Puncture-healing thermoplastic resin carbon-fiber reinforced composites

Abstract

A composite comprising a combination of a self-healing polymer matrix and a carbon fiber reinforcement is described. In one embodiment, the matrix is a polybutadiene graft copolymer matrix, such as polybutadiene graft copolymer comprising poly(butadiene)-graft-poly(methyl acrylate-co-acrylonitrile). A method of fabricating the composite is also described, comprising the steps of manufacturing a pre-impregnated unidirectional carbon fiber preform by wetting a plurality of carbon fibers with a solution, the solution comprising a self-healing polymer and a solvent, and curing the preform. A method of repairing a structure made from the composite of the invention is described. A novel prepreg material used to manufacture the composite of the invention is described.

Claims

1. A method of fabricating a composite comprising the steps of: dissolving a self-healing polymer with a solvent to form a solution; manufacturing a pre-impregnated unidirectional carbon fiber preform by wetting a plurality of carbon fibers with the solution; and curing the preform with heat and pressure to form the composite.

2. The method of claim 1, wherein the self-healing polymer comprises a polybutadiene graft copolymer.

3. The method of claim 2, wherein the self-healing polymer comprises poly(butadiene)-graft-poly(methyl acrylate-co-acrylonitrile).

4. The method of claim 2, wherein the solvent comprises N-methylpyrrolidone.

5. The method of claim 1, wherein the curing step comprises processing the wetted fibers in a vacuum press.

6. The method of claim 5, wherein the curing step comprises the steps of: increasing the temperature in the vacuum press from about 25° C. to about 150° C. at about 2° C. per minute under full vacuum and holding at about 150° C. for about 60 minutes under full vacuum; increasing the temperature in the vacuum press to about 225° C. at about 2 CC per minute under full vacuum and holding at about 225° C. for about 60 minutes under full vacuum and about 1.7 MPa compaction pressure; and cooling the vacuum press to about 25° C. at about 2° C. per minute under full vacuum.

7. The method of claim 6, wherein the sell-healing polymer is a polybutadiene graft copolymer.

8. The method of claim 7, wherein the self-healing polymer is poly(butadiene)-graft-poly(methyl acrylate-co-acrylonitrile).

9. The method of claim 6, wherein the solvent is N-methylpyrrolidone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The organization and manner of the structure and operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, wherein like reference numerals identify like elements in which:

(2) FIG. 1 is a diagram of the chemical structure of the polybutadiene graft copolymer of the preferred embodiment of the invention.

(3) FIG. 2 is a graph of a differential scanning calorimeter (DSC) scan of pristine polybutadiene graft copolymer (PBg) amorphous thermoplastic.

(4) FIG. 3 is a graph of dynamic temperature taken using a thermo-gravimetric analyzer (TGA) of pristine PBg amorphous thermoplastic.

(5) FIG. 4 is a graph of mass evolution in dynamic temperature TGA scan of structural reinforcing IM7 carbon fiber (IM7)/PBg prepreg containing anhydrous N-Methylpyrrolidone (NMP) solvent.

(6) FIG. 5 is a graph of isothermal mass evolution of pristine PBg and IM7/PBg prepreg.

(7) FIG. 6 is a dynamic temperature viscosity profile of the pristine PBg thermoplastic.

(8) FIG. 7 is a graph of the isothermal temperature and dynamic viscosity of PBg-NMP solution.

(9) FIG. 8 is a photomicrograph at 50× of IM7 5-harness satin biaxial/PBg matrix composite by resin film infusion (RFI).

(10) FIG. 9A is an optical micrograph at 100× of IM7/PBg.

(11) FIG. 9B is an optical micrograph at 100× of IM7/977-3.

(12) FIG. 10A is a through-transmission c-scan of IM7/PBg panel post-impact.

(13) FIG. 10B is a through-transmission c-scan of the panel of FIG. 10A, post-healing cycle.

(14) FIG. 11 is a graph of IM7/PBg testing results of pristine coupon compression, barely visible impact damage (BVID) coupon compression after impact (CAI) and BVID-Healed coupon compression prepared from two batches of experimental prepreg (TM340 and TM341).

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

(15) While the invention may be susceptible to embodiment in different forms, there is shown in the drawings, and herein will be described in detail, specific embodiments with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that as illustrated and described herein. Therefore, unless otherwise noted, features disclosed herein may be combined together to form additional combinations that were not otherwise shown for purposes of brevity.

(16) In one embodiment, a polybutadiene graft copolymer (PBg) was selected as a matrix in carbon-fiber-reinforced composites due to its higher mechanical and thermal properties as compared to the other self-healing thermoplastics which have been studied. According to material suppliers, PBg has a glass transition temperature (Tg)=80 degrees Celsius (° C.), room temperature (RT) tensile strength of 37 MPa, RT tensile modulus of 2.47 GPa, and a 7.5 percent elongation at break. The tensile modulus of the neat polymer is about 10 percent lower than the 2.76 GPa required of matrix polymers typically used in aerospace primary structural applications. In one embodiment, the matrix is a polybutadiene graft copolymer matrix, such as a polybutadiene graft copolymer comprising poly(butadiene)-graft-poly(methyl acrylate-co-acrylonitrile). In an embodiment, the self-healing polymer comprises poly(butadiene)-graft-poly(methyl acrylate-co-acrylonitrile). In some embodiments, the matrix is a soluble matrix that allows for processing of the self-healing thermoplastic composite. In further embodiments, the solvent used to process the matrix is N-methylpyrrolidone.

(17) Structures using self-healing polymer matrices such as, for example, PBg provide the following advantages: (1) increased damage tolerance compared to thermosetting polymer matrices which incur a greater extent of impact damage as compared to PBg composites, based upon C-scan data; (2) Self-repairing polymer matrices provide a route for recovery of a large proportion of the pristine mechanical properties, thus extending the life of the structure; (3) Since self-healing is an intrinsic property of the matrix material, this matrix can be treated a direct substitute to conventional thermosetting matrices that do not possess self-healing characteristics; (4) The intrinsic healing of PBg does not require the introduction of microcapsules needed by other healing polymer matrices described in the prior art section; it therefore has the advantage of not needing microcapsules which can act as defect initiators in the composite; (5) As long as there is no loss of matrix material incurred in the damage event, PBg can self-heal repeatedly, compared to the microcapsule approach, where healing is limited by the amount of monomer present at the site where damage occurs. In some embodiments, the self-healing polymer is able to heal crack(s) and/or puncture(s) in the structure.

(18) A composite fabrication process cycle was developed from composite precursor materials to fabricate composite laminates for aerospace structures. The precursor material is a pre-impregnated unidirectional carbon-fiber preform, also known as a prepreg. In the prepregging process, the high-strength, structural reinforcing IM7 carbon fiber is wetted by a solution containing the self-healing PBg polymer dissolved in N-Methylpyrrolidone (NMP). Precursor materials in the form of unidirectional prepreg are widely used in the aerospace industry to fabricate and manufacture aerospace composite structures for both commercial and military aviation and space launch vehicle systems. The experimental prepreg containing the puncture-self-healing polymer PBg was characterized and a cure cycle developed to be used in fabrication of composite laminate test coupons from this novel prepreg material.

(19) A self-healing thermoplastic PBg, poly(butadiene)-graft-poly(methyl acrylate-co-acrylonitrile), a polybutadiene graft copolymer, was obtained in pellets from Sigma-Aldrich. The chemical structure of this PBg is shown in FIG. 1. Initially CFRC processing attempts with the PBg thermoplastic were conducted using IM7-6K 5-harness satin woven fabric (GP sizing, 280 gsm) from Textile Products, Inc., Anaheim, Calif., USA. Experimental batches of PBg prepreg were developed using IM7-12K unsized fiber tow from Hexcel and anhydrous N-Methylpyrrolidone (“NMP”) solvent supplied by BASF Chemical Co, Florham Park, N.J. Quasi-isotropic panels were fabricated for the purpose of consolidation quality comparison using Cycom® M7/977-3 prepreg supplied by Cytec Engineered Materials, Woodland Park, N.J.

(20) As a first step in determining the CFRC processing temperature window, the glass transition (Tg) of the PBg polymer pellets was verified by conducting a dynamic temperature scan in nitrogen from 25° C. to 300° C. at 5° C./min in a Netzsch 204-F1 Phoenix® differential scanning calorimeter (DSC). In addition to the dynamic scan in DSC, dynamic temperature scans in nitrogen from 25° C. to 300° C. at 5° C./min were conducted using a Netzsch TG-209-F1 Libra thermo-gravimetric analyzer (TGA) to determine the decomposition temperature of the pristine PBg polymer.

(21) Residual solvent content in IM7/PBg prepreg was obtained from end-roll specimens of the prepreg batch, designated tape machine run #(TM-341), using a dynamic temperature scan from a thermo-gravimetric analyzer (TGA) under a nitrogen purge from 25° C. to 300° C. at five ° C./min. The TGA was a Netszch TG-209-F1 Libra® TGA. This data was also used to determine the temperature which would be required to remove the volatiles (NMP) from the IM7/PBg prepreg during CFRC processing. Using these results to initially select an isothermal dwell temperature in the CFRC processing prior to the compaction step, the time duration of this proposed dwell was determined by performing an isothermal scan at 150° C. and 225° C. in TGA of the IM7/PBg prepreg. Using a specimen from the TM341 roll of IM7/PBg prepreg, the mass evolution of the material during the proposed CFRC process cycle was determined in the Netzsch TGA by heating to 150° C. at 5° C./min and holding for one hour under nitrogen purge and then heating from 150° C. to the mold compaction temperature of 225° C. at five ° C./min and holding for two hours.

(22) The PBg polymer was molded into neat resin disks 2.54 cm in diameter by 1.5 millimeters (mm) in thickness for rheological characterization in a Rheometrics ARESV® parallel plate rheometer. All of the rheology results presented in this study were collected using a cyclic strain of two percent. A dynamic temperature scan in nitrogen was conducted from 25 TC to 285° C. at 5° C./min. Based on the results of the TGA thermogram, an isothermal temperature scan at 150° C. and 225° C. was performed on the solution of NMP containing 31 percent solids PBg to determine the change in the dynamic viscosity as the matrix material devolatilizes during the proposed processing cycle.

(23) Pellets of the PBg polymer were dissolved in anhydrous NMP by continuously stirring for 48 hours at 25° C. under nitrogen purge. The resulting uniform mixture contained 31 percent solids in NMP by weight. The Brookfield viscosity of the resulting solution at 25° C. was determined to be 21.12 Pa*sec (211.20 poise). This viscosity was acceptable for the solution prepregging process. Two experimental batches of PBg prepreg were fabricated with prepregging equipment. The 22 to 25 centimeters (cm) wide prepregs were fabricated using 90 unsized IM7-12K tows by introducing the PBg-NMP polymer in solution to the unsized IM7 fiber via the dip tank in the prepregging process. Using an established procedure of weighing, oven drying, and reweighing samples of the prepreg, the resulting fiber areal weight (FAW), PBg resin and NMP solvent content of these two experimental batches were determined.

(24) The processing cycle determined following the above tests was then used to fabricate three [45/0/−45/90].sub.4S IM7/PBg panels including geometries of 7.6 cm×7.6 cm, 15.2 cm×15.2 cm, and 30.5 cm×30.5 cm. Material from the first prepreg batch was processed in stainless steel closed molds using a TMP® three-ton vacuum press with a layer of breather and release cloth separating the stack of prepreg from the stainless steel mold base and plunger. A 15.2 cm×15.2 cm [45/0/−45/90].sub.4S panel was fabricated in the same mold and vacuum press using Cytec IM7/977-3 toughened epoxy prepreg and the Cytec recommended processing cycle, C-49. Both the IM7/PBg and the IM7/977-3 15.2 cm×15.2 cm panels were cross-sectioned at the center using a wet saw and then potted and polished for optical microscopy in a Reichert MEF4 M microscope. Following ASTM D3171, FVF/VVF analysis by acid digestion were conducted for this IM7/PBg panel and three subsequently processed 15.2 cm×15.2 cm IM7 PBg panels. Based on these results, six additional [45/0/−45/90].sub.4S 15.2 cm×15.2 cm IM7/PBg panels were fabricated from prepreg batch (TM-340) and six from prepreg batch (TM-341) in the vacuum press for the purpose of determining the compression after impact (CAI) strength of these composite materials after low-velocity impact damage.

(25) Nine IM7/PBg panels were prepared as test coupons and subjected to low-velocity impact according to ASTM D7136. A spherical tup was used to impact each 15.2 cm×10.1 cm coupon at the center. The average coupon thickness of the six panels fabricated from prepreg batch TM340 was 5.40 mm, and an impact energy of 36.08 J was used to damage these coupons. The six panels fabricated from prepreg batch TM-341 had an average thickness of 4.62 mm. Four of these six panels were damaged using an impact energy of 31.09 J. Non-destructive evaluation (NDE) by through-transmission, time-of-flight c-scan of these impacted panels was conducted using a Sonotek® c-scan with a 10 MHz transducer. After c-scan of all of the damaged coupons, one of the coupons from the panels fabricated using the TM-340 prepreg batch and three of those fabricated using the TM-341 batch were randomly selected and subjected to an elevated temperature/pressure healing cycle in the vacuum press using the following cycle:

(26) 25° C. to 225° C. at five ° C./min under full vacuum;

(27) hold at 225° C. for 30 minutes under full vacuum and 1.7 MPa pressure; and

(28) cool down the vacuum press to 25° C. at five ° C./min under full vacuum. In some embodiments, the vacuum press is cooled such that the sample cools slowly while still under full vacuum.

(29) Both pristine and damaged IM7/PBg quasi-isotropic laminates were tested according to ASTM D7137 using a CAI test fixture in an MTS 250KN Load Frame. In addition, the pristine compression strength of the IM7/PBg was determined by mounting these pristine 15.2 cm×10.1 cm coupons in the CAI fixture and loading them in axial compression.

(30) The T.sub.g of the polymer was determined at the inflection in the heat vs temperature curve shown in FIG. 2. This measured value of 75° C. is very close to the vendor-specified value of 80° C. A significant reduction in the modulus of the polymer is associated with this transition. For example, the tensile modulus of the PBg polymer at 25° C. of 2.5 GPa is reduced to 2.2 MPa at 100° C. as reported by the material supplier.

(31) The results of the dynamic temperature scan in TGA shown in FIG. 3 indicates a pristine PBg sample mass loss of 2 percent at 300° C. as the decomposition temperature and indicates that the polymer can be processed at temperatures up to about 300° C. without significant degradation.

(32) Residual solvent trapped in the prepreg will result in composite parts with high void content. The thermogravimetric scan shown in FIG. 4 indicates that about 7 percent solvent evolved from the prepreg between 100° C. and 200° C.

(33) Using these results, an isothermal dwell temperature of 150° C. in the CFRC processing cycle was initially selected to devolatilize the IM7/PBg prepreg prior to the compaction step at 225° C. The temperature of 150° C. also coincides with the reduced viscosity determined by rheological analysis. The time required in the proposed devolatilization dwell was investigated by isothermal TGA of the IM7/PBg prepreg. Shown in FIG. 5 are the mass evolution of both the pristine PBg polymer from pellet and the IM7/PBg prepreg during the proposed one-hour isothermal drying step at 150° C., as well as the two-hour compaction step at 225° C. The isothermal TGA scans in FIG. 5 indicate a mass loss of one percent in the pristine PBg polymer after one-hour hold at 150° C., while the IM7/PBg prepreg lost up to 5 percent mass. This result indicates a net devolatilization of NMP solvent of about 4 percent by weight leaving a possible residual six percent by weight (w) solvent in the prepreg, based on the total amount of solvent left in the prepreg after the prepregging process, going into the temperature ramp to the compaction and consolidation step at 225° C. This level of solvent content could result in void entrapment during the compaction phase. The high viscosity of the PBg polymer might prevent the full removal of NMP, regardless of the devolatilization step duration.

(34) Having determined the T.sub.g and the decomposition temperature of the PBg polymer, the CFRC processing temperature window exists between about 75° C. and about 300° C. Thermoplastic polymers are typically difficult to process as matrix in CFRC because the long molecular chains of these polymers make the bulk material highly viscous. Knowing the thermal processing window, the dynamic viscosity versus temperature of the PBg polymer was determined via parallel plate rheology. The dynamic-temperature rheology scan in FIG. 6 indicates that the PBg polymer exhibits two separate events where the heat introduced to the material results in significant reduction of the dynamic viscosity, η*. Following the stick-slip phenomenon in the first 20 minutes of the test, the first event occurs near 150° C. where the dynamic viscosity decreased from a maximum of 5,000 Pa*sec (50,000 poise) to 1,500 Pa*sec (15,000 poise). The second event occurs at about 260° C. where the PBg polymer reaches its minimum dynamic viscosity of 360 Pa*sec (3,600 poise). However, the rapid increase in the viscosity at temperatures above about 275° C., combined with the two percent mass loss observed in TGA indicates that the polymer is beginning to degrade at this elevated temperature. Therefore a maximum molding temperature of 225° C. with viscosity of 1,300 Pa*sec (13,000 poise) was selected to process the PBg as matrix in CFRC composites.

(35) After determining the mass evolution in TGA of the IM7/PBg prepreg during the proposed CFRC processing devolatilization step at 150° C. and compaction step at 225° C., the effect of these processing dwells on the dynamic viscosity of the PBg polymer was investigated in parallel plate rheology. The results of this isothermal temperature scan performed using the PBg-NMP prepregging solution are shown in FIG. 7. During the isothermal hold at 150° C. the PBg containing residual NMP exhibited a relatively stable viscosity of 1,500 Pa*sec (15,000) poise. A significant decrease in dynamic viscosity was observed during the five ° C./min temperature ramp to 225° C. During the 60-minute hold at 225° C., the viscosity of the material was less stable, increasing from 6,700 poise at the beginning to 12,300 poise by the end of the isothermal hold. The change in viscosity may be due to devolatilization of the NMP solvent, which has a boiling point of 200° C. at atmospheric pressure or the increase may be due to some initial degradation of the PBg molecule.

(36) The resin film infusion (RFI) process is preferable to alternative methods that require an intermediate step of pre-impregnating carbon fiber tows with resin. However, the photomicrograph in FIG. 8 shows that this method was not suitable for PBg.

(37) It is noted that the PBg resin was able to flow around the IM7-6K fiber tows in the biaxial fabric, but does not penetrate the tows. In fact the tows which are 90-degrees to the plane are clearly surrounded by a region of voids. Based on these results, the effort to fabricate CFRC with PBg thermoplastic matrix focused on the development of a processing method with an intermediate prepreg material.

(38) Two experimental batches of PBg prepreg were fabricated using prepregging equipment. The fiber areal weight (FAW), PBg resin and NMP solvent content of these two experimental batches are displayed in Table 1.

(39) TABLE-US-00001 TABLE 1 Characteristics of NASA LaRC IM7/PBg unidirectional prepreg. Resin Resin Solvent Run Viscosity, FAW, Content, Content, Width, Length, Number Poise g/m.sup.2 wt % Dry wt % Wet cm m TM-340 210 159 41-43 15-20 25 66 TM-341 211 146-150 34-35 10-11 22 59

(40) Based on the results obtained in the thermal and rheological analysis of the IM7/PBg experimental prepreg, the processing cycle in the vacuum press for the IM7/PBg composite was selected to be: 1. 25° C. to 150° C. at two ° C./min under full vacuum, hold at 150° C. under full vacuum for 60 minutes; 2. 150° C. to 225° C. at two ° C./min under full vacuum, hold at 225° C. for 60 minutes under full vacuum and 1.7 MPa compaction pressure during entire temperature hold; 3. Cool down to 25° C. at two ° C./min under full vacuum.

(41) Using these conditions, three [45/0/−45/90].sub.4S panels measuring 7.6 cm×7.6 cm, 15.2 cm×15.2 cm, and 30.5 cm×30.5 cm were fabricated. Upon visual inspection, all three of these panels exhibited higher consolidation quality than the previous RFI panels. Both the IM7/PBg and the IM7/977-3 15.2 cm×15.2 cm panels were cross-sectioned at the center using a wet saw and then potted and polished for photo microscopy. The resulting images are shown side-by-side for comparison in FIG. 9.

(42) The micrographs in FIG. 9 of the center plies of the 32-ply quasi panels indicate that the IM7/977-3 composite was very well consolidated and essentially void free, or consistent with VVF less than two percent. The plies had uniform thickness of 0.127 mm and the fibers were uniformly dispersed with the epoxy matrix in each of the plies. The micrograph of the IM7/PBg composite at the same stack location show that the plies were fairly uniform with an average thickness of about 0.183 mm, but there were some resin fiber discontinuities, or resin-rich regions. There were several small voids evident in this small sampling of the overall composite. Photomicrographs of additional IM7/PBg composites also contained voids, especially near panel edges. These were likely formed by the NMP volatiles trapped in the highly viscous polymer. Void content analysis by acid digestion, using a polymer density of 1.31 g/cc, of three subsequent 32 ply quasi-isotropic 15.2 cm×15.2 cm IM7/PBg panels revealed an average FVF>60 percent, and average VVF<two percent. Based on these results, additional IM7/PBg panels from both experimental batches of prepreg were fabricated in the vacuum press for the purpose of determining the compression after impact (CAI) strength of these novel composite materials.

(43) Nine IM7/PBg panels were subjected to low-velocity impact resulting in an average damage dent depth of 1.9 mm (0.075 in). The damage regions of all the impacted coupons were analyzed by c-scan. A representative image of the damage incurred in the IM7/PBg panels is shown in FIG. 10A. NDE of these IM7/PBg coupons indicated an average planar delamination area of 15.3 cm.sup.2. The damage area and dent depth are consistent with barely visible impact damage (BVID).

(44) Four of the BVID IM7/PBg panels were subjected to an elevated temperature/pressure healing cycle described above and then tested to failure in compression to determine the influence of the cycle on the IM7/PBg composite CAI failure strength. The time-of-flight c-scan image of one of these IM7/PBg panels before and after, healing is shown in FIGS. 10A and 10B. As a result of the elevated temperature/pressure healing cycle, no apparent damage was evident in the c-scan. After the healing cycle, the 1.9 mm deep dent on the surface of the panel was no longer visible.

(45) The results of the axial compression of the two pristine IM7/PBg coupons, the five BVID coupons and the four BVID coupons subjected to an elevated temperature/pressure healing cycle are shown in FIG. 11.

(46) All of the coupons failed due to fiber micro-buckling. In the five BVID coupons, this failure initiated at the site of the impact damage and propagated across the width of the coupons. The four IM7/PBG coupons containing BVID which were subjected to an elevated temperature/pressure healing cycle also failed due to fiber microbuckling initiating at the original impact site and propagating across the 10.2 cm width of the coupons. The pristine compressive strength resulting from this limited sample of coupons of quasi-isotropic laminates was about 52 percent of the compressive strength of the 675 MPa reported for a typical toughened epoxy 32-ply quasi-isotropic CFRC intended for aerospace structure.

(47) The CAI strength of the BVID coupons appears to be independent of the prepreg batch. This is expected since the failure of these coupons is dominated by the impact damage and in this study the impact energy used for BVID was varied to account for the panel thickness variation resulting from prepreg batch inconsistency. The inconsistency of the prepreg batches was not of significant concern at this initial phase in the study given that all of the IM7/PBg prepreg were considered experimental. The differences in the batch to batch quality is noted in Table 1 and demonstrates that there is considerable room for improvement in both the quality of the prepreg and the resulting mechanical performance of CFRC panels fabricated from the intermediate material. In addition to the inconsistencies in the experimental prepreg, the fiber/matrix interface is not optimized with fiber sizing used to optimize the interfacial adhesion in toughened epoxy CFRCs.

(48) Regardless of prepreg batch, there was a significant improvement in the failure strength of the BVID coupons subjected to an elevated temperature/pressure healing cycle vs the BVID panels which were not healed. The TM340 batch of damaged IM7/PBg panels exhibited a 64 percent retention of compressive strength. The retention of compressive strength of coupons fabricated from this same batch of prepreg, which were subjected to BVID and then the elevated temperature/pressure healing cycle was found to be 80 percent. However, the large error associated with the BVID-Heal coupons from the second prepreg batch (B2) indicates that this notable improvement in compressive strength may be just as dependent on the quality of the CFRC laminate as it is on the healing capability of the matrix. The non-autonomic healing cycle utilized in this initial study amounts to reprocessing of the amorphous thermoplastic PBg matrix in the IM7/PBg composite.

(49) The composite of the present invention will be useful for structures in applications such as, but not limited to, structural components that can be incorporated into vehicles such as aircraft, rotorcraft, and spacecraft. Particular applications that can include these structural components are by way of example and not limitation, combat aircraft, large military transport and bomber aircraft, commercial transport, small transport, general aviation, military rotorcraft, commercial and general aviation rotorcraft, spacecraft, and missiles.

(50) While preferred embodiments of the present invention are shown and described, it is envisioned that those skilled in the art may devise various modifications of the present invention without departing from the spirit and scope of the appended claims.