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Transcript of fatigue life prediction of thermo-mechanically loaded engine

  • 1

    FATIGUE LIFE PREDICTION OF THERMO-MECHANICALLY LOADED

    ENGINE COMPONENTS

    Csaba Halszi, Christian Gaier, Helmut Dannbauer

    MAGNA Powertrain, Engineering Center Steyr GmbH & Co. KG

    Steyrer str. 32, A-4300 St.Valentin, Austria

    Keywords: thermo-mechanical fatigue, engine, Sehitoglu

    INTRODUCTION

    Nowadays engine components are subjected to higher loads at elevated temperatures than before,

    due to the increasing requirements regarding weight, performance and exhaust gas emission. Thus,

    fatigue due to simultaneous thermal and mechanical loading became determinant among the

    damage forms.

    At the same time, there is the need to reduce development times and costs to handle the growing

    number of model variants. Therefore, the development of suitable simulation tools, which reduce

    the number of necessary component tests, seems to be very rewarding.

    The problem of thermo-mechanical fatigue (TMF) life prediction has received considerable

    attention in recent years, with efforts principally concentrated on the prediction of TMF under

    uniformly repeated loading conditions. Several researchers have developed models to treat this

    problem, generally based on isothermal (IT) considerations. However, isothermal tests do not

    capture all damage mechanisms that operate under variable strain-temperature conditions. As

    Sehitoglu emphasizes, a deeper understanding of the different micro mechanisms affecting the

    behavior of materials under isothermal and thermo-mechanical loading conditions is needed.

    THERMO-MECHANICAL FATIGUE

    Thermo-mechanical fatigue (TMF) is the case of fatigue failure due to simultaneous thermal and

    mechanical loading. The life prediction of TMF loading cases has received considerable attention in

    recent years mainly in engine and gas turbine development. The fluctuation of complex thermal and

    mechanical strains is usually determinant for fatigue life of machine parts. The mechanical strain

    arises either from external constraints or externally applied loadings.

    Thermo-mechanical and low cycle fatigue (LCF) can show a lot of similarity, mainly because of

    the presence of cyclic plastic strain. The cyclic thermal load occurs by nature in a small number of

    cycles, but the stresses generated by the restrained thermal expansion may be far beyond the elastic

    limit. In engine parts the superposition of a LCF/TMF effect due to start-stop cycles and a HCF

    effect due to the combustion cycle is to be observed.

    Throughout a thermo-mechanical cycle, one crosses different temperatures. The temperature

    dependent processes that occur during a common TMF cycle are plastic deformation, cyclic aging,

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    creep and oxidation effects, coarsening of the microstructure and crack initiation and propagation.

    The main damage mechanisms are fatigue (mechanical) damage, environmental (oxidation) damage

    and creep damage. These may act independently or can interact depending on various material

    characteristics like thermal conductivity, thermal strain coefficient, Youngs modulus, mechanical

    properties and operating conditions, as maximum and minimum temperatures, mechanical strain

    range, strain rate, phasing of temperature and mechanical strain, dwell time, environmental factor.

    TMF testing methodology

    In TMF tests a specimen is subjected to a desired thermal and mechanical strain with different

    phasings. Two baselines of TMF tests are conducted in the laboratory with proportional phasings:

    in-phase (IP), where the maximum strain (the strain is signed: tension has a positive, and

    compression has a negative sign) occurs at maximum temperature, and out-of-phase (OP), with

    maximum strain at the minimum temperature. The variations of thermal, mechanical and total strain

    components with time as well as schematics of stress-strain behavior corresponding to OP and IP

    cases are illustrated in Figure 1 and Figure 2.

    Figure 1 Out-of-phase loading

    Figure 2 In-phase loading

    The two types of phasing reproduce many of the mechanisms that develop under TMF. The

    mechanical strain ( mech ) is the sum of the elastic ( el ) and inelastic ( in ) strain components, while

    the total strain ( tot or t ) is the sum of thermal ( th ) and mechanical ( mech ) strain components,

    Eqn (1).

    ( ) inelmechthtot TT ++=+= 0 (1)

    where , 0T is the reference temperature where the test was begun, T is the test temperature, and

    is the coefficient of thermal expansion.

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    SEHITOGLUS TMF LIFE PREDICTION MODEL

    Prof. Huseyin Sehitoglu and his colleagues at the University of Illinois, Urbana-Champaign

    developed a method for life prediction of thermo-mechanically loaded components. This method

    includes a unified constitutive visco-plastic material model to describe the material behavior at

    different temperatures and strain rates in the stress-strain analysis, and a damage model.

    The damage model is based on three separate damage mechanisms (fatigue, environmental

    attack, creep) that are acting simultaneously. Depending on the strain rate, strain amplitude,

    temperature and phasing these damage mechanisms can have different parts in the total damage,

    which is the sum of these components. Formally:

    creepoxfattot DDDD ++= or creepoxfattot

    NNNN

    1111++= (2)

    where totD , fatD , oxD and creepD are total damage, fatigue (mechanical) damage, environmental

    (oxidation) and creep damage, respectively, and totN , fatN , oxN , creepN are total cycles to failure,

    cycles to failure due to mechanical fatigue, oxidation and creep damage, respectively.

    Fatigue (mechanical) damage

    Fatigue damage is represented by the classical fatigue mechanisms, which normally occur at

    ambient temperature. These include crack nucleation and propagation due to the strain amplitude.

    Since the cyclic plastic strain is remarkable, strain-life approach is used to describe the fatigue

    behavior. The fatigue life term, fatN , is estimated with the Manson-Coffin-Basquin relationship:

    ( ) ( )cfatfbfatf

    pl

    mech

    e

    mechmech NNE

    2'2'

    222

    +=

    +

    =

    (3)

    Tests performed with steel and aluminum showed that the room temperature (RT) strain-life curve

    can be considered as an upper bound of all strain-life curves. Thus, the constants ( cbE ff ,,,, ) are

    determined from isothermal room-temperature fatigue tests. As the temperature increases, the

    damage increases due to oxidation and creep.

    Environmental (oxidation) damage

    Oxidation damage mechanism includes crack nucleation in surface oxides and oxide-induced crack

    growth. Crack nucleation is defined as the rupture of the first oxide layer. Oxide-induced crack

    growth is described as the repeated formation of an oxide layer at the crack tip and its rupture,

    exposing fresh metallic surface to the environment.

    Upon the examination of oxide-induced crack growth two major growth types can be differrenti-

    ated. These crack initiation types are shown on Figure 3 and denoted as Type I and Type II crack

    formation.

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    Figure 3 left: Type I growth, right: Type II growth

    All oxide-induced crack growth process starts with an initial oxide layer formed on the surface of

    the specimen. When this layer reaches a critical thickness, the oxide ruptures, and the crack

    nucleation has occurred. Then a fresh metallic surface is exposed to the environment, which rapidly

    oxidizes. When the thickness of this newly formed oxide reaches its own critical thickness, the

    oxide ruptures again. The process continues as shown in Figure 3.

    Type I growth is characterized by a continuous oxide layer. This layer results in oxide

    intrusion without any visible stratification. Therefore, it will be distinguished from Type II, which is

    characterized by multilayered or stratificated oxide growth. It is important to note, that the

    progression of both growth types is similar until the layer reaches its critical thickness. The

    difference comes up with the first rupture, as in Type II growth the oxide layer detaches from the

    surface of the substrate. This results in exposition of a larger fresh surface area to the environment,

    accounting for a wider intrusion,

    The oxidation damage is calculated using following equation:

    ( )( )( )

    a

    mech

    mech

    eff

    p

    ox

    cr

    ox

    ox

    KB

    h

    ND

    +

    ==

    1

    121

    0 21

    & (4)

    where hcr, 0, , and B are material parameters. mech& is the mechanical strain rate and eff

    pK is the

    effective oxidation constant,

    ( )dt

    tT

    QD

    tK

    Ct

    C

    eff

    p

    =

    0

    0R

    exp1

    (5)

    where D0 is a constant, Q is the activation energy for oxidation and R is the universal gas constant.

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    The Oxidation Phasing Factor

    ( )dt

    tox

    mechtht

    c

    ox c

    +=

    2

    0

    1/

    2

    1exp

    1

    && (6)

    where ox measures the relative amount of damage associated with different phasing. th& is the

    thermal strain rate.

    The form of ox is chosen to represent the behavior of the oxidation damage observed in the tests. The most detrimental TMF loading case is the fully const