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"%!(!)Abstract 

Several studies have indicated that selective reinforcement offers the 

potential to significantly improve the performance of metallic structures for 

aerospace applications. Applying high-strength, high-stiffness fibers to the 

high-stress regions of aluminum-based structures can increase the structural 

load-carrying capability and inhibit fatigue crack initiation and growth. 

This paper discusses an investigation into potential methods for applying 

reinforcing fibers onto the surface of aluminum and aluminum-lithium plate. 

Commercially-available alumina-fiber reinforced aluminum alloy tapes were 

used as the reinforcing material. Vacuum hot pressing was used to bond the 

reinforcing tape to aluminum alloy 2219 and aluminum-lithium alloy 2195 

base plates. Static and cyclic three-point bend testing and metallurgical 

analysis were used to evaluate the enhancement of mechanical performance 

and the integrity of the bond between the tape and the base plate. The tests 

demonstrated an increase in specific bending stiffness. In addition, no issues 

with debonding of the reinforcing tape from the base plate during bend 

testing were observed. The increase in specific stiffness indicates that 

selectively-reinforced structures could be designed with the same 

performance capabilities as a conventional unreinforced structure but with 

lower mass. 

Introduction 

Selective reinforcement of high-stress regions of metallic airframe fuselage and wing structures 

offers the potential for increased structural efficiency. Previous analytical and experimental 

studies of selectively-reinforced aluminum specimens have shown great potential for improving 

structural performance as well as for enhancing toughness and fatigue crack growth behavior 

(ref. 1-3). In one study, 7075 aluminum alloy open-hole compression panels were reinforced in 

selected locations using alumina fibers embedded in a thin aluminum tape (ref. 1). The 

selectively-reinforced panel exhibited 23% - 68% improvement in specific buckling load 

depending on the reinforcement architecture and loading conditions. In another study, selective 

reinforcement of 7075 aluminum alloy compact tension specimens was shown to improve the 

toughness and fatigue crack growth behavior (ref. 2). The selectively-reinforced specimens in 

these studies were prepared by either soldering or adhesively bonding the reinforcing tape into 

grooves machined into the specimens. 

In addition to aircraft structural applications, selective reinforcement can offer performance 

improvements of other types of structural components such as integrally-stiffened propellant 

tanks and space exploration pressurized vessels. For the past several years, NASA has been 

investigating processes for fabricating near-net-shape integrally-stiffened unitized metallic 

structural shapes such as barrels, domes, and cones. Some examples of these efforts are 

described in references 4-5. Adding a reinforcing material to the stiffeners can lead to more 
2 

efficient structural designs for these types of components. The manufacturing efficiency would 

be improved by developing a method to bond the reinforcing material to the structure in-situ 

during the near-net-shape fabrication process. 

This paper describes research efforts to assess the feasibility of selectively reinforcing aluminum 

structural components with fiber-reinforced metallic tapes. Exploratory processing experiments 

were conducted using vacuum hot press techniques to directly embed a commercially-available 

reinforcing material into aluminum and aluminum-lithium alloy plates. The goal of these 

experiments was not process optimization but rather to determine typical pressures and 

temperatures needed to establish adequate bonding between the reinforcing material and the base 

material. The integrity of these bonds was evaluated using microstructural analysis and threepoint bend testing. In addition, in-situ bonding methods that can incorporate the reinforcing 

materials into structures during near-net-shape fabrication processes were explored and are 

discussed. 

Materials 

 

Two different base plate materials were used for these processing experiments: aluminum alloy 

2219-T851 plate with thickness of 0.25 inch and aluminum-lithium alloy 2195-T8 with thickness 

of 0.190 inch. Base plates with thickness ranging from 0.18 inch to 0.25 inch were machined 

from these plates. 

The reinforcing material was MetPregTM tape, a commercially-available fiber-reinforced 

aluminum material. This tape consisted of a commercially-pure aluminum (Al-1100) matrix 

reinforced with 50 volume percent continuous NextelTM 610 alumina fibers. The tape thickness 

was nominally 0.018 inch and the width was either 0.375 inch or 0.48 inch. Detailed information 

about this fiber-reinforced aluminum tape can be found in references 6-9. In addition to this 

MetPregTM tape, two more variants of the tape were examined. One variant had the same fiber 

volume fraction but used an aluminum alloy matrix with 2 weight percent copper (Al-2Cu) 

instead of Al-1100. This tape was 0.018-inch thick by 0.375-inch wide. The other variant used 

the Al-1100 matrix but the tape thickness was increased to 0.180 inch. This thicker tape had a 

nominal width of 0.45 inch. 

Experimental Procedures 

Selective Reinforcement Process 

A 190-ton vacuum hot press with temperature capability of 2300°F was used to consolidate the 

selectively-reinforced panels. Base plates were machined to the desired dimensions. Base plate 

width was in the range of 1 inch to 3 inches. The length varied from 2.75 inches to 6 inches. 

Some of the base plates had a groove machined into the surface deep enough to accommodate 

the reinforcing tape. The base plates and reinforcing tapes were chemically cleaned prior to 

consolidation processing. The base plate and tape stacking sequence was assembled. In some 

cases, stainless steel dies were used to limit the outward flow of the base plate material during 
3 

hot pressing. Boron nitride anti-seize compound and molybdenum foils were used to protect the 

hot press platens and any dies that were used to support the assembly. The hot press chamber 

was evacuated and heated to the target processing temperature. The platens were engaged to 

apply the consolidation load to the assembly for the desired length of time. The platens were 

then disengaged to remove the load and the consolidated panel was allowed to furnace cool. 

Static 3-Point Bend Tests 

Three-point bend tests were conducted on specimens machined from some of the consolidated 

panels to evaluate the mechanical integrity of the bond between the base plate and the 

reinforcing tape. ASTM Standards D7264 and E85 (ref. 10 and 11) were used as guides for the 

tests. However, the specimen dimensions did not meet the dimensional requirements from the 

standards due to size limitations of the consolidated panels. The 3-point bend test apparatus is 

shown in figure 1. The base of the load fixture was attached to the load cell mounted to the test 

machine. Specimens were tested using a span of 3 inches. The mid-span load was applied to the 

specimen using the test machine's hydraulic ram at a constant deflection rate of 0.01 inch/minute. 

An extensometer was located beneath the specimen to measure deflection at the mid-span 

location. An automated data acquisition system collected the load and deflection data. Multiple 

tests were conducted on each specimen at low loads to evaluate the stability of the loaddeflection behavior for the selectively-reinforced material. Testing was performed with the 

reinforced surface in either tension or compression (see figure 2). Bending stiffness was defined 

as the slope of the load-deflection curve and was calculated by linear regression. Eventually, the 

specimens were loaded to failure to investigate the fracture behavior of the selectively-reinforced 

material and the integrity of the bond line. 

3-Point Bend Fatigue Tests 

Following static 3-point bend testing at low loads, two of the specimens were selected for fatigue 

testing. Tests were conducted with a span of 3 inches using the apparatus shown in figure 1. 

The specimens were configured such that the reinforced surface was loaded in tension. An R 

ratio of 0.1 was used. The target frequency was 5 Hz. 

The fatigue test load cycle sequence is illustrated in figure 3 . The initial maximum fatigue load 

was 50 lbs and the specimen was fatigued for 50,000 cycles. The test was paused and the 

maximum fatigue load was increased by 10 lbs. The specimen was then fatigued for another 

50,000 cycles. The maximum fatigue load was incremented by 10 lbs following each set of 

50,000 cycles until the specimen failed. In addition, static bend tests up to the maximum fatigue 

load were conducted before and after each set of fatigue cycles to determine if the loaddeflection behavior was affected by fatigue cycling. 

Microstructural Analysis 

Microstructures and test specimen fracture surfaces were analyzed using optical and scanning 

electron microscopy. 
4 

Results and Discussion 

Selectively-Reinforced Al-2219 Processing Experiments 

Exploratory processing experiments were conducted to investigate selective reinforcement of 

thin Al-2219 plate. The panel components and processing parameters are summarized in table 1. 

The effects of the vacuum hot press parameters on the bond between the base plate and the 

reinforcing tape were evaluated with microstructural analysis. 

Figure 4 shows a photograph of the components used for one of the selective reinforcement 

experiments. Figure 5 shows a diagram of the cross section of this panel assembly. A 0.48-inch 

wide by 0.03-inch deep groove was machined into the top surface of a 0.25-inch thick Al-2219 

base plate. The MetPregTM tape with the Al-1100 matrix was inserted in the groove. A 0.050-

inch thick sheet of Al-2024 was positioned on top of the assembly to facilitate even distribution 

of the hot press load over the whole part. (Al-2024 sheet was used for this particular experiment 

because it was readily available in thin sheet form whereas the Al-2219 plate was thicker than 

desired for a thin cover plate.) The overall length and width of the panel assembly were 2.75 

inches and 1.0 inch, respectively. The assembly was processed in the vacuum hot press at 930°F 

with a pressure of 15 ksi for 1 hour. Figure 6 shows a photograph of the consolidated panel 

(VHP-387) compared to a representative assembly. The processing temperature was very high 

and thus the materials exhibited a large degree of plastic flow. The hot-pressed panel had a 

thickness of 0.110 inch. The microstructure of the reinforced region is shown in figure 7. A 

good bond was formed between the reinforcing tape and both the Al-2219 base plate and the Al2024 cover sheet, but the tape exhibited excessive lateral flow due to the applied pressure at high 

temperature. A second assembly was hot pressed for one hour at a slightly lower temperature 

(890°F) and a much lower pressure (0.35 ksi). This panel (VHP-388) did not have the extreme 

deformation that was observed in the previous panel. However, due to the low consolidation 

pressure, the tape did not adhere to the base plate. 

Subsequent experiments to bond the tape to the Al-2219 surface involved applying the hot press 

load directly to the tape instead of distributing the load over the whole panel surface. Figure 8 

shows a photograph of the panel assembly and a schematic of the cross section for this loading 

configuration. Two pieces of Al-2219 plate were used. A 0.48-inch wide by 0.04-inch deep 

groove was machined into the surface of the base plate. The top plate had the surface machined 

down such that a 0.48-inch wide by 0.06-inch tall stub was left on the surface that would fit into 

the groove on the base plate. The tape with the Al-1100 matrix was pre-placed in the groove and 

the stub of the top plate was inserted into the groove on top of the tape. The assembly had a gap 

between the top plate and base plate of approximately 0.04 inch. The overall length and width of 

the assembly were 6 inches by 2 inches, respectively. The length and width of the tape over 

which the hot press load was applied were 6 inches by 0.48 inch, respectively. This assembly 

was processed at 570°F for 15 minutes at a constant load such that the pressure applied to the 

surface of the tape was 60 ksi. Figure 9 shows a photograph of the consolidated panel (VHP391). The materials in the vicinity of the reinforcing tape were well consolidated. A high 

consolidation pressure was maintained on the reinforcing tape because the gap between the two 

plates did not close up enough to cause significant load redistribution. The microstructure of the 

interface region (figure 10) shows some signs of cracking between the tape and the base plate. 
5 

 

Selectively-Reinforced Al-2195 Processing Experiments 

Exploratory processing experiments were conducted to investigate selective reinforcement of 

thin Al-2195 aluminum-lithium alloy plate. The panel components and processing parameters 

are summarized in table 2. All of these experiments had the reinforcing tape pre-placed on the 

flat surface of the base plate such that the consolidation load was applied directly to the tape. 

The effect of the vacuum hot press parameters on the bond between the base plate and the 

reinforcing tape was evaluated with microscopy and 3-point bend testing. 

Figure 11 shows a diagram of the assembly used to fabricate selectively-reinforced Al-2195 

panels. Base plates with thickness of 0.185 inch were machined from a thicker plate of Al-2195-

T8. The base plates were 2.5 inches long by 1 inch wide. No grooves were machined into the 

plates. A strip of the MetPregTM tape with either the Al-2Cu matrix or the Al-1100 matrix was 

pre-placed onto the surface. The tape width and thickness was 0.375 inch by 0.018 inch. Plates 

were processed in the vacuum hot press using the parameters shown in table 2. The processing 

temperatures were significantly higher than those for which the Al-2219 panels exhibited a good 

bond with the reinforcement. These higher temperatures were selected to allow plastic 

deformation in the base plate material such that the tape could be embedded into the plate. 

Figure 12 shows photographs of the consolidated panels as well as the microstructure of the 

interface between the tape and the base plate. The base plate material deformed enough to allow 

the tape to become embedded into the base plate such that the top surface of the tape was flush 

with the top surface of the plate. The consolidation pressure decreased after enough deformation 

occurred to allow the top platen to come into contact with the top surface of the base plate. The 

microstructures show that the panels were well consolidated with no apparent cracks or defects at 

the bond lines. 

Based on these successful bonding experiments, several more panels were fabricated for 3-point 

bend testing. Figure 13 shows the configuration used for panel fabrication. Panels were 

fabricated with either one strip or a stack of two strips of tape pre-placed onto the surface of the 

Al-2195 base plates. The two strips of wider tape were used to increase the volume fraction of 

selective reinforcement in the panel. The base plates were nominally 5 inches long by 1 inch 

wide by 0.17 inch thick (see table 2). Figure 14 shows the panel assembly with one strip of tape 

in the hot press chamber. Four panels with the Al-2Cu matrix were processed simultaneously at 

800°F and 11 ksi (with respect to the tape surface) for 5 minutes (VHP-412-1, -2, -3, and -4). In 

addition, four panels with a stack of two strips of tape with Al-1100 matrix were processed 

simultaneously in a second hot press run using the same parameters (VHP-423-1, -2, -3, and -4). 

Seven of the eight panels appeared to be well bonded. One panel with one strip of tape (VHP412-4) did not exhibit good bonding between the tape and the base plate. The tape fell off after 

removal of the panel from the hot press. It is likely that the platens did not exert the full force on 

this specimen and thus the tape did not experience the required bonding pressure. This specimen 

was used to measure the mass increase associated with adding reinforcing tape to the base plate. 

The mass of the components of this panel was measured following the tape delamination. The 

base plate mass was 30.8439 grams while the tape mass was 1.1506 grams. Thus, the single 
6 

layer of tape increased the mass of the base plate by approximately 4%. This measurement can 

be used to calculate specific mechanical properties of selectively-reinforced specimens on a 

mass-normalized basis. 

Figure 15 shows representative microstructures for panels fabricated with one and two layers of 

tape. In all cases, the reinforcing tapes were embedded into the base plate such that the top 

surface of the tape was flush with the top surface of the base plate. The specimen with two 

layers of tape (VHP-423-1) showed a pronounced bond line between the two pieces of tape. The 

higher-magnification view shows porosity along the bond line between the two tape layers, 

which was typical for the specimens produced with two layers of tape. The cause of the porosity 

has not yet been determined. 

With the exception of the panel in which the tape was not bonded, all of the panels exhibited 

significant distortion due to thermal expansion mismatch between the tape and base plate (see 

figure 16). The coefficient of thermal expansion (CTE) for Al-2195 alloy is approximately 14 

in/in/°F (ref. 12) over the processing temperature range. The MetPregTM tape has a significantly 

lower CTE of 4 in/in/°F (ref. 6). During cool-down from the processing temperature, the tape 

and base plate constrain each other such that the resultant consolidated panel has the tape in a 

state of residual compression and the base plate in residual tension. 

Selectively-Reinforced Al-2195 Bend Testing 

Static 3-point Bend Tests 

Each of the eight Al-2195 panels selectively-reinforced with one and two strips of tape was 

machined to produce 3-point bend specimens. The ends of the panels were trimmed off to be 

used for microstructural analysis and the edges were machined to produce specimens that were 4 

inches long by 1 inch wide with the embedded tape centered on the top surface of the base plate 

(see figure 17). Following low-load bend testing, some of the specimens had the edges 

machined down such that there was no excess base plate on the sides of the specimen (see figure 

18). The de-bonded specimen (VHP-412-4) was used as a baseline to evaluate the bending 

behavior of the unreinforced base plate. This specimen had a shallow groove in the top surface 

where the tape had been placed. 

Figure 19 shows 3-point bend load-deflection curves from the first three tests on specimen VHP412-3 with one layer of reinforcing tape. The specimen was configured such that the reinforced 

side of the specimen was loaded in compression. The specimen was loaded to 100 lbs and 

unloaded back to zero during the three separate tests. The bending behavior was very stable 

with no hysteresis. The same load-deflection curve was generated during loading and unloading 

for each test. The bending stiffness was approximately 9400 lb/in. 

The specimen was also tested three times with the reinforced side loaded in tension. Figure 20 

shows the results. The loading portion of the load-deflection curve for the first test exhibited a 

large degree of non-linearity while the curve was linear during unloading. The second and third 

tests generated linear load-deflection curves during loading and unloading. The bending 

stiffness calculated from these curves was approximately 9000 lb/in. The nonlinearity during the 
7 

first loading was most likely a result of base plate yielding due to residual stresses near the 

interface between the tape and the base plate. The Al-2195 alloy yielded at a relatively low load 

as the bending load superimposed additional tensile stress onto the residual tensile stress in the 

base plate. During subsequent tests, the specimen accommodated the 100-lb bending load 

without yielding due to the work hardening from the first cycle. In the initial set of tests in 

which the specimen was loaded such that the reinforced side was placed in compression, the 

load-deflection curve was linear because the residual tensile stress in the base plate allowed it to 

accommodate higher applied compressive stresses from the bending load without yielding. The 

specimens reinforced with two strips of tape had similar results. 

Figure 21 shows a comparison of the bending stiffness of specimens with one and two layers of 

tape reinforcement. Also shown are stiffness data for the specimen without tape reinforcement 

(VHP-412-4). This is the specimen from which the tape failed to bond to the base plate during 

vacuum hot pressing. All of the specimens had nominal width and thickness dimensions of 1.00 

inch and 0.17 inch, respectively, and were tested with a 3-inch span. Each data point represents 

one test. The unreinforced specimen had an average bending stiffness of 8370 lb/in over 6 tests 

with a tight scatter band. The standard deviation (SD) was 43 lb/in. The three specimens with 

one layer of reinforcing tape had much greater variability in the stiffness measured from test-totest as well as from specimen-to-specimen. Two of these specimens (VHP-412-1 and 2) had 

average bending stiffness values of 8490 lb/in (SD = 409 lb/in) and 8360 lb/in (SD = 160 lb/in), 

respectively, which were similar to the stiffness of the unreinforced specimen. Specimen VHP412-3 had an average bending stiffness of 9410 lb/in (SD = 256 lb/in). For this particular 

specimen, increasing the mass by 4% by adding reinforcing tape resulted in a 12% increase in 

bending stiffness. Thus, selective reinforcement increased the specific stiffness of the Al-2195 

base plate. The specimen with two layers of tape (VHP-423-1) had results similar to those for 

specimen VHP-412-3 with only one layer of tape. It had a bending stiffness of 9230 lb/in (SD = 

303 lb/in). Although all of the specimens had the same nominal cross-section dimensions, part of 

the specimen-to-specimen variation can be attributed to small differences between the measured 

specimen thickness. The bending stiffness is proportional to the specimen thickness raised to the 

3rd power. Thus, small thickness differences can result in significant stiffness differences. 

Following multiple low-load bending tests to assess stiffness behavior, two of the specimens 

with one layer of tape reinforcement were tested to failure. Figure 22 shows the load-deflection 

curve and post-test photographs of specimen VHP-412-1. This specimen was loaded such that 

the reinforced side was in tension. The specimen exhibited tensile fracture of the reinforcing 

tape at a load of 225 lbs. This fracture compromised the load-carrying capability of the 

specimen and the load decreased rapidly to about 180 lbs. At this point, the base plate was able 

to carry the load and the load began increasing again. Eventually the test was stopped without 

further fracture and the specimen was unloaded. 

Figure 23 shows the load-deflection curve and post-test photographs of specimen VHP-412-3, 

which was tested such that the reinforced side was in compression. The specimen exhibited 

buckling of the tape at 440 lbs. This tape buckling compromised the load-carrying capability of 

the specimen and the load decreased rapidly to about 300 lbs. At this point, the base plate was 

able to carry the load and the load began increasing again. Eventually the test was stopped 

without further fracture and the specimen was unloaded. 
8 

In addition, two of the specimens with two layers of tape reinforcement were tested to failure. 

These specimens had the same failure modes as did the specimens with one layer of tape 

reinforcement. The specimen loaded with the reinforced side in tension exhibited tensile failure 

of the tape while the specimen loaded with the reinforced side compression exhibited localized 

buckling of the tape. Neither specimen showed signs of delamination at the base plate-to-tape 

interface or the tape-to-tape interface. 

3-point Bend Fatigue Tests 

Specimen VHP-412-2 with one layer of reinforcing tape and specimen VHP-423-2 with two 

layers of reinforcing tape were selected for fatigue testing. The edges of the specimens were 

trimmed off to remove the excess Al-2195 base plate in order to have the bond line between the 

base plate and reinforcing tape exposed along the entire length of the specimen. Figure 24 shows 

a cross-section photograph of the specimens. Once the specimens were modified, they were 

renamed VHP-412-2-MOD and VHP-423-2-MOD. The final width of the two specimens was 

0.40 inch and 0.35 inch, respectively. The specimens were tested such that the reinforced side 

was loaded in tension. Several static 3-point bend tests were conducted on the specimens to a 

maximum load of 50 lbs to establish baseline load-deflection curves prior to fatigue testing. 

Figures 25 and 26 show the load-deflection curves for the two specimens before and after 50,000 

fatigue cycles at a maximum fatigue load of 50 lbs. No change in bending behavior was 

observed due to fatigue. This result was typical for each of the sets of 50,000 fatigue cycles at 

the incrementally-increased maximum fatigue loads. Table 3 shows the bending stiffness for 

both specimens measured before and after each set of fatigue cycles. The bending stiffness after 

each set of fatigue cycles was within 5% of that measured prior to fatigue testing. 

Specimen VHP-412-2-MOD with one layer of tape was subjected to 5 sets of 50,000 fatigue 

cycles at maximum fatigue loads of 50 lbs to 90 lbs in increments of 10 lbs without failure or 

changes in load-deflection behavior. The specimen was inadvertently overloaded during test 

setup for the 100-lb maximum fatigue load test. The tape fractured in tension but remained 

bonded to the Al-2195 base plate (see figure 27). 

Specimen VHP-423-2-MOD with two layers of tape was subjected to 8 sets of 50,000 fatigue 

cycles at maximum fatigue loads of 50 lbs to 120 lbs in increments of 10 lbs without failure or 

changes in load-deflection behavior. During fatigue testing at a maximum load of 130 lbs, the 

specimen failed after approximately 13,000 cycles. Figure 28 shows a photograph of the overall 

view of the failed specimen and an scanning electron micrograph of the fracture region. The 

outer layer of tape delaminated from the inner layer of tape. This loss of load-carrying capability 

resulted in overload of the specimen and tensile fracture of the inner tape. The inner tape 

remained bonded to the base plate. 

Selective Reinforcement of Al-2195 Stiffeners 

An experiment was conducted to simulate the in-situ selective-reinforcement of Al-2195 during 

near-net-shape processing of integrally-stiffened structure. A stainless steel die was fabricated 
9 

with a channel that was 0.45 inch wide, and 0.5 inch deep. A 0.180-inch thick strip of customfabricated MetPregTM tape was positioned in the bottom of the channel. A 0.7-inch tall block of 

Al-2195 was placed in the die and hot pressed at 800°F and 10 ksi pressure for 5 minutes (VHP444). 

Figure 29 shows a photograph of the specimen/die assembly prior to hot press consolidation. 

The specimen was wrapped in molybdenum foil to protect the die and platens. The consolidated 

specimen is shown in figure 30. The reinforcing tape appeared to be well bonded to the top of 

the simulated stiffener. The stiffener is bowed due to residual stress. Microstructural analysis 

(figure 31) indicated a defect-free bond between the tape and the base plate. There were no signs 

of delamination. This experiment showed that bonding the tape directly to the face of the 

stiffener during the stiffener forming process is feasible. 

Concluding Remarks 

Exploratory processing experiments successfully demonstrated the feasibility of selectively 

reinforcing aluminum and aluminum-lithium based materials with continuous fiber-reinforced 

metallic tapes. These commercially-available reinforcing tapes, which consisted of 50 volume 

percent alumina fibers in an aluminum matrix, were bonded to Al-2219 and Al-2195 base plates. 

Vacuum hot press processing techniques were used to reinforce these base plates with the 

reinforcing tape. These techniques were not optimized but did show the feasibility of selective 

reinforcement. 

Processing experiments were conducted in which the consolidation load was applied directly to 

the reinforcing tape as well as being distributed over the entire surface of the base plate. 

Application of the consolidation load directly to the tape embedded the tape into the base plate, 

formed good bonding between the tape and base plate, and limited the extent of lateral flow of 

the tape. 

Static three-point bend tests and three-point bend fatigue tests were used to assess the mechanical 

performance of the selectively-reinforced Al-2195 materials and to evaluate the integrity of the 

bond between the reinforcement and the base plates. The bond between the reinforcing tape and 

the Al-2195 base plate performed very well. No debonding occurred during multiple low-load 

bending tests. In addition, bend tests were conducted to failure in which the reinforced side of 

the specimen was loaded in tension and in compression. At the failure load, the reinforcing tape 

failed in tension or buckled in compression, but the tape remained bonded to the base plate. In 

addition, the base plate was able to continue carrying load following failure of the reinforcing 

tape. 

Selectively-reinforced Al-2195 plate specimens showed improvements in performance. 

Although embedding a layer of reinforcing tape into the surface of the plate added 4% more 

mass, the bending stiffness was increased by 12%. This increase in specific stiffness indicated 

that selectively-reinforced structures could be designed with the same performance capabilities 

as a conventional unreinforced structure but with significantly lower mass. 
10 

A vacuum hot press processing experiment was also conducted that successfully simulated the 

in-situ bonding of reinforcing tape to the top surface of a stiffener. This demonstration suggests 

that incorporating selective reinforcement into stiffened structures during near-net shape 

fabrication processes is feasible. 

References

1. Farley, Gary L.: “Selective Reinforcement to Enhance Structural Performance of Metallic 

Compression Panels”; 45th AIAA/ASME/ASCE/AHS/ASC Structures, Structural 

Dynamics & Materials Conference, Palm Springs, CA, April 19-22, 2004. 

2. Farley, Gary L.; Newman, John A.; and James, Mark A.: “Selective Reinforcement to 

Improve Fracture Toughness and Fatigue Crack Growth Resistance of Metallic 

Structures”; 45th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & 

Materials Conference, Palm Springs, CA, April 19-22, 2004. 

3. Abada, Christopher H.; Farley, Gary L.; and Hyer, Michael W.: “Fracture Response 

Enhancement of Aluminum Using In-Situ Selective Reinforcement”; 47th 

AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials 

Conference, Newport, RI, May 1-4, 2006. 

4. Wagner, John A.; Domack, Marcia S.; and Hoffman, Eric K.: “Recent Advances in 

Near-Net-Shape Fabrication of Al-Li 2195 Alloy Launch Vehicles”; National Space & 

Missile Materials Symposium, Keystone, CO, June 2007. 

5. Troeger, Lillianne P.; and Wagner, John A.: “Microstructure and Mechanical Property 

Characterization of Shear Formed Al-Li 2195 Alloy for Launch Vehicle Applications”; 

Journal of Materials Processing & Manufacturing Science, Vol. 9, 2001. 

6. Irick, Virgil; and Gordon, Brian L.; and Cohen, David: “MetPregTM Metallic Prepregs 

for the Composites Industry”; SAMPE Journal, March/April 2004. 

7. MetPregTM Metallic Prepregs for the Composites Industry. Data Sheet from Touchstone 

Research Laboratory, Ltd. 

8. Gordon, Brian L.; and Cohen, David: “Metal Matrix Composite Filament Winding”; 

Composites 2006 Convention and Trade Show, St. Louis, MO, October 18-20, 2006. 

9. Gordon, Brian: “Metal Matrix Composite Filament Winding”; Composites & Polycon 

2006, St. Louis, MO, October 18-20, 2006. 

10. “Standard Test Method for Flexural Testing of Polymer Composite Materials.” Annual 

Book of ASTM Standards. Designation D7264-07, American Society for Testing and 

Materials, West Conshohocken, PA. 
11 

11. “Standard Test Methods for Bend Testing of Metallic Flat Materials for Spring 

Applications Involving Static Load.” Annual Book of ASTM Standards. Designation 

E85-08, American Society for Testing and Materials, West Conshohocken, PA. 

12. Aerospace Structural Metals Handbook – Volume 3, Code 3228. Brown, Mindlin, and 

Ho, eds. 39th Edition, CINDAS/USAF CRDA Handbook operation, Purdue University, 

West Lafayette, Indiana, 2005. 
12 

Tables 

Table 1. Vacuum hot press processing parameters for selectively-reinforced Al-2219 panels. 

Table 2. Vacuum hot press processing parameters for selectively-reinforced Al-2195 panels. 

 

Vacuum 

Hot Press 

Run No.

Tape 

Matrix Temp. Load Pressure (tape)

Pressure 

(base 

plate)

Time Comments

width thick. length width thick.

in. in. in. in. in. °F tons ksi ksi min.

VHP-387 Al-1100 0.48 0.018 2.75 1.0 0.25 930 20.6 --- 15.0 60 Al-2024 cover plate

VHP-388 Al-1100 0.48 0.018 2.75 1.0 0.25 890 0.5 --- 0.35 60 Al-2024 cover plate

VHP-391 Al-1100 0.48 0.018 6.0 2.0 0.25 570 90.0 60.0 --- 15 Load applied directly to 

tape surface

Tape Dimensions Base Plate Dimensions

Vacuum 

Hot Press 

Run No.

Tape 

Matrix Temp. Load Pressure (tape)

Pressure 

(base 

plate)

Time Comments

width thick. length width thick.

in. in. in. in. in. °F tons ksi ksi min.

VHP-398 Al-1100 0.375 0.018 2.5 1.0 0.185 730 22.0 46.9 17.6 5

VHP-399 Al-2Cu 0.375 0.018 2.5 1.0 0.185 710 5.2 11.1 4.2 5

VHP-412 Al-2Cu 0.375 0.018 5.0 1.0 0.170 800 40.0 10.7 4.0 5 Four panels fabricated 

simultaneously

VHP-423 Al-1100 0.48 0.018 4.6 1.0 0.170 800 47.9 10.8 5.2 5 Four panels fabricated 

simultaneously

Tape Dimensions Base Plate Dimensions
13 

Table 3. Three-point bending fatigue test parameters and stiffness data following sets of 

50,000 fatigue cycles at incrementally increasing maximum fatigue loads for 

selectively-reinforced Al-2195 specimens. 

Fatigue Set Fatigue Conditions Cumulative No. of 

Fatigue Cycles

VHP-412-2-MOD 

(one layer of tape)

VHP-423-2-MOD 

(two layers of tape)

------- prior to fatigue testing 0 4,010 4,827

1 50,000 cycles at Pmax = 50 lbs 50,000 3,987 4,783

2 50,000 cycles at Pmax = 60 lbs 100,000 4,085 4,830

3 50,000 cycles at Pmax = 70 lbs 150,000 4,048 4,927

4 50,000 cycles at Pmax = 80 lbs 200,000 4,031 4,784

5 50,000 cycles at Pmax = 90 lbs 250,000 4,068 5,037

6 50,000 cycles at Pmax = 100 lbs 300,000 ------- 4,952

7 50,000 cycles at Pmax = 110 lbs 350,000 ------- 4,933

8 50,000 cycles at Pmax = 120 lbs 400,000 ------- 4,996

Post-Fatigue Bending Stiffness 

(lb/in)
14 

Figures 

Figure 1. Photograph of three-point bend test apparatus. 

Figure 2. Three-point bend test specimen loading configurations. 

 

Extensometer

Load 

Ram

Load 

Cell

Specimen

P

P/2 P/2

Reinforced Surface

In Compression

Reinforced Surface

In Tension

P

P/2 P/2

Base plate

Tape

Tape

Base plate
15 

Figure 3. Three-point bend fatigue load cycle sequence. 

Figure 4. Photograph of panel components for selective reinforcement of Al-2219 plate. 

 

0

10

20

30

40

50

60

70

80

Load,

lbs

Time

50,000 

fatigue

cycles

R = 0.1

f = 5 Hz (target)

5 cycles

(0.01 in/min)

1 cyc.

1 cyc.

1 cyc.

5 cycles

(0.01 in/min)

5 cycles

(0.01 in/min)

50,000 

fatigue

cycles

50,000 

fatigue

cycles

Series 1 Series 2 Series 3

Al-2219 base plate

Tape

Al-2024 cover plate

2.75 in. 

1.0 in.
16 

Figure 5. Diagram of panel configuration for selective reinforcement of Al-2219 plate. 

Figure 6. Photographs of representative panel assembly and selectively-reinforced Al-2219 

panel (VHP-387) after consolidation at 930°F and 15 ksi for 1 hour. 

 

Panel Assembly Consolidated Panel

2.75 in. 
17 

Figure 7. Photomicrograph of selectively-reinforced Al-2219 panel (VHP-387). 

Figure 8. Photograph and diagram of Al-2219 panel configuration with stub for applying 

consolidation load directly to reinforcing tape (VHP-391). 

 

Tape

Al-2024 cover sheet

Al-2219 base plate

Bond line

Bond line

100 m 

Mo 

Foil Al-2219

Al-2219

Tape in groove

2 in. 

Stub

~0.04 in. 

Tape (0.018 in.)

Al-2219

Al-2219

0.21 in.

0.25 in. 0.19 in.

0.25 in.
18 

Figure 9. Photograph of Al-2219 panel (VHP-391) consolidated at 570°F and 60 ksi for 15 

minutes with load applied directly to reinforcing tape. 

Figure 10. Photomicrographs of Al-2219 panel (VHP-391) with selective reinforcement. 

 

Al-2219

Al-2219

Tape

2 in. 

Stub

Al-2219 Al-2219

Tape

100 m 100 m 

Bond

line

Cracks
19 

Figure 11. Diagram of panel assembly used to evaluate selective reinforcement of Al-2195. 

Figure 12. Photographs of consolidated selectively-reinforced Al-2195 panels and 

microstructures of the tape/base plate interface. 

 

Tape

2195 

0.185 in.

0.018 in.

VHP Platen

VHP Platen

VHP-398 

730°F/47ksi/5 min.

VHP-399 

710°F/11ksi/5 min.

2195

Tape

2195

Tape

10 m 10 m 

Tape with

Al-1100 

matrix

Tape with

Al-2Cu 

matrix
20 

Figure 13. Diagram of assemblies for fabricating selectively-reinforced Al-2195 panels with 

one and two tape layers for bend testing. 

Figure 14. Photograph of vacuum hot press set-up for simultaneously consolidating four 

selectively-reinforced Al-2195 panels with one layer of tape (VHP-412). 

 

2195 

(0.17-in. thick)

Tape with Al-2Cu matrix

(0.375-in. wide / 

0.018-in. thick)

(VHP-412)

VHP Platen Load

2195 

(0.17-in. thick)

(VHP-423)

VHP Platen Load

Tape with Al-1100 matrix

(0.48-in. wide / 

0.018-in. thick)

Tape

VHP Platen

VHP Platen

Mo Foil

5 in.

#1

#4

#3

#2
21 

Figure 15. Photomicrographs showing representative microstructures of Al-2195 panels 

selectively-reinforced with one layer (VHP-412-1) and two layers (VHP-423-1) of 

tape. 

Figure 16. Side view photograph of selectively-reinforced Al-2195 panel showing distortion 

due to thermal expansion mismatch between tape and base plate. 

 

VHP-412-1

2 mm

Tape

VHP-423-1

2 mm Tape

Tape

Tape

Tape

Porosity

2195

2195

4 in. 

SIDE VIEW

Warped due to 

residual stresses 

from thermal 

0.18 in. expansion mismatch

Reinforced Surface
22 

Figure 17. Top view photograph of three-point bend specimen machined from selectivelyreinforced Al-2195 panel. 

Figure 18. Top view photograph of selectively-reinforced Al-2195 three-point bend specimen 

with excess Al-2195 base plate edges trimmed off. 

 

Tape

2195

2195

4 in. 

TOP VIEW

Trim

each end

1.0 in.

TOP VIEW 0.35 in. 

4 in. 
23 

Figure 19. Three-point bend load-deflection curves for Al-2195 specimen VHP-412-3 with one 

layer of reinforcing tape loaded and unloaded three separate times (reinforced side 

loaded in compression). 

Figure 20. Three-point bend load-deflection curves for Al-2195 specimen VHP-412-3 with one 

layer of reinforcing tape loaded and unloaded three separate times (reinforced side 

loaded in tension). 

 

0

50

100

150

0.00 0.01 0.02

Load,

lbs

Deflection, inches

Reinforced side in Compression Span = 3 inches

9400 lb/in

0

50

100

150

0.00 0.01 0.02

Load,

lbs

Deflection, inches

Reinforced side in Tension Span = 3 inches

1st test

2nd and

3rd tests

9000 lb/in
24 

Figure 21. Bending stiffness measured from selectively-reinforced Al-2195 specimens. (Each 

data point represents one 3-point bending test.) 

Figure 22. Three-point bend load-deflection curve and photographs of failure mode for 

selectively-reinforced Al-2195 specimen VHP-412-1 with one layer of tape loaded 

to failure. (Tape side loaded in tension.) 

 

6,000

7,000

8,000

9,000

10,000

VHP-412-4 VHP-412-1 VHP-412-2 VHP-412-3 VHP-423-1

no tape 1 layer of tape 2 layers of tape

Bending

Stiffness,

lb/in

Specimen

Tape

2195

1 in.

0.17 in.

0

50

100

150

200

250

0.0 0.1 0.2

Load,

lbs

Deflection, in.

Tape failed in tension

3-inch Load Span
25 

Figure 23. Three-point bend load-deflection curve and photographs of failure mode for 

selectively-reinforced Al-2195 specimen VHP-412-3 with one layer of tape loaded 

to failure. (Tape side loaded in compression.) 

Figure 24. Photographs of cross sections of Al-2195 three-point bend fatigue specimens 

selectively reinforced with one and two layers of tape. 

 

Tape

2195

0

100

200

300

400

500

0.0 0.1 0.2 0.3

Load,

lbs

Deflection, in.

Tape failed in compression

3-inch Load Span

1 in.

0.17 in. 

Tape

2195

Tape

2195

0.17 in. 0.18 in.

VHP-423-2-MOD

(two layers of tape)

VHP-412-2-MOD

(one layer of tape)

0.40 in. 0.35 in.
26 

Figure 25. Three-point bend load-deflection curves for Al-2195 specimen VHP-412-2-MOD 

selectively reinforced with one layer of tape before and after 50,000 fatigue cycles 

with a maximum fatigue load of 50 lbs. (Reinforced side loaded in tension.) 

Figure 26. Three-point bend load-deflection curves for Al-2195 specimen VHP-423-2-MOD 

selectively reinforced with two layers of tape before and after 50,000 fatigue cycles 

with a maximum fatigue load of 50 lbs. (Reinforced side loaded in tension.) 

 

0

10

20

30

40

50

60

0.00 0.01 0.02

Load,

lbs

Deflection , inch

Reinforced side in Tension

Before Fatigue

After Fatigue

Span = 3 inches

0

10

20

30

40

50

60

0.00 0.01 0.02

Load,

lbs

Deflection, inch

Reinforced side in Tension

Before Fatigue

After Fatigue

Span = 3 inches
27 

Figure 27. Photograph showing failure mode of Al-2195 specimen VHP-412-2-MOD 

selectively reinforced with one layer of tape inadvertently loaded to failure between 

sets of fatigue cycles. (Reinforced side loaded in tension). 

Figure 28. Photograph and electron photomicrograph showing failure mode of Al-2195 

specimen VHP-423-2-MOD selectively reinforced with two layers of tape that 

failed during fatigue cycling at 130-lb maximum fatigue load. (Reinforced side 

loaded in tension.) 

 

Tape

Tape

2195

2195

Tape fracture

Inner Tape Layer

Outer Tape Layer

0.18 in. 

1 mm
28 

Figure 29. Photograph of end view of vacuum hot press set-up for simulating selective 

reinforcement of an Al-2195 stiffener (VHP-444). 

Figure 30. Photograph of a simulated selectively-reinforced Al-2195 stiffener (VHP-444). 

 

Tape

2195

Die

0.5 in.

Mo Foil

6.7 in.

Tape

2195

Side View

0.6 in.
29 

Figure 31. Electron photomicrographs showing the interface between the tape and stiffener for 

simulated selectively-reinforced Al-2195 stiffener (VHP-444). 

50 m 50 m 

Al-2195

Tape

Transverse Longitudinal
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Investigation of Methods for Selectively Reinforcing Aluminum and 

Aluminum-Lithium Materials 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Bird, R. Keith; Alexa, Joel A.; Messick, Peter L.; Domack, Marcia S.; 

Wagner, John A.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

L-20348

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

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Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

Several studies have indicated that selective reinforcement offers the potential to significantly improve the performance of metallic structures for aerospace 

applications. Applying high-strength, high-stiffness fibers to the high-stress regions of aluminum-based structures can increase the structural load-carrying 

capability and inhibit fatigue crack initiation and growth. This paper discusses an investigation into potential methods for applying reinforcing fibers onto the 

surface of aluminum and aluminum-lithium plate. Commercially-available alumina-fiber reinforced aluminum alloy tapes were used as the reinforcing material. 

Vacuum hot pressing was used to bond the reinforcing tape to aluminum alloy 2219 and aluminum-lithium alloy 2195 base plates. Static and cyclic three-point 

bend testing and metallurgical analysis were used to evaluate the enhancement of mechanical performance and the integrity of the bond between the tape and 

the base plate. The tests demonstrated an increase in specific bending stiffness. In addition, no issues with debonding of the reinforcing tape from the base 

plate during bend testing were observed. The increase in specific stiffness indicates that selectively-reinforced structures could be designed with the same 

performance capabilities as a conventional unreinforced structure but with lower mass. 

15. SUBJECT TERMS

Aluminum alloys; Aluminum-lithium alloys; Bend tests; Fatigue; Reinforcing materials

18. NUMBER

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34

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(443) 757-5802

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01- 12 - 2013