ASTM E1221 Test for Determining Plane-Strain Crack-Arrest Fracture Toughness, KIa, of Ferritic Steels

ASTM E1221 Test for Determining Plane-Strain Crack-Arrest Fracture Toughness, KIa, of Ferritic Steels

ASTM E1221 test method is used to determine the plane-strain crack-arrest fracture toughness, KIa , of ferritic steels. Crack-arrest fracture toughness, KA , is the ability of a region in a material to halt a crack.

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    Scope:

    When a fast-running crack arrests, the stress intensity factor is a measure of the ability of the material to arrest such a crack. The stress intensity factor provides a value for the crack-arrest fracture toughness, KA. ASTM E1221 test method provides a static analysis determination of the stress intensity factor in a short time after the crack arrest. The estimate is denoted as Ka. When certain size requirements are met, the test result provides an estimate, termed KIa, of the plane-strain crack-arrest toughness of the material. KA cannot be determined if the specimen does not show rapid crack propagation and arrest. This test method is used in a material’s research and development and to determine the effects of metallurgical variables such as composition or heat treatment on the ability to arrest running cracks. 

    Test Procedure:

    ASTM E1221 method is used to calculate the stress intensity factor, K. In this test, a wedge is forced into a split-pin. This applies a force across the crack starter notch, provoking a run and arrest crack extension. This run-arrest crack propagation needs a dynamic analysis of the test results. However, an adjusted static analysis of test results provides a useful estimate of the stress intensity factor at the time of the crack arrest. KA is determined from dynamic analysis; KIa is determined from static analysis. 

    K0 is the stress intensity factor, K, at the onset of crack propagation. 

    Video 01: Method to measure fracture toughness

    Specimen size:

    • This ASTM E1221 test method uses a side-grooved, crack-line-wedge-loaded specimen to obtain a rapid run-arrest segment of flat-tensile separation with a nearly straight crack front. 
    • The thickness should be either full product plate thickness or a thickness sufficient to produce a condition of plane strain. 
    • Certain size requirements must be met to limit plastic deformation in the specimen before crack initiation. These requirements depend upon the material yield strength, and Ka and K0 are needed to achieve an appropriate run-arrest event. 
    • The in-plane specimen dimensions must be large enough to allow for the linear elastic analysis employed. 
    • A starting notch should be machined that will permit crack propagation. Different materials require different starter notch preparation procedures. 

    Data:

    From the autographic force-displacement record, several displacement values are determined. 

    The following data is calculated in ASTM E1221:

    Calculation of KA and KQa

    Where:

    E = Young’s modulus,

    a = slot size,

    W = specimen width, 

    B = specimen thickness, 

    BN = specimen thickness at the crack plane, 

    δ = crack mouth opening displacement, 

    To calculate K0, use a = a0 and δ = d0

    To calculate Ka, use a = aa and δ = da. 

    The quantities d0 and da are given as follows:

    Young modulus: 

    Y = (38.3 × f2 × ρ × ℓ4)/d2

    Y = Young’s Modulus

    f = resonance frequency of oscillations.

    ρ = density

    ℓ = length

    d = width

    Conclusion:

    ASTM E1221 test method provides a static analysis determination of the stress intensity factor in a short time after the crack arrest.

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