Composite Structures 111 (2014) 1–12

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Composite Structures

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Cryogenic strength of adhesive bridge joints for thermal insulationsandwich constructions

0263-8223/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compstruct.2013.12.020

⇑ Corresponding author. Tel.: +82 42 350 3221; fax: +82 42 350 5221.E-mail address: dglee@kaist.ac.kr (D.G. Lee).

Soon Ho Yoon a, Ki Hyun Kim b, Dong Young Lee b, Dai Gil Lee b,⇑a Composite Materials Group, Korea Institute of Materials Science, 797 Changwondae-ro, Seongsan-gu, Changwon-shi, Gyeongnam 642-831, Republic of Koreab School of Mechanical Aerospace & Systems Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon-shi 305-701, Republic of Korea

a r t i c l e i n f o

Article history:Available online 26 December 2013

Keywords:Cryogenic liquidsAdhesive bridge jointPeel strengthGlass fiber reinforcement

a b s t r a c t

Cryogenic containment systems for LH2 (Liquid Hydrogen) and LNG (Liquefied Natural Gas) generallyrequire two barriers: the primary barrier made of stainless steel or Invar which contacts directly the cryo-genic liquids, the secondary barrier made of composite or metal foil which functions as the faces of sand-wich constructions for thermal insulation.

The faces of sandwich constructions are bridged with metal strips which are usually adhesively bondedfor gas tightness. When cryogenic liquids are loaded in the containment system, the adhesive bridgejoints are subjected to both the thermal load and deformation due to the shrinkage of the sandwich con-structions.

Since the maximum deformation in the sandwich construction systems may occur in the metal bridgebetween the two sandwich constructions, the cryogenic performances of adhesive bridge joint with leveldifference, which would be generated due to manufacturing tolerance, were evaluated using both thenew appropriate test method and FE analysis.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

CCS (Cryogenic Containment Systems) for LH2 (Liquid Hydro-gen) and LNG (Liquefied Natural Gas) generally require two barri-ers: the primary barrier made of stainless steel or INVAR steelwhich contacts directly the cryogenic liquids, the secondary barriermade of composite or metal foil which functions as the faces ofsandwich constructions for thermal insulation as shown in Fig. 1.

The primary barrier, which contacts the cryogenic liquids, ismade of welded stainless steel (304L) or INVAR steel plates be-cause these materials have face-centered cubic crystal structuresthat do not exhibit a nil ductility temperature.

For the high thermal insulation of the CCS, two insulated sand-wich constructions (upper and lower constructions) composed ofplywood faces and foam cores of low thermal conductivity are gen-erally placed serially between the hull of the containment and theprimary barrier [1–5]. The faces of sandwich constructions arebridged with metal strips which are usually adhesively bondedfor gas tightness [6]. When cryogenic liquids are loaded in the con-tainment system, the adhesive bridge joints are subjected to boththe thermal load and deformation due to the shrinkage of the sand-wich constructions.

The adhesive bridge joints of the secondary barrier are con-structed by the specially designed hot pad system as shown inFig. 2. The hot pad system is composed of a thin stainless steel foilheater and butyl rubber air pressure bag to give a uniform pressureand heat on the adhesive joint of the secondary barrier as shown inFig. 2(a). The upper part of the hot pad was fabricated with thecomposite sandwich construction composed of a glass fiber epoxycomposite face for its light weight and a foam core for its low ther-mal conductivity, as shown in Fig. 2(a) [7]. After the film typeepoxy adhesive was backed-up with the secondary barrier of stain-less steel strip, it is placed on the aluminum face of the sandwichconstruction, which is another part of the secondary barrier, asshown in Fig. 2(b). The hot pad system is mounted on the second-ary barrier and film type epoxy adhesive and fixed with settingbars by tightening bolts into the inserts which were mounted inthe plywood of the sandwich constructions. The adhesive is curedunder the uniform pressure of the hot pad system as shown inFig. 2(c) [7].

The adhesive joints of the secondary barrier have different lev-els due to the non-uniform flatness of the hull structure of CCS aswell as the manufacturing tolerances of sandwich constructions.The level difference condition of maximum 3 mm between thesandwich constructions of the secondary barriers is reported tobe the harshest condition for the adhesive joints, which would gen-erate a peel stress at the edge of the lower sandwich constructionsdue to the shrinkage in the opposite direction when the cryogenic

Fig. 1. Schematic diagram of the CCS (Cryogenic Containment System): (a) thermal gradient and thermal shrinkage of the components for CCS, (b) magnified sectional view ofthe secondary barrier with the level difference.

2 S.H. Yoon et al. / Composite Structures 111 (2014) 1–12

liquids are loaded in the CCS as shown in Figs. 1 and 3 [8]. Since thebridge part is not supported during the curing operation of adhe-sive, it would be bent by the curing pressure and the adhesivethickness would change as shown in Fig. 1, which makes it indis-pensable to evaluate the performances of adhesive bridge jointwith respect to curing pressure imposing appropriate boundaryconditions.

Most researches have been focused on the performance of theadhesive joints at the room or higher temperatures, and only fewinvestigations have studied adhesive joints at cryogenic tempera-tures [9–12]. Therefore, in this work, to verify the adhesion perfor-mance of the adhesive bridge joints with level difference withrespect to curing pressure, a new peel test method considering le-vel differences of the secondary barrier was used [8]. Also, themaximum peel stresses were numerically calculated with FE (Fi-nite Element) analysis under thermal and tensile loading conditionto verify the effects of the curing pressure on the adhesion perfor-mance of the adhesive joints with the level difference.

2. Evaluation method for the secondary barrier with the leveldifference

Although there are several peel test standards in the ASTM(American Society for Testing and Materials) standards for theadhesive joint, no appropriate test standard exists for the adhesivebridge joint specimens with the level difference such as the sec-ondary barrier composed of the sandwich constructions as shownin Fig. 3 [13–17]. Recently, in order to verify the bonding perfor-mance of the secondary barrier with the level difference, a specialtest method was developed by Yoon et al. as shown in Fig. 4 [8].

The peel test specimens have been prepared with the level dif-ference of 3 mm, which is the harshest condition in the practicalbuilding process of CCS. Since the adhesive bridge joint of

secondary barrier at the cryogenic temperature was subjected totensile deformation, the test specimen was designed with the dou-ble lap joint type to give a pure tensile deformation without bend-ing stress on the adhesive layer. Fig. 4 shows the drawing of thepeel test specimen with the level difference of 3 mm. The thick-nesses of the right side and left side aluminum adherends were16 and 22 mm, respectively, which generates 3 mm level differ-ence between the adhesive bonds. The width of the specimenwas 50 mm, and the total bonding length was 70 mm which isthe similar bonding length of the secondary barrier of CCS [8].

3. Experiments

3.1. Specimen preparation

Fig. 4(a) shows the dimensions of the peel test specimen tomeasure the peel load with respect to the curing pressure at thecryogenic temperature of �150 �C. The film type epoxy adhesive(FM8210-M, Cytec, USA) reinforced with 2 plies of plain weavetype glass fabrics (1080-HET, Hankuk Fiber, Korea) was used forthe adhesive of the peel test specimen. The properties of the spec-imen components are listed in Table 1. All the surfaces of the stain-less steel and aluminum adherends were treated with the flameand silane treatment methods which were the same surface treat-ment methods for the secondary barrier of the CCS [3,18,19].

For flame surface treatments of the aluminum and stainlesssteel, the flame burners of propane (C3H8) gas were used to elimi-nate the surface contaminants such as absorbed machine oil, lubri-cants and debris on the metal foil surfaces. The flame treatmentwas performed under the pressure of 0.025 MPa and treatmentdistance of 50 mm with the feeding speed of 0.030–0.035 m/s,which generated the flame temperature of about 900 �C [18]. Afterthe flame treatment, the aluminum and stainless steel sheets were

Insert

Inner hull of LNG shipInsulation board

(a)

Film type adhesive

(b)

Secondary barrier strip

Inner hull of LNG shipEpoxy adhesive

Bolt Insulation foam Setting bar Hot pad

Secondary barrier

Air inlet

Foil heater Teflon film Butyl rubber air bag Glass wool

Velcro

Sandwich panel with foam core

(c) Fig. 2. Construction method of the secondary barrier: (a) hot pad system for the secondary barrier, (b) attaching process, and (c) installation of the hot pad using setting bars.

Thermal deformation

Peel stressStainless steel

Film adhesive

Aluminum

Insulation board

Stainless steel

Film adhesive

Aluminum

Insulation board

30 mm

3 mm

Fig. 3. Generation of the peel stress due to the level difference condition of theinsulation boards during the loading of LNG.

S.H. Yoon et al. / Composite Structures 111 (2014) 1–12 3

treated using a silane coupling agent, c-glycidoxypropyltrimethox-ysilane (c-GPS) solution (KBM-403, Shinetsu, Japan) to coat theirsurfaces. The acidic solution of pH 5 was prepared by mixing dis-tilled water and acetic acid (CH3COOH). Then, the silane couplingagent of 1.0 wt% was prepared by mixing the agent into the acidicsolution using a magnetic stirrer at the rotating speed of 500 rpmfor 60 min. The aluminum and stainless steel were immersed inthe hydrolyzed silane solution for 10 min at 25 �C, which saturatedthe adsorption of the silane solution onto the aluminum and stain-less steel surfaces. Finally, the silane treated aluminum and stain-less steel were dried at 110 �C for 60 min in an oven to promote thecondensation of silanols to siloxanes [3,18,19].

Fig. 5 shows the fabricating process of the peel test specimen,where the two aluminum adherends of 16 and 22 mm thick-nesses were placed with 30 mm gap. The stainless steel foil of0.2 mm thickness was attached with the film type epoxy adhesive(FM8210-M, Cytec, USA) reinforced with 2 plies of plain weavetype glass fabric (1080-HET, Hankuk Fiber, Korea) as shown inFig. 5(a). Two Teflon tapes were placed on the sides of the alumi-num adherends to eliminate the effects of adhesive fillets asshown in Fig. 5(a). The specimen was cured using a vacuumbag degassing curing method at 90 �C for 4 h under the vacuum

(a)

(b)

AluminumadherendStainless steel

70

16

50

30

50 70

120 120

22

Aluminumadherend Film adhesive

Fig. 4. Drawing and photograph of the peel test specimen with the level difference of 3 mm: (a) drawing (Unit: mm), (b) photograph (width = 50 mm).

Table 1Mechanical properties of the components of the peel test specimen.

Material Test temperature(�C)

Tensile modulus(GPa)

Shear modulus (GPa) Coefficient of the thermal expansion(�10�6 m/m �C)

Poisson’sratio

Aluminum – 69 26 24 0.3Stainless steel (SUS304-O) – 193 74 17 0.3

Film adhesive (FM8210-M + 2 plies of 1080-HETa) 25 Ex = 7.10 Gxy = 1.72 ax = 36 mxy = 0.24Ey = 5.50 Gxz = 1.23 ay = 51 mxz = 0.35Ez = 2.40 Gyz = 1.24 az = 53 myz = 0.12

�150 Ex = 10.60 Gxy = 3.14 ax = 8 mxy = 0.32Ey = 8.70 Gxz = 2.32 ay = 15 mxz = 0.35Ez = 6.10 Gyz = 2.18 az = 23 myz = 0.19

a 1080-HET: Plain weave type E-glass fabric.

Vacuum bag

AluminumadherendStainless steel

Aluminumadherend Film adhesive

(a)

Teflon tape

AluminumadherendStainless steel

Aluminumadherend Film adhesive

(b)Fig. 5. Fabricating processes for the peel test specimen depicting the secondary barrier with the level difference: (a) attaching the strip of stainless steel with film adhesive,(b) curing the strip using the vacuum bag molding method.

4 S.H. Yoon et al. / Composite Structures 111 (2014) 1–12

pressures of 0.025, 0.050, 0.075 and 0.10 MPa to give uniformpressures on the specimens as like the newly developed hotpad system as shown in Fig. 5(b). After curing the adhesive,adhesive burrs were removed by abrading them with 120 and

2000 mesh sandpapers to make fine surfaces. Since the peel stressin the adhesive layer is dependent on the peeling angle, thespecimens have been fabricated with the precise control ofspecimen dimensions [20].

Fig. 6. Maximum depth of the peel test specimen with respect to curing pressure: (a) curing pressure of 0.025 MPa, (b) curing pressure of 0.050 MPa, (c) curing pressure of0.075 MPa, (d) curing pressure of 0.10 MPa.

Stainless steel

Film adhesive

Magnified part

(a) (b)

0.5mm

(c) (d)

0.5mm0.5mm

0.5mm

Fig. 7. Magnified view of the lower part at the edge of the peel test specimen: (a) curing pressure of 0.025 MPa, (b) curing pressure of 0.050 MPa, (c) curing pressure of0.075 MPa, and (d) curing pressure of 0.10 MPa.

S.H. Yoon et al. / Composite Structures 111 (2014) 1–12 5

(a)

(b)

Fan

Material testing system

Liquid nitrogen

tank

Insulation chamber

Controller

Specimen

Fig. 8. Experimental setup for the peel test: (a) photograph, (b) schematic diagram.

6 S.H. Yoon et al. / Composite Structures 111 (2014) 1–12

Since the stainless steel bridge strip between the two aluminumadherends was not supported during the curing operation, it be-came curved by the curing pressure as shown in Fig. 6. As expected,

(a)

(b)

(d) Fig. 9. FE analysis model of the peel test specimen with the level difference of 3 mm: (a) 20.025 MPa, (c) curing pressure of 0.050 MPa, (d) curing pressure of 0.075 MPa, and (e) c

the maximum depth and the angle of edge of the stainless steelbridge strip from the horizontal axis of the stainless steel increasedas the curing pressure was increased as shown in Figs. 6 and 7.Also, the adhesive thickness around the edge of the aluminumadherend decreased as the curing pressure was increased by thestress increase due to stress concentration as shown in Fig. 7.

3.2. Test method

The tensile tests of the peel test specimens were performed atthe cryogenic temperature of �150 �C which was generally harshercondition than the temperature of the secondary barrier of CCS be-cause the temperature of secondary barrier should be maintainedhigher than about �100 �C to avoid nil ductility temperature(NDT) of steel hull of CCS. The test rate of 1.0 mm/min was appliedby the material testing system (INSTRON 4206, Instron, USA) thatwas equipped with an insulation chamber as shown in Fig. 8.Liquid nitrogen of �196 �C was circulated by a fan in order tomaintain a uniform target temperature of �150 �C. As the temper-ature reached the cryogenic temperature of �150 �C, thermalstresses were generated by the contraction of the jigs and the testspecimen, which was removed by adjusting the grip positions ofspecimens.

4. Finite element analysis models and boundary conditions

The effects of the curing pressure on the level difference partwere numerically analyzed with the commercial software (ABA-QUS 6.9, SIMULIA, USA). In this study, the deformation behaviorsand changes in local stresses of the adhesive layer were investi-gated with respect to the applied load and curing pressure.Fig. 9(a) shows the FE analysis model according to the dimensionsof the peel test specimen with the level difference of 3 mm. Twoaluminum adherends of 16 and 22 mm thicknesses were placedwith 30 mm gap. The curved stainless steel bridge parts of second-ary barriers between the aluminum adherends were modeled

(c)

(e)D full model of specimen with the level difference (Unit: mm), (b) curing pressure ofuring pressure of 0.10 MPa.

Fig. 10. FE analysis model of the peel test specimen with the level difference of 3 mm: (a) curing pressure of 0.025 MPa, (b) curing pressure of 0.050 MPa, (c) curing pressureof 0.075 MPa, and (d) curing pressure of 0.10 MPa.

(a) (b)

(c) (d)Fig. 11. Adhesive thickness and mesh size of the FE analysis model: (a) curing pressure of 0.025 MPa, (b) curing pressure of 0.050 MPa, (c) curing pressure of 0.075 MPa, and(d) curing pressure of 0.10 MPa.

S.H. Yoon et al. / Composite Structures 111 (2014) 1–12 7

0

200

400

600

800

1000

1200

1400

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Stre

ss (M

Pa)

Strain (m/m)

FEMTrueEngineering

Fig. 12. Mechanical properties of the stainless steel 304-O for the FE analysis.

0

2

4

6

8

10

12

14

16

18

0 0.5 1 1.5 2 2.5 3 3.5 4

Displacement (mm)

Loa

d (k

N)

: Crack initiation

0.025 MPa

0.050 MPa

0.075 MPa

0.100 MPa

(a)

0

2

4

6

8

10

12

14

16

0.025 0.050 0.075 0.100

Curing pressure (MPa)

Peel

load

(kN

)

(b)Fig. 13. Tensile test results of the peel test specimen at the cryogenic temperatureof �150 �C with respect to curing pressure: (a) load–displacement curves, (b)measured maximum peel loads.

8 S.H. Yoon et al. / Composite Structures 111 (2014) 1–12

according to the test specimens as shown in Fig. 9(b–e). The initialangles of the stainless steel foil from the horizontal axis were mod-eled like the test specimens as shown in Fig. 10. Also, the adhesivethickness was modeled using the thickness of the test specimen asshown in Fig. 11. The 2D full model was used to consider the50 mm width of the level difference specimen with the CPE4RT(4-node plane strain thermally coupled quadrilateral, reduced inte-gration) elements of ABAQUS 6.9. To characterize the deformationbehaviors, the profiles of the deformation of the 30 mm gap andthe stress changes of the adhesive layer on the edge of the alumi-num adherend were calculated with respect to the magnitude ofapplied loads and compared with the experiment results. The meshsize of the adhesive layer of 0.025 mm (Length) � 0.005 mm(Thickness) was used to compare the local stress on the adhesivelayer around the aluminum adherend as shown in Fig. 11.

Table 1 lists the mechanical properties of the componentswhich were used in the FE analysis. In the cases of the stainlesssteel 304-O, the true stress–strain relationships were fit using amulti-linear strain hardening approximation, as shown in Fig. 12,to simulate the elasto-plastic behavior for the FE analysis.

The FE analysis was performed according to boundary condi-tions of the test environments as listed in Table 2. Firstly, the tem-perature of the specimen was decreased 90 �C to �150 �C toconsider the effects of the curing temperature of the specimen.Then the axial tensile load was applied on the FE analysis modelof the peel test specimen as shown in Fig. 9(a).

5. Test and analysis results

5.1. Test results and discussions

Fig. 13(a) shows the load–displacement curve of the peel testspecimen with the level difference of 3 mm at the cryogenic tem-perature of �150 �C with respect to the curing pressure. Althoughthe adhesion performance normally increased as the curingpressure was increased, the maximum peel load of the test speci-

Table 2Processes for the FE analysis of the peel test specimen.

Step 1 Step 2

Analysis condition 90 �C ? �150 �C Apply tensile load of 2 kN

men with the level difference decreased when the curing pressurewas larger than 0.050 MPa as shown in Fig. 13(b). The peel load ofthe test specimen bonded under the curing pressure of 0.050 MPawas 37% higher than that of the test specimen bonded under thecuring pressure of 0.075 MPa as shown in Fig. 13(b).

The peel test specimen with the level difference of 3 mm failedwith the partial cohesive failure mode as shown in Fig. 14(a). Thecracks initiated in the adhesive around the edge of the aluminumadherend of the peel test specimen, which was the stress concen-tration point, and propagated along the surface of the stainlesssteel for all the test specimens as shown in Fig. 14(a). The stainlesssteel and adhesive of the peel test specimen between the alumi-num adherends which were curved, became straightened as shownin Fig. 14(b) when a tensile load was applied to the specimen asshown in Fig. 14(c), which generated a concentrated tensile stressin the adhesive at around the edge of the aluminum adherend dueto the bending motion of the stainless steel and tensile load.

Therefore, it might be concluded that the initial angle increaseof the stainless steel strip from the horizontal axis and the adhesive

Step 3 Step 4

Apply tensile load of 10 kN Apply tensile load of 13 kN

(a)

Stainless steel

Aluminum

Adhesive

l

(b)

Stainless steel

Adhesive

Crack

Aluminum

l l

(c)l: Initial length of the gap (mm)l: Deformation of the gap when the stainless steel was stretched (mm)

Fig. 14. Failure behavior of the peel test specimen with the level difference of 3 mm:(a) failure mode of the peel test specimen at the cryogenic temperature of�150 �C, (b)deformation of the bent stainless steel strip and adhesive under the tensile load of thestainless steel, (c) crack propagation trend of the peel test specimen.

(a)

(b)Fig. 15. Measuring and calculating parts for the load–displacement curves: (a) FEanalysis, (b) experiment.

0

2

4

6

8

10

12

14

16

18

0 0.5 1 1.5 2 2.5 3 3.5

25oC-150oC

Loa

d (k

N)

Displacement (mm)

Fig. 16. Typical load–displacement curves of the peel test specimen with the leveldifference of 3 mm, fabricated under the curing pressure of 0.025 MPa at the room(25 �C) and cryogenic (�150 �C) temperatures using the material testing system.

0

2

4

6

8

10

12

0 0.5 1 1.5 2 2.5 3 3.5 4

Loa

d (k

N)

Displacement (mm)

ExperimentFE analysis

0.025 MPa 0.050 MPa 0.075 MPa 0.100 MPa

Fig. 17. Measured and calculated load–displacement curves of the specimen withthe level difference of 3 mm using the extensometer and the material testingsystem.

S.H. Yoon et al. / Composite Structures 111 (2014) 1–12 9

thickness decrease at around the edge of the aluminum adherenddecreased the performance of the adhesive bridge joints with thelevel difference at the cryogenic temperature.

5.2. Deformation behavior of the peel specimen with a level difference

To verify the deformation behaviors of the peel test specimenswith the level difference of 3 mm, the deformations of the30 mm bridge part of the secondary barrier were calculated withrespect to the applied loads as shown in Fig. 15(a), and comparedwith the measured results under the tensile loading conditionusing a material testing system (INSTRON 4469, Instron, USA)and an extensometer (Extensometer 2630-107, Instron, USA) atthe room temperature of 25 �C as shown in Fig. 15(b) becausethe tensile load–displacement behaviors before failure were al-most same at the room (25 �C) and cryogenic (�150 �C) tempera-tures when the displacements were measured using the materialtesting system as shown in Fig. 16.

The load–displacement curves which were calculated using theFE analysis method with a multi-linear strain hardening approxi-mation of the stainless steel well matched to those of the measuredusing the extensometer and material testing system as shown inFig. 17, where the stainless steel strips yielded at around 8.0 kNdue to the tensile deformation. The initial stretching point ofthe specimen with the curvature was estimated using theload–displacement curve of the calculated results as shown inFig. 17, where the initial nonlinear curve represented the flatteningregion of the specimen with the initial curvature. All the specimenshad linear regions at around the applied load of 2.0 kN as shown in

(a) (b)

(c) (d)Fig. 18. FE analysis results of the peel test specimen with the level difference of 3 mm when the applied load was 2 kN: (a) curing pressure of 0.025 MPa, (b) curing pressureof 0.050 MPa, (c) curing pressure of 0.075 MPa, and (d) curing pressure of 0.10 MPa.

10 S.H. Yoon et al. / Composite Structures 111 (2014) 1–12

Fig. 17, where the specimens have been flattened as explained inFig. 18. The stretching displacement increased as the curing pres-sure was increased because the length of the curved part of thestainless steel increased as the curing pressure was increased asshown in Fig. 18. Since the stainless steel bridge strip was sub-jected to tension after stretching of the curved stainless steel bythe tensile load, the actual peel angle of the peel test specimen de-creased as the curing pressure was increased.

5.3. Local stress in the adhesive layer of the peel test specimen with alevel difference

Although the local stress in the adhesive layer might not be pre-dicted accurately when the specimen had singularity points in theFE analysis model, the FE analysis was used to investigate the effectsof the curing pressure on the sandwich construction parts with thelevel difference because the calculated stress at the singularitypoints might be compared when the mesh sizes on the singularitypoints were same. The local tensile stresses, r11, were calculatedthrough the interface between the adhesive and aluminum facewith respect to curing pressure according to the peel test specimensas shown in Fig. 19, because the first crack on the adhesive layer wasgenerated due to the axial tensile load as shown in Fig. 14. At thetensile load of 2.0 kN which was the stretching point of the peel testspecimen with the level difference of 3 mm, the local tensile stress,r11, increased as the curing pressure of the specimen was increasedas shown in Fig. 19. Because the decrease of adhesive thickness atthe singularity point increased the stress concentration, the in-crease of the initial angle of the stainless steel strip from the hori-zontal axis increased the bending deformation of the adhesivelayer as shown in Fig. 14. However, under the tensile load of10.0 kN, which was enough to fail the peel test specimens bondedboth under the curing pressures of 0.075 and 0.10 MPa, the tensilestress in the adhesive layer of the peel test specimen bonded underthe curing pressure of 0.075 MPa was a little higher (5%) than that ofthe peel test specimen bonded under the curing pressure of

0.10 MPa. Also, at the tensile load of 13.0 kN, which was more thanenough to fail the peel test specimens bonded both under the curingpressures of 0.025 and 0.050 MPa, the tensile stress in the adhesivelayer of the peel test specimen bonded under 0.025 MPa wasslightly higher (2%) than that of the 0.050 MPa. Since the axial ten-sile strain was subjected to the stainless strip after stretching thecurved stainless steel bridge strip, the actual peel angle of the peeltest specimen decreased as the curing pressure was increased dueto the increase of length of the curved part of the stainless steel strip.Therefore, it might be concluded that the length of the curved stain-less steel might affect slightly the peel load.

From the FE analysis, it was found that the increase of curingpressure on the stainless steel bridge strip with the level differenceof 3 mm had both the positive effect (increase of the length of thestainless steel strip) and the negative effect (increases of the initialangle and adhesive thickness around the edge of the aluminumadherend).

6. Conclusion

In this work, the effects of the curing pressure on the stainlesssteel strip (0.2 mm thickness) adhesive bridge joint of sandwichconstructions for thermal insulation were investigated using a spe-cially devised test specimen and FE analysis method at the cryo-genic temperature of �150 �C when the level difference of was3 mm with the gap of 30 mm between the sandwich constructions,which were the harshest conditions during the construction ofcryogenic containment systems.

Although the adhesion strength normally increased as the cur-ing pressure was increased, the measured maximum peel load ofthe specimen with the level difference decreased when the curingpressure was larger than 0.050 MPa. The peel load of the test spec-imen bonded under the curing pressure of 0.050 MPa was 37%higher than that of the test specimen bonded under the curingpressure of 0.075 MPa.

0

500

1000

1500

2000

2500

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

0.10 MPa0.075 MPa0.050 MPa0.025 MPa

Stre

ss,

11(M

Pa)

X (mm)

0

500

1000

1500

2000

2500

Stre

ss,

11(M

Pa)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

X (mm)

0.10 MPa0.075 MPa0.050 MPa0.025 MPa

(a) (b)

0

500

1000

1500

2000

2500

Stre

ss,

11(M

Pa)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

X (mm)

0.10 MPa0.075 MPa0.050 MPa0.025 MPa

(c)Fig. 19. Calculated local tensile stress, r11, in the adhesive layer with respect to the curing pressure of the peel test specimen with the level difference of 3 mm: (a) at tensileload of 2.0 kN, (b) at tensile load of 10.0 kN, (c) at tensile load of 13.0 kN.

S.H. Yoon et al. / Composite Structures 111 (2014) 1–12 11

From the FE analysis results, the local stress in the adhesivelayer around the edge of the aluminum adherend increased asthe curing pressure of the peel test specimen was increased whenthe axial tensile load was 2.0 kN, at which the peel test specimenstarted to stretch because the bending deformation of the stainlesssteel bridge strip and the stress concentration were dominant fac-tors for the performance. At the tensile load of 10.0 kN, which wasthe enough to fail the peel test specimens bonded both under thecuring pressures of 0.075 and 0.10 MPa, the tensile stress in theadhesive layer of the peel test specimen bonded under 0.075 MPawas a little higher (5%) than that of the peel test specimen bondedcured under the pressure of 0.10 MPa. Also, at the tensile load of13.0 kN, which was more than enough to fail the peel test speci-mens bonded both under the curing pressures of 0.025 and0.050 MPa, the tensile stress of the adhesive layer of the peel test

specimen bonded under the curing pressure of 0.025 MPa wasslightly higher (2%) than that of the peel test specimen bonded un-der the curing pressure of 0.050 MPa. Since the axial tensile strainwas subjected to the stainless strip after stretching the curvedstainless steel bridge strip, the actual peel angle of the peel testspecimen decreased as the curing pressure was increased due tothe increase of length of the curved part of the stainless steel strip.Therefore, it might be concluded that the length of the curvedstainless steel might affect slightly the peel load.

From the calculated with FEM and the experimental results forthe peel test specimen with the level difference of 3 mm, it has beenconcluded that the high curing pressure cannot guarantee the highadhesion performance due to the bending of the stainless steelbridge foil between the sandwich constructions and the stress con-centration in the adhesive layer under the high curing pressures.

12 S.H. Yoon et al. / Composite Structures 111 (2014) 1–12

Acknowledgements

This research was supported by WCU (World Class University)program through the National Research Foundation of Koreafunded by the Ministry of Education, Science and Technology(R31-2008-000-10045-0) and BK21.

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