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  • 8/10/2019 An Initial and Progressive Failure Analysis for Cryogenic Composite Fuel Tank Design

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    http://jcm.sagepub.com/Materials

    Journal of Composite

    http://jcm.sagepub.com/content/41/21/2545The online version of this article can be found at:

    DOI: 10.1177/0021998307076492

    2007 41: 2545Journal of Composite MaterialsJaehyung Ju, Brent D. Pickle, Roger J. Morgan and J.N. Reddy

    n Initial and Progressive Failure Analysis for Cryogenic Composite Fuel Tank Design

    Published by:

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    On behalf of:

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    An Initial and Progressive Failure Analysisfor Cryogenic Composite Fuel Tank Design

    JAEHYUNGJU,* BRENTD. PICKLE, ROGER J. MORGAN ANDJ. N. REDDY

    Department of Mechanical Engineering, Texas A&M University

    College Station, TX 77843-3123, USA

    ABSTRACT: Thermal residual stresses, internal pressure stresses, and acceleration

    stresses during launch were evaluated and quantified for cryogenic composite fueltank design. Both failure initiation and progression of graphite/epoxy laminatesystem (IM7/977-2) [0/90/90/0/0/90]s and graphite/BMI laminate system(IM7/5250-4) [0/90/90/0/0/90]s were investigated using the non-isothermal classicallaminate and plate theory (CLPT) and the maximum stress failure criterion.The thermal residual stresses in the transverse direction are the dominant stresses oneach ply in the launch stage. After initial ply cracking, through-the-thicknesstemperature change of a laminate related to fuel leakage as well as a laminatestiffness matrix change was applied to the progressive failure analysis. The fuelleakage-based progressive analysis shows a higher number of initial ply crackingdoes not necessarily mean a higher chance of matrix cracking in all plies. Thegraphite/BMI laminate has such an advantage as transverse thermo-mechanical

    resistance over the graphite/epoxy laminate at an initial exposure to253C and1500 kPa. In terms of complete laminate matrix cracking, however, the graphite/epoxy laminate is more resistant to transferring stresses to other plies than thegraphite/BMI laminate.

    KEY WORDS: cryogenic composite fuel tank, progressive failure analysis,fuel leakage, IM7/977-2 (graphite/epoxy laminate system), IM7/5250-4 (graphite/BMI laminate system).

    INTRODUCTION

    FOR MORE THAN two decades, design and manufacturing of cryogenic composite fueltanks has been one of the most challenging technologies for achieving drastic weight

    reduction of reusable launch vehicles (RLV) [1]. However, the development of the fuel

    tank has not easily progressed. In July 1996, NASA and Lockheed Martin Corporation

    and its industry partners entered into a cooperative agreement for the design,

    development, and flight-testing of the X-33 RLV. But, a failure occurred during testing

    of the X-33 prototype. An investigation teams study on the failure of the fuel tank

    revealed the causes of the failure to be (i) inner face-sheet cracking which leads to

    *Author to whom correspondence should be addressed. E-mail: [email protected] 18 appear in color online: http://jcm.sagepub.com

    Journal ofCOMPOSITE MATERIALS, Vol. 41, No. 21/2007 2545

    0021-9983/07/21 254524 $10.00/0 DOI: 10.1177/0021998307076492 SAGE Publications 2007

    Los Angeles, London, New Delhi and Singapore

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    permeation of liquid and gaseous hydrogen, (ii) high core pressure resulting from liquid

    and gaseous hydrogen, (iii) reduced bond-line strength and toughness, and (iv)

    manufacturing flaws and defects [2]. The wall of the X-33 fuel tank consists of two

    layers of IM7/977-2, graphite/epoxy composite around a honeycombed Kevlar core.

    The major cause of failure has been thought to be extensive transverse cracking of the

    inside IM7/977-2 face-sheet. The cracking occurred due to a combination of mechanical

    and thermal residual stresses as the tank was filled with cryogenic liquid hydrogen.

    Interconnection of the cracks allows the hydrogen to infiltrate the honeycombed Kevlar

    core. When the tank was subsequently emptied and temperatures began to rise, the cracks

    closed and trapped the hydrogen between the two layers of IM7/977-2 laminate.

    The hydrogen expanded as the temperature continued to increase, building up pressure in

    the honeycombed Kevlar core until the outside layer of graphite/epoxy composite

    ruptured. While the rupture was not directly caused by cracking, cracking was

    the underlying reason why the failure occurred. Extensive cracking does also weaken

    the laminate and can lead to an ultimate failure as well.

    The program ran for 56 months with an overall cost of approximately $1.3 billion.The redesign of the fuel tank was proposed to cost about $100 million [3]. Therefore, it is

    significant to develop more accurate analytical and numerical tools for cryogenic fuel tank

    design to save time and cost. In order to detect the onset and extent of cracking in a

    laminate, a stress analysis of each ply is required. While looking at the laminate from the

    individual ply level, the onset of cracking will be determined by checking which ply

    experiences cracking first. The extent of cracking will also be judged by counting failed

    plies while applying loads.

    Numerous studies on the design of composite pressure vessels subjected to internal and

    external pressure have been conducted at room temperature [411]. Several stress analyses

    for cryogenic fuel tank design have been performed [1220]. Glaessgen et al. presented theinitial ply level thermal and mechanical stress analyses to determine the causes of the X-33

    liquid-hydrogen tank failure [14]. Abumeri et al. performed a finite element stress analysis

    of IM7/977-2 laminate at253C. (423F) temperature, 207 kPa (30 psi) internal fuelpressure, and 2.53 g launch inertia force [15]. However, a detailed computational

    algorithm was not given in either paper. Moreover, thermal gradients through the tank

    skin were not considered in their studies. Instead, the same temperature assumption

    through the thickness was made, which may give inaccurate stress results. Using

    conventional damage mechanics, Mallick et al. showed damage evolution by deriving

    modulus reduction as a function of crack density and applied stress in a filament wound

    fuel tank laminate [/90/] [16]. But, they did not show ply level stress components. The

    amount of the stresses applied on a composite by thermo-mechanical cycle, temperature,

    acceleration induced force, and fuel pressure was addressed. However, detailed algorithms

    and ply-by-ply stress analysis were not presented in their study.

    In order to simulate damage propagation, a progressive failure analysis method is

    required. Most progressive analyses check ply-cracking, update modulus reduction, and

    use iterative decision making to cracking at applied load increments [2127]. The present

    study suggests a new progressive failure analysis by changing through-the-thickness

    temperatures associated with fuel leakage as well as equivalent laminate stiffness matrix,

    whenever damaged plies are detected. Through-the-thickness temperatures are also

    updated with an increasing number of failed plies and their connectivity at every step.

    The equivalent laminate stiffness matrix is updated with an increasing number of damagedplies and lay-ups.

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    In this study, ply-by-ply failure analysis will be performed and used to determine the

    state of cracking in IM7/977-2 (graphite/epoxy laminate system) and IM7/5250-4

    (graphite/BMI laminate system) for [0/90/90/0/0/90]s lay-up. The scope of the study is

    on the deterministic conceptual design approach for cryogenic composite tank

    applications.

    MATERIALS

    The [0/90/90/0/0/90]slay-up of IM7/5250-4, provided by NASA Glenn Research Center

    for our previous project for both experimental and theoretical analyses, was used in the

    present analytical study. The lay-up is different from that of the liquid hydrogen composite

    tank used in the X-33. In this study, the feasibility of IM7/5250-4 laminates for cryogenic

    fuel tank design will be investigated by comparing damage analysis of IM7/5250-4

    laminates to the analysis of IM7/977-2 laminates which were already used in the X-33 fuel

    tank. The laminate thickness is 40 mm, which is approximately same as that of the X-33sandwich structure including inner face-sheet, core, and outer face-sheet. The thermal

    and mechanical properties of IM7/977-2 and IM7/52504 lamina are shown in Tables 13

    [2830].

    STRESS ANALYSIS

    The stress analysis of the cryogenic fuel tank at a launch stage will be divided into three

    separate calculations: thermal residual stresses, stresses due to internal fuel pressure,

    Table 2. Material property coefficients for IM7/977-2 carbon fiber/epoxy lamina (Vf 0.65)obtained from References [29,30]. Property (T) C0 C1T C2T2, T inC.Properties C0 C1 C2

    Longitudinal tensile modulus, E11 (GPa) 180.63

    Transverse tensile modulus, E22, E33 (GPa) 9.9424 0.0189 2.00E-05In-plane shear modulus, G12 (GPa) 6.1311 0.01 Poissons ratio v12, v13 0.33

    Longitudinal thermal expansion coefficient, 11 (E-6/C) 0.1839 0.0001 1.00E-06Transverse thermal expansion coefficient, 22 (E-6/C) 23.055 0.0411

    Transverse tensile strength, S22T(MPa) 72.84 0.0365 In-plane shear strength, S12 (MPa) 127.26 0.0548 Longitudinal tensile strength, S11T(MPa) 2953.03 Longitudinal compressive strength, S11C (MPa) 1447.67

    Table 1. Thermal properties for IM7/977-2 and IM7/5250-4 laminas (Vf 0.65)obtained from Reference [28].

    Properties IM7/977-2 IM7/5250-4

    Specific heat, Cp (J/kgC) 942.0 812.5

    Transverse thermal conductivity, k(W/mC) 0.464 0.540Density, (kg/m3) 1594.15 1571.60

    Thermal diffusivity, (m2/s) 3.10E-07 4.10E-07

    Failure Analysis for Cryogenic Composite Fuel Tank Design 2547

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    and stresses caused by accelerated forces. A section of the tank wall will be approximated

    by flat rectangular plates of 12 layer [0/90/90/0/0/90]sof IM7/977-2 and IM7/5250-4. The

    flat plate approximation can be used because the radius of the cylindrical tank is large

    enough, and a slight curvature in a small section of the tank wall can be neglected. The

    plate will be oriented in a coordinate system as can be seen in Figure 1. As shown in

    Figure 1, in the local laminate coordinate, the xy plane is the mid-plane of the laminate

    and the positive z-direction is toward the outside of the tank along with the r-direction.

    Total stresses applied on each lamina, k, are expressed as a summation of thermal,

    pressure, and launch induced stresses, as shown in Equation (1). The three stress

    components are assumed to be decoupled; therefore, the total stresses are expressed as a

    linear combination of the three stresses.

    Totalij k Tijk Pijk Lijk: 1

    r

    qt

    Z

    x

    2(T): Transverse direction

    1(F): Fiber direction

    1 2 3

    8.687 m

    6 m

    y

    z

    40 mm

    Figure 1. Schematic of a cylindrical cryogenic composite fuel tank.

    Table 3. Material property coefficients for IM7/5250-4 carbon fiber/BMI lamina (Vf 0.65)obtained from References [29,30]. Property (T) C0 C1T C2T2, T in C.

    Properties C0 C1 C2

    Longitudinal tensile modulus, E11 (GPa) 181.01

    Transverse tensile modulus, E22, E33 (GPa) 11.158 0.0066 1.00E05In-plane shear modulus, G12 (GPa) 6.368 0.0129 Poissons ratio v12, v13 0.36

    Longitudinal thermal expansion coefficient, 11 (E-6/C) 0.25

    Transverse thermal expansion coefficient, 22 (E-6/C) 21.902 0.0307

    Transverse tensile strength, S22T(MPa) 85.984 0.0815 0.0002In-plane shear strength, S12 (MPa) 100.42 0.105 Longitudinal tensile strength, S11T(MPa) 2959.2

    Longitudinal compressive strength, S11C (MPa) 1450.70

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    resulting in thermal bending stresses. The temperature profiles in the laminate can be

    described by the thermal diffusion equation:

    @T

    @t @

    2T

    @r2 3

    where thermal diffusivity, k/Cp. A solution of Equation (3) is given inReference [32] as

    Tr, t Ti T1 2ffiffiffi

    pZ c

    0

    et2

    dt T1 4

    where c r=2ffiffiffi

    p t [32]. Ti is the initial temperature, which is the laminate temperature

    before fuel is filled. T1

    is the fuel temperature. Thermal properties of IM7/977-2 and

    IM7/5250-4 laminas are shown in Table 1 [28]. Based on the thermal properties and

    Equation (4), temperature profiles as a function of time of IM7/977-2 and IM7/5250-4 at

    20 and 40 mm, which are mid-and outer-surfaces, are shown in Figure 2. It takes more

    than 600 h for mid-surface and the outer one to reach the LH2fuel temperature. According

    to the final report of the X-33 liquid hydrogen tank, it took about 3 h for the composite

    fuel tank to be filled with LH2and tested before the tank vent was initiated [2]. Therefore,

    the 3-h period after the fuel filling is an interesting time frame in this study. Figure 3

    shows temperature profiles through the thickness after the 3-h exposure to cryogenic fuel

    on the inside laminate surface, r RI (z 0:5 N tply). Different thermal gradientsexist depending on laminates thermal diffusivities. Thermal gradients for IM7/977-2 and

    IM7/5250-4, after the 3-h LH2 exposure on the inner surface, are 2.58 and 2.27C/mm,respectively (Figure 3). Through-the-thickness temperature profiles are applied to

    250.00

    200.00

    150.00

    100.00

    50.00

    0.00

    50.00

    0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0

    Time (h)

    Temperature(C)

    IM7/977-2 at 40mm

    IM7/5250-4 at 40mm

    IM7/977-2 at 20mm

    IM7/5250-4 at 20mm

    at 40mm

    at 20mm

    r

    Thickness = 0mm Thickness = 40mm

    Figure 2. Temperature profiles through thickness as a function of time when the inner surface is exposed

    to LH2.

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    determine T for each lamina in Equation (2), and accurate thermal residual stresses on

    each lamina are obtained.

    Internal Stresses Induced by Fuel Pressure

    The internal pressure required to store the cryogenic fuel in the tank also causes

    stresses in the wall of the fuel tank; stresses in the longitudinal and hoop directions.According to the ASME boiler and pressure vessel code, the applied longitudinal and

    hoop stresses of isotropic materials by pressure, corresponding to Zand directions in

    the global cylindrical coordinate (Figure 1), respectively, are defined as Equation (5)

    and (6) [33].

    PZZ P

    R0=R1 215

    and:

    Pk

    P 1 R0=rk2

    R0=R1 21: 6

    The axial stresses are all the same through the layers of a laminate wall; however, the

    hoop stresses are shown as a distributed function ofr(k). Therefore, there exists a pressure

    gradient through the hoop direction but the gradient is low enough to neglect.

    For accurate computation in this study, the average hoop stress is calculated as:

    P 1

    RO RIZ RO

    RIPk dr: 7

    After 3 hours

    260.00

    210.00

    160.00

    110.000 5 10 15 20 25 30 35 40

    Thickness (mm)

    Temperature(C)

    IM7/977-2 IM7/5250-4

    r

    Thickness= 0mm Thickness=40mm

    Figure 3. Through-thickness temperatures after 3-h cryogenic exposure at the inner surface.

    Failure Analysis for Cryogenic Composite Fuel Tank Design 2551

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    r(k) appears as the radius at the point in which the stress is being calculated. Ther-direction

    in the global cylindrical coordinate is along the z-direction in the local laminate

    coordinate. In this study, the points at which the hoop stress will be calculated are centers

    of each lamina in the z-direction. From the flat rectangular plate approximation,pZZand

    p

    are rewritten as p

    xx

    and p

    yy

    , respectively.

    Internal stresses of each lamina are different depending on their orientation. In order to

    derive the individual lamina stresses, we need to start from the global stresses by pressures

    applied on a laminate. Under the assumption that the pressure loading does not cause

    bending, in-plane stresses on individual plies of a laminate by pressure can be expressed as:

    fPijgkPxxPyyPxy

    8; 8

    where:

    "0xx"0yy0xy"1xx"1yy1xy

    8>>>>>>>>>>>>>:

    9>>>>>>>=>>>>>>>;

    P

    A11 A12 A16A12 A22 A26A16 A26 A66

    24

    35 B11 B12 B16B12 B22 B26

    B16 B26 B66

    24

    35

    B11 B12 B16B12 B22 B26B16 B26 B66

    24

    35 D11 D12 D16D12 D22 D26

    D16 D26 D66

    24

    35

    26666664

    37777775

    1 NPxxNPyyNPxyMPxxMPxxMPxy

    8>>>>>>>>>>>>>:

    9>>>>>>>=>>>>>>>;

    AijXNk1

    Qkijzk1 zk

    Bij1

    2

    XNk1

    Qkijz2k1 z2k

    Dij1

    3

    XNk1

    Qkijz3k1 z3k

    NP tply

    N

    Pxx

    Pyy

    0

    8

    >:

    9

    >=>;

    MP

    0

    0

    0

    8>:

    9>=>;

    It should be noted that temperature gradients and temperature-dependent material

    properties result in a [0/90/90/0/0/90]s laminate that is no longer symmetrical when faced

    to the cryogenic fuel at one surface. Therefore, the extension-bending coupling stiffness,

    Bij should be considered in the computation of the fuel pressure induced strains in

    Equation (8). Therefore,f"1ijg is induced by the internal pressures even though momentresultants do not exist under the pressure loading.

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    Stresses by Launch

    The maximum applied load will occur at take off when the fuel tank is completely

    full and the acceleration is the greatest. During take off, the fuel tank will experience

    acceleration in both the axial and hoop directions, az and a. The in-plane stresses due to

    the launch acceleration are:

    LZZLLZ

    8>>>>>>>>>>>>>>>>:

    9>>>>>>>>>=>>>>>>>>>;

    L

    A11 A12 A16

    A12 A22 A26

    A16 A26 A66

    24 35 B11 B12 B16B12 B22 B26

    B16 B26 B66

    24 35B11 B12 B16

    B12 B22 B26

    B16 B26 B66

    24

    35 D11 D12 D16D12 D22 D26

    D16 D26 D66

    24

    35

    266666664

    377777775

    1 NLxx

    NL

    yy

    NLxy

    MLxxMLxxMLxy

    8>>>>>>>>>>>>>>>:

    9>>>>>>>>=>>>>>>>>;

    AijXNk1

    Qkijzk1 zk

    Bij1

    2XNk1

    Qk

    ijz2k1 z

    2k

    Dij 13

    XNk1

    Qkijz3k1 z3k

    NL tply N

    Lxx

    Lyy

    0

    8>:

    9>=>;

    ML 0

    0

    0

    8>:

    9>=>;

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    While computing launch acceleration induced stresses, Bij are also considered because

    the [0/90/90/0/0/90]s lay-up is not symmetric any more due to thermal gradients and

    temperature dependent ply properties.

    Failure Analysis Using Maximum Stress Criterion

    The maximum stress failure criterion is applied to investigate ply level damage related

    to transverse matrix cracking and longitudinal tensile and compressive failure:

    Rk ijSij

    k11

    where ij

    is the lamina stress applied and Sij

    the lamina strength. If R(k) is equal to or

    higher than 1, damage on ply k occurs.

    Initial and Progressive Failure Model Depending on Failure Mechanism

    For two decades, a number of progressive failure models of composite laminates

    have been developed. The main concept of the progressive failure model is that non-

    homogenous stresses within a composite structure give rise to a complicated failure

    scenario. For example, when one ply experiences damage, subsequent plies may also

    experience damage at the same load conditions due to redistribution of stresses, thusleading to overall reduction in laminate stiffness.

    Conventional progressive models deal with the stiffness reduction caused by matrix

    cracking as a function of microcrack densities, resulting in strength reduction of a

    laminate [2127]. If transverse matrix cracking is encountered in layers,Q12,Q22, and Q66components that are in the layers stiffness matrix are reduced to be near zero, and

    iterative computations are performed for updating Aij, Bij, and Dij. In this study, each

    laminas thermal, pressure, and launch stresses are updated based on the updated Aij, Bij,

    and Dij.

    In addition to considering the updated laminas stiffness reduction, the fuel leakage

    effect through the thickness direction is considered in modeling a progressive failure

    model. Microscopic interface defects at an overlapping region of two matrix cracks in

    adjacent layers have been assumed to provide the connecting path of fluid between

    transverse matrix cracks [34,35]. The overlapping regions are thought to be attacked by

    cryogenic fuel, resulting in complicated laminate modulus update and lamina stress

    redistribution associated with local temperature changes. Inner surface plies, if cracked,

    experience a through-the-thickness temperature change by cryogenic fuel leakage.

    The crack induced temperature change also affects the thermal gradient over the rest of

    plies. If cracking does not occur on inner surface plies, temperature and thermal gradient

    changes are not expected (Figure 4).

    Fuel leakage, due to the transverse matrix cracks in conjunction with inter-ply

    delaminations resulting in an intersecting network of passages, has been an important issuefor designing cryogenic fuel tank. Some researchers have studied the leakage

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    or permeation of cryogenic fuel through composite laminates by experiment or

    modeling [3440].

    The overall computational algorithm is shown in a flowchart (Figure 5). Thermal,

    pressure, launch and total stresses are obtained in ply level based on temperature dependent

    material properties and temperature distribution through the laminate thickness. Ply leveldamage is checked based on the failure criterion. The progressive failure analysis is

    Tin, Interior Temperature,

    = (Fuel Temperature)

    Tout, Exterior Temperature

    (a) Pressure, launch and thermal stresses as a function of failed plies

    ( )syP

    eyP

    (k)

    (k)

    (k) (k)

    (k)

    (k)

    (k) (k)

    (k)

    (k)

    (k)

    (k)

    [ ]Q=

    ( )syT

    eyT[ ]Q=

    ( )syL

    eyL[ ]Q=

    ( )sxP

    exP[ ]Q=

    ( )sxT

    exT[ ]Q=

    ( )sxL

    exL[ ]Q=

    z

    q

    Tin

    Tin

    TinTi

    Tout

    Tout Tout

    (b) Ply temperature change as a function of number of failed plies and their connectivity

    Cracking and interconnection: Thermal gradientupdate

    Inside ply cracking: No thermal gradient update

    Figure 4. Schematic of progressive failure analysis using temperature gradient change by crack

    interconnection.

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    is assumed to be 253C and fuel mass 95,250 kg. Space vehicle mass (launch mass),ms, isassumed to be 129,250 kg. Internal fuel pressure is assumed to be under various conditions

    from 65 to 1500 kPa. Diameter of the cylindrical fuel tank is set to be 6 m. A laminate

    thickness of 40 mm and 12-layer oriented [0/90/90/0/0/90]sof the two different composites

    were used for the stress analysis.

    Figures 6 and 7 show thermal, pressure, and launch acceleration induced stresses of

    IM7/977-2 and IM7/5250-4, respectively, in the principal material coordinate at253Cand 65, 132, 290, 600, 1000, and 1500 kPa (10, 20, 44, 91, 152, and 227 psi). Thermal

    residual stresses in the fiber direction are compressive and those in the transverse direction

    are tensile. Thermal residual stresses are dominant compared to pressure and launch

    induced stress components at 65, 132 and 290 kPa in both the fiber and the transverse

    directions. Thermal residual stresses in the transverse direction seem to be a leading source

    causing ply damage due to the lower transverse ply strength than the longitudinal one.

    Thermal residual stresses of IM7/977-2 plies are 6064 MPa in the transverse direction and62 to65 MPa in the fiber direction. The absolute value of thermal residual stresses of

    (a)at 253C and 65 KPa (IM7/977-2)

    Ply and direction (F-Fiber direction, T-Transverse direction)

    F

    T

    (b)at 253Cand 132 KPa (IM7/977-2)

    Ply and direction (F-Fiber direction, T-Transverse direction)

    F

    T

    (c)at 253Cand 290KPa (IM7/977-2)

    Ply and direction (F-Fiber direction, T-Transverse direction)

    F

    T

    (d)at 253Cand 600 KPa (IM7/977-2)

    Ply and direction (F-Fiber direction, T-Transverse direction)

    F

    T

    Ply and direction (F-Fiber direction, T-Transverse direction)

    (e)at 253Cand 1000 KPa (IM7/977-2)

    F

    T

    (f)at 253Cand 1500 KPa (IM7/977-2)

    Ply and direction (F-Fiber direction, T-Transverse direction)

    Thermal Pressure LaunchThermal Pressure Launch

    Thermal Pressure Launch

    Thermal Pressure Launch Thermal Pressure Launch

    Thermal Pressure Launch

    F

    T

    70

    20

    30

    80

    130

    180

    230

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    S

    tress(MPa)

    70

    20

    30

    80

    130

    180

    230

    S

    tress(MPa)

    70

    20

    30

    80

    130

    180

    230

    Stress(MPa)

    70

    20

    30

    80

    130

    180

    230

    Stress(MPa)

    70

    20

    30

    80

    130

    180

    230

    Stress(MPa)

    70

    20

    30

    80

    130

    180

    230

    Stress(MPa)

    Figure 6. Thermal, pressure, and launch induced stresses of IM7/977-2 [0/90/90/0/0/90]s in the fiber and

    transverse directions at253C under various pressures.

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    IM7/5250-4 are about 10 MPa higher in both fiber and transverse directions than those of

    IM7/977-2, which are 7276 MPa in the transverse direction and

    73 to

    77 MPa in the

    fiber direction. The ply 1 shows the highest thermal residual stress values in both the fiber

    and the transverse directions, due to the direct cryogenic fuel contact. Thermal residual

    stresses on plies gradually decrease when the location of the plies moves away from the

    inside surface. In Bechel and Kims paper, after submerging both IM7/5250-4 [0/90]2sand

    IM7/977-2 [0/90]2s of 5cm 5 cm 0.112 cm flat plates into LN2 and 1000 cryogeniccycles, interior plies of both plates have a less matrix crack density than ply 1 and 8, which

    are exterior plies, indicating that there exists thermal gradients though the thickness of

    the plates [30].

    The thermal residual stress gradients of IM7/977-2 in the fiber and the transverse

    directions are 0.08 and

    0.09 MPa/mm, respectively. The magnitudes of IM7/5250-4

    thermal residual stress gradients are a little higher than those of IM7/977-2, which are 0.10and0.11 MPa/mm in the fiber and the transverse directions, respectively.

    (a)at 253C and 65 KPa (IM7/5250-4)

    70

    20

    30

    80

    130

    180

    230

    70

    20

    30

    80

    130

    180

    230

    70

    20

    30

    80

    130

    180

    230

    70

    20

    30

    80

    130

    180

    230

    70

    20

    30

    80

    130

    180

    230

    70

    20

    30

    80

    130

    180

    230

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    1(F)

    2(F)

    3(F)

    4(F)

    5(F)

    6(F)

    7(F)

    8(F)

    9(F)

    10(F)

    11(F)

    12(F)

    1(T)

    2(T)

    3(T)

    4(T)

    5(T)

    6(T)

    7(T)

    8(T)

    9(T)

    10(T)

    11(T)

    12(T)

    Ply and direction (F-Fiber direction, T-Transverse direction)

    Stress(MPa)

    Stress(MPa)

    Stress(MPa)

    Str

    ess(MPa)

    Stress(MPa)

    Stress(MPa)

    Thermal Pressure Launch

    Thermal Pressure Launch

    Thermal Pressure Launch

    Thermal Pressure Launch

    Thermal Pressure Launch

    Thermal Pressure Launch

    F

    F

    F

    T

    T

    F

    T

    F

    T

    T

    (b)at 253C and 132 KPa (IM7/5250-4)

    Ply and direction(F-Fiber direction, T-Transverse direction)

    (c)at 253C and 290 KPa (IM7/5250-4)

    Ply and direction (F-Fiber direction, T-Transverse direction)

    (d)at 253C and 600 KPa (IM7/5250-4)

    Ply and direction (F-Fiber direction, T-Transverse direction)

    (e)at 253C and 1000 KPa (IM7/5250-4)

    Ply and direction (F-Fiber direction, T-Transverse direction)

    F

    T

    (f)at 253C and 1500 KPa (IM7/5250-4)

    Ply and direction (F-Fiber direction, T-Transverse direction)

    F

    T

    F

    T

    Figure 7. Thermal, pressure, and launch induced stresses of IM7/5250-4 [0/90/90/0/0/90]s in the fiber and

    transverse directions at253C under various pressures.

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    As internal fuel pressure increases, pressure stresses become dominant in the fiber

    directions. Pressure induced stresses are higher in the fiber direction than those in the

    transverse direction due to the higher lamina stiffness in the fiber direction. IM7/977-2 and

    IM7/5250-4 have almost the same pressure stresses in the fiber direction due to the fiber

    dominant modulus. Transverse pressure stresses of both laminates are low and similar in

    both laminates. As the internal fuel pressure increases, the longitudinal directions loading

    mode is changed from compression to tension. Pressure stresses begin to exceed thermal

    residual stresses at 600 kPa in the fiber direction of 90 plies. At 600 kPa, tensile andcompressive stresses coexist in the fiber direction at IM7/977-2 laminate. Total stresses of

    IM7/977-2 in the fiber direction experience tensile stresses at 1000 and 1500 kPa. In the

    case of IM7/5250-4, all total stresses in the fiber direction start to be tensile at 1500 kPa

    due to the relatively higher compressive thermal residual stresses than those of IM7/977-2.

    Pressure stresses in the fiber direction of 90 plies are higher than those of 0 plies, becausehoop stresses in the global coordinate are applied in the fiber direction of 90 plies. On thecontrary, pressure stresses in the transverse direction of 0 plies are higher than those of

    90 plies because hoop stresses in the global coordinate are also applied in the transversedirection of 0 plies.

    Launch stresses are also higher in the fiber direction than in the transverse direction due

    to the higher modulus in the fiber direction. But, the launch stresses are not high compared

    to the thermal and pressure stresses. Launch stresses of 5.66.8 MPa are applied on both

    laminate plies in the fiber direction, which are 63160% and 2.76.9% of pressure stresses

    in the fiber direction at 65 and 1500 kPa internal pressures, respectively. Launch stresses

    applied in the transverse direction are negligible, which are about 0.6 MPa.

    Thermal residual stresses in the transverse direction contribute to about 80 to 98% of

    the total stresses in both laminates, which means the thermal residual stress is the

    dominant factor that should be critically considered for cryogenic composite fuel tankdesign.

    Grenoble and Gates showed, in their experiment with IM7/977-2 [45/903/45/03]slaminates, higher leak rates at cryogenic temperature with increasing levels of uni-axial

    tensile strain than at room temperature with the same levels of strains [36]. Their results

    show that the thermal residual stresses at cryogenic temperature made an additional

    contribution to matrix cracking compared with the mechanical loading at room

    temperature.

    Yokozeki et al. also showed that the thermal residual stress is a major reason causing

    ply damage of a IM600/#133 carbon fiber/toughened epoxy system with [45/45/90]s,[45/

    45/902]s, [902/

    45/45]s, and [902/0/902]s lay-ups by investigating gas leakage data at

    two conditions; room temperature and LN2temperature, under static mechanical loading

    [40]. Their results exhibit higher leak rates under LN2 conditions than under room

    temperature for all cases, indicating that gas leakage increases under cryogenic conditions

    because of the increase of the leak path size due to the thermal residual strains, which is

    exactly explained in the present study.

    Cracking Conditions of Each Lamina under Different Loading Modes

    Ply-by-ply total stress to strength ratio, (ij/Sij)(k), was computed and plotted in

    Figure 8. Figure 8 shows transverse tensile stress to strength ratio, (22T/S22T)(k),longitudinal compressive stress to strength ratio, (11C/S11C)

    (k), and longitudinal tensile

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    through the width direction and indicates a strong possibility of fuel leakage associated

    with networking of the fully developed matrix cracks through the width direction.

    At253C and 1500 kPa, for IM7/977-2, after two updates of equivalent laminatestiffness related to individual ply damage, six plies are left non-cracked. Combining

    laminate temperature redistribution by fuel leakage with the updated laminate stiffness

    matrix also did not show a distinctive change compared to the conventional progressive

    failure model, leaving six plies undamaged (Tables 4 and 5).

    IM7/5250-4 laminate originally has one ply transverse matrix cracking at253C and1500 kPa. Six updates of laminate stiffness matrix and iteration left three plies undamaged

    using the conventional progressive failure model (Table 6). However, if the updated

    laminate stiffness matrix is combined with thermal redistribution associated with the inner

    surface matrix cracking-induced fuel leakage, all plies are transversely damaged as shownin Table 7.

    Table 5. (r22T/S22T)(k) values of each layer of IM7/977-2 [0/90/90/0/0/90]s for progressive

    failure analysis considering lamina stiffness reduction and fuel leakageat253C and 1500 kPa.

    Number of iteration

    Ply 0 1 2

    1 1.001 (Failure)

    2 0.940 0.990 0.981

    3 0.944 0.992 0.986

    4 1.011 (Failure) 5 1.012 (Failure)

    6 0.947 0.990 0.992

    7 0.945 0.986 0.991

    8 1.007 (Failure)

    9 1.003 (Failure)

    10 0.934 0.969 0.982

    11 0.928 0.961 0.977

    12 0.984 1.034 (Failure)

    Table 4. (r22T/S22T)(k) values of each layer of IM7/977-2 [0/90/90/0/0/90]s for progressive

    failure analysis considering lamina stiffness reduction at253C and 1500 kPa.Number of iteration

    Ply 0 1 2

    1 1.001 (Failure)

    2 0.940 0.995 0.986

    3 0.944 0.995 0.989

    4 1.011 (Failure)

    5 1.012 (Failure)

    6 0.947 0.989 0.991

    7 0.945 0.985 0.990

    8 1.007 (Failure)

    9 1.003 (Failure)

    10 0.934 0.964 0.977

    11 0.928 0.955 0.971

    12 0.984 1.028 (Failure)

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    Note that IM7/977-2 had a greater number of plies damaged in the matrix than

    IM7/5250-4 at the initial damage stage, but IM7/977-2 had three plies un-cracked when

    IM7/5250-4 plies were all cracked at the final stage at 253C and 1500 kPa. Plies 2 and 3seem to be crucial plies to determine whether the laminate will undergo complete laminate

    damage or not. IM7/977-2 shows no transverse cracking on plies 2 and 3, resulting in no

    direct thermal attack by fuel and no severe further damage transfer to other plies. On the

    other hand, IM7/5250-4 experiences the transverse cracking on plies 2 and 3, after two and

    three updates of laminate stiffness matrix and temperature redistribution, respectively.

    A lower ply temperature of IM7/5250-4 than IM7/977-2 by a higher thermal conductivity

    of IM7/5250-4 might contribute the higher thermal stress, resulting in matrix cracking inall plies.

    Table 6. (r22T/S22T)(k) values of each layer of IM7/5250-4 [0/90/90/0/0/90]s

    for progressive failure analysis considering lamina stiffnessreduction at253C and 1500 kPa.

    Number of iteration

    Ply 0 1 2 3 4 5 6

    1 1.003 (Failure)

    2 0.947 0.971 1.000 (Failure)

    3 0.943 0.964 0.990 1.024 (Failure)

    4 0.990 1.020 (Failure)

    5 0.985 1.011 (Failure)

    6 0.929 0.941 0.960 0.983 1.006 (Failure)

    7 0.923 0.933 0.950 0.970 0.991 1.003 (Failure)

    8 0.969 0.979 1.004 (Failure)

    9 0.963 0.969 0.989 1.006 (Failure)

    10 0.907 0.907 0.917 0.929 0.943 0.954 0.968

    11 0.901 0.898 0.907 0.915 0.928 0.938 0.951

    12 0.945 0.937 0.945 0.958 0.978 0.983 0.992

    Table 7. (r22T/S22T)(k) values of each layer of IM7/5250-4 [0/90/90/0/0/90]s for

    progressive failure analysis considering fuel leakage and lamina stiffnessreduction at253C and 1500 kPa.

    Number of iteration

    Ply 0 1 2 3 4 5

    1 1.003 (Failure)

    2 0.947 0.976 1.005 (Failure)

    3 0.943 0.969 0.995 1.034 (Failure)

    4 0.990 1.026 (Failure)

    5 0.985 1.016 (Failure)

    6 0.929 0.946 0.965 0.994 1.033 (Failure)

    7 0.923 0.938 0.955 0.980 1.017 (Failure)

    8 0.969 0.985 1.010 (Failure)

    9 0.963 0.975 0.995 1.017 (Failure)

    10 0.907 0.913 0.923 0.940 0.971 1.016 (Failure)

    11 0.901 0.904 0.912 0.926 0.955 1.001 (Failure)

    12 0.945 0.943 0.951 0.970 1.007 (Failure)

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    For R(k) values, (22T/S22T)(k) at253C and 1500 kPa of IM7/977-2 and IM7/5250-4

    are between 0.9 and 1 (Tables 47), which means it is hard to say that plies are safe to

    avoid the transverse matrix cracking against the thermal stresses applied on each ply.

    Preventing the cryogenic temperature contact with the inside wall of the fuel tank could be

    accomplished by insulating the laminate in order to minimize thermal residual stresses

    on each ply. If the inner surface temperature of IM7/977-2 and IM7/5250-4 laminates can

    be maintained to LN2 temperature which is196C, (22T/S22T)(k) at 1500 kPa of bothlaminates can be reduced to 0.750.8. Reducing thermal residual stresses using an electron-

    beam curing is another way to alleviate the thermal stresses in the transverse direction of

    the laminates. Assuming composites cured at room temperature using the electron-beam

    curing, (22T/S22T)(k) of the IM7/977-2 and IM7/5250-4 laminates can be reduced to

    0.430.51 and 0.380.47, respectively, which might be a potential benefit to the cryogenic

    composite fuel tank design.

    The initial stage of ply failure may cause an entire laminate failure by the fuel leakage

    effect associated with crack connectivity. That was clearly shown in the case of IM7/5250-4

    at253C and 1500 kPa. The fuel leakage problem could be resolved by using a liner,but there is still a trade-off between structural weight increase and protection of fuel

    leakage.

    It has been known that the transverse ply thickness plays a role in damage initiation and

    progression. Generally, as the ply thickness decreases, the strain necessary to initiate

    transverse matrix cracks increases and saturated transverse crack density decrease.

    Bechel et al. showed about 1228% decrease of exterior plies crack density of IM7/5250-4

    [0/90]2s after 1000 LN2 cryogenic cycles when the ply thickness decreased from 0.21 to

    0.14 mm [30]. The present model, based on the classical laminate and plate theory, has the

    limitation in predicting accurate ply matrix damage as a function of ply thickness.

    CONCLUSIONS

    An appropriate stress analysis was developed for cryogenic fuel tank design based on the

    classical plate laminate theory using three key environmental conditions: (i) thermal

    residual stress, (ii) internal fuel pressure, and (iii) acceleration during launch. Thermal

    residual stresses applied on IM7/977-2 and IM7/5250-4 laminates are the most significant

    source to determine initial ply transverse matrix cracking and complete laminate matrix

    cracking. The thermal residual stresses are 8098% of the total stresses in the transverse

    direction while internal pressure varies from 65 to 1500 kPa. Therefore, lowering the stress

    free temperature, e.g., by using E-beam cured laminates, or protecting composite from

    directly contacting cryogenic fuel by placing thermal protection layers will be the efficient

    ways to reduce the total stresses. For accurate design, an exact measurement of CTE is

    required as a function of temperature. Thermal residual stresses are very sensitive to CTE,

    which might lead to imperfect designs if used with imprecise CTE.

    Of the remaining stress sources, the stress by internal fuel pressure at 1500 kPa ranges

    from 21 to 46% of the total stresses in the fiber direction depending on ply angles.

    Lowering the internal pressure would reduce the total stress, but the pressure should be

    determined based on the requirements for the design of fuel storages. The fuel pressure

    itself does not seem to be a devastating factor for the cryogenic fuel tank design, compared

    to the thermal residual stress. However, once cracks initiate, causing stiffness decreases inthese plies, the fuel pressure plays an important role in the progressive failure by raising

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    pressure stresses on undamaged plies. The acceleration induced launch stress ranges from 2

    to 8% of the total stresses in the fiber direction. The launch stresses applied in the

    transverse direction are negligible, which are about 0.6 MPa. This could be reduced by

    lightening the whole weight of the launch vehicle structure.

    A fuel leakage-based progressive failure analysis was suggested in this study.

    An instantaneous temperature and temperature gradient updates caused by the fuel

    leakage as well as laminate stiffness matrix updates affect significantly the progressive

    failure behavior of laminates. For IM7/5250-4 at253C and 1500 kPa, the fuel leakagebased progressive analysis shows that an initial stage of the ply failure may cause a

    complete laminate cracking, which cannot be shown using the conventional laminate

    stiffness matrix update method.

    NOMENCLATURE

    Qkij plane stress reduced stiffness components ofkth lamina;i,j 1, 2, 6

    xx, yy, xy coefficient of thermal expansion (CTE) of a lamina in the globalcoordinate consisting ofx and y directions

    Totalij k total stress applied on kth laminaTijk thermal residual stress applied on kth laminaPijk pressure induced stress applied on kth laminaLijk launch induced stress applied on kth lamina

    NTxx, NT

    yy, NTxy in-plane thermal force resultants

    NPxx, NP

    yy, NPxy in-plane pressure force resultants

    NLxx, NL

    yy, NLxy in-plane launch force resultants

    MT

    xx, MT

    yy, MT

    xy moment resultants by thermal forcesMPxx, M

    Pyy, M

    Pxy moment resultants by pressure forces

    MLxx, ML

    yy, MLxy moment resultants by launch forcesN number of plies

    "0xx , "0

    yy , 0xy the membrane strains

    "1xx , "1

    yy , 1xy the flexural (bending) strains, known as the curvaturesT temperature of materials

    T the difference between the operated temperature and the stress freetemperature of a laminate

    P internal fuel pressureRO

    outside radius of the fuel tank

    RI inside radius of the fuel tankaZZ, a launch acceleration to Z and directions in the global cylindrical

    coordinate

    AZZ, A cross-sectional area to Zand directions in the global cylindricalcoordinate

    ms mass of the space vehiclet time

    tply lamina thicknessk thermal conductivity (W/mC)

    Cp

    specific heat (kJ/kgC)

    density (kg/m3) thermal diffusivity (m2/s)

    Failure Analysis for Cryogenic Composite Fuel Tank Design 2565

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    Subscript T, C tensile, compressiveSuperscriptT,P,L thermal, pressure, launch

    ACKNOWLEDGMENTS

    This research was kindly funded by Dr Charles Lee of the Air Force Scientific Office

    of Research (AFSOR) and the Advanced Technology Program sponsored by the State

    of Texas.

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