Standard Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement
This test method characterizes the fracture toughness of materials through the determination of crack-tip opening displacement (CTOD) at one of three events: (a) onset of unstable crack extension without significant prior stable crack extension, or (b) onset of unstable crack extension with significant prior stable crack extension, or (c) the end-of-test after significant slow stable crack extension. This test method may also be used to characterize the toughness of materials for which the properties and thickness of interest preclude the determination of K lc fracture toughness in accordance with Test Method E 399.
The different values of CTOD determined by this test method can be used to characterize the resistance of a material to crack initiation and early crack extension at a given temperature.
The values of CTOD may be affected by specimen dimensions. It has been shown that values of CTOD determined on SE(B) specimens using the square section geometry may not be the same as those using the rectangular section geometry, and may differ from those obtained with either the C(T) or A(B) specimens.
The values of CTOD determined by this test method may serve the following purposes:
In research and development, CTOD testing can show the effects of certain parameters on the fracture toughness of metallic materials significant to service performance. These parameters include material composition, thermo-mechanical processing, welding, and thermal stress relief.
CTOD testing may be used in specifications of acceptance and manufacturing quality control of base materials, weld metals, and weld heat affected zones. Previous versions of Test Method E 1290 made effective use of the value of CTOD at the first attainment of a maximum force plateau for such purposes. Qualitative comparisons of this type can only be made if a consistent specimen geometry is used and the materials compared have similar constitutive properties. The value of CTOD at the first attainment of a maximum force plateau was removed from this test method because is was not associated with a measurement of crack extension and therefore cannot be considered a measurement of fracture toughness. The δeot value may be used in place of the value of CTOD at the first attainment of a maximum force plateau for quality control and specifications.
The δc and δeot values from CTOD testing can be used for inspection and flaw assessment criteria, when used in conjunction with other standards such as Test Methods E 1921 and E 1820 and informed fracture mechanics analyses. Awareness of differences that may exist between laboratory test and field conditions is required to make proper flaw assessment (see 4.3 and 4.4).
1.1 This test method covers the determination of critical crack-tip opening displacement (CTOD) values at one or more of several crack extension events, and may be used to measure cleavage crack initiation toughness for materials that exhibit a change from ductile to brittle behavior with decreasing temperature, such as ferritic steels. This test method applies specifically to notched specimens sharpened by fatigue cracking. The recommended specimens are three-point bend [SE(B)], compact [C(T)], or arc-shaped bend [A(B)] specimens. The loading rate is slow and influences of environment (other than temperature) are not covered. The specimens are tested under crosshead or clip gage displacement controlled loading.
1.1.1 The recommended specimen thickness, B, for the SE(B) and C(T) specimens is that of the material in thicknesses intended for an application. For the A(B) specimen, the recommended depth, W, is the wall thickness of the tube or pipe from which the specimen is obtained. Superficial surface machining may be used when desired.
1.1.2 For the recommended three-point bend specimens [SE(B)], width, W, is either equal to, or twice, the specimen thickness, B, depending upon the application of the test. (See 4.3 for applications of the recommended specimens.) For SE(B) specimens the recommended initial normalized crack size is 0.45 ≤ ao/W ≤ 0.70. The span-to-width ratio (S/W) is specified as 4.
1.1.3 For the recommended compact specimen [C(T)] the initial normalized crack size is 0.45 ≤ ao/W ≤ 0.70. The half-height-to-width ratio (H/W) equals 0.6 and the width to thickness ratio W/B is specified to be 2.
1.1.4 For the recommended arc-shaped bend [A(B)] specimen, B is one-half the specimen depth, W. The initial normalized crack size is 0.45 < ao/W< 0.70. The span to width ratio, S/W, may be either 3 or 4 depending on the ratio of the inner to outer tube radius. For an inner radius, r1, to an outer radius, r2, ratio of > 0.6 to 1.0, a span to width ratio, S/W, of 4 may be used. For r1/r2 ratios from 0.4 to 0.6, an S/W of 3 may be used.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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