of 8 /8
ISSN: 2277-3754 ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT) Volume 3, Issue 9, March 2014 222 Abstractthe article presents the results of an experimentalanalytical study of the cyclic fracture process in a locally austempered ductile iron (LADI). A heat treatment is realized in such a way as to form three layers with different microstructure, mechanical properties and resistance to cyclic loading. The fracture criterion, the stress intensity factor K, dependent on stresses of cyclic loading and induced residual stresses, was chosen as a criterion for analyzing the process of cyclic fracturing. The limiting values of threshold stress intensity factor range ΔK th were determined for the compact tension (CT) specimens after application of loading and they were found to be different. It was determined that residual stresses increase the ΔK th factor when the crack propagation rate is less than 10 -11 m/cycle. Index Termscracking threshold, residual stresses. I. INTRODUCTION Austempered ductile iron (ADI) is a specially heat-treated cast iron with spheroid graphite. It has good properties of strength, plasticity, fracture toughness, fatigue and resistance to wear, and is therefore widely used in engineering practice to produce the engineering elements in various fields of industry, e.g., transportation and the mineral mining industries, for the manufacturing of gears, shafts, etc. [1], [2]. The process of heat treatment involves three stages, the duration of which depends on the shape and size of the element to be produced. For large gears and supports, needed when producing mining equipment in the mineral industry, austempering with induction heating of a localized zone is applied. This technique replaces the conventional heat-treating process and provides the desired ADI microstructure in the required zone and depth [3]-[5]. The locally austempered ductile iron (LADI) is obtained by using high frequency induction heating or by applying direct electric current [6]. Application of the LADI makes it possible to obtain almost the same properties as conventional ADI [7]. However, due to the different micro structural transformations in heat- affected and unaffected zones, residual stresses are induced. These zones significantly influence the fatigue, i.e., the fracturing indicators [8]-[10]. The conventional method in this experimental and analytical investigation is replaced by another one, aiming to evaluate several factors of the separate layers, namely the static and cyclic properties of the material, its microstructure, in homogeneity and other factors. II. SPECIMENS SG cast iron was chosen as the basic material for producing the layered cast iron specimens. The mechanical properties of SG cast iron are presented in Table 1. Table 1. Cast iron mechanical properties Cast iron R 0.2 , MPa R m , MPa Z, % SG 630-634 863-933 3.7-6.7 The threshold stress intensity factor ranges of ΔK th = 7.9-9.1 MPa·m 1/2 were determined for the compact tension (CT) specimens by applying the known techniques [11]. Eight work-pieces of larger bars for preparing the CT specimens (dimensions: 65 mm 62.5 mm 25 mm) were produced for performing the cyclic tests. The work-pieces for the austenitizing were placed in an induction heating furnace and heated up to the temperature required for austenitizing (900 o C). Then the work-pieces were placed in an isothermal transformation chamber where they were kept for the conventional duration for the austempering. The heat treatment was applied in such a way as to form three different layers in the specimen. Cuts were made in the CT specimens to ensure that the fatigue crack started in the hard layer (heat-treated layer), further propagating through the subsequent perpendicular to the intermediate and mild layers of the crack front (Fig. 1). The arrangement of layers was determined by the measurements of etching and hardness. The mechanical properties of the separate layers were quite different. The properties of the mild layer during heat treatment changed insignificantly. Table 2 shows that the plasticity of the hard layer slightly increased. The properties of the intermediate layer were conditional. It was difficult to establish them precisely enough. The heat treatment process is hard to control, and therefore the size and the location of the intermediate layer in the investigated CT specimens are different. The thicknesses of the layers are shown in Table 2. The thickness of the hard layer L h varies within the bounds of 27.5 mm and 36.5 mm when analyzing all the specimens, while that of the intermediate layer varies within the bounds of 1 mm and 4 mm. The remaining CT specimens can be described as the mild (thermally unaffected) layer. The different locations and thicknesses of the layers described above result in different impacts on the cyclic fracturing indicators. Fatigue Crack Propagation in Locally Heat-Treated Ductile Iron Mindaugas Leonavičius, Algimantas Krenevičius, Gediminas Petraitis, Arnoldas Norkus Vilnius Gediminas Technical University, Saulėtekio al. 11, 10223, Vilnius, Lithuania

Volume 3, Issue 9, March 2014 Fatigue Crack Propagation in ... 3/Issue 9/IJEIT1412201403_37.pdf · Volume 3, Issue 9, March 2014 ... the crack stabilizes for the fixed number of cycles

  • Author
    others

  • View
    0

  • Download
    0

Embed Size (px)

Text of Volume 3, Issue 9, March 2014 Fatigue Crack Propagation in ... 3/Issue 9/IJEIT1412201403_37.pdf ·...

  • ISSN: 2277-3754

    ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT)

    Volume 3, Issue 9, March 2014

    222

    Abstract—the article presents the results of an

    experimental–analytical study of the cyclic fracture process in a

    locally austempered ductile iron (LADI). A heat treatment is

    realized in such a way as to form three layers with different

    microstructure, mechanical properties and resistance to cyclic

    loading. The fracture criterion, the stress intensity factor K,

    dependent on stresses of cyclic loading and induced residual

    stresses, was chosen as a criterion for analyzing the process of

    cyclic fracturing. The limiting values of threshold stress intensity

    factor range ΔKth were determined for the compact tension (CT)

    specimens after application of loading and they were found to be

    different. It was determined that residual stresses increase the

    ΔKth factor when the crack propagation rate is less than 10-11

    m/cycle.

    Index Terms—cracking threshold, residual stresses.

    I. INTRODUCTION

    Austempered ductile iron (ADI) is a specially heat-treated

    cast iron with spheroid graphite. It has good properties of

    strength, plasticity, fracture toughness, fatigue and resistance

    to wear, and is therefore widely used in engineering practice

    to produce the engineering elements in various fields of

    industry, e.g., transportation and the mineral mining

    industries, for the manufacturing of gears, shafts, etc. [1], [2].

    The process of heat treatment involves three stages, the

    duration of which depends on the shape and size of the

    element to be produced. For large gears and supports, needed

    when producing mining equipment in the mineral industry,

    austempering with induction heating of a localized zone is

    applied. This technique replaces the conventional

    heat-treating process and provides the desired ADI

    microstructure in the required zone and depth [3]-[5]. The

    locally austempered ductile iron (LADI) is obtained by using

    high frequency induction heating or by applying direct

    electric current [6]. Application of the LADI makes it

    possible to obtain almost the same properties as conventional

    ADI [7]. However, due to the different micro structural

    transformations in heat- affected and unaffected zones,

    residual stresses are induced. These zones significantly

    influence the fatigue, i.e., the fracturing indicators [8]-[10].

    The conventional method in this experimental and analytical

    investigation is replaced by another one, aiming to evaluate

    several factors of the separate layers, namely the static and

    cyclic properties of the material, its microstructure, in

    homogeneity and other factors.

    II. SPECIMENS

    SG cast iron was chosen as the basic material for

    producing the layered cast iron specimens. The mechanical

    properties of SG cast iron are presented in Table 1.

    Table 1. Cast iron mechanical properties

    Cast iron R0.2, MPa Rm, MPa Z, %

    SG 630-634 863-933 3.7-6.7

    The threshold stress intensity factor ranges of ΔKth =

    7.9-9.1 MPa·m1/2

    were determined for the compact tension

    (CT) specimens by applying the known techniques [11].

    Eight work-pieces of larger bars for preparing the CT

    specimens (dimensions: 65 mm 62.5 mm 25 mm) were

    produced for performing the cyclic tests. The work-pieces for

    the austenitizing were placed in an induction heating furnace

    and heated up to the temperature required for austenitizing

    (900 oC). Then the work-pieces were placed in an isothermal

    transformation chamber where they were kept for the

    conventional duration for the austempering. The heat

    treatment was applied in such a way as to form three different

    layers in the specimen. Cuts were made in the CT specimens

    to ensure that the fatigue crack started in the hard layer

    (heat-treated layer), further propagating through the

    subsequent perpendicular to the intermediate and mild layers

    of the crack front (Fig. 1). The arrangement of layers was

    determined by the measurements of etching and hardness.

    The mechanical properties of the separate layers were quite

    different. The properties of the mild layer during heat

    treatment changed insignificantly. Table 2 shows that the

    plasticity of the hard layer slightly increased. The properties

    of the intermediate layer were conditional. It was difficult to

    establish them precisely enough. The heat treatment process

    is hard to control, and therefore the size and the location of

    the intermediate layer in the investigated CT specimens are

    different. The thicknesses of the layers are shown in Table 2.

    The thickness of the hard layer Lh varies within the bounds of

    27.5 mm and 36.5 mm when analyzing all the specimens,

    while that of the intermediate layer varies within the bounds

    of 1 mm and 4 mm. The remaining CT specimens can be

    described as the mild (thermally unaffected) layer. The

    different locations and thicknesses of the layers described

    above result in different impacts on the cyclic fracturing

    indicators.

    Fatigue Crack Propagation in Locally

    Heat-Treated Ductile Iron Mindaugas Leonavičius, Algimantas Krenevičius, Gediminas Petraitis, Arnoldas Norkus

    Vilnius Gediminas Technical University, Saulėtekio al. 11, 10223, Vilnius, Lithuania

  • ISSN: 2277-3754

    ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT)

    Volume 3, Issue 9, March 2014

    223

    Hard layer (temperad layer)

    Intermediate layer

    Mild layer

    LmLh15

    a

    W

    1.2

    W

    Li Fig. 1 Arrangement of layers in CT specimens

    Table 2. Thicknesses of layers in CT specimens after

    austempering

    Specimen

    Hard layer

    size Lh,

    mm

    Intermediate

    layer size Li,

    mm

    Mild layer

    size Lm,

    mm

    1 32.3 1.0 16.7

    2 27.5 2.5 20.0

    3 27.7 3.0 19.3

    4 31.0 3.0 16.0

    5 31.8 1.0 17.2

    6 31.5 2.0 16.5

    7 36.5 4.0 9.5

    8 33.5 1.0 15.5

    The microstructure and the hardness were investigated for

    specimen 5: the thickness of the hard layer is Lh = 31.8 mm,

    that of the intermediate layer is 1 mm and that of the mild

    layer is 17.0 mm. The properties were determined at five

    points, namely at 5 mm, 17 mm, 33 mm, 47 mm, and 59 mm

    from the side of the cut, as shown in Fig. 2.

    Fig. 2 Microstructure and hardness measurement points

    Cylindrical specimens (Fig. 3) for identifying the

    mechanical properties were produced from the CT

    specimens. Then tension tests were performed with the

    specimens. Analysis of the determined mechanical properties

    shows that the microstructure, hardness and mechanical

    properties of the heat-treated parts differ significantly from

    the properties of the basic cast iron and those of the

    non-heat-treated part.

    Fig. 3 Cutting scheme and dimensions of tension specimens

    The determined mechanical properties are presented in

    Table 3.

    Table 3. Mechanical properties

    Layer Rpr,

    MPa

    R0.2,

    MPa

    Rm,

    MPa

    E,

    GPa

    Z,

    %

    mild 492 607 869 165 3.3

    mild 490 602 864 164 2.9

    intermediate 490 603 864 163 2.8

    intermediate 494 608 874 163 3.2

    hard 748 957 1261 162 3.6

    hard 756 963 1254 159 4.1

    The microstructures are shown in Fig. 4. An initial

    microstructure of the SG cast irons is shown in Fig. 4a. It

    consists of pearlite with slight inclusions of ferrite and

    spherical graphite. The microstructure of the heat-treated part

    is shown in Figs. 4b, 4c, 4d, and 4e. A coarse-needle

    martensite, spheroid graphite, several cementite inclusions

    and a residual austenite are resident for the microstructure.

    The microstructure at the level of 59 mm, corresponding to

    the heat-unaffected part of the specimen, is shown in Fig. 4f.

    It consists of pearlite, ferrite and graphite. The microstructure

    of the intermediate layer (thickness 1 mm) consists of trostite,

    ferrite, and pearlite and spheroid graphite. The microstructure

    of the heat-treated part is characteristic of that of austempered

    ductile iron. The heat-unaffected part corresponds to that of

    normalized iron. The microstructure of the other specimens is

    common. The microstructure of the non-treated layers is the

    same for all specimens except for specimen 3, where more

    ferrite was detected compared with the other specimens. The

    coarse-needle martensite in the heat-treated part was detected

    only for specimens 4 and 7. The martensite of specimen 6 is

    medium-sized. Fine-needle martensite was identified for

    specimen 1. Residual cementite was found in specimens 4, 5,

    6 and 7 but was missing in specimens 1, 3 and 6. The

    intermediate layer usually consists of trostite, ferrite

    (specimens 3, 4, 5, 8), in some cases with inserts of cementite

    (specimens 1, 2, 6, 7) and fine plate pearlite (specimen 4).

    The hardness was determined at the same locations where the

    microstructure was determined, as mentioned above (Fig. 2).

    At the distances of 5 mm, 17 mm, and 33 mm (hard layer) it

    was 388 BHN, 401 BHN, and 401 BHN, at 47 mm

    (intermediate layer) it was 302 BHN, and at 59 mm (mild

    layer) it was 285 BHN. The hardness of the other specimens

    was similar. The large magnitude of hardness is conditioned

    by the martensite.

    5 3 1

    4 2 6

    5

    31

    14

    12

    M1

    0

  • ISSN: 2277-3754

    ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT)

    Volume 3, Issue 9, March 2014

    224

    (a) (b)

    (c) (d)

    (e) (f)

    Fig. 4 Structures of layers: initial (a), heat treated layer at 5 mm (b), 17 mm (c), 33 mm (d), 47 mm (e),

    Non-heat-treated layer at 59 mm (f)

    III. EXPERIMENTS

    Cyclic loading tests were performed to determine the

    threshold stress intensity factor range ΔKth in different parts

    of the specimens. CT specimens made from heat-treated

    work-pieces were tested. The test procedures conform to the

    standard ASTM E647 [11], but introducing a modification to

    determine the threshold crack propagation rates,

    corresponding to their lower magnitudes (identified during

    the exploitation of real structures) and their durability

    (serviceability) exceeding 25 years. The determined

    thresholds correspond to the cases of crack propagation rates

    being less than 10-11

    m/cycle. The procedure for determining

    the threshold involves an initial pre-cracking with a

    subsequent gradual decreasing of the load magnitude until

    the crack stabilizes for the fixed number of cycles. The

    procedure is repeated several times for the tested specimens,

    i.e., to determine the various thresholds corresponding to the

    different crack sizes. Such an approach makes it possible to

    identify the crack propagation through the separate layers

    under investigation. The relationships of crack propagation

    rates vs. the stress intensity factor range were developed by

    processing the test data of eight CT specimens (Fig. 5).

    Referring to ASTM E647 to calculate the stress intensity

    factor K, the following formula was employed:

    ,minmax KKK

    .6.572.14

    32.1364.4866.01/2)(

    ,/

    ,/

    43

    22/3

    2/1

    f

    Wa

    fBWFK

    (1)

    where K is the stress intensity factor, F is the tension force,

    B = 25 mm is the specimen width, W = 50 mm is the specimen

    base length, f() is the geometrical function, and a is the

    crack size (depth) (Fig. 1).

    100 μm 100 μm

    100 μm

    100 μm 100 μm

    100 μm

  • ISSN: 2277-3754

    ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT)

    Volume 3, Issue 9, March 2014

    225

    (a)

    1e-12

    1e-11

    1e-10

    1e-9

    1e-8

    1e-7

    5 10 15 20 25 30

    crack size a, mm

    19,0

    27,5

    5 10 15 20 25 30 ΔKth (MPa∙√m)

    10-8

    10-10

    10-12

    da

    /dN

    (m

    /cy

    cle)

    a, mm

    □ – 19.0

    ○ – 27.5

    (b)

    1e-12

    1e-11

    1e-10

    1e-9

    1e-8

    1e-7

    5 10 15 20 25 30

    crack size a, mm

    19,0

    27,5

    5 10 15 20 25 30

    ΔKth (MPa∙√m)

    10-8

    10-10

    10-12

    da

    /dN

    (m

    /cy

    cle)

    a, mm

    ◊ – 17.0

    Δ – 18,9

    (c)

    1e-12

    1e-11

    1e-10

    1e-9

    1e-8

    1e-7

    5 10 15 20 25 30

    crack size a, mm

    15.6

    18.0

    22.9

    34.1 mild

    37.4 mild

    5 10 15 20 25 30

    ΔKth (MPa∙√m)

    10-8

    10-10

    10-12

    da

    /dN

    (m

    /cy

    cle)

    a, mm

    ○ – 15.6

    □ – 18.0

    ◊ – 22.9

    ▲ – 34.1

    27.5ggfd

    fdfghdfg

    h34dfgh

    dfgh34,1 (d)

    1e-12

    1e-11

    1e-10

    1e-9

    1e-8

    1e-7

    5 10 15 20 25 30

    crack size a, mm

    15.1

    20.3

    24.1

    27.5

    30.4

    44.7 mild

    5 10 15 20 25 30

    ΔKth (MPa∙√m)

    10-8

    10-10

    10-12

    da

    /dN

    (m

    /cy

    cle)

    a, mm

    ○ – 15.1

    □ – 20.3

    ◊ – 24.1

    Δ – 27.5

    + – 30.4

    (e)

    1e-12

    1e-11

    1e-10

    1e-9

    1e-8

    1e-7

    5 10 15 20 25 30

    crack size a, mm

    15.4

    23.9

    26.6

    27.4

    33.9 mild

    5 10 15 20 25 30

    ΔKth (MPa∙√m)

    10-8

    10-10

    10-12

    da

    /dN

    (m

    /cy

    cle)

    a, mm

    ○ – 15.4

    □ – 23.9

    ◊ – 26.6

    Δ – 27.4

    ● – 33.9

    (f)

    1e-12

    1e-11

    1e-10

    1e-9

    1e-8

    1e-7

    5 10 15 20 25 30

    crack size a, mm

    15.3

    18.3

    28.8

    5 10 15 20 25 30

    ΔKth (MPa∙√m)

    10-8

    10-10

    10-12

    da

    /dN

    (m

    /cy

    cle)

    a, mm

    ○ – 15.3

    □ – 18.3

    ◊ – 28.8

    (g)

    1e-12

    1e-11

    1e-10

    1e-9

    1e-8

    1e-7

    5 10 15 20 25 30

    crack size a, mm

    17.6

    24.3

    32.6

    36.1

    40.1 mild

    5 10 15 20 25 30

    ΔKth (MPa∙√m)

    10-8

    10-10

    10-12

    da

    /dN

    (m

    /cy

    cle)

    a, mm

    ○ – 17.6

    □ – 24.3

    ◊ – 32.6

    Δ – 36.1

    ● – 40.1

    (h)

    1e-12

    1e-11

    1e-10

    1e-9

    1e-8

    1e-7

    5 10 15 20 25 30

    crak size a, mm

    17.5

    21.2

    25.0

    31.8

    38.3 mild

    39.6 mild

    5 10 15 20 25 30

    ΔKth (MPa∙√m)

    10-8

    10-10

    10-12

    da

    /dN

    (m

    /cy

    cle)

    a, mm

    ○ – 17.5

    □ – 21.2

    ◊ – 25.0

    Δ – 31.8

    ● – 38.3

    Fig. 5 Relationships for crack propagation rates (da/dN) vs. stress intensity factor range (ΔKth) of specimens: (a) specimen 1, (b)

    specimen 2, (c) specimen 3, (d) specimen 4, (e) specimen 5, (f) specimen 6, (g) specimen 7, (h) specimen 8.

  • ISSN: 2277-3754

    ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT)

    Volume 3, Issue 9, March 2014

    226

    The thresholds of all the specimens depending on crack

    size and corresponding to a rate of less than 10-11

    m/cycle

    were determined for different layers. These were different:

    for the hard layer ΔKth = 7.72-16.72 MPa m , for the

    intermediate layer ΔKth = 10.1 MPa m , for the mild layer

    ΔKth = 9.5-14.62 MPa m . The significant difference in the

    determined magnitudes of the thresholds shows the influence

    of the residual stresses and microstructure. The stresses in the

    hard layer change when the notches in the CT specimens are

    made. At an initial stage of cyclic loading, when the crack

    size is 15.6-20 mm, the ΔKth for the layer and the base

    material are similar. When the crack exceeds 20 mm, the ΔKth

    increases to 16.72 MPa m , as the compressed mild layer

    constrains the opening of the crack. When the crack passes

    the intermediate layer, the ΔKth quickly declines until the

    magnitude of 9.5 MPa m .

    IV. DISCUSSION

    The residual stresses have a great influence on fatigue

    fracturing and its indicators, and one can find many

    techniques for determining these stresses [12]-[14]. The

    relationships (Fig. 6) of the thresholds vs. crack size were

    developed on the basis of the performed experimental data to

    evaluate the influence of the residual stresses.

    a)

    10 20 30 40 a (mm)

    20

    16

    12

    8

    ΔK

    th (

    MP

    a√m

    )

    notch crack

    b)

    10 20 30 40 a (mm)

    20

    16

    12

    8

    Δ

    Kth

    (M

    Pa√

    m)

    c)

    10 20 30 40 a (mm)

    20

    16

    12

    8

    ΔK

    th (

    MP

    a√m

    )

    d)

    10 20 30 40 a (mm)

    20

    16

    12

    8

    ΔK

    th (

    MP

    a√m

    )

    e)

    10 20 30 40 a (mm)

    20

    16

    12

    8

    ΔK

    th (

    MP

    a√m

    )

    f)

    10 20 30 40 a (mm)

    20

    16

    12

    8

    ΔK

    th (

    MP

    a√m

    )

    g)

    10 20 30 40 a (mm)

    20

    16

    12

    8

    ΔK

    th (

    MP

    a√m

    )

    h)

    10 20 30 40 a (mm)

    20

    16

    12

    8

    ΔK

    th (

    MP

    a√m

    )

    Fig. 6 Relationships of threshold stress intensity factor range (ΔKth) vs. crack size (a) for specimens : (a) specimen 1, (b)

    specimen 2, (c) specimen 3, (d) specimen 4, (e) specimen 5, (f) specimen 6, (g) specimen 7, (h) specimen 8; ○ – crack tip before

    intermediate layer; ● – crack tip beyond intermediate layer

  • ISSN: 2277-3754

    ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT)

    Volume 3, Issue 9, March 2014

    227

    Fig. 7 shows the generalized relationship (obtained via

    processing the experimental data of all specimens, given in

    Fig. 5) of the threshold vs. crack size.

    Fig. 7 Generalized relationship for specimens 3, 5, 7 and 8 of

    threshold stress intensity factor range (ΔKth) vs. crack

    size (a): ○ – hard layer; ● – mild layer

    Analysis of the relationships of the thresholds vs. crack

    sizes shows that the threshold increases when the crack

    approaches the intermediate and mild layers, and reduces

    when it enters the mild layer. The maximum magnitude of

    ΔKth is reached just prior to the intermediate layer (Figs. 6b,

    6g, 6h). The processed experimental data resulted in the

    following relations:

    mean: ,503.1423.0th aK (2)

    upper limit: ,734.343.0th aK (3)

    lower limit: .032.143.0th aK (4)

    The images of the fractures in specimens 3 and 5 are

    presented in Fig. 8. It can be seen that at the first stages

    of cyclic loading many focuses of cracks appear in the

    vicinity of the notch. Further, when increasing the number of

    loading cycles and when the crack size reaches approx. 2-3

    mm from the top, the main crack is formed. One can also

    identify the separate layers of different structure in the

    fractures.

    (a)

    (b)

    Fig. 8 Fractures of CT specimens 3 (a) and 5 (b)

    The images of the separate zones, obtained by scanning

    electronic microscopy (SEM), are presented in Fig. 9. It can

    be seen that the microstructures (presented in Fig. 4) of the

    different layers have a great influence on the fracturing

    process.

    (a) (b) (c)

    Fig. 9 SEM fracture images of 5th specimen at hard (a), intermediate (b) and mild layers (c)

    Fig. 10 shows the crack propagation model with regard to

    the influence of residual stresses. When residual stresses σr

    do not develop (Fig. 10a), the characteristic pre-cracking

    zone forms in the material and the crack opening gradually

    increases through the specimen. In the presence of residual

    compressive stresses (Fig. 10b), the pre-cracking zone and

    the crack opening are smaller as residual stresses reduce the

    effective stress intensity factor at the crack tip. As a result, the

    threshold stress intensity factor increases, i.e., the fracture

    process slows down. The tension stresses must be increased

    up to σ', as shown in Fig. 10c, to continue the cracking

    process.

    Hard (40.2mm)

    Mild (19.3mm)

    Intermediate (3.0 mm)

    Intermediate (1.0 mm)

    Hard (46.8 mm) Mild

    (17.3 mm)

    a (mm)

    ΔK

    th (

    MP

    a·√

    m)

  • ISSN: 2277-3754

    ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT)

    Volume 3, Issue 9, March 2014

    228

    (a)

    σ

    σpre-crack

    a

    pre-fracture

    zone

    (b)

    σ

    σ

    pre-fracture

    zone

    transitional

    layer

    σr

    σr

    mild layer

    hard

    layer

    (c)

    σ'

    σ'

    pre-fracture

    zone

    σr

    σr

    a

    Fig. 10. Crack growth model: (a) – at stresses σ, without

    residual stresses; (b) – at stresses σ, with residual stress, crack is

    closed (does not spread); (c) – for crack propagation the stresses

    σ‘ =σ + σr are necessary

    The threshold stress intensity range decreases when the

    crack crosses the heat-affected layer.

    V. CONCLUSIONS

    1. During the localized austempering three layers with

    different microstructures, hardnesses and mechanical

    properties are formed comparing with a basic cast iron. The

    developed residual stresses, which distribute unevenly along

    the crack propagation direction, influence the loading cycle

    and affect the process of cyclic fracturing.

    2. For evaluation of the influences of cyclic loading, micro

    structural changes and residual stresses on fracturing process

    the stress intensity factor K was chosen. For the obtaining the

    dependency ΔK-da/dN the experimental research methodizes

    had involved the crack size and layers arrangement.

    3. The thresholds determined for separate layers vary

    within the wide range of 7.72 and 16.72 MPa·m1/2

    in the case

    when crack propagation rate is less the 10-11

    m/cycle and it

    rapidly decreases having passed the intermediate layer.

    4. The test results yield the variation of ΔKth = 7.72-10.55

    MPa·m1/2

    related to the crack depth variation of 15.6-20 mm.

    The increment of crack enlarges the ΔKth. This means that the

    microstructure and the residual stresses change stress strain

    state at crack tip so reducing the crack propagation.

    VI. ACKNOWLEDGMENT

    The equipment and infrastructure of the Civil Engineering

    Scientific Research Center of Vilnius Gediminas Technical

    University was employed for investigations.

    REFERENCES [1] R. A. Harding, ―The production, properties and automotive

    applications of austempered ductile iron,‖ Kovove Mater, vol.

    45, pp. 1-16, 2007.

    [2] K. L. Hayrynen, K. R. Brandenberg, and J. R. Keough, ―Applications of austempered cast irons,‖ AFS Transactions,

    vol. 110, pp. 929-938, 2002.

    [3] V. Rudnev, D. Loveless, R. Cook, and M. Black, ―Handbook of Induction Heating,‖ New York: Marcel Dekker, pp. 11-14,

    2003.

    [4] E. S. Davenport, and E. C. Bain, ―Transformation of austenite at constant subcritical temperatures,‖ Trans Am Inst Min

    Metall Eng, vol. 90, pp. 117-44, 1930.

    [5] E. Dorazil, ―High strength austempered ductile cast iron,‖ Prague, Ellis Horwood, p. 244, 1991.

    [6] S. Tada, T. Abe, T. Takahashi, and H. Nemoto, ―Local Austempering Treatment by Electric Current Zone Heating,‖

    Journal of Japan Foundry Engineering Society, vol. 69(9), pp.

    725-730, 1997.

    [7] C. Bixler, K. Hayrynen, J. Keough, G. Pfaffmann, et al. ―Locally Austempered Ductile Iron (LADI),‖ SAE Technical

    Paper 2010-01-0652, doi: 10.4271/2010-01-0652, 2010.

    [8] D. H. Herring, ―A Discussion of Retained Austenite,‖ Industrial Heating, vol. 72(3), pp. 14-16, 2005.

    [9] T. J. Marrow, H. Centinel, S. H. Macdonald, A. Venslovas, and M. Leonavičius, ‖X–ray Tomography of Short Fatigue Cracks

    in Ductile Iron,‖ In: Proceedings of the 14th Biennial

    Conference on Fracture (ed Niemetz A, Rokach IV, Kocanda

    D, Golos K), Krakow, Poland (ECF 14), Fracture Mechanics

    Beyond 2000, vol. II/III, pp. 443–450, 2002.

    [10] K. B. Rundman, ―Heat Treating of Ductile Irons,‖ ASM Handbook, vol. 4, pp. 682-692, 1991.

    [11] ASTM E 647-00. ―Standard Test Method for Measurement of Fatigue Crack Growth Rates‖.

    [12] K. Xu, J. He, and H. Zhou, ―Effect of residual stress on fatigue behavior of notches,‖ Fatigue, vol. 16, pp. 337-343, 1994.

    [13] M. Benedetti, V. Fontanari, B. R. Hohn, P. Oster, and T. Tobie, ―Influence of Shot Peening on Bending Tooth Fatigue Limit of

    Case Hardened Gears,‖ Int. J. Fatigue, vol 24, pp. 1127-1136,

    2002.

    [14] Y. Sun, H. Choo, P. K. Liaw, Y. Lu, B. Yang, D. W. Brown, and M. A. Bourke, ―Neutron diffraction studies on lattice strain

    intermediate

    https://www.jstage.jst.go.jp/AF06S010ShsiKskGmnHyj?chshnmHkwtsh=Shuji+Tadahttps://www.jstage.jst.go.jp/AF06S010ShsiKskGmnHyj?chshnmHkwtsh=Toshihiko+Abehttps://www.jstage.jst.go.jp/AF06S010ShsiKskGmnHyj?chshnmHkwtsh=Toshio+Takahashihttps://www.jstage.jst.go.jp/AF06S010ShsiKskGmnHyj?chshnmHkwtsh=Hidemi+Nemoto

  • ISSN: 2277-3754

    ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT)

    Volume 3, Issue 9, March 2014

    229

    evolution around a crack-tip during tensile loading and

    unloading cycle,‖ Scripta Materialia, vol. 53, pp. 971-975,

    2005.

    AUHTOR’S PROFILE

    Prof. Mindaugas Leonavičius. Professor of Dept.

    of Strength of Materials of Vilnius Gediminas Technical University. Research interests: strength of materials, experimental mechanics, fracture

    mechanics.

    Prof. Algimantas Krenevičius. Professor of Dept.

    of Strength of Metarials of Vilnius Gediminas Technical University.

    Research interests: fatigue of structures, fracture mechanics.

    Assoc. Prof. Gediminas Petraitis. In 2002 graduated Vilnius Gediminas Technical university. Since 2006 head of

    Research Laboratory of Strength Mechanics of Vilnius Gediminas Technical

    University. Research interests: high cycle fatigue, fracture mechanics.

    Prof. Arnoldas Norkus. Professor at Vilnius Gediminas Technical University, Dept. of Geotechnical Engineering. Since

    2007 Head of Dept. Research interests: soil mechanics, modeling of mechanical properties of materials, mechanics of structures