Composite Structures 94 (2012) 462–468

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Composite Structures

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Cryogenic reliability of composite insulation panels for liquefied naturalgas (LNG) ships

Young Ho Yu a, Bu Gi Kim a, Dai Gil Lee b,⇑a School of Mechanical Aerospace & Systems Engineering, Korea Advanced Institute of Science and Technology, ME3221, Guseong-dong, Yuseong-gu,Daejeon 305-701, Republic of Koreab KAIST Institute for Design of Complex Systems (KIDCS), Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Available online 27 August 2011

Keywords:LNG shipComposite insulation panelSecondary barrierCrack retardationGlass fabric reinforcement

0263-8223/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compstruct.2011.08.009

⇑ Corresponding author.E-mail addresses: younghomj@kaist.ac.kr (Y.

(B.G. Kim), dglee@kaist.ac.kr (D.G. Lee).URLs: http://scs.kaist.ac.kr (Y.H. Yu), http://kidcs.k

The major carrier of liquefied natural gas (LNG) is LNG ships, whose containment system is composed ofdual barriers and composite insulation panels. The LNG containment system should have cryogenic reli-ability and high thermal insulation performance for safe and efficient transportation of LNG. The second-ary barrier composed of adhesive bonded aluminum strips should keep tightness for 15 days, when thewelded stainless primary barrier fails. However, cracks are generated in the composite insulation panelsdue to the local stress concentration and the brittleness of insulation materials at the cryogenic temper-ature of �163 �C. If cracks generated in the insulation panel propagate into the secondary barrier, LNGleakage problem might occur, which is a remaining concern in ship building industries.

In this study, crack retardation capability in the composite insulation panel was investigated with glassfabric reinforcement. Finite element analysis was conducted to estimate the thermal stress at the cryo-genic temperature and a new experimental method was developed to investigate the failure of secondarybarrier of composite insulation panel. From the experimental results, it was found that the glass fabricreinforcement was effective to retard the crack propagation into the aluminum secondary barrier fromthe polyurethane insulation foam at the cryogenic temperature.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

As the consumption of liquefied natural gas (LNG) has been in-creased much specially due to nuclear power plant disaster inJapan, the LNG ships have been spotlighted as major carriers ofLNG [1–4]. Since LNG is normally carried in ships at �163 �C, thefunctional requirements of LNG ship are to have cryogenic reliabil-ity due to thermal cyclic stresses as well as to have high thermalinsulation performance for safe and efficient transportation ofLNG [5,6]. For efficient transportation, recently, the membranetype LNG containment systems are usually employed [7]. Themembrane type LNG containment system has octagonal pillarshape with high capacity (larger than 39,000 m3), which is com-posed of dual barriers and composite insulation panels as shownin Fig. 1 [8,9]. The secondary barrier composed of adhesivelybonded metal strips should keep tightness for 15 days when thewelded stainless primary barrier fails. The insulation foam shouldhave very low thermal conductivity for high thermal insulationperformance, which limits the density of foam less than 120 kg/

ll rights reserved.

H. Yu), willy4@kaist.ac.kr

aist.ac.kr (D.G. Lee).

m3. Since the foam density is not high enough for structural pur-pose and brittle at cryogenic temperatures, the reliability of con-ventional containment systems are not fully guaranteed due tothe crack propagation in insulation foams, which is a remainingconcern in ship building industries [10].

The composite insulation panels are composed of aluminumstrip of 0.3 mm thickness, plywood and fiber reinforced polyure-thane foam as shown in Fig. 1. The size of the insulation panel is1 m (Width) � 3 m (Length) � 0.27 m (Height). The aluminumstrip secondary barrier and polyurethane foam are adhesivelybonded with polyurethane adhesive as shown in Fig. 2. Duringthe operation of the cryogenic containment system (CCS) of LNGships at the cryogenic temperature, cracks are generated in thecomposite insulation panels by the local stress concentrationsdue to the cyclic thermal stresses and the low fracture toughnessof polyurethane foam at the cryogenic temperature [11–13]. In or-der to retard the crack generation, the polyurethane foam (PUF) isreinforced with glass fibers as shown in Fig. 2. However, the uni-form glass fiber reinforcement with high volume fraction in PUFis not only possible in the mass production, but also it increasesthe thermal conductivity of polyurethane foam, which decreasesthe efficiency of the LNG transportation. When the cracks aregenerated, they propagate into the aluminum secondary barrieras shown in Fig. 2, which will induce the leakage of LNG.

Primary barrier

Plywood

Polyurethane foamSecondary barrier

LNG

Aluminum

Stainless steel

Composite insulation panel

LNG

Containment systemContainment system

Fig. 1. Schematic diagrams of LNG containment system composed of dual barriersand composite insulation panels.

Primary barrier

Plywood

Polyurethane foamSecondary barrier

LNG

Aluminum

Stainless steel

Composite insulation panel

Glass fiber Polyurethane foam

Aluminum barrier

Polyurethane adhesive

Crack

Local elongation due to the crack propagation

Tension due to thermal shrinkage

Fig. 2. Schematic diagrams of crack propagation from the polyurethane foam intoaluminum secondary barrier due to the local elongation with thermal shrinkage ofcomposite insulation panels.

Flat insulation panel

Corner insulation panel

Fig. 3. Photograph of the large scale mock-up to investigate the cryogenic reliabilityof LNG containment system.

Flat panel

Corner panel

(b)

(a)

Section A

Crack

Aluminum

Plywood

Polyurethane foam

Stainless steel

Aluminum

Polyurethane adhesive

Fig. 4. Composite insulation panels used in mock-up test: (a) schematic diagram offlat and corner of composite insulation panels in mock-up test and (b) photographand schematic diagram of crack on the aluminum barrier generated on section Aduring mock-up test.

Y.H. Yu et al. / Composite Structures 94 (2012) 462–468 463

In this study, the crack generation was investigated with finiteelement analysis. Also a new cryogenic experimental methodwas developed to investigate the cryogenic reliability of the com-posite insulation panels. For the crack retardation from the insula-tion panel into the aluminum secondary barrier, the adhesivebetween the aluminum barrier and the polyurethane foam wasreinforced with glass fabric. From the experimental results, itwas found that the glass fabric reinforced adhesive of composite

insulation panels retarded the crack propagation into the alumi-num secondary barrier, which improved the cryogenic reliabilityof composite insulation panel.

2. Experimental

The large scale mock-up has been usually constructed to inves-tigate the cryogenic reliability of the CCS with various types ofcomposite insulation panels as shown in Fig. 3 [14]. The compositeinsulation panels can be classified into the flat panel and corner pa-nel as shown in Fig. 4a. From the cryogenic experimental resultswith large scale mock-up with composite insulation panels, crackswere detected on the aluminum secondary barrier of the section Aas shown in Fig. 4b. Although the fully annealed aluminum strip of1000 series has high failure strain of 34%, local strain concentra-tions in the aluminum strip due to the local failures of polyure-thane foam, led to the crack propagations in the aluminumsecondary barrier as shown in Fig. 4b. The crack propagation intothe aluminum secondary barrier will cause the safety problemssuch as LNG leakage and explosion.

Table 1Properties of polyurethane foam used for insulation foam.

20 �C �170 �CModulus Ex, Ez (MPa) 136 185Modulus Ey (MPa) 73 117Shear modulus (Gxy,Gxy,Gxy) 11 21Poisson’s ratio vxy 0.24 0.20

vyz 0.39 0.32vzx 0.09 0.09

Coefficient of thermal expansion (�10�5) ax 3.09ay 5.86az 3.05

Failure strain at �170 �C x, z direction 2.1%Tensile strength at �170 �C (MPa) x, z direction 3.8

Table 2Properties of aluminum used for secondary barrier.

Modulus (MPa) 70Poisson’s ratio 0.3Coefficient of thermal expansion

(10�5 m/m K)2.4

Failure strain at �170 �C 34%Tensile strength at �170 �C (MPa) 98

(b)

-163°Cx-symmetry

y-symmetry

Fixed condition, 25°C

xy

xy

xy

xy

xy

(a)

Stainless steelAdhesives Aluminum Polyurethane foam Section A

x

y

z

Fig. 5. Finite element model of flat panel and corner panel: (a) boundary conditionswith coordinates and (b) mesh configuration of finite element model with finemesh.

464 Y.H. Yu et al. / Composite Structures 94 (2012) 462–468

2.1. Finite element analysis of the composite insulation panels

The stress and strain distributions in the composite insulationpanels at the cryogenic temperature were calculated with finiteelement analysis with ABAQUS 6.10 (SIMULIA, USA). The proper-ties of polyurethane foam and aluminum are listed in Tables 1and 2, where x, z directions are in-plane direction and y directionis out of plane direction as shown in Fig. 5a.

The flat panel and corner panel were modeled with the bound-ary condition in Fig. 5a. Fig. 5b shows the mesh configuration withfine meshes on the section A, where the cracks were detected inthe aluminum secondary barrier during the mock-up test.

Since the cracks were propagated into the aluminum secondarybarrier from the insulation foam in section A of a corner panel inthe mock up test, the stress and strain distributions induced bythe thermal shrinkage at the cryogenic temperature were esti-mated with finite element analysis.

Since the aluminum strip can function as the secondary barriereven though it yields until its rupture, the strain criterion ratherthan the yield criterion was employed for the failure of aluminumbarrier. From the calculated results as shown in Fig. 6a, the maxi-mum strain of the aluminum strip was 5.9%, which was 5.8 timeslower than the maximum failure strain of the aluminum of 34%at the cryogenic temperature. Fig. 6a shows the distribution ofmaximum strain along the aluminum barrier.

Since the polyurethane foam was brittle at the cryogenic tem-perature, the maximum strain criterion was employed for the fail-ure criterion of polyurethane foam as follows [15]:

ecx < ex < et

x ð1:1Þ

ecy < ey < et

y ð1:2Þ

ecz < ez < et

z ð1:3Þ

�Cyz < cyz < Cyz ð1:4Þ

�Czx < czx < Czx ð1:5Þ

�Cxy < cxy < Cxy ð1:6Þ

where eti and ec

i (i = x,y,z) represent the failure tensile strain andcompressive strain respectively, Cyz, Czx and Cxy represent the fail-ure shear strains in the planes yz, zx and xy, respectively. The calcu-lated maximum strain ex in the polyurethane foam (PUF) was 3.2%at the cryogenic temperature as shown in Fig. 6b. Fig. 6b showsthe strains along the insulation foam under the aluminum barrier,where the strain concentrations occurred at the cryogenic temper-ature. Since the failure strain of polyurethane foam in x-direction is2.1% as in Table 1, the polyurethane foam failed at the cryogenictemperature and the cracks propagated through the aluminum sec-ondary barrier although the strain of aluminum was much lowerthan that of its failure strain as illustrated in Fig. 2.

Since the aluminum secondary barrier failed at the low straindue to the crack propagated from the adhesively bonded PUF asshown in Fig. 4, the strain in the aluminum secondary barrierwas calculated by the finite element analysis to estimate the refer-ence strain for the safety of the aluminum barrier in the compositeinsulation panel. In Fig. 7, the section A is pulled 0.935(0.321 + 0.614) mm along the x-direction due to the thermalshrinkage of flat panel and the corner panel at the cryogenic tem-perature. Also, the section A should extend to accommodate thethermal shrinkage at the cryogenic temperature, whose magnitudeDL can be calculated as follows:

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Failure strain of polyurethane foam: 2.1% St

rain

, (%

)

Distance (mm)

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35 400 5 10 15 20 25 30 35 40

Distance (mm)

Stra

in (

%)

Failure strain of aluminum barrier: 34%

(a)

Path along aluminum barrier(secondary barrier)

Path along polyurethane foamunder the aluminum barrier

Section A

x

y

z

(b)

Maximum value: 5.9%

Maximum value: 3.2%(strain concentration)

Distance

Fig. 6. Strain distributions of the aluminum barrier and polyurethane foam: (a) strain of the aluminum along the path of aluminum barrier (secondary barrier) and (b) strainof polyurethane foam along the path of polyurethane foam under the aluminum barrier.

Flat panel

Corner panel

0.614 mm0.321 mm35 mm

Shrinkage of corner panel

Shrinkage of flat panel

Original position

0.321 mm

Fig. 7. Schematic diagram of extension of section A along x-direction at cryogenictemperature due to the thermal shrinkage of flat panel and corner panel atcryogenic temperature.

Y.H. Yu et al. / Composite Structures 94 (2012) 462–468 465

DL ¼ a� L0 � DT ð2Þ

where L0 is original length, a is coefficient of thermal expansion,and DT is temperature change [16,17]. The original length of sectionA is 35 mm and temperature difference is �188 �C when the panelcools down from 25 �C to �163 �C. From Eq. (2), the elongation ofsection A due to thermal shrinkage at the cryogenic temperatureis �0.203 mm. Therefore, the total elongation of section A at thecryogenic temperature of �163 �C can be calculated to be1.138 mm by summing up 0.935 mm and 0.203 mm and the strainof section A is expected to be 3.3% with the gage length of 35 mm.

2.2. Cryogenic experiments

A new test method was developed to evaluate the failure strainof aluminum secondary barrier in the foam insulation panel. Theeffect of reinforcement was also investigated with the developedmethod. The configuration of the specimen is shown in Fig. 8,where the specimen has a symmetric shape to avoid bending atthe cryogenic temperature due to the difference of coefficient ofthermal expansion. The gage length of the specimen was 35 mmin order to simulate the section A as shown in Fig. 4. The tab lengthwas 60 mm for the uniform stress distribution at the gage lengthaccording to the Saint–Venant’s principle [18,19]. Since the crack

RPUFSteel tab Aluminum

Crack (3 mm)

205

155

60 35

35

RPUF

Crack (3 mm)

Aluminum

Polyurethane adhesive

10

(unit: mm)

Fig. 8. Shape of specimen to investigate the failure strain of aluminum barrier withpolyurethane foam.

Insulation chamber

Thermocouple

Liquid nitrogen tank

Specimen

Liquidnitrogen

tank

Insulation chamber

Controller

Specimen

Fan

Material testing system

(a)

(b)Fig. 9. Experimental setup for cryogenic experiment: (a) schematic diagram of theexperimental setup and (b) photograph of the experimental setup.

Table 3Thickness and areal density of plain weave E-glass fabric.

Thickness (mm) Areal density (kg/m2)

Fabric A 0.06 0.05Fabric B 0.18 0.17Fabric C 0.23 0.25

Table 4Adhesive thickness and volume fraction of E-glass fabric.

Curingpressure(MPa)

Withoutreinforcement

Glass fabricreinforcement

FabricA

FabricB

FabricC

0.001 Adhesivethickness(mm)

0.50 0.54 0.61 0.64

Volumefraction (%)

– 3.4 10.5 15.4

0.05 Adhesivethickness(mm)

0.24 0.36 0.47 0.49

Volumefraction (%)

– 5.1 13.5 20.3

0.1 Adhesivethickness(mm)

0.17 0.22 0.33 0.35

Volumefraction (%)

– 8.2 19.3 28.0

466 Y.H. Yu et al. / Composite Structures 94 (2012) 462–468

would propagate from the polyurethane foam to the aluminumbarrier, a pre-crack of 3 mm depth was fabricated on the PUF atthe center of specimen with the razor blade tapping method[20]. The aluminum strip was bonded on the PUF with the polyure-thane adhesive, which was cured at 25 �C for 12 h. The tests wereconducted with material testing system (INSTRON 4206, Instron,USA) with a thermal insulation chamber, as shown in Fig. 9a. Thetensile load was applied to the specimen to measure the failure

strain of the specimens at the cryogenic temperature. The cross-head speed was 1 mm/min and the chamber was cooled down to�150 �C, which was the lowest temperature with the material test-ing system available in this work. The temperature of a specimenwas measured with a thermocouple as shown in Fig. 9b.

The failure strain was measured with the rupture of aluminum.The moment of failure was detected by the load displacementcurve.

Since the crack generated in the PUF propagated into the alumi-num barrier, the PUF below the aluminum secondary barrier wasreinforced with plain weave E-glass fabrics. Several types of glassfabrics listed in Table 3 were impregnated with polyurethaneadhesive. Different curing pressures up to 0.1 MPa, the maximumpressure for the curing of larger panels, were applied to avoidwrinkling of the secondary aluminum barrier during the bondingprocess.

3. Results and discussion

The adhesive thickness and volume fraction of glass fiber weremeasured with respect to curing pressures and fabric types aslisted in Table 4. The measured failure strains of the aluminum bar-riers without reinforcement were 3.0%, 3.9%, 3.7% when the curingpressures were 0.001 MPa, 0.05 MPa and 0.1 MPa, respectively asshown in Fig. 10. Since the average value of the failure strain with-out reinforcement was 3.5%, which was slightly larger than thestrain value of 3.3% obtained from the finite element analysis,the cracks might start in the aluminum secondary barrier at thecryogenic temperature of �163 �C. Therefore, without reinforce-ment, the aluminum barrier could be failed due to the local elonga-tion of panels and the aluminum strip itself at cryogenictemperature of �163 �C.

For the specimens with the reinforcement of fabric type of A inTable 3, the failure strains of the aluminum barrier were 3.7%, 4.0%,3.9% when the curing pressures were 0.001 MPa, 0.05 MPa and0.1 MPa, respectively. The average value of the failure strain of

0

2

4

6

8

10

12

14

16

18

Failu

re s

trai

n of

alu

min

um b

arri

er (

%)

0.001 MPa 0.05 MPa 0.1 MPa

1: without reinforcement

3: with glass fabric reinforcement (fabric B) 4: with glass fabric reinforcement (fabric C)

2: with glass fabric reinforcement (fabric A)

Curing pressure

1 2 3 4

Fig. 10. Failure strain of aluminum barrier with respect to the curing pressure forthe specimens reinforced with fabric A, fabric B and fabric C.

(b)

(a)

Small yielding area

Fiber reinforced polyurethane foam

Aluminum

Fiber reinforced polyurethane foam

Aluminum

Polyurethane adhesive

Fig. 11. Failure of the aluminum secondary barrier by local elongation: (a)schematic diagram of failure mode and (b) photograph of failure mode.

(b)

(a)

Local interfacialdebonding

Large yielding area

Fiber reinforced polyurethane foam

Fiber reinforced polyurethane foam

Aluminum

Reinforced polyurethane adhesive

Aluminum

Fig. 12. Failure of aluminum barrier after local interfacial debonding: (a) schematicdiagram of failure mode and (b) photograph of failure mode.

Y.H. Yu et al. / Composite Structures 94 (2012) 462–468 467

specimen was 3.9%. For the specimens with the reinforcement offabric type B in Table 3, the failure strains of the aluminum barrierwere 3.7%, 4.0%, 13.1% when the curing pressures were 0.001 MPa,0.05 MPa and 0.1 MPa, respectively. For the specimens with thereinforcement of fabric type C in Table 3, the failure strains of alu-minum barrier were 14.0%, 14.6%, 14.3% when the curing pressureswere 0.001 MPa, 0.05 MPa and 0.1 MPa, respectively. The averagevalue of the failure strain of specimen was 14.3% with the rein-forcement of fabric type C, which was 4.3 times larger than themaximum strain of 3.3% obtained from the finite element analysis.However, the reinforcement with fabric type B hardly improvedthe failure strain of the aluminum secondary barrier when the cur-ing pressures were 0.001–0.05 MPa. From the results as shown inFig. 10 and Table 4, it was found that the glass fabric reinforcementwas much effective to increase the failure strain of the aluminumbarrier on the PUF when the volume fraction of glass fabric inpolyurethane adhesive was larger than 15.4%. Therefore, thereinforcement with the glass fabric type C in Table 4 has been em-

ployed for the cryogenic reliability of the aluminum secondary bar-rier adhesively bonded to the PUF core in the composite sandwichinsulation panels.

The increase of the failure strain of the aluminum secondarybarrier with reinforcement of glass fabric was due to the changeof failure mode. For the cases at the low volume fraction of glassfabric in Fig. 10, the aluminum barrier was failed with less than5% strain due to the both failures of the polyurethane adhesiveand the PUF by the local strain concentration as shown in Fig. 11.However, for the cases in Fig. 10 with high fiber volume fraction,whose failure strain of the aluminum was higher than 10%, the lo-cal interfacial debonding between the aluminum barrier and thepolyurethane adhesive was followed by the failure of polyurethaneadhesive and the PUF. The local interfacial debonding caused theyielding of large area of the aluminum barrier before failure asshown in Fig. 12. The typical load displacement graphs of two dif-ferent failure modes are shown in Fig. 13. For the failure mode withthe local debonding, the load decreased suddenly when the PUFfailed, but maintained some load carrying capability, which pre-vented the leakage of LNG until the final rupture of the aluminumsecondary barrier.

With the high volume fraction of glass fabric, the glass fabriccould retard the crack propagation from the PUF to the aluminumbarrier because the interface between the aluminum barrier andthe adhesive layer could not transfer the extra load after debond-ing. Also, at the high volume fraction of glass fabric, the interfacialbonding strength between aluminum barrier and adhesive woulddecrease due to the direct contact between some of the glass fiberswithout adhesive and the aluminum barrier. Although the glassfabric reinforcement which is not fully impregnated with adhesivecould decrease the bonding strength, the bonding strength wasmuch higher than the strength of polyurethane foam of 3.8 MPain Table 1. Therefore, the glass fabric reinforcement could be em-ployed in order to increase the failure strain of the aluminum bar-rier adhesively bonded to the PUF of the composite sandwichinsulation panels.

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5

Loa

d (k

N)

Displacement (mm)

Failure by local elongation

Failure after local interfacial debonding

Aluminum yielding with large area

Failure of aluminum by local elongation (low failure strain of aluminum)

Local interfacial debonding (high failure strain of aluminum)

Fig. 13. Typical load–displacement curves for two different failure modes, failureby local elongation and failure after local interfacial debonding with glass fabricreinforcement at cryogenic temperature of �150 �C.

468 Y.H. Yu et al. / Composite Structures 94 (2012) 462–468

4. Conclusions

In this study, the cryogenic reliability of composite thermalinsulation panels of the cryogenic containment system (CCS) of liq-uefied natural gas (LNG) ships was investigated. It was found thatthe aluminum secondary barrier adhesively bonded to the polyure-thane foam (PUF) failed by the propagation of the crack generatedfrom the PUF under the aluminum secondary barrier. Although thealuminum barrier had a much high failure strain of 34% at the cryo-genic temperature, the local strain concentration in the aluminumbarrier bonded to the PUF reduced the failure strain of the alumi-num to 3.3%.

For the crack retardation, the aluminum secondary barrier wasreinforced by the plain weave E-glass fabrics between the alumi-num secondary barrier and the PUF. From the experiments, thefailure strain of aluminum with polyurethane foam improved to14.3% when the volume fraction of glass fabric was higher than15.4% and the fabrics impregnated with adhesive were cured underthe pressure of 0.001–0.1 MPa, which was 4.3 times higher thanthe failure strain of the aluminum at the cryogenic temperature.Therefore, the glass fabric reinforcement is one of the promisingmethod to improve the cryogenic reliability of composite insula-tion panels.

Acknowledgments

This research was supported by WCU (World Class University)program through the National Research Foundation of Korea

funded by the Ministry of Education, Science and Technology(R31-2008-000-10045-0), Grant No. EEWS-2011-N01110017 fromEEWS Research Project of the office of KAIST EEWS Initiative(EEWS: Energy, Environment, Water, and Sustainability) and BK21.

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