Damage Analysis of a Type 3 Cryogenic Propellant Tank...

Damage Analysis of a Type 3 CryogenicPropellant Tank After LN2 Storage Test

SANG-GUK KANG, MYUNG-GON KIM, SANG-WUK PARK

AND CHUN-GON KIM*Division of Aerospace Engineering, Korea Advanced Institute of Science and

Technology 373-1, Kuseong-dong, Yuseong-gu, Daejeon, 305-701, Republic of Korea

CHEOL-WON KONG

Department of Structures and Materials, Korea Aerospace Research Institute

45 Eoeun-dong, Yuseong-gu, Daejeon, 305-333, Republic of Korea

ABSTRACT: The application of composites to cryotanks has been one of the majorconcerns for lightweight launch vehicles. In this study, a prototype of a Type 3cryotank was fabricated with the composite developed for cryogenic application andaluminum liner, and the cryogenic conditions were applied by filling the prototypewith liquid nitrogen and then pressurizing it with gaseous nitrogen. During theexperiment, delamination inside the cryotank happened. This article describes severalattempts made to investigate failure through both analytical approach with thermo-elastic analysis accompanied by progressive failure and experimental approach withLN2 immersion of composite/aluminum ring specimens.

KEY WORDS: cryogenic environment, type 3 tank, liquid nitrogen storage, thermoelastic analysis, progressive failure, delamination.

INTRODUCTION

NOWADAYS, SPACE TECHNOLOGY is important enough to determine the strength of acountry, so active researches have focused on reusable launch vehicles (RLV) in each

country which plays a key roles in this area. Liquid propellants have been adopted inrecent launch vehicles because they have higher specific impulse than solid ones and aresuitable for reusability. Metal alloys have been used as materials for storage systems inorder to have higher factors of safety, which makes the launch vehicles a lot heavier [1].Therefore, studies on lightweight propellant tanks have interested many researchers.For this purpose, composite materials which have high specific strength and stiffness andlow coefficients of thermal expansion (CTEs) have recently been investigated for theirapplicability to the material systems of propellant tanks [1–17].

*Author to whom correspondence should be addressed. E-mail: [email protected] 1–5, 7, 9, 10, 12–14 and 16 appear in color online: http://jcm.sagepub.com

Journal of COMPOSITE MATERIALS, Vol. 42, No. 10/2008 975

0021-9983/08/10 0975–18 $10.00/0 DOI: 10.1177/0021998308088619� SAGE Publications 2008

Los Angeles, London, New Delhi and Singapore

Under cryogenic environment, different CTEs between reinforcing fibers and matrixcause extreme thermal stress, which leads eventually to microcracks in matrix [14–17].As the microcrack density increases, they can be propagated in thickness direction andemerge as paths for leakage as well as mechanical degradation of the tank structure.Therefore, a Type 3 composite tank, which is lined with metal for the purpose ofmaintaining permeability, is generally preferred to a Type 4 tank fabricated only withcomposite materials. Considerable weight reduction of Type 4 tanks, however, is still ofinterest to many researchers through the addition of toughening materials like CNTs,elastomers, and so on, although some failures occur as shown in the case of X-33.

In previous researches, authors have been trying to characterize cryogenic behaviors ofcomposites and adhesives for cryotank application [18–20], from which the resinconstitution with suitable cryogenic characteristics, Type B, could be obtained. It couldbe concluded that it showed 7% higher strength at cryogenic temperatures than a baselinematerial [18]. In addition, the cryogenic performance of adhesive between composite andaluminum was influenced by means of double-lap shear tests for Type 3 application [19].Thermo-mechanical response of metal lined composite structures was investigated usingcomposite/aluminum ring specimens [20]. For the next step, a trial to fabricate and test aprototype of a Type 3 tank with the developed resin and aluminum liner was made becauseit was important to look into the problems with the developed composite during thestorage of cryogenic liquids and to investigate its feasibility. This is described in this article.The cryogenic conditions were applied by liquid nitrogen storage and consecutive gaseousnitrogen pressurization. Temperature distribution and strain field were predicted throughthermo elastic finite element analysis. During the test, however, the composite laminate ofthe cryotank delaminated. Several attempts were made to investigate this phenomenonthrough both an analytical approach with thermo-elastic analysis in consideration of theprogressive failure and experimental approach with a LN2 immersion test of composite/aluminum ring specimens which are known to be suitable for simulating a Type 3 tankstructure. It was shown that the main reason for the delamination was voids caused byevaporation of acetone during the fabrication procedure, and some complementarymeasures to establish the optimal curing cycle in filament winding process were suggested.

DESIGN AND FABRICATION OF THE PROTOTYPE

OF A TYPE 3 CRYOTANK

A prototype of a Type 3 cryotank was designed and fabricated to apply the developedcryogenic resin system to a cryotank structure. The dimension of the prototype was setconsiderably small compared to the actual cryotank for the purpose of easy comparison ofthe results with ring specimens as well as convenience of experiment. Ring specimens arediscussed in a later section. The resin system used was named as ‘Type B’, and containsmore bisphenol-A than phenol novolac in its resin composition. Carboxyl terminatedbutadiene acrylonitrile (CTBN) modified rubber was added for the purpose oftoughening the resin at cryogenic temperatures. Type B had been proven to be excellentunder cryogenic environment in a previous study [18]. T700 fiber was adopted forreinforcement.

It is generally known that hoop and helical layers of composite are wound onto a metalliner in the Type 3 tank structure. In this study, aluminum liner, which was of isotensoiddome shape, was manufactured through welding two symmetric pieces at the middle of the

976 S.-G. KANG ET AL.

cylinder as shown in Figure 1. Close examination of the welding part with X-ray showedno defect in it. The cylinder part of the liner is 150mm in diameter and 140mm in length,and the boss part is 50mm in diameter. The thickness and the total length of the liner are4mm and 300mm, respectively.

Design parameters are winding angle and thickness of the composite. In the designprocess of semi-geodesic dome, winding angle can be obtained once radii of cylinder andboss are determined. Equation (1) shows a semi-geodesic path equation, where �, x, r, andl are winding angle, axial coordinate, radial coordinate, and slippage tendency betweenfiber and mandrel, respectively [21]. In addition, r0 and r00 mean dr/dx and d2r=dx2,respectively. After integration of Equation (1), winding angle (�) can be determined for theentire surface of the liner:

d�

dx¼

lðA2 sin2 �� rr00 cos2 �Þ � r0A2 sin �

rA2 cos�ðA ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffi1þ r02

pÞ: ð1Þ

From the winding angle, thickness can also be calculated from Equation (2) [22]:

t ¼rc cos�c

r cos�� tc ð2Þ

where, r, �, and t are radius, winding angle, and thickness, respectively. Subscript ‘c’ meansthe cylinder part. In this equation, it was assumed that the fiber volume fraction was fixedand the number of fibers in a cross-section was uniform. Using in-house code, the helicalwinding angle in the cylinder part was determined to be 19.58 to the axis of the tank. Totalnumbers of hoop and helical layers were set to have sufficiently high burst pressure takingprecautions against evaporation of liquid nitrogen even though pressure is not the mainconcern due to very low pressure of liquid nitrogen and low test pressure (1.7MPa).Final hoop and helical layers were set to be 1.2mm in thickness in the cylinder part. In thedome part, the thickness of helical layers increase as expected in Equation (2).

The different steps in fabricating the cryotank are illustrated in Figure 2. At first, theadhesive film, Bondex606, was wrapped onto the aluminum liner. This adhesive waschosen for cryogenic use through the previous study [19]. Next, sequential helical winding

300

150

Figure 1. Aluminum liner of an isotensoid dome shape (unit: mm).

Damage Analysis of a Type 3 Cryogenic Propellant Tank 977

of two layers and hoop winging of four layers were done to prepare for curing. Bandwidthand winding tension were 10mm and 1.5 kgf, respectively. In this filament windingprocess, acetone was used to lower viscosity of the resin. Thus, properties that had beenobtained through autoclave in Reference [18] might not be reproduced exactly in thisstudy. After the winding of the composite was over, it was cured in the oven at 1308C for2 h, and the final prototype was obtained.

EXPERIMENTAL PROCEDURE

Electric strain gauges (ESGs) for cryogenic use and a K-type thermocouple were usedfor the measurement of strain and temperature on the cryotank surface. Half bridgecircuits using titanium silicate as a dummy gauge were made up to compensate for theapparent strains of active gauges on the cryotank surface. Titanium silicate was set onthe cryotank surface in order that it could experience similar temperature to that of thecryotank surface but the deformation of the cryotank could not be transferred to it.The strains and the temperature on the cryotank surface were obtained with regard to theelapsed time. The locations of ESGs are shown in Figure 3.

Schematic diagram for the experiment is shown in Figure 4. First, the cryotank washung from the ceiling using fixing devices and flanges attached to it. Next, the GN2 supplyline was connected through the upper boss of the cryotank, and the pressure of thecryotank was built up to 1.7MPa. While this pressure was held constant for 5min, it couldbe checked whether the leakage occured through the cryotank wall and the sealing of theconnecting parts or not. After disconnecting the GN2 supply line from the cryotank,the LN2 injection line was linked to the lower boss of the cryotank and started to fill the

<adhesive wrapping> <helical winding>

<completion of helical winding> <completion of hoop winding>

Figure 2. Fabrication procedure of the prototype of a Type 3 cryotank before curing.


CL

ES

G 2

ESG

3

ESG

4

T/C

CL

ES

G 1

20 40

27

40

Figure 3. Locations of ESGs and a thermocouple.

GN2pressurization

GN2 vent

LN2 injection

LN2 discharge

Figure 4. Schematic diagram for LN2 injection and GN2 pressurization.


cryotank to simulate the storage of a cryogenic propellant. As soon as the cryotank wasfilled up with LN2, the valve was shut to stop LN2 injection and the GN2 supply linewas connected again to increase the pressure inside the cryotank up to 1.7MPa. Thetemperature and the strains on the cryotank surface were obtained from LN2 injection toGN2 pressurization.

THERMO-ELASTIC ANALYSIS IN CONSIDERATION

OF THE PROGRESSIVE FAILURE

In this research, MSC/PATRAN was used not only as a preprocessor for modeling butas a postprocessor for displaying results. ABAQUS was used for heat transfer andelasticity analysis. DC3D20 and C3D20 were adopted for elastic and thermal analysesrespectively. These are three-dimensional cubic elements with 20 nodes. The total numberof nodes and elements used was 9145 and 1212, respectively. The finite element model andboundary conditions of the Type 3 cryotank and the coordinate system are shown inFigure 5. The origin is located at the point where the axis of the tank and the line startingfrom the junction part meet perpendicularly.

An uncoupled method was used for thermo-elastic analysis in this study. That is, a heattransfer analysis for the finite element model was done to get nodal temperatures, whichwere used as boundary conditions for the elastic analysis. This method is suitableespecially for steady-state analysis and reduced time and cost. In the heat transfer analysis,�1968C, which is the boiling point of liquid nitrogen, was applied to the inner surface ofthe tank and free convection condition by air of 208C to the outer surface. It is known that

Inner surface−196°C

20°C

+ AF pressure

Outer surface:

Free convection

YZ plane symmetric

Cyclic symmetric

Z

X

Figure 5. Finite element model and boundary conditions of the Type 3 cryotank.


convection coefficient of air is not fixed and it is difficult to obtain this exactly in thermalanalysis [23]. In this article, the convection coefficient was obtained as 150W/m2K throughmatching the surface temperature from finite element analysis with that obtained in theexperiment. Usually the convection coefficient is 20–200W/m2K [23]. This scheme is a typeof averaging method and thought to be suitable because winds, icing, etc., vary theconvection coefficient continuously during the test. As a result, temperature distributionthrough the thickness was obtained.

In the elastic analysis, the pressure was applied to the inner surface according to tests,while the temperature field was maintained as obtained from the heat transfer problem.For the convenience of the analysis for axisymmetric structures such as the cryotank in thisresearch, the cyclic symmetric condition was applied. In addition, the Y–Z planesymmetric condition was applied at the middle line of the cylinder.

On the other hand, for a more accurate analysis of the Type 3 cryotank, the progressivefailure analysis was considered usingmodifiedHashin’s failure theory, whichwas introducedby Chang [24]. In the progressive failure analysis, the stiffness reduction coefficient (SRC)was set to 0.1, which meant that the material properties of failed elements were degraded to10% of their originals according to the failure mode [25]. This was materialized through auser subroutine USDFLD in ABAQUS. Mechanical properties of T700/Type B andaluminum are fromReference [18] and adhesives fromReference [19]. Thermal conductivityand expansions are from References [23] and [26], respectively. As shown in the tables,material nonlinearity with respect to temperature was considered in the analysis. Materialproperties with respect to temperatures are summarized in Tables 1 and 2. Materialproperties at temperatures other than the designated ones in Tables 1 and 2 are interpolated.

RESULTS AND DISCUSSION

Strain and Temperature Response with Respect to the Elapsed Time

Since there was no pressure reduction inside the cryotank during the first pressurizationand holding, it was confirmed that no leakage through the cryotank wall occurred andconnecting parts were sealed tightly.

Table 1. Material properties of T700/Type B at low temperature.

Temperature (8C) 25 �50 �100 �150 �196

E1 (GPa) 143.6 – – 155.1 158.1E2 (GPa) 8.9 – – 11.9 12.7G12 (GPa) 4.5 – – 7.5 8.3�12 0.3 – – 0.3 0.3�1 (�"/8C) �1.2 �0.39 0.29 1.5 2.5�2 (�"/8C) 26 29 25 18 11X (MPa) 2930 – – 2930 2930Y (MPa) 54 – – 67 70S (MPa) 65 – – 132 150k1 (W/mK) 11.1 8.7 at �738C – 5.0

5.7 at �1738Ck2 (W/mK) 0.87 0.68 at �738C – 0.41

0.46 at �1738C


When LN2 injection started, the temperature inside the cryotank decreased abruptly andeventually the surface temperature reached �1108C in steady state as shown in Figure 6.To investigate the temperature distribution of the cryotank wall in steady state, a heattransfer analysis was done. As mentioned above, convection coefficient of air in theanalysis was determined in order that the surface temperature of the cryotank obtainedfrom the heat transfer analysis could be the same as that measured during the test. Theresult of the heat transfer analysis is shown in Figure 7. Since there was no insulation forthe cryotank, its wall operated at extremely low temperatures, especially the aluminumliner whose thermal conductivity was very high and showed almost the same temperatureas that of liquid nitrogen. The temperature field obtained in Figure 7 was used as nodalboundary conditions to perform elastic analysis. The result appears in Figure 8, whichshows the predicted fiber directional strain on the outmost surface of the cryotank thatstores LN2. The strains in the middle of the cylinder part and the junction part are about�2200 and �2700 me respectively.

Figure 9 shows the experimental strain results of the cryotank surface from the timewhen LN2 injection started. Most of the strains dropped sharply and abruptly jumped

0 10 20 30 40 50 60 70 80−140

−120

−100

−80

−60

−40

−20

0

20

Sur

face

tem

pera

ture

(°C

)

Time (min)

Figure 6. Surface temperature of the cryotank with respect to elapsed time.

Table 2. Material properties of Aluminum-6061 at low temperature.

Temperature (8C) 25 250 2100 2150 2196

E (GPa) 68.5 70.6 72.0 74.1 76.0� 0.33 0.33 0.33 0.33 0.33�yield (MPa) 294.6 313.5 321.3 350.5 377.0�al (�"/8C) 23 21 20 17 13k (W/mK) 237 237 at �738C – 317

302 at �1738C


after 5min, which meant some problem had happened inside the tank. It was thought thatdelamination occurred somewhere, and additional investigations were followed tofind the reason for this phenomenon. The cryotank was pressurized using GN2 50minafter the liquid nitrogen storage test had begun. This was reflected in the strain resultsin Figure 9.

−110°C on the surface

MSC.Patran 2003 20-Apr-06 23:00:05

Fringe:Default step, total time=1., Temperature (Nodal), Layer or section points, At SECTION_POINT_1

1.96+002

−7.14+001

−7.97+001

−8.80+001

−9.63+001

−1.05+002

−1.13+002

−1.21+002

−1.30+002

−1.38+002

−1.46+002

−1.54+002

−1.63+002

−1.71+002

−1.79+002

−1.88+002

−1.96+002

Default_Fringe:Max−7.14+001 @Nd 4026 Min−1.96+002 @Nd 3

Z

X

Figure 7. Temperature distribution of the Type 3 cryotank.

−60 −40 −20 0 20 40 60

−50

−1000

−1500

−2000

−2500

−3000

0

Str

ain

(µε)

x (mm)

ZX

Figure 8. Predicted fiber directional strain in the outermost surface of the cryotank storing LN2.


To investigate what happened in the cryotank, it was cut into a few slices in the cylinderpart. From microscopy, it could be observed that some interfaces between the hoop andhelical layers were detached locally (delamination) as shown in Figure 10. However, nodebonding between the composite and aluminum was found, which means strong adhesionof adhesives at cryogenic temperatures.

Investigation into the Cause of the Cryotank Damage

Both analytical and experimental approaches were used to investigate the reason for thedelamination that occurred in the cryotank. First, thermo-elastic analysis for the cryotankin consideration of the progressive failure was carried out to check stress components at

0 10 20 30 30 50 60 70 80

0

−500

−1000

−1500

−2000

Str

ain

(µε)

ESG1 ESG2 ESG3 ESG4

ESG1

ESG2

ESG3

ESG4

Time (min)

Figure 9. Strains on the Type 3 cryotank surface during LN2 storage.

VoidsHelical layer

Delamination

Hoop layer

Figure 10. Microscopy of the cross-section cut from the cylinder part of the cryotank.


the interface between hoop and helical layers that were directly related to thedelamination, and the failure criterion was evaluated. Next, composite/aluminum ringspecimens, which are suitable to simulate the Type 3 tank easily, were immersed in liquidnitrogen, and the behaviors were observed and compared to the cross-section of thecryotank. Microscopy at the interface was followed for the cases before and after the liquidnitrogen immersion test.

To begin with, thermo-elastic analysis for the cryotank with the progressive failure beingconsidered was performed to evaluate the interlaminar stress components (�xz, �yz, and �zz)and the results are shown in Figure 11. As shown in the figure, their maximum stresseswere �3.7, �3.4, and 17.6MPa, respectively. These values were used to calculate thefailure index at the interface between hoop and helical layers to evaluate whetherthe loading condition the cryotank had undergone was fatal enough to cause thedelamination.

In general, there are two methods to predict the delamination inside the compositestructure; the material mechanics approach and the fracture mechanics approach. In thisstudy, the tensor polynomial failure theory [27] for anisotropic materials, which wasproposed by Tsai and Wu, was adopted as the material mechanics approach. In thistheory, the failure surface in the stress space is described by the tensor polynomial asexpressed in Equation (3):

Fi�i þ Fij�i�j ¼ 1 ði, j ¼ 1, 2, . . . , 6Þ ð3Þ

where Fi and Fij are experimentally determined strength tensors of the second and fourthranks, respectively [28]. This criterion can judge only whether the structure failed or not,but cannot distinguish the progressive direction of failure. When this equation is applied toevaluate the delamination, it can be expressed as Equation (4) with an assumption that thedelamination occurs only by the interlaminar stress components (�xz, �yz, and �zz).

Fzz�2z þ Fuu�

2yz þ Ftt�

2xz þ Fz�z ¼ 1

Fzz ¼1

ZZ0 , Fuu ¼

1

S223

, Ftt ¼1

S213

, Fz ¼1

Z�

1

Z0 :

ð4Þ

Interlaminar strengths of T700/Type B which are used to determine the strengthcoefficients are as follows. Here, Z and Z0 represent the tensile and compressive strengthsin axis 3, that is, out-of-plane direction, respectively. S23 and S13 are shear strengths in the2–3 plane and in the 1–3 plane, respectively. On the other hand, since interlaminar tensilestrength is very hard to measure, it is generally replaced by the transverse strength (ST) ofthe composite. From the experimental results of Brewer and Lagace [29], it was confirmedthat S13 can be replaced by in-plane shear strength (S12). Besides, S23 and Z0 wereproportionally obtained from the literature [30]. Table 3 shows the interlaminar strengthcomponents used in the calculation of Equation (4).

Using the result of finite element analysis in Figure 11 and material data in Table 3, themaximum Tsai–Wu index in Equation (4) was calculated to be 0.182, which guaranteedthe safety of the structure. From the analytical approach above, it was thought thatthere would be another reason that caused the delamination in the cryotank.

For the experimental approach to understand the damage mechanism of the cryotank,composite/aluminum ring specimens were fabricated so that they could have the same


dimension and the winding pattern as those of the cylinder part of the cryotank.Reference [31] explains that ring specimens have almost the same mechanical propertieswith those of a full tank structure and can be easily used to predict its behaviors, althoughthey are not able to strictly represent the boundary conditions of a full tank.

The width of the ring specimens is 12.7mm in thickness. They were immersed in thevessel containing LN2 to predict the behavior of the Type 3 cryotank for the more extreme

−60 −50 −40 −30 −20 −10 0−4

−2

0

2

4(a)

t xz

(MP

a)

x (mm)

ZX

Max: −3.7 MPa

0−60 −50 −40 −30 −20 −10−4

−2

0

2

4(b)

t xz

(MP

a)

x (mm)

Max: −3.4 MPa

ZX

0

3

6

9

12

15

18(c)

s z (

MP

a)

0−60 −50 −40 −30 −20 −10

x (mm)

ZX

Max: 17.6 MPa

Figure 11. Interlaminar stress components at the interface between hoop and helical layers of theType 3 cryotank: (a) Interlaminar shear stress (�xz); (b) Interlaminar shear stress (�yz) and (c) Interlaminarnormal stress (�zz).


cryogenic case. As a result, the delamination occurred between hoop and helical layers asshown in Figure 12, which was in accordance with the local delaminations that wereobserved in the cross-section of the cryotank.

Therefore, microscopy was followed to check the status of the composite/aluminum ringspecimen both before and after the LN2 immersion test as shown in Figure 13. As shown inthe figure, even before the LN2 immersion test, many defects near the interface betweenhoop and helical layers such as voids and microcracks could be observed in manylocations, which showed the problem during the fabrication procedure. It is obvious thatmicrocracks propagated from these defects under cryogenic environment.

The reason why many defects were formed is that since Type B resin has relatively highviscosity, acetone was used to lower its viscosity to avoid the difficulty in impregnating theroving fiber. When the filament wound structures were cured in the oven with the curingcycle shown in Figure 14, acetone that had been mixed with the resin began to boil andevaporated, because it had a boiling point at 568C. It brought about the formation ofvoids inside the matrix after curing, which expanded to formation and propagation ofmicrocracks under cryogenic environment. This problem is especially critical to the regionnear interface between hoop and helical layers. This is thought to have been the mainreason for the delamination of the cryotank. Therefore, the optimal curing cycle in thefilament winding process could be established for Type B resin that had been developed forcryogenic use to have the uniform quality with little defect during the fabricationprocedure. For this purpose, the amount of acetone needed to lower the viscosity of the

Aluminum

Delamination

Hoop layer

Helical layer

Figure 12. Delamination between hoop and helical layers after LN2 immersion test.

Table 3. Interlaminar strength components of T700/Type B at low temperature.

Temperature (8C) 25 268 2150

Z (MPa) 54 59 67Z0 (MPa) 124 136 154S23 (MPa) 57 80 116S13 (MPa) 65 91 132


resin should be as low as possible and some measures to fully evaporate it are necessarythrough lowering the temperature increasing rate and keeping the temperature near theboiling point of acetone longer before the first holding of the total curing cycle. Thepossibility of autoclave curing should be examined through adding the pressure and

MicrocracksMicrocracksMicrocracks

(a)

(b)

Crack propagationCrack propagation

Voids

Crack propagation

Figure 13. Microscopy of the interface between hoop and helical layers of the composite/aluminum ringbefore (a) and after LN2 immersion test (b).

∆T=1~2°C/min

Tem

pera

ture

(°C

)

130

80

20

2 hours

0.5 hour

56 Acetone boiling

Figure 14. Curing cycle of the prototype of the cryotank.


forming a vacuum inside the composites during the curing [32–34]. Another method is torevise the winding machine to have a device that is able to dry acetone when the rovingfiber passes by the final groove of the machine. Through this method, it is thought thatmost acetone inside the resin could be removed before curing.

Finally, the strains appearing on ESGs were predicted for both the case before thedelamination and the one after it respectively, which were compared to the experimentalresult shown in Figure 15. The strains before the delamination were the same as beforeand the ones after the delamination were predicted through subtracting the ones beforethe delamination from the thermal strain of the composite only, which was obtainedfrom the ESG attached on the composite ring specimen whose inner and outer diameterwere the same as those of the cryotank. Its thermal strain was �55�" from RT to thecryotank surface temperature, �1108C. ESG1 and ESG2 in Figure 15 indicate the strainsafter delamination during the test, and it could be predicted that about 40% of the perfectdelamination had happened inside the cryotank wall.

The stress distribution of the cryotank after the delamination between the hoop andhelical layers was obtained to investigate its safety by means of thermo-elastic analysis.For this purpose, some assumptions were made as follows; first, temperature distributionwas constant after delamination, second, the composite hoop layer could not carry theload any more, and finally, the perfect delamination happened between hoop and helicallayers without any bonding region left. In the analysis, a pressure of 1.7MPa was appliedas in the case of the cryotank experiment.

Figure 16 shows the fiber stress in the outmost surface before and after thedelamination. The outmost surface is the hoop layer in the cases before delaminationand the helical layer after delamination. Before delamination, some discontinuityof fiber stress in the outmost surface was observed near the junction where thecylinder and the dome meet because of geometric discontinuity. After delamination, the

3000

2000

ESG 1ESG 2

70%

50%

40%

30%

No delamination

Perfect delamination

1000

0

Sur

face

_fib

er s

trai

n (µ

ε)

−1000

−2000

−3000−80 −60 −40

x (mm)

−20 0

Figure 15. Strain results of ESGs in the cylinder part before and after the delamination.


geometric discontinuity was removed resulting in little noticeable difference in fiberstress compared to the case before delamination. Therefore, the delamination betweenhoop and helical layers had little influence on the LN2 storage function of the cryotankfor this case.

CONCLUSIONS

In this study, a prototype of a Type 3 cryotank was fabricated with the compositedeveloped for cryogenic application and an aluminum liner, and the cryogenic conditionswere applied to it. For this purpose, liquid nitrogen was stored in the cryotank ascryogenic medium and gaseous nitrogen was used to pressurize it. During the experiment,delamination inside the cryotank happened. Several attempts were made to investigate thisphenomenon through both as analytical approach, with thermo-elastic analysis inconsideration of the progressive failure, and as an experimental one, with a LN2 immersiontest of composite/aluminum ring specimens which are suitable for simulating a Type 3 tankstructure. As a result, it was shown that the main reason for delamination was theformation of voids caused by evaporation of acetone during the fabrication procedure.These voids inside the matrix were extremely dangerous especially at cryogenictemperature because cracks initiated from them due to a large CTE difference betweenfiber and matrix. In addition, some complementary measures to establish the optimalcuring cycle in a filament winding process were suggested. The contribution of this study isboth to report experimental results of a Type 3 cryotank and to investigate the behaviorand safety of the cryotank structure by means of ring specimen tests which can effectivelyreduce time and cost even under harsh test conditions and progressive finite elementanalysis accompanied with thermal analysis.

0

z

x

Before delaminationAfter delamination

−100LN2 injection + pressure (1.7 MPa)

−200

−300

−400

−500

−600

−700

−800−60 −40 −20 0 20

x (mm)

Sur

face

_fib

er s

tres

s (M

Pa)

40 60

Figure 16. Fiber directional stress in the outermost surface before and after the delamination.


ACKNOWLEDGMENTS

The authors would like to thank Korea Aerospace Research Institute, Korea, for theirfinancial support and Hankuk Fiber Glass Co., Ltd., Korea, for the material developmentand the supply of specimens.

REFERENCES

1. Heydenreich, R. (1998). Cryotanks in Future Vehicles, Cryogenics, 38: 125–130.

2. Vendroux, G., Auberon, M. and Dessaut, J. (1997). Cryogenic Composite Tanks: StructuralAnalysis and Manufacturing Concepts, 42nd International SAMPE Symposium, 42(2): 828–838.

3. Johnson, T.F., Natividad, R., Rivers, H.K. and Smith, R. (1998). Thermal StructuresTechnology Development for Reusable Launch Vehicle Cryogenic Propellant Tanks, SpaceTechnology and Application International Forum (STAIF), 3rd Conference on Next GenerationLaunch Systems, January 25–29.

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