Designation: E1921 − 18Standard Test Method forDetermination of Reference Temperature, To, for FerriticSteels in the Transition Range1This standard is issued under the fixed designation E1921; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the determination of a referencetemperature, To, which characterizes the fracture toughness offerritic steels that experience onset of cleavage cracking atelastic, or elastic-plastic KJcinstabilities, or both. The specifictypes of ferritic steels (3.2.1) covered are those with yieldstrengths ranging from 275 to 825 MPa (40 to 120 ksi) andweld metals, after stress-relief annealing, that have 10 % orless 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-shapedcompact tension specimens, C(T) or DC(T). A range ofspecimen sizes with proportional dimensions is recommended.The dimension on which the proportionality is based isspecimen thickness.1.3 Median KJcvalues tend to vary with the specimen typeat a given test temperature, presumably due to constraintdifferences among the allowable test specimens in 1.2. Thedegree of KJcvariability among specimen types is analyticallypredicted to be a function of the material flow properties (1)2and decreases with increasing strain hardening capacity for agiven yield strength material. This KJcdependency ultimatelyleads to discrepancies in calculated Tovalues as a function ofspecimen type for the same material. Tovalues obtained fromC(T) specimens are expected to be higher than Tovaluesobtained from SE(B) specimens. Best estimate comparisons ofseveral materials indicate that the average difference betweenC(T) and SE(B)-derived Tovalues is approximately 10°C (2).C(T) and SE(B) Todifferences up to 15°C have also beenrecorded (3). However, comparisons of individual, small data-sets may not necessarily reveal this average trend. Datasetswhich contain both C(T) and SE(B) specimens may generateToresults which fall between the Tovalues calculated usingsolely C(T) or SE(B) specimens. It is therefore stronglyrecommended that the specimen type be reported along withthe derived Tovalue in all reporting, analysis, and discussion ofresults. This recommended reporting is in addition to therequirements in 11.1.1.1.4 Requirements are set on specimen size and the numberof replicate tests that are needed to establish acceptablecharacterization of KJcdata populations.1.5 Tois dependent on loading rate. Tois evaluated for aquasi-static loading rate range with 0.12MPa√m/s) in Annex A1. Note that this threshold loading ratefor application of Annex A1 is a much lower threshold than isrequired in other fracture toughness test methods such as E399and E1820.1.6 The statistical effects of specimen size on KJcin thetransition range are treated using the weakest-link theory (4)applied to a three-parameter Weibull distribution of fracturetoughness values. A limit on KJcvalues, relative to thespecimen size, is specified to ensure high constraint conditionsalong the crack front at fracture. For some materials, particu-larly those with low strain hardening, this limit may not besufficient to ensure that a single-parameter (KJc) adequatelydescribes the crack-front deformation state (5).1.7 Statistical methods are employed to predict the transi-tion toughness curve and specified tolerance bounds for 1Tspecimens of the material tested. The standard deviation of thedata distribution is a function of Weibull slope and median KJc.The procedure for applying this information to the establish-ment of transition temperature shift determinations and theestablishment of tolerance limits is prescribed.1.8 This test method assumes that the test material ismacroscopically homogeneous such that the materials haveuniform tensile and toughness properties. The fracture tough-ness evaluation of nonuniform materials is not amenable to thestatistical analysis methods employed in the main body of thistest method. Application of the analysis of this test method toan inhomogeneous material will result in an inaccurate esti-mate of the transition reference value Toand non-conservativeconfidence bounds. For example, multipass weldments can1This test method is under the jurisdiction of ASTM Committee E08 on Fatigueand Fracture and is the direct responsibility of E08.07 on Fracture Mechanics.Current edition approved Jan. 15, 2018. Published March 2018. Originallyapproved in 1997. Last previous edition approved in 2017 as E1921 – 17a. DOI:10.1520/E1921-18.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1create heat-affected and brittle zones with localized propertiesthat are quite different from either the bulk material or weld.Thick section steels also often exhibit some variation inproperties near the surfaces. Metallography and initial screen-ing may be necessary to verify the applicability of these andsimilarly graded materials. An appendix to analyze the cleav-age toughness properties of nonuniform or inhomogeneousmaterials is currently being prepared. In the interim, users arereferred to (6-8) for procedures to analyze inhomogeneousmaterials.1.9 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.10 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:3E4 Practices for Force Verification of Testing MachinesE8/E8M Test Methods for Tension Testing of Metallic Ma-terialsE23 Test Methods for Notched Bar Impact Testing of Me-tallic MaterialsE74 Practice of Calibration of Force-Measuring Instrumentsfor Verifying the Force Indication of Testing MachinesE111 Test Method for Young’s Modulus, Tangent Modulus,and Chord ModulusE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE208 Test Method for Conducting Drop-Weight Test toDetermine Nil-Ductility Transition Temperature of Fer-ritic SteelsE399 Test Method for Linear-Elastic Plan-Strain FractureToughness KIcof Metallic MaterialsE436 Test Method for Drop-Weight Tear Tests of FerriticSteelsE561 Test Method for KRCurve DeterminationE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodE1820 Test Method for Measurement of Fracture ToughnessE1823 Terminology Relating to Fatigue and Fracture Testing2.2 ASME Standards:4ASME Boiler and Pressure Vessel Code, Section II, Part D3. Terminology3.1 Terminology given in Terminology E1823 is applicableto this test method.3.2 Definitions:3.2.1 ferritic steels—typically carbon, low-alloy, and higheralloy grades. Typical microstructures are bainite, temperedbainite, tempered martensite, and ferrite and pearlite. Allferritic steels have body centered cubic crystal structures thatdisplay ductile-to-cleavage transition temperature fracturetoughness characteristics. See also Test Methods E23, E208and E436.3.2.1.1 Discussion—This definition is not intended to implythat all of the many possible types of ferritic steels have beenverified as being amenable to analysis by this test method.3.2.2 stress-intensity factor, K [FL– 3/2]—the magnitude ofthe mathematically ideal crack-tip stress field coefficient (stressfield singularity) for a particular mode of crack-tip regiondeformation in a homogeneous body.3.2.2.1 Discussion—In this test method, Mode I is assumed.See Terminology E1823 for further discussion.3.2.3 J-integral, J [FL–1]—a mathematical expression; aline or surface integral that encloses the crack front from onecrack surface to the other; used to characterize the localstress-strain field around the crack front (9). See TerminologyE1823 for further discussion.3.3 Definitions of Terms Specific to This Standard:3.3.1 control force, Pm[F]—a calculated value of maximumforce, used in 7.8.1 to stipulate allowable precracking limits.3.3.2 crack initiation—describes the onset of crack propa-gation from a preexisting macroscopic crack created in thespecimen by a stipulated procedure.3.3.3 effective modulus, Eeff[FL–2]—an elastic modulus thatallows a theoretical (modulus normalized) compliance tomatch an experimentally measured compliance for an actualinitial crack size, ao.3.3.4 effective yield strength, σY[FL-2], — an assumed valueof uniaxial yield strength that represents the influence of plasticyielding upon fracture test parameters.3.3.4.1 Discussion—It is calculated as the average of the0.2 % offset yield strength σYS, and the ultimate tensilestrength, σTSas follows:σY5σYS1σTS23.3.5 elastic modulus, E [FL–2]—a linear-elastic factorrelating stress to strain, the value of which is dependent on thedegree of constraint. For plane stress, E = E is used, and forplane strain, E/(1 – v2) is used, with E being Young’s modulusand v being Poisson’s ratio.3.3.6 elastic plastic Jc[FL–1]—J-integral at the onset ofcleavage fracture.3.3.7 elastic-plastic KJ[FL–3/2]—An elastic-plastic equiva-lent stress intensity factor derived from the J-integral.3.3.7.1 Discussion—In this test method, KJalso implies a3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at

[email protected] For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.4Available from American Society of Mechanical Engineers (ASME), ASMEInternational Headquarters, Two Park Ave., New York, NY 10016-5990, http://www.asme.org.E1921 − 182stress intensity factor determined at the test termination pointunder conditions that require censoring the data by 8.9.2.3.3.8 elastic-plastic KJc[FL–3/2]—an elastic-plastic equiva-lent stress intensity factor derived from the J-integral at thepoint of onset of cleavage fracture, Jc.3.3.9 equivalent value of median toughness, KJc~med!eq[FL-3/2]—an equivalent value of the median toughness for amulti-temperature data set.3.3.10 Eta (η)—a dimensionless parameter that relates plas-tic work done on a specimen to crack growth resistance definedin terms of deformation theory J-integral (10).3.3.11 failure probability, pf—the probability that a singleselected specimen chosen at random from a population ofspecimens will fail at or before reaching the KJcvalue ofinterest.3.3.12 initial ligament length, bo[L]— the distance from theinitial crack tip, ao, to the back face of a specimen.3.3.13 load-line displacement rate,∆˙LL[LT-1]—rate of in-crease of specimen load-line displacement.3.3.14 pop-in—a discontinuity in a force versus displace-ment test record (11).3.3.14.1 Discussion—A pop-in event is usually audible, andis a sudden cleavage crack initiation event followed by crackarrest. The test record will show increased displacement anddrop in applied force if the test frame is stiff. Subsequently, thetest record may continue on to higher forces and increaseddisplacements.3.3.15 precracked Charpy, PCC, specimen—SE(B) speci-men with W = B = 10 mm (0.394 in.).3.3.16 provisional reference temperature, (ToQ) [°C]—Interim Tovalue calculated using the standard test methoddescribed herein. ToQis validated as Toin 10.5.3.3.17 reference temperature, To[°C]—The test temperatureat which the median of the KJcdistribution from 1T sizespecimens will equal 100 MPa√m (91.0 ksi√in.).3.3.18 SE(B) specimen span, S [L]—the distance betweenspecimen supports (See Test Method E1820 Fig. 3).3.3.19 specimen thickness, B [L]—the distance between theparallel sides of a test specimen as depicted in Fig. 1–3.3.3.19.1 Discussion—In the case of side-groovedspecimens, the net thickness, BN, is the distance between theroots of the side-groove notches.3.3.20 specimen size, nT—a code used to define specimendimensions, where n is expressed in multiples of 1 in.3.3.20.1 Discussion—In this method, specimen proportion-ality is required. For compact specimens and bend bars,specimen thickness B=ninches.3.3.21 temperature, TQ[°C]—For KJcvalues that are devel-oped using specimens or test practices, or both, that do notconform to the requirements of this test method, a temperatureat which KJc (med)= 100 MPa√m is defined as TQ.TQis not aprovisional value of To.3.3.22 time to control force, tm[T],—time to Pm.3.3.23 Weibull fitting parameter, K0—a scale parameterlocated at the 63.2 % cumulative failure probability level (12).KJc=K0when pf= 0.632.3.3.24 Weibull slope, b—with pfand KJcdata pairs plotted inlinearized Weibull coordinates obtainable by rearranging Eq18, b is the slope of a line that defines the characteristics of thetypical scatter of KJcdata.3.3.24.1 Discussion—A Weibull slope of 4 is used exclu-sively in this method.3.3.25 yield strength, σYS[FL−2]—the stress at which amaterial exhibits a specific limiting deviation from the propor-tionality of stress to strain at the test temperature. Thisdeviation is expressed in terms of strain.3.3.25.1 Discussion—It is customary to determine yieldstrength by either (1) Offset Method (usually a strain of 0.2 %is specified) or (2) Total-Extension-Under-Force Method (usu-ally a strain of 0.5 % is specified although other values of strainmay be used).3.3.25.2 Discussion—Whenever yield strength is specified,the method of test must be stated along with the percent offsetor total strain under force. The values obtained by the twomethods may differ.4. Summary of Test Method4.1 This test method involves the testing of notched andfatigue precracked bend or compact specimens in a tempera-ture range where either cleavage cracking or crack pop-indevelop during the loading of specimens. Crack aspect ratio,a/W, is nominally 0.5. Specimen width in compact specimensis two times the thickness. In bend bars, specimen width can beeither one or two times the thickness.4.2 Force versus displacement across the notch at a speci-fied location is recorded by autographic recorder or computerdata acquis