# ASTM E1457-15

Designation: E1457 − 15Standard Test Method forMeasurement of Creep Crack Growth Times in Metals1This standard is issued under the fixed designation E1457; 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 creep crackinitiation (CCI) and creep crack growth (CCG) in metals atelevated temperatures using pre-cracked specimens subjectedto static or quasi-static loading conditions. The solutionspresented in this test method are validated for base material(i.e. homogenous properties) and mixed base/weld materialwith inhomogeneous microstructures and creep properties. TheCCI time, t0.2, which is the time required to reach an initialcrack extension of δai= 0.2 mm to occur from the onset of firstapplied force, and CCG rate, a˙ or da/dt are expressed in termsof the magnitude of creep crack growth correlated by fracturemechanics parameters, C*orK, with C* defined as the steadystate determination of the crack tip stresses derived in principalfrom C*(t) and Ct(1-17).2The crack growth derived in thismanner is identified as a material property which can be usedin modeling and life assessment methods (17-28).1.1.1 The choice of the crack growth correlating parameterC*, C*(t), Ct,orK depends on the material creep properties,geometry and size of the specimen. Two types of materialbehavior are generally observed during creep crack growthtests; creep-ductile (1-17) and creep-brittle (29-44). In creepductile materials, where creep strains dominate and creep crackgrowth is accompanied by substantial time-dependent creepstrains at the crack tip, the crack growth rate is correlated bythe steady state definitions of Ctor C*(t), defined as C* (see1.1.4). In creep-brittle materials, creep crack growth occurs atlow creep ductility. Consequently, the time-dependent creepstrains are comparable to or dominated by accompanyingelastic strains local to the crack tip. Under such steady statecreep-brittle conditions, Ctor K could be chosen as thecorrelating parameter (8-14).1.1.2 In any one test, two regions of crack growth behaviormay be present (12, 13). The initial transient region whereelastic strains dominate and creep damage develops and in thesteady state region where crack grows proportionally to time.Steady-state creep crack growth rate behavior is covered bythis standard. In addition specific recommendations are madein 11.7 as to how the transient region should be treated in termsof an initial crack growth period. During steady state, a uniquecorrelation exists between da/dt and the appropriate crackgrowth rate relating parameter.1.1.3 In creep ductile materials, extensive creep occurswhen the entire un-cracked ligament undergoes creep defor-mation. Such conditions are distinct from the conditions ofsmall-scale creep and transition creep (1-10). In the case ofextensive creep, the region dominated by creep deformation issignificant in size in comparison to both the crack length andthe uncracked ligament sizes. In small-scale-creep only a smallregion of the un-cracked ligament local to the crack tipexperiences creep deformation.1.1.4 The creep crack growth rate in the extensive creepregion is correlated by the C*(t)-integral. The Ctparametercorrelates the creep crack growth rate in the small-scale creepand the transition creep regions and reduces, by definition, toC*(t) in the extensive creep region (5). Hence in this documentthe definition C* is used as the relevant parameter in the steadystate extensive creep regime whereas C*(t) and/or Ctare theparameters which describe the instantaneous stress state fromthe small scale creep, transient and the steady state regimes increep. The recommended functions to derive C* for thedifferent geometries shown in AnnexA1 is described in AnnexA2.1.1.5 An engineering definition of an initial crack extensionsize δaiis used in order to quantify the initial period of crackdevelopment. This distance is given as 0.2 mm. It has beenshown (41-44) that this initial period which exists at the start ofthe test could be a substantial period of the test time. Duringthis early period the crack tip undergoes damage developmentas well as redistribution of stresses prior reaching steady state.Recommendation is made to correlate this initial crack growthperiod defined as t0.2at δai= 0.2 mm with the steady state C*when the crack tip is under extensive creep and with K forcreep brittle conditions. The values for C* and K should becalculated at the final specified crack size defined as ao+ δaiwhere aois initial size of the starter crack.1.1.6 The recommended specimens for CCI and CCG test-ing is the standard compact tension specimen C(T) (see Fig.A1.1) which is pin-loaded in tension under constant loadingconditions.The clevis setup is shown in Fig.A1.2 (see 7.2.1 for1This test method is under the jurisdiction of ASTM Committee E08 on Fatigueand Fracture and is the direct responsibility of Subcommittee E08.06 on CrackGrowth Behavior.Current edition approved June 1, 2015. Published October 2015. Originallyapproved in 1992. Last previous edition approved in 2000 as E1457 – 07ε4. DOI:10.1520/E1457-15.2The 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 States1details). Additional geometries which are valid for testing inthis procedure are shown in Fig. A1.3. These are the C-ring intension CS(T), middle crack specimen in tension M(T), singleedge notched tension SEN(T), single edge notched bendSEN(B), and double edge notched tension DEN(T). In Fig.A1.3, the specimens’ side-grooving-position for measuringdisplacement at the force-line displacement (FLD) and crackmouth opening displacement (CMOD) and also positions forthe potential drop (PD) input and output leads are shown.Recommended loading for the tension specimens is pin-loading. The configurations, size range are given in TableA1.1of Annex A1, (43-47). Specimen selection will be discussed in5.9.1.1.7 The state-of-stress at the crack tip may have aninfluence on the creep crack growth behavior and can causecrack-front tunneling in plane-sided specimens. Specimen size,geometry, crack length, test duration and creep properties willaffect the state-of-stress at the crack tip and are importantfactors in determining crack growth rate. A recommended sizerange of test specimens and their side-grooving are given inTable A1.1 in Annex A1. It has been shown that for this rangethe cracking rates do not vary for a range of materials andloading conditions (43-47). Suggesting that the level ofconstraint, for the relatively short term test durations (less thanone year), does not vary within the range of normal data scatterobserved in tests of these geometries. However it is recom-mended that, within the limitations imposed on the laboratory,that tests are performed on different geometries, specimen size,dimensions and crack size starters. In all cases a comparison ofthe data from the above should be made by testing the standardC(T) specimen where possible. It is clear that increasedconfidence in the materials crack growth data can be producedby testing a wider range of specimen types and conditions asdescribed above.1.1.8 Material inhomogeneity, residual stresses and materialdegradation at temperature, specimen geometry and low-forcelong duration tests (mainly greater that one year) can influencethe rate of crack initiation and growth properties (42-50).Incases where residual stresses exist, the effect can be significantwhen test specimens are taken from material that characteris-tically embodies residual stress fields or the damaged material,or both. For example weldments, or thick cast, forged,extruded, components, plastically bent components and com-plex component shapes, or a combination thereof, where fullstress relief is impractical. Specimens taken from such com-ponent that contain residual stresses may likewise containresidual stresses which may have altered in their extent anddistribution due to specimen fabrication. Extraction of speci-mens in itself partially relieves and redistributes the residualstress pattern; however, the remaining magnitude could stillcause significant effects in the ensuing test unless post-weldheat treatment (PWHT) is performed. Otherwise residualstresses are superimposed on applied stress and results incrack-tip stress intensity that is different from that based solelyon externally applied forces or displacements. Not taking thetensile residual stress effect into account will produce C*values lower than expected effectively producing a fastercracking rate with respect to a constant C*.This would produceconservative estimates for life assessment and non-conservative calculations for design purposes. It should also benoted that distortion during specimen machining can alsoindicate the presence of residual stresses.1.1.9 Stress relaxation of the residual stresses due to creepand crack extension should also be taken into consideration.No specific allowance is included in this standard for dealingwith these variations. However the method of calculating C*presented in this document which used the specimen’s creepdisplacement rate to estimate C* inherently takes into accountthe effects described above as reflected by the instantaneouscreep strains that have been measured. However extra cautionshould still be observed with the analysis of these types of testsas the correlating parameters K and C* shown in Annex A2even though it is expected that stress relaxation at hightemperatures could in part negate the effects due to residualstresses. Annex A4 presents the correct calculations needed toderive J and C* for weldment tests where a mis-match factorneeds to be taken into account.1.1.10 Specimen configurations and sizes other than thoselisted in Table A1.1 which are tested under constant force willinvolve further validity requirements. This is done by compar-ing data from recommended test configurations. Nevertheless,use of other geometries are applicable by this method provideddata are compared to data obtained from standard specimens(as identified in Table A1.1) and the appropriate correlatingparameters have been validated.1.2 The values stated in SI units are to be regarded as thestandard. The inch-pound units given in parentheses are forinformation only.1.3 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 and health practices and determine the applica-bility of regulatory limitations prior to use.2. Scope of Material Properties Data Resulting from ThisStandard2.1 This test method covers the determination of initialcreep crack extension (CCI) times and growth (CCG) in metalsat elevated temperature using pre-cracked specimens subjectedto static or quasi-static loading conditions. The metallic mate-rials investigated range from creep-ductile to creep-brittleconditions.2.2 The crack growth rate a˙ or da/dt is expressed in terms ofthe magnitude of CCG rate relating parameters, C*(t), Ctor K.The resulting output derived as a˙vC* (as the steady stateformulation of C*(t)), or Ctfor creep-ductile materials or asa˙vK(for creep-brittle materials) is deemed as material propertyfor CCG.2.3 In addition for CCI derivation of crack extension timet0.2vC* (for creep-ductile materials) or t0.2vK (for creep-brittlematerials) can also be used as a material property for thepurpose of modeling and remaining life assessment.2.4 The output from these results can be used as ‘Bench-mark’material properties data which can subsequently be usedE1457 − 152in crack growth numerical modeling, in component design andremaining life assessment methods.3. Referenced Documents3.1 ASTM Standards:3E4 Practices for Force Verification of Testing MachinesE74 Practice of Calibration of Force-Measuring Instrumentsfor Verifying the Force Indication of Testing MachinesE83 Practice for Verification and Classification of Exten-someter SystemsE139 Test Methods for Conducting Creep, Creep-Rupture,and Stress-Rupture Tests of Metallic MaterialsE220 Test Method for Calibration of Thermocouples ByComparison TechniquesE399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic MaterialsE647 Test Method for Measurement of Fatigue CrackGrowth RatesE813 Test Method for JIc,AMeasure of Fracture ToughnessE1152 Test Method for Determining-J-R-CurvesE1820 Test Method for Measurement of Fracture ToughnessE1823 Terminology Relating to Fatigue and Fracture TestingE2818 Practice for Determination of Quasistatic FractureToughness of Welds4. Terminology4.1 Terminology related to fracture testing contained inTerminology E1823 is applicable to this test method. Addi-tional terminology specific to this standard is detailed in 4.2and 4.3. For clarity and easier access within this documentsome of the terminology in E1823 relevant to this standard isrepeated below (see Terminology E1823, for further discussionand details).4.2 Definitions:4.2.1 creep crack growth (CCG) rate, da/dt, ∆a/∆at [L/t]—the rate of crack extension caused by creep damage andexpressed in terms of average crack extension per unit time.[E1823]4.2.2 C*(t)-integral, C*(t) [FL-1T-1]—a mathematical ex-pression a line or surface integral that encloses the crack frontfrom one crack surface to the other, used to characterize thelocal stress-strain rate fields at any instant around the crackfront in a body subjected to extensive creep conditions4.2.2.1 Discussion—The parameter relevant to creep crackgrowth is given as the C*(t)-Integral consisting of a line orsurface integral that encloses the crack front from one cracksurface to the other. C*(t) is used to characterize the localstressstrain rate fields at any instant around the crack front in abody subjected to extensive creep conditions.4.2.2.2 Discussion—The C*(t) expression for a two-dimensional crack, in the x-z plane with the crack front parallelto the z-axis, is the line integral:C* 5*ΓSW˙dy 2 Ti] u˙i]xdsD(1)where:W˙= instantaneous stress-power or energy rate per unitvolume,Γ = path of the integral, that encloses (that is, contains)the crack tip contour,ds = increment in the contour path,T = outward traction vector on ds,u˙ = displacement rate vector at ds,x, y, z = rectangular coordinate system, andTi]u˙i]xis the rate of stress-power input into the area enclosed byΓ across the elemental length ds.4.2.2.3 Discussion—The value of C*(t) from this equation ispath-independent for materials that deform according to con-stitutive law that may be separated into single-value time andstress functions or strain and st