fast repair of laminated beams using uv curing composites

Fast repair of laminated beams using UV curing composites

Guoqiang Li *, Neema Pourmohamadian, Adam Cygan, Jerry Peck,Jack E. Helms, Su-Seng Pang

Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803, USA

Abstract

In this study, laminated beam samples were predamaged by low velocity impact. The damaged samples were repaired using either

ambient environment curing epoxy, heat activated curing prepreg, or ultraviolet (UV) curing resin. Environmental conditioning

using UV radiation and a seawater bath was conducted to investigate the durability of the repaired samples. A uniaxial tension test

was conducted on a total of 45 effective samples to evaluate the strength recovered by the repair materials and the strength lost by

environmental conditioning. A finite element analysis was conducted to understand the failure modes of the repaired samples. The

test results show that fiber reinforced UV curing resin is a fast, strong, durable, and cost effective method to repair low velocity

impact damaged composite laminates.

� 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Laminated beam; Low velocity impact; Damage; UV curing resin; Repair; Conditioning

1. Introduction

Over the years, there have been mounting concerns

over the safety of laminated composites subjected to lowvelocity impact. This kind of impact is not uncommon––

the simple drop of a tool during routine inspection can

create damage in laminated plates. This force on a

laminated composite can cause various types of dam-

ages, including delamination, matrix cracking, fiber

breakage, and fiber/matrix interfacial debonding. These

types of damage can cause structural failure at a load

well below the design level [1–4].With the increased use of laminated composite ma-

terials in various industries, it is becoming more im-

portant to develop techniques and materials to repair

laminated composites damaged by low velocity impact.

Field level repair generally requires an adhesive bonded

approach to provide the load transfer and restore the

original design strength of the composite laminate.

Currently, two techniques are available for the repair,along with two types of repair materials [5,6]. The two

techniques are scarf repair and lap (single or double)

repair. The two materials are wet lay-up material, which

is cured at a room temperature, and prepreg material,

which is cured at an elevated temperature. However, the

currently used repair materials have limitations. The wet

lay-up material usually requires three to seven days forcomplete curing of the resin. In many applications,

however, a seven-day wait is unacceptable. Heat acti-

vated curing prepreg can reduce the repair time to sev-

eral hours. However, these materials generally require

freezer storage and have a limited shelf life. Heat and

pressure are required to cure the adhesive and patch

materials in order to obtain a uniform, nonporous ad-

hesive layer. The most common heating method for fieldrepair requires heat blankets that are controlled by a

programmable temperature controller. The heat blan-

kets are a series of electrical resistance wires embedded

in silicone rubber. There are several disadvantages to

this heating method. First, prepreg curing requires a

curing temperature with a narrow tolerance. However,

due to thermally complex structures, achieving curing

temperatures within the required range is often difficult.Second, heating large areas using heat blankets requires

large amounts of energy that can easily exceed available

power sources. Third, for structures with complicated

geometries, the required curing pressure is generally

difficult to apply. Finally, the time required for field level

repair is still too long. This is because the curing cycle

includes heating the blanket to the curing temperature

*Corresponding author. Fax: +1-225-578-5924.

E-mail address: [email protected] (G. Li).

0263-8223/03/$ - see front matter � 2003 Elsevier Science Ltd. All rights reserved.

PII: S0263-8223 (02 )00292-1

Composite Structures 60 (2003) 73–81

www.elsevier.com/locate/compstruct

mail to: [email protected]

and, after soaking, cooling down to the ambient tem-

perature, which may take several hours. New and

innovative repair materials and repair approaches must

be developed.During the 1980s, ultraviolet (UV) curing aerobic

acrylic adhesive was developed to structurally bond

plastics in seconds [7,8]. Recent photocatalyst inventions

have led to the development of formulations that cure

with UV light, visible light, or both. Advances in photo-

catalysts now allow the bonding of UV blocked clear

plastics. Light curing adhesive bonds have the same

strength as plastic welding or solvent bonding. Thisrepresents a significant advance for industries in which

product reliability, safety, and fast repair are required

[7].

The purpose of this study was to experimentally in-

vestigate the effectiveness of using UV curing resin to

fast repair laminated beams damaged by low velocity

impact. A total of 45 effective specimens were prepared,

damaged, repaired, conditioned, and tested. A finite ele-ment analysis was conducted to understand the failure

modes of the repaired samples. The effectiveness of the

fast repair resin was evaluated based on the test results.

2. Test procedure

2.1. Raw materials

Three types of resins were used. One was a two-part

high strength epoxy adhesive that can be cured in 24 h at

room temperature. It was used as a control resin in this

study. The second was an E-glass fabric reinforcedprepreg that can be cured in 1.5 h at 135 �C. It was also

used as a control resin in this study. The third was a UV

curing resin. It can be cured by UV light or sunlight in

about 20 min. For comparison purposes, the fibers used

to reinforce the epoxy and the UV curing resin were the

same type of E-glass fabric used in the prepreg. The

physical/mechanical properties of the composites used

are shown in Table 1.The laminated beams were cut from a laminated plate

using a diamond saw. It was a crossply [(0/90)5]s lami-

nated plate made of an E-glass fiber reinforced epoxy.

The lamina thickness was 0.158 mm, its mechanical

properties are also shown in Table 1.

2.2. Predamage of specimens

Forty-five 203.2 mm� 50:8 mm� 3:175 mm cross-

ply laminated beam samples were cut from the lami-nated plate. Among the 45 samples, 36 samples were

predamaged by low velocity impact using a DynaTup

8250 HV impact machine. The laminated beam samples

were simply supported with a span length of 101.6 mm.

The impactor nose shape was a hemisphere with a dia-

meter of 12.7 mm. The total hammer weight was 35 N.

The impact velocity was 2.0 m/s. The details of the

damages were similar to those found in Refs. [1,3].

2.3. Repair of specimens

After predamage, the samples were ready to be re-

paired. As mentioned previously, two repair configura-tions are commonly used: scarf repair and single or

double lap repair. For the low velocity impact damaged

laminated beams, a hybrid scarf/single lap repair was

used. The reason is that the damaged sublaminate con-

tains various types of damage including delamination,

matrix cracking, fiber breakage, and fiber/matrix inter-

facial debonding. Because of this, the capability for the

damaged sublaminate to transfer load is significantlyreduced. If the sublaminate is not removed, it will not

transfer load effectively. Also, it will occupy some struc-

tural space, which is undesirable. Therefore, the dam-

aged sublaminate has to be removed and patched. If the

removed sublaminate is repaired using scarf configura-

tion only, the lost load carrying capacity cannot be re-

covered. The reason is that the fibers in the original

undamaged laminate are continuous and so is the inter-lamina bonding. However, the fibers become discon-

tinuous at the interface where the repair patch meets

with the boundary of the remaining substrate. Because

the load carrying capacity of a laminate is mainly pro-

vided by the fibers, an additional load transfer path has

to be provided in order to fully recover the lost strength.

A single lap repair patch can be used to satisfy the re-

quirement. The additional single lap repair, togetherwith the scarf repair, forms a hybrid scarf/single lap

repair. A schematic of the hybrid repair is shown in Fig.

1. This configuration was used in the study.

For the hybrid repair procedure, three layers at

the back surface underneath the impacted area were

Table 1

Physical/mechanical properties of the raw materials used

Materials Viscosity at 25 �C (cps) Tensile strength (MPa) Modulus of elasticity (GPa)

Epoxy 300,000 83 3.8

UV curing resin 500 70 4.2

Prepreg – 330 24.8

E-glass fabric – 3,000 70.0

A lamina from the laminated beam – 960 38.6

74 G. Li et al. / Composite Structures 60 (2003) 73–81

removed using a rotary tool with a cutting wheel. The

removed layers were squares with a size of 645.16 mm2.Two repair procedures were used in this study. For the

epoxy resin and the UV curing resin, the wet lay-up

technique was used. This technique started with appli-

cation of a layer of resin (about 300 g/m2 for epoxy and

about 200 g/m2 for UV curing resin) to the surface of the

laminated beam. Next, an E-glass fabric of 25.4 mm

long � 25.4 mm wide was laid on top of the resin. A

roller was then used to repeatedly roll over the fabric inorder remove the entrapped air bubbles and press the

resin to penetrate into the fabric. The roller was con-

tinuously rolled until the resin was reflected on the top

surface of the fabric, a sign of complete wetting. On top

of the fabric, another layer of resin was applied, thus

completing one layer of repair. The procedure was re-

peated to apply the subsequent two layers with dimen-

sions of 50.8 mm� 25:4 mm for the second layer and76.2 mm� 38:1 mm for the top layer. The second layer

and the third layer formed the single lap repair. Fig.

2(a)–(d) shows the process of completing one layer of

epoxy repair. It is noted that when using the UV curing

resin, no roller was used, the viscosity of the resin was

Fig. 1. A schematic of hybrid repair.

Fig. 2. (a) Remove the damaged layers, (b) apply a layer of epoxy and lay down a layer of fabric, (c) roller roll over the fabric and (d) apply another

layer of epoxy.

Fig. 3. (a) Epoxy repaired samples, (b) UV curing resin repaired

samples and (c) prepreg repaired samples.

G. Li et al. / Composite Structures 60 (2003) 73–81 75

adequate to wet through the fabric without excessive

running. After repairing, the epoxy repaired samples

remained in the lab for curing for a total of 24 h. The

UV curing resin repaired samples were brought out-doors and exposed to direct sunlight. Considering that

the glass fabric may block the penetration of UV light

and it was cloudy that day, the curing time was extended

from 20 to 50 min for proper curing. For the repair

using heat activated curing prepreg, the procedure

developed in joining composite beams was used [5]. In

this technique, the three repair pieces of 25.4 mm � 25.4

mm, 50.8 mm � 25.4 mm, and 76.2 mm � 38.1 mm werecut from a roll of prepreg. Next, the three pieces were

put one on the top of another onto the laminated beam.

Then, on the top of the prepreg layer, a polyester shrink

tape was wrapped in a spiral pattern to apply pressure

during curing. Finally, the assembled sample was put

into a programmed oven for curing at 135 �C for 1.5 h.Fig. 3(a)–(c) shows the repaired samples using the three

resins.

2.4. Conditioning of specimens

After repair, the samples were divided into three

groups: the control group, the conditioned group using

UV radiation, and the conditioned group using seawater

and UV radiation. The details for each sample areshown in Table 2. Among them, sample nos. 1–9 were

Table 2

Details of each sample

Sample no. Predamage by low velocity impact Repair materials Conditioning

1 No – No

2 No – No

3 No – No

4 No – UV

5 No – UV

6 No – UV

7 No – UVþ seawater



10 Yes – No

11 Yes – No

12 Yes – No

13 Yes – UV

14 Yes – UV

15 Yes – UV

16 Yes – UVþ seawater



19 Yes UV curing resin No



22 Yes UV curing resin UV



25 Yes UV curing resin UVþ seawater



28 Yes Prepreg No

29 Yes Prepreg No

30 Yes Prepreg No

31 Yes Prepreg UV

32 Yes Prepreg UV

33 Yes Prepreg UV

34 Yes Prepreg UVþ seawater



37 Yes Epoxy No

38 Yes Epoxy No

39 Yes Epoxy No

40 Yes Epoxy UV

41 Yes Epoxy UV

42 Yes Epoxy UV

43 Yes Epoxy UVþ seawater




control samples without predamage, nos. 10–18 were

control samples with predamage, nos. 19–27 were sam-

ples repaired using UV curing resin; nos. 28–36 were

samples repaired using prepreg; and nos. 37–45 weresamples repaired using wet lay-up resin.

In order to investigate the durability and the feasi-

bility of the various repairs in maritime structures, sea-

water and UV radiation were used to condition the

specimens. The seawater was taken from the Gulf of

Mexico. A 609:6� 203:2� 304:8 mm glass box was used

as the conditioning chamber. A 300 w Mog Base UV

lamp was used as the UV radiation source, which had awavelength ranging from 280 to 340 nm. The distance

between the UV source and the sample surfaces was

about 50 cm. For the samples soaked in seawater, the

distance between the water surface and the sample sur-

faces was 25.4 mm. The total duration of the condi-tioning was 10 days. Fig. 4 shows the samples under

conditioning.

2.5. Uniaxial tension test

After conditioning, all 45 samples were uniaxially

tensioned in an Instron MTS 810 machine to determine

the tensile strength of the various control samples and

repaired samples. The tension test was conducted per the

ASTM D3096-76 standard. Fig. 5 shows a sample under

tensile test.

3. Results and discussion

The averaged tensile strength of the various control

samples and repaired samples is shown in Fig. 6. From

Fig. 6, the following analyses can be conducted.

3.1. Effect of low velocity impact on the residual load

carrying capacity

From Fig. 6, it is seen that low velocity impact sig-

nificantly reduced the residual tensile strength of the

laminated beams. About 12.9% of the original tensile

strength was lost. This is because the delaminated sub-

laminate was heavily damaged and could not transfer a

tensile load effectively. This means that the damagedsublaminate needs to be removed and repair of the

laminated beams is of paramount importance. AfterFig. 4. Samples under conditioning.

Fig. 5. Tension test.

Und

amag

ed

Pred

amag

ed

UV

resi

nre

pair

Prep

reg

repa

ire

Epox

yre

paire

d

Tens

ilest

reng

th(M

Pa)

0

100

200

300

400

500

600

UnconditionedUV conditionedUV + seawater conditioned

Fig. 6. Tensile strength of various control samples and conditioned

samples.


conditioning, the strength was only slightly reduced.

About 13.7% of the original tensile strength was lost for

UV radiation conditioned samples and this number

became 13.8% for UV and seawater conditioned sam-ples. This means the crossply laminated beams were

reasonably stable when subjected to environmental con-

ditioning.

3.2. Enhancement of the load carrying capacity by various

repairs

From Fig. 6, the repairs significantly increased the

tensile strength of the damaged laminated beams. Actu-

ally, the strength of the UV curing resin repaired samples

was 1.08 times the strength of the unrepaired samples.

This number became 1.07 times and 1.23 times for the

prepreg repaired samples and epoxy repaired samples,

respectively. Obviously, ambient environment curingepoxy had the highest reinforcing efficiency. This can be

contributed to the chemical compatibility of the epoxy

repairs with the E-glass fiber reinforced epoxy substrate

beams. However, this does not mean that the epoxy is the

most effective repair material. The effectiveness of a re-

pair material also depends on its cost, workability, and

durability.

3.3. Degradation of the load carrying capacity due to

environmental conditioning

As shown in Fig. 6, the repairs still considerably in-

creased the tensile strength of the predamaged laminated

beams when subjected to environmental conditioning.For the UV radiation conditioned samples, the residual

tensile strength for the UV curing resin repaired samples

was 1.25 times the strength of the predamaged un-

repaired samples. This number became 1.17 times and

1.05 times for the prepreg repaired samples and epoxy

repaired samples, respectively. After UV conditioning,

UV curing resin became the most effective repair mate-

rial and the epoxy became the least. Since laminatedcomposite structures will inevitably be subjected to UV

radiation attacks, the UV curing resin is the most suit-

able repair material among the three resins. The reason

may be that the UV light strengthened rather than

weakened the resin bonding. Subjected to UV radiation,

the UV curing resin was further cured and strengthened,

while the prepreg and the epoxy were damaged. Con-

sequently, the UV curing resin repair became the mosteffective of the three.

For the UV and seawater conditioned samples, the

effectiveness of the various repairs changed. The residual

tensile strength for the UV curing resin repaired samples

was 1.21 times the strength of the predamaged un-

repaired samples. This number became 1.11 times and

1.21 times for the prepreg repaired samples and epoxy

repaired samples, respectively. For the UV curing resin

and prepreg, the existence of seawater slightly reduced

the reinforcing efficiency, while it was increased for the

epoxy repaired samples. This can be attributed to theepoxy being very sensitive to UV radiation attacks. With

the seawater, some of the UV radiation energy was

consumed by the seawater, thus the adverse effect of UV

radiation was slightly reduced. Actually, the reinforcing

efficiency of the UV curing resin and the epoxy was

nearly the same. An economic analysis is required in

order to determine the most effective repair method.

The reasons for the degradation of fiber reinforcedplastics (FRPs) may be first attributable to the water

absorption, leading to deterioration of the resin matrix

and fiber/matrix interfacial bonding. Penetration of

water into the FRP occurs by diffusion through the

matrix resin and capillary flow via microcracks and

voids. This moisture absorption results in development

of residual stress and plasticization of the resin. It also

leads to debonding at the fiber/matrix interface. Second,the salt may further weaken the fiber/matrix interfacial

bonding and attack the E-glass fibers. Third, the UV

radiation may break the chemical bond of organic

molecules. Consequently, the matrix may become brittle

and matrix cracking and fiber/matrix interfacial de-

bonding may be increased, reducing the mechanical

strength of the FRPs. Because the undamaged lami-

nated beams were least affected by the environmentalconditioning, the degradation of the repaired samples is

most likely due to the degradation of the repair layers.

This can be validated through the test of the repair

layers alone. For this purpose, 203.2 mm � 50.8 mm

laminated beams were prepared using the three repair

materials. The laminated beams consisted of two layers.

The samples were divided into two groups, one for

control and the other for conditioning. The conditioningwas achieved through UV radiation and seawater im-

mersion for 10 days. A uniaxial tension test was con-

ducted on the control and conditioned samples. The

peak loads for the three repair materials are shown in

Table 3. From Table 3, about 11.5% of the peak load

was lost for the UV curing FRP after conditioning. The

load reduction was 13.8% and 12.6% for the prepreg and

epoxy FRPs, respectively.

Table 3

Residual mechanical properties of FRPs

Materials Conditioning Peak load (KN)

Epoxy FRP No 6.12

UVþ seawater 5.35

UV curing resin

FRP

No 5.50

UVþ seawater 4.87

Prepreg FRP No 6.20

UVþ seawater 5.34


3.4. Failure mode analysis

Two failure modes were observed during the test of

repaired samples. The first failure mode was a two-stepfailure. When the load was about one-half to two-thirds

of the peak load, there was a sudden drop in the load,

corresponding to a debonding of the lap layers from the

substrate beam. From there, the load increased again

until the fracture of the substrate beam, which corre-

sponded to the second sudden drop of the load. The UV

curing resin repaired samples and the prepreg repaired

samples showed such failure modes. For the epoxy re-paired samples, the same type of failure mode was ob-

served when subjected to environmental conditioning.

Without conditioning, however, the epoxy repaired

samples exhibited another type of failure mode––a one-

step failure mode. In this failure mode, the applied load

smoothly increased until a simultaneous fracture of the

repairs and the substrate beam occurred. Only one drop

in load was observed. Fig. 7(a) and (b) shows the failuremode of epoxy repaired samples with and without en-

vironmental conditioning. The repair layers were broken

for the unconditioned sample, while they were not

broken for the conditioned sample, whose repair layers

were debonded from the substrate. The stress–strain

distributions corresponding to the two failure modes are

shown in Fig. 8.

The different failure modes may result from thevarying interfacial stress concentrations and interfacial

bonding strengths of the various repairs. Subjected to a

uniaxial tension load, a portion of the load was trans-

ferred from the substrate to the repair layers through

interfacial shear stresses. In addition, owing to the small

nonalignment of the lap repair layers with respect to the

axis of the substrate beam, there was also a considerable

peel stress at the interface between the lap layers and thesubstrate. Once this stress concentration exceeded the

shear strength of the repair materials, the repair lap

layers debonded from the substrate, leading to a sud-

den drop in the load on the stress–strain curve, corre-

sponding to the two-step failure mode. When, however,

the interfacial bonding was strong enough, the lap layers

would not debond from the substrate. They would

continue to take the load until there was a tensile frac-ture of the entire structure. This corresponded to the

one-step failure mode. Obviously, the one-step failure

mode had a higher tensile strength because it fully uti-

lized the load carrying capacity of the repair layers. This

was the case from the test results. From Fig. 6, the epoxy

repaired samples without conditioning had the highest

tensile strength, corresponding to the one-step failure

mode.The above analysis was validated by a finite element

analysis of the test sample. In this analysis, COSMOS/M

(version 2.7) software package with eightnode composite

element SOLIDL were employed. A total of 13,056 ele-

ments were used to mesh the sample. A 480 MPa uni-

axial tensile load was applied to the two ends of the

repaired sample and proper boundary conditions were

applied. The shear stress and peel stress at the interfaceof the lap repairs and the substrate beam are shown in

Fig. 9(a) and (b), respectively. From Fig. 9(a), the

maximum interfacial shear stress is 40.94 MPa, which is

larger than the lap shear strength of most resins. Deb-

onding of the lap layers from the substrate beam was

inevitable. The interfacial peel stress concentration

shown in Fig. 9(b) assisted in the debonding failure. This

was why the UV repaired samples, the prepreg repairedsamples, and the epoxy repaired samples with environ-

mental conditioning failed by the two-step failure mode.

For the epoxy repaired samples without conditioning,

Fig. 7. (a) Failure mode of epoxy repaired sample without condi-

tioning and (b) failure mode of epoxy repaired sample with condi-

tioning.

Strain (mm/mm)

0.000 0.005 0.010 0.015 0.020 0.025

Tens

ile s

tress

(MPa

)

0

100

200

300

400

500

Two-step failure modeOne-step failure mode

First load drop

Second loaddrop

Fig. 8. Typical stress–strain relations corresponding to the two-step

failure mode and the one-step failure mode.


the repair epoxy was chemically compatible with the

substrate epoxy. A good epoxy/epoxy interfacial bond

existed. It was possible that this epoxy/epoxy bond had

shear strength larger than 40.94 MPa, leading to the

one-step failure mode.

From the above stress analysis, it is seen that in-

creasing the interfacial shear strength is very effective in

enhancing the strength of repaired samples. Reducingthe nonalignment, for instance using double lap repairs,

will reduce the interfacial peel stress and thus result in

stronger repairs.

3.5. Cost/benefit analysis

When comparing the UV repaired samples with

epoxy repaired samples, the former performed betterwith UV conditioning, while the latter performed better

without UV conditioning. Subjected to UV and seawater

conditioning, the two performed virtually the same. The

prepreg repaired samples were slightly lower in strength.

Therefore, only an economic analysis can give a sub-

stantial difference between the UV curing resin and the

epoxy resin. A cost/benefit analysis then would fall al-

most exclusively on the cost of the materials and thelabor. A break down of the cost per repaired sample is

shown in Table 4. Table 4 considers only the material

costs and the labor costs, not including the reduced cost

on the shorter repair time for the UV curing resin. The

UV curing resin not only has nearly the same strength-

ening efficiency as the ambient environment curing

epoxy, but it is also very cost-effective. Considering the

cost and the ease of construction, the 20-min UV curingresin is more desirable than the 24-h curing epoxy.

4. Conclusion

In this study, laminated beams were prepared. A low

velocity impact test was used to predamage the samples.

Three types of resins ranging from 20-min curing to 24-h

curing were used to repair the predamaged samples. Ahybrid scarf/single lap repair configuration was used.

Environmental conditioning using UV radiation and

seawater immersion was conducted on a portion of the

repaired samples to evaluate their durability in maritime

structures. A uniaxial tensile strength test was conducted

on the various control samples and conditioning sam-

ples. A finite element analysis was implemented to assist

in understanding the two failure modes observed. Based

Fig. 9. (a) Interfacial shear stress concentration in the lap repair layers

and (b) interfacial peel stress concentration in the lap repair layers.

Table 4

Raw materials and labor costs for repairing per sample

Repair method Item Cost per unit Units used per sample Cost per item

Epoxy Epoxy $148/L 0.00193 L $0.28

Fabric $3.60/m2 0.0048 m2 $0.02

Labor $15/h 0.25 h $3.75

Total cost per sample $4.05

Prepreg Prepreg sheet $15.79/m2 0.0048 m2 $0.08

Shrink tape $2/roll 0.05 roll $0.10

Electricity $0.08/kWh 9 kWh $0.72

Labor $15/h 0.083 h $1.25


UV Curing Resin $7.93/L 0.00161 L $0.02

Fabric $3.60/m2 0.0048 m2 $0.02

Labor $15/h 0.083 h $1.25



on the test results and the finite element analysis results,

the following preliminary conclusions were obtained:

(1) The UV curing resin is a fast, strong, durable, andcost-effective repair method. It can replace the ambi-

ent environment curing resin and the heat activated

prepreg resin in repairing composite laminates dam-

aged by low velocity impact.

(2) The hybrid scarf/single lap repair configuration can

be used to repair damaged composite laminates.

(3) Increasing the interfacial shear strength between the

repair layers and the substrate can increase thestrength of repaired samples. Using double lap re-

pair can reduce the interfacial peel stress and con-

tribute to enhancing the repair efficiency.

(4) Environmental conditioning had an adverse effect

on the control samples and repaired samples except

for UV curing resin repaired samples. For UV cur-

ing resin repaired samples, conditioning by UV radi-

ation increased the residual tensile strength.

Acknowledgements

This investigation was partially sponsored by the

Louisiana Board of Regents, NASA, and NSF under

contract no. LEQSF(2000-03)-RD-B-05, LEQSF(2001-

04)-RD-B-03, NASA/LEQSF (01-05)-LaSPACE, and

NSF/REU (EEC-9820369). Dr. Samuel Ibekwe from

Southern University assisted in the test.

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