcing value (reduced the measured load. Better bonds (i.e., joints) could be prepared using higher pressure during cycle.
Childress 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, associates 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.
FIGS. 13 & 14 illustrate a susceptor 50 for thermoplastic welding. The susceptor 50 can be used as the padup strip or can be part of my precured strip. The susceptor 50 has a metal mesh 55 and a resin 60 that encapsulates the mesh. Generally the resin is uncured, but, in my preferred application, it would be precured. In FIG. 13, the susceptor includes selvage edge strips 65.
The precured strip 99 constitutes a plurality of plies of fiber-reinforced resin or a laminated resin-metal foil composite or any of the other constructions discussed for the core of the sandwich structure in a pin-carrying foam. The strip Z-pins 130 extend in stubble fields on opposed faces of the strip 99. The strip should be thin enough to remain flexible so that it can assume the contour of the bond line. Otherwise, the strip needs to be manufactured with the intended contour and the pieces need to be matched for assembly. Versatility of the strip is diminished.
While fiber waviness can be overcome by conservative overdesign of the skin 18, adding plies to be sure of adequate strength introduces a significant weight penalty and increases the part cost.
For joining graphite-epoxy detail parts, I recommend that the body of the precured strip 99 be two plies of cured graphite/epoxy fabric. The plies are cured at the same time the pins are inserted with heat and pressure. FIG. 18 schematically illustrates how I make the precured strip. I place the prepreg for the body 175 on a silicone backing 176 atop a hard tooling surface 177. I place a pin-carrying foam 188 atop the body 175 and complete the assembly with a rigid caul plate 193 before enclosing all the layers with a vacuum bagging film 196 that I seal 199 to the tooling surface 177. I force the pins from the pin-carrying foam 188 into the fabric 175 generally in an autoclave under heat and pressure. Of course, I can use any way to apply pressure to the caul plate and to heat the fabric. The thickness of the silicone backing 176 controls the penetration of the pins and produces a precured strip 99 with the desired, opposing stubble fields.
Avila described an alternative method for inserting the pins in his patent application Ser. No. 08/657,859 entitled "Tooling for Inserting Z-pins." Avila's insertion machine is shown in FIG. 19 and includes a sliding piston 1915 and cure tool 1919 to ensure that pins in the pin-carrying foam 188 are accurately positioned in the body 175. For simplicity of illustration, I have omitted the silicone backing and vacuum bagging film from FIG. 19.
While shown as a flat sheet with opposed pin stubble fields, the precured strip can take other configurations such as a T or cruciform. The strip might even be a pinned radius filler. McCarville et al. described titanium and composite radius fillers or radius inserts of conventional, T, and cruciform (cross) design in U. S. patent application Ser. No. 08/648,825 entitled "Titanium Radius Filler for use in Composite Interfaces," which I incorporate by reference. I can add pins to any of these inserts or fillers.
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U.S. Classification 156/91, 156/272.4, 428/223
International Classification B29C65/00, B64C1/00, B29C70/24
Cooperative Classification B29C66/61, B29C66/81821, B29C66/8122, B29C66/73921, B29C66/7212, B29C66/7394, B29C66/71, B29C66/727, B29C66/72525, B29C66/8322, Y10T428/249923, B29C66/43441, Y10T428/24058, Y10T428/24182, Y10T428/24116, Y10T428/24033, Y10T428/24174, B29C66/4344, B29C65/3648, B29C66/474, B29C66/532, B29C66/524, B29C65/3476, B29C65/368, B29C65/3676, B29C65/564, B29C65/76, B64C2001/0072, B29C66/721, B29C66/1122, B29L2031/3076, B64C2001/0081, B29C66/81455, Y02T50/433, B64C1/06, B29C65/364, B29C70/24, B29C65/344, B29C66/54
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:PENNELL, SHAWN D.;REEL/FRAME:008131/0246