ASTM E1921-24 - Standard Test Method for Determination of Reference Temperature, <emph type="bdit">T<inf >0</inf></emph>, for Ferritic Steels in the Transition Range

1.1 This test method covers the determination of a reference temperature, T₀, which characterizes the fracture toughness of ferritic steels that experience onset of cleavage cracking at elastic, or elastic-plastic K_Jc instabilities, or both. The specific types of ferritic steels (3.2.2) covered are those with yield strengths ranging from 275 MPa to 825 MPa (40 ksi to 120 ksi) and weld metals, after stress-relief annealing, that have 10 % or less strength mismatch relative to that of the base metal.

1.2 The specimens covered are fatigue precracked single-edge notched bend bars, SE(B), and standard or disk-shaped compact tension specimens, C(T) or DC(T). A range of specimen sizes with proportional dimensions is recommended. The dimension on which the proportionality is based is specimen thickness.

1.3 Median K_Jc values tend to vary with the specimen type at a given test temperature, presumably due to constraint differences among the allowable test specimens in 1.2. The degree of K_Jc variability among specimen types is analytically predicted to be a function of the material flow properties (1)2 and decreases with increasing strain hardening capacity for a given yield strength material. This K_Jc dependency ultimately leads to discrepancies in calculated T₀ values as a function of specimen type for the same material. T₀ values obtained from C(T) specimens are expected to be higher than T₀ values obtained from SE(B) specimens. Best estimate comparisons of several materials indicate that the average difference between C(T) and SE(B)-derived T₀ values is approximately 10°C (2). C(T) and SE(B) T₀ differences up to 15 °C have also been recorded (3). However, comparisons of individual, small datasets may not necessarily reveal this average trend. Datasets which contain both C(T) and SE(B) specimens may generate T₀ results which fall between the T₀ values calculated using solely C(T) or SE(B) specimens. It is therefore strongly recommended that the specimen type be reported along with the derived T₀ value in all reporting, analysis, and discussion of results. This recommended reporting is in addition to the requirements in 11.1.1.

1.4 Requirements are set on specimen size and the number of replicate tests that are needed to establish acceptable characterization of K_Jc data populations.

1.5 T₀ is dependent on the K-rate. T₀ is evaluated for a quasi-static K-rate range with 0.5 < < 2 MPa√m/s. T₀ values for slowly loaded specimens ( < 0.5 MPa√m) can be considered valid if environmental effects are known to be negligible. Provision is also made for higher K-rates ( > 2 MPa√m/s) in Annex A1. Note that this threshold K-rate for application of Annex A1 is a much lower threshold than is required in other fracture toughness test methods such as E399 and E1820.

1.6 The statistical effects of specimen size on K_Jc in the transition range are treated using the weakest-link theory (4) applied to a three-parameter Weibull distribution of fracture toughness values. A limit on K_Jc values, relative to the specimen size, is specified to ensure high constraint conditions along the crack front at fracture. For some materials, particularly those with low strain hardening, this limit may not be sufficient to ensure that a single-parameter (K_Jc) adequately describes the crack-front deformation state (5).

1.7 Statistical methods are employed to predict the transition toughness curve and specified tolerance bounds for 1T specimens of the material tested. The standard deviation of the data distribution is a function of Weibull slope and median K_Jc. The procedure for applying this information to the establishment of transition temperature shift determinations and the establishment of tolerance limits is prescribed.

1.8 The procedures described in this test method assume that the data set represents a macroscopically homogeneous material, such that the test material has uniform tensile and toughness properties. Application of this test method to an inhomogeneous material will result in an inaccurate estimate of the transition reference value T₀ and nonconservative confidence bounds. For example, multi-pass weldments can create heat-affected and brittle zones with localized properties that are quite different from either the bulk or weld materials. Thick-section steels also often exhibit some variation in properties near the surfaces. Metallography and initial screening may be necessary to verify the applicability of these and similarly graded materials. Section 10.6 provides a screening criterion to assess whether the data set may not be representative of a macroscopically homogeneous material, and therefore, may not be amenable to the statistical analysis procedures employed in this test method. If the data set fails the screening criterion in 10.6, the homogeneity of the material and its fracture toughness can be more accurately assessed using the analysis methods described in Appendix X5.

1.9 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.10 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.