Creep fatigue interaction testing
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Creep.fatigue test cycles.
ators. The cycles are simplifications of actual complex
service cycles. Experimental replication of service
histories would require complex programming achiev-
able through a computer-controlled servo system.
Usually, life prediction models are relied upon to link
single lah_,to_ ¢_e.s _th mlggi¢ se_,d¢.¢t'_,_.s.
The deformation, crack initiation, and early growth
mechanisms, in general, will be different for each of
these discrete cycles. Schematic models developed
to illustrate these differences have been compared to
experimental observations for a variety of alloys
(Manson and Halford [I]_. Of the more than 100
creep-fatigue life predicuon models proposed since the
1950s,onlyabout10% havebeenappliedextensively
enoughtowarranttheirreview(Miller[_ Halford
et al. _. The three most widely used-aTffthe time-
and cyc--'ffd-fractionrule (ASME _), strai_ange
partitioning (SLIP), and damage--_-echanics. Few
specitically address inevitable oxidation interactions.
The ASME Code Case is extremeIy conservative and
does not directlyaddresswhen hilurewould occur.
Thatconditionisacceptablebecausepowerplantsdo
not havethesame stringentweightand performance
requirementsas,forexample,aerospacepropulsion
sx0055
. .-_: .?.:_;.;_.:,.. , -" 7, .](https://image.slidesharecdn.com/creepfatigueinteractiontesting-161128162845/85/Creep-fatigue-interaction-testing-4-320.jpg)
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C:[_an/emr409037 Apr04-pro p lc- lm 6 (X 5)
systems. The Inner require mtr.h g_mm:r life .1_
accuracy to conserve weight while m2,_._, z a
combination of durability and performance.
3. Co_ Re_ks
Creep-fa6gue interactionis reasonably well uader-
stood at the phenomeuological level.A sisni_:ant
short-termexperimentaldatabase existsand several
satisfactory analytic models are in use for estir_fing
cyclic lives. Modeling of oxldatioo interactlom with
creep-fatigue, lack of long-term databases, and verified
long-term extrapolation procedures remain as im-
portant areas of research.
See also: Creep-Fatigue: Oxidation Interactions
Ix0060
Creep-Fatigue: Oxidation Interactions
Biblioftaph¥
_'_ricaa Societyof Mechaaical Engineers1986 Code Case/¢-
47-2$. ASME, New York
l:I'_ord G R, Lerch B A, McC_w M A 2000 Fatigue, ¢_-eep-
fatigue, and thermomechanical fatigue life testingof alloys In:
(ed.}ASM Han_ook. Vol.8, MechanlcalTeJgln$and
Evaluation. ASM International, Materials Park, OH,Sect. 8B
a_l'_'nson S S, Halford G R 1984 Relation of cyclic loading
pattern to mlcrostmctural fracture in creep-fatigue In: C J
Beevets (ed.) Proc. 2nd Int. Conf. Fatlfw and FaHfue
' Threzho!d_ (Fatigue 84). Engineering Materials Adv Serv Ltd,
Wartey, UK, Vol. 3, pp. 1237-55
l_[il]'erD A, Priest R H, EUison E G 1984 A review of material
response and life prediction techniques under fatigue-creep
loading conditions. High Temp. Mater. Process. 6, 155-94
G. R. Halford
!QA: What is_,dv Serv! short for?](https://image.slidesharecdn.com/creepfatigueinteractiontesting-161128162845/85/Creep-fatigue-interaction-testing-5-320.jpg)
Creep fatigue interaction testing
- 1. Creep-fatigue Interaction Testing Fatigue lives in metals are nominally time independent below 0.5 To.,,. A: higher temperatures, fatigue lives are altered due to rime-dependent, thermally activated creep. Conversely, creep rates are altered by super. imposed fatigue loading. Creep and fatigue generally interact synergistically to reduce material lifetime. Their interaction, therefore, L_ of £mpbrtanc_) to structural durability of high-temperature structures such as nuclear reactors, reusable rocket engines, gas turbine engines , terrestrial steam turbines, pressure vessel and piping components, casting dies, molds for plastics, and pollution control devices. Safety and life- cycle costs force designers to quantify these inter- actions. Analytical and experimental approaches to creep-fatigue began in the era following World War 1I. In this article experimental and life prediction approaches ate reviewed for assessing creep-fatigue interactions of metallic materials. Mechanistic models are also discussed briefly. 1. I'est Facilities Modern creep-fatigue testing facilities are sophisti- cated closed-loop,servo-con_olled, electro-hydraulic fatiguetestingmachines('I-Ialfordetal._ adapted to high-temperature operation (Fig.---l_. Typical machinesconsistoftwo orfourposts,a rigidloading frame, and a hydraulic cylinder mounted in the center ofthebase.The cylindertransmitsh_'draulic pressure from a pump throughan electromcscrvovalve.A specimengripismounted on theupperend of the cylinderram, and anotherisattachedto a loadcell mounted to the upper cross member to measure axial loadtransmittedthroughthespecimen.Modern grips utilizehydraulicsto alignand securethespecimen. Alignment of the specimen and loadingaxes is importantto minimizebending loads and avoid buckling.Specimenshavea uniformlyreducedcross- sectionthatisas uniformlyheatedas possibleand experiencesthe cyclicstressesand strains.Cyclic strains are measured and controlled with commercially availableextensometers with resolutionson the order of I0microstrain, A specimenheatingsystemisthe core of high- temperature testing. Test machine components such as the extensometer sensing element, specimen grips, load cell, and all hydraulic components must be kept cool.The mostversatile heating systemusesinduction coilssurroundingthespecimen.High-frequencycur. rentpassingthroughthecoilsinduceseddycurrentsat the surfaceof the specimen. Specimen heating results. The high thermal conductivity of most alloys results in a specimen with minimal thermal gradients in the radial direction. Coil spacing controls axial thermal gradients.Threeindependentlycontrolledcoilscanbe sx0015 sx0020 This report is a preprint of an article submitted to a journal for publication. Because of changes that may be made before formal publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author.
- 2. / C:/jan/emr409037 Apr04-pro p lc- lm 3 _ 2) i C I i/ Error Specimen_ Gain I f I [ .. ve ,o.v, I [ ,(Servovalve Figure 1 Modern creep-fatigue testing machine. employed to minimize the axial thermal gradient. Coil spacing will allow visual observation of the specimen and permits attachment of an extensonjgter sporting ceramic extension rods that transmiq_gage section displane.mmz_ to the more distant sensini[ element. A thermocouple held in direct contact with the specimen surface supplies feedback for temperature control. Commercial noncontacfing optical pyrometers are also available. Alternate heating techniques, such as a combination of induction and radiation, are required for low conductivity materials such as ceramics. A small metallic susceptor surrounds the specimen and is heated by induction. The susceptor then radiates heat to the specimen. The low thermal mass of this system permits reasonably fast temperature changes. Radi- ation furnaces are quite common. They use coiled Nichrome wire or siliconcarbideelements or even banks of quartz lamps. Furnaces offerhigh thermal stability because of the large thermal mass, but are slow to respond to intentional temperature changes during thermomechanical testing. Direct resistance heating also has been used successfully, but remains uncommon. High current (low voltage) passed sxO025 i |$
- 3. / C:/jan/emr409037 Apr04-pro p Ic-lm 4 (X 31 "_'.rough the specimen results in seE-heating due to the specimen's electrical resistivity. This technique is the =ost di_cult to conuol t_causeoftbe _l"enfly high £_e:mal response. Heating of the test section does not reiy on the thermal conductivity of the specimen; cooling, however, does. Maintaining an accurate constanttemperatureisdilr_-ult. Modern testfacilities arc typicallyinappropriatefor long-term(_ 1 month)testing.Lessexpensive,more reliabl¢, screw-driven machines are sul:_riorfor purpose. If long-termcreep-fatigue resistanceis a designrequirement,long-termtestresultsarenecess- ary to verifylifepredictionmodels for accurate extrapolation. _0030 2. MeehanDma and Life Prediction Modela sx0035 Fatigue crack initiation and early growth mechanisms differ substantially from those for creep cracking. Fatigue damage is time-lndependent to-and-fro crys- tallographic slip, most evident at free surfaces. Creep damage occurs throughout the volume of stressed material and requires the thermally activated diffusion of atomic vacancies. Creep damage due to grain boundary sliding, however, intersects free surfaces in much thesameway as fatigueslip. The additionofcreeptoan otherwisepuretension/ sx0040 compressionfatiguecyclecan be accomplishedin threedistinctlydifferentways as shown below.A baselineconditionof purefatiguecyclingwillhave negligiblecreep presentif a high enough cyclic frequency(_>IHz) isimposed. - Constantstrainingratelow enoughto introduce creepinadditiontocrystallographicslip. - Constantstressascreepoccurswithtime. - Constanttotalstrainwhilestressrelaxes,con- vertingelastictocreepstrain(leftpanelsof Fig.2). Sinceany one of thesethreeways of introducingsx0045 creepintoa cyclecouldbe appliedto tensiononly, compressiononly,or bothtenslonand compression, thereemerge,in theextreme,onlyfourbasiccom- binationsofcompletelyreversedcreep-fatiguestress- straincycles: - Tensileplasticstrain(p)reversedby compressive plasticstrain(p),i.e.,(pp) - Tensileplasticstrain(p)reversedby compressive creepstrain(c),i.e.,(pc) - Tensilecreepstrain(e)reversedby compressive creepstrain(c),i.e.,(cp) - Tensilecreepstrain(c)reversedby compressive plasticstrain(p),i.e.,(cc) The rightpanelsof Fig.2 illustratetypicalstress- sx0050 strainhysteresisloopsfor strainhold typecycles. Invariablycreep-fatiguecyclesinvolve(p.v)straining in additionto (cp),(pc),or (cc)components.Such cyclesdemand use ofservo-controlledsystemswith extensometersand state-of-the-artfunctiongener-
- 4. / C:/jan/emr409037 Apr04-pro p It- lm 5 (X 4) ,',Slnm c, $1_u , , .' _., / ", / ', , _ 84 B2 81 ,:%/ :Y___.a' /'qtarc 2 Creep.fatigue test cycles. ators. The cycles are simplifications of actual complex service cycles. Experimental replication of service histories would require complex programming achiev- able through a computer-controlled servo system. Usually, life prediction models are relied upon to link single lah_,to_ ¢_e.s _th mlggi¢ se_,d¢.¢t'_,_.s. The deformation, crack initiation, and early growth mechanisms, in general, will be different for each of these discrete cycles. Schematic models developed to illustrate these differences have been compared to experimental observations for a variety of alloys (Manson and Halford [I]_. Of the more than 100 creep-fatigue life predicuon models proposed since the 1950s,onlyabout10% havebeenappliedextensively enoughtowarranttheirreview(Miller[_ Halford et al. _. The three most widely used-aTffthe time- and cyc--'ffd-fractionrule (ASME _), strai_ange partitioning (SLIP), and damage--_-echanics. Few specitically address inevitable oxidation interactions. The ASME Code Case is extremeIy conservative and does not directlyaddresswhen hilurewould occur. Thatconditionisacceptablebecausepowerplantsdo not havethesame stringentweightand performance requirementsas,forexample,aerospacepropulsion sx0055 . .-_: .?.:_;.;_.:,.. , -" 7, .
- 5. v I C:[_an/emr409037 Apr04-pro p lc- lm 6 (X 5) systems. The Inner require mtr.h g_mm:r life .1_ accuracy to conserve weight while m2,_._, z a combination of durability and performance. 3. Co_ Re_ks Creep-fa6gue interactionis reasonably well uader- stood at the phenomeuological level.A sisni_:ant short-termexperimentaldatabase existsand several satisfactory analytic models are in use for estir_fing cyclic lives. Modeling of oxldatioo interactlom with creep-fatigue, lack of long-term databases, and verified long-term extrapolation procedures remain as im- portant areas of research. See also: Creep-Fatigue: Oxidation Interactions Ix0060 Creep-Fatigue: Oxidation Interactions Biblioftaph¥ _'_ricaa Societyof Mechaaical Engineers1986 Code Case/¢- 47-2$. ASME, New York l:I'_ord G R, Lerch B A, McC_w M A 2000 Fatigue, ¢_-eep- fatigue, and thermomechanical fatigue life testingof alloys In: (ed.}ASM Han_ook. Vol.8, MechanlcalTeJgln$and Evaluation. ASM International, Materials Park, OH,Sect. 8B a_l'_'nson S S, Halford G R 1984 Relation of cyclic loading pattern to mlcrostmctural fracture in creep-fatigue In: C J Beevets (ed.) Proc. 2nd Int. Conf. Fatlfw and FaHfue ' Threzho!d_ (Fatigue 84). Engineering Materials Adv Serv Ltd, Wartey, UK, Vol. 3, pp. 1237-55 l_[il]'erD A, Priest R H, EUison E G 1984 A review of material response and life prediction techniques under fatigue-creep loading conditions. High Temp. Mater. Process. 6, 155-94 G. R. Halford !QA: What is_,dv Serv! short for?