ASTM_E_1457_-_15.pdf

Designation:E145715Standard 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 ai=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*or K,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,or K 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 aiis 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 ai=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+aiwhere 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 jurisdi