ASTM E1681 Test for Determining Threshold Stress Intensity Factor for Environment-Assisted Cracking of Metals

ASTM E1681 Test for Determining Threshold Stress Intensity Factor for Environment-Assisted Cracking of Metals

ASTM E1681 test method is used to determine the environment-assisted cracking threshold stress intensity factor parameters, KIEAC and KEAC, for metals from constant-force testing of fatigue pre-cracked beam or compact fracture specimens and constant-displacement testing of fatigue pre-cracked bolt-load compact fracture specimens.

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    Scope:

    ASTM E1681 method determines the resistance of a pre-cracked metal crack growth under certain environmental conditions and loading conditions in which the crack-tip plastic region is small compared with the crack depth and the uncracked ligament. This test method is used in environment-assisted cracking in aqueous or other aggressive environments.

    Test Procedure:

    In ASTM E1681, the specimens tested are single-edge notched specimens, compact specimens, or bolt-load compact specimens. They are precracked by fatigue. The single-edge notched specimen is tested by deadweight loading. An environmental chamber is provided, which is either attached to the specimen, or the specimen is contained within the chamber. Specified environmental conditions are maintained within the chamber during the test. In ASTM E1681, the specimens are either deadweight loaded, held under constant force, or held under constant displacement for a specified length of time. Crack growth or fracture may or may not happen. 

    KIEAC and KEAC are defined as the highest value of stress intensity factor at which neither failure nor crack growth occurs. The KIEAC value calculated is valid only if the specimen meets the size requirements to ensure plane strain conditions. The KEAC value calculated depends on meeting the size requirements to ensure plane strain conditions, as stated in Test Method E399. The validity of the KEAC value depends on meeting the size requirements for linear elastic behavior, as stated in Test Method E647. 

    Specimen size:

    For the ASTM E1681 test results to be valid, the specimen should be predominantly elastic in its behavior.

    This test involves testing single-edge notched [SE(B)] specimens, compact [C(T)] specimens, or bolt-load compact [MC(W)] specimens, precracked in fatigue.

    Metals tested are not limited by thickness or by strength as long as their specimens are of sufficient thickness and planar size to meet the size requirements of this test. 

    A range of specimen sizes with proportional planar dimensions are provided, but the size may be variable and adjusted for yield strength and applied force. Specimen thickness is a variable independent of planar size.

    Data:

    The following data is calculated in ASTM E1681:

    The Stress Intensity Factor, K: 

    The stress intensity factor formula for the beam specimen is:

    α = 1 – (a0/W), 

    M = bending moment on the crack plane, 

    M = WaLa + Wt L, 

    Wa = weight of the arm, 

    La = distance from the notch plane to the center of gravity of the arm, 

    Wt = total weight of the platen, platen support, and added weight, 

    L = moment arm,

    B = specimen thickness,

    W = specimen width, 

    a0 = original crack size.

    The stress intensity factor formula for the compact specimen, taken from Test Method E399, is:

    a0 = original crack size

    B = specimen thickness

    W = specimen width, and 

    P = force.

    The stress intensity factor formula for the bolt-load compact specimen is:

    Vm = crack-mouth opening displacement on the specimen face, 

    E = Young’s modulus, 

    a = original or final crack size,

    B = specimen thickness, and 

    W = specimen width.

    For the beam and compact specimens, the value of KIEAC and KEAC determined by this ASTM E1681 test method is the highest applied K level that does not cause a fracture or evidence of subcritical crack growth in a specimen after reaching the recommended test duration. 

    For the bolt-load compact specimen, the value of KIEAC or KEAC, determined by this test method is the lowest applied K level that shows evidence of subcritical crack growth in a specimen after reaching the recommended test duration.

    Conclusion:

    ASTM E1681 method determines the resistance of a pre-cracked metal crack growth under certain environmental conditions and loading conditions in which the crack-tip plastic region is small compared with the crack depth and the uncracked ligament. 

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