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NASA Technical M'mmrandum 87225 9 ¸ Low-Cycle Thermal Fatigue _ATIGUE N86-2£651 [NASA-TM-8722 5) IOW-CYCLE CSCL 20K [[qASA) ]14 p HC AC6/N]_ AOI Unclas G3/39 43421 Gary R. Halford Lewis Research Center Cleveland, Ohio February 1986 f w CONTENTS Page INIRDDUCTION • " ° • ° • ° • " " ° " " " • " " " " " • • • - • • - • • • • ° l NOMENC LAIURE • ° • " • • ° " " " ° " " ° " ° • " " " • • • • ° • • • • • ° • 4 HISTORICAL PERSPEClIVE • • • • " " " " " • " " " " " " • • - • • - • • • • • 5 THERMAL AND THEMOMECHANICAL FATIGUE--UNBALANCED DEFORMATION AND CRACKING PROCESSES • • " " • " • " • " • • • " • • • • • • • - • • - • • • • 7 A• CYCLIC IMBALANCES IN BULK BEHAVIOR .................. 8 I. Cyclic Stress-Straln Response .................. 8 a. Temperature-Dependence of Strength and Mean Stress Development ......................... 8 b. Ratchettlng ......................... 9 c. Multlaxlal Stress Straln States ............... g d. Metallurgical Instabilities ................. TO 2. Cyclic Crack Initiation ..................... IO a. Temperature-Dependent Fatigue Resistance ........... II I. Stress (Elastic Strain) ................. 12 2. Inelastic Strain ..................... 12 3. Total Strain ....................... 12 4. Damage Parameters .................... 12 5. Strainrange Partitioning ............... . . 13 b. Mean Stress Effect on Life .................. 13 l. Elastic Cyclllng ..................... 13 2. Inelastic Cycling .................... 15 c. Ratchetting (exhaustion of ductility) ............ 16 d. Multlaxlal Factors ...................... 17 e. Metallurgical Instabilities and Mechanisms .......... 17 ° Cyclic Crack Propagation ..................... 19 a. Temperature Dependent Propagation Rates ........... 20 b. Unbalanced Aspects ...................... 20 LOCALIZED BEHAVIOR B° I. Environmental Effects ...................... 21 2. Inhomogeneous Constituents of Material .............. 22 a. Surface Oxides and Coatings ................. 22 b. Internal Particles, Fiberous Elements, and Interfaces .... 23 THERMAL FATIGUE RESISIANCE .......................... 24 A TES1 MEIHODS AND EQUIPMEN1 • " ..................... 24 B• TRENDS IN BEHAVIOR I. Crack Initiation.... " " " " " " ................ 26 ......................... 26 2 Crack Propagation • " ....................... 27 THERMOMECHANICAL FATIGUE (TMF) ....................... 2B 28 A. EXPERIMENIAL PROCEDURES ....................... TRENDS IN BEHAVIOR .......................... 29 B• I. Crack Initiation ......................... 29 2. Crack Propagation ........................ 31 32 BITHERMAL FAlIGUE ............................. 32 A. RATIONALE FOR BITHERMAL FATIGUE .................. 34 B. BITHERMAL FAlIGUE RESULTS ...................... ISOIHERMAL LIFE PREDICTION MODELS ..................... 36 PREDICTION OF 1MF LIVES USING IDEALIZED ISOTHERMAL CHARACIERISIICS ..... 3B CONSIAN1 COFFIN-MANSON FAILURE CRIIERION ............... 40 Ao I. TMF Life Prediction for Time-Independent Constitutive Behavior 40 a Total Strain Range Criterion ................ 41 • 42 b. Ostergren Approach ..................... TMF Life Prediction for Time-Dependent Constitutive Behavior 43 , a. Total Strain Range Criterion ................ 43 b. Ostergren Approach ...................... 43 B. CONSIAN1 TOIAL STRAIN RANGE FAILURE CRIIERION ............ 45 45 CONSIAN1 OSTERGREN FAILURE CRITERION ................ C° I. Time-Independent Constitutive Behavior .............. 45 2. Time-Dependent Constitutive Behavior ............... 46 3. TMF Life Pred_ctlon for llme-Independent and Time-Dependent Cases ....................... 46 a. Ostergren Approach ...................... 46 b. Manson--Coffln Failure Criterion ............... 46 c. Total Strain Rango Criterion ................. 4? D. INIERPREIAIION OF ANALYSES ...................... 47 THERMAL FATIGUE LIFE PREDICIIONS OF ENGINEERING SIRUCIURAL COMPONENIS . 48 A. HEAl EXCHANGER INLEI NOZZLE ..................... 49 50 B. TURBINE ROIOR ............................ 50 C. HIGH PRESSURE TURBINE BLADE ..................... 51 D. COMBUSIDR LINER ........................... DO'sANDDON'Is IN DESIGNAGAINSTTHERMAFLATIGUE............. 52 A. THETHERMAL FORCING FUNCITON ..................... 52 B. GEOMETRY OF STRUCTURAL COMPONENTS AND THERMAL/PHYSICAL PROPERTIES 53 C. MATERIAL MECHANICAL RESPONSE CHARACIERISTICS ............. 55 REFERENCES .°-J....,°.°,.J.......o.°..,°°_.,56 TABLES ................................... LOW-CYCLTEHERMAFLAIIGUE Gary R. Halford National Aeronautics and SpaceAdministration Lewis ResearchCenter Cleveland, Ohio 44135 INTRODUCIION Low-cycle thermal fatigue (LClF) Is one of the dominant failure modes in h_gh temperature structural components. In fact, the problem is intensifying in aeronautical gas turbine blades and vanes as greater emphasis is placed on internal cooling of hollow airfoils to permit higher gas temperatures. The resultant thermal gradients at thin leading and trailing edges become greater, and the propensity for thermal fatigue cracking increases. As greater perform- ance is demanded of gas turbines, operating temperatures increase, rotational speeds increase, thermal transients Intensify, and durability suffers. As a 0 case in point, the United States Space Shuttle Main Engine utilizes turbo- O0 machinery that extracts over seven hundred horsepower per turbine blade. Yet, I the blade airfoil is only about the size of a thumb. Their durability is measured in Just thousands of seconds of operation during which time low-cycle thermal fatigue cracks initiate and grow to readily detectable proportions. Pressure vessel and piping components in the electric power industry also must be designed to resist thermal fatigue cracking. This is done at great expense, because the consequences of suffering low-cycle thermal fatigue cracking would be many times more expensive than the cost of initial analyses. Structural safety, particularly when high pressure steam and radioactive sub- stances are involved, is of paramount importance. It demands that structural integrity be malntalned even under the most severely imaginable circumstances. Thermal fatigue cracking problems also abound in a multitude of less sophisti- cated but equally important hlgh-temperature hardware. The cost of all thermal fatigue failures, and of the prevention of those that might have occurred, is quite high. Costs of replacement parts alone are estimated at several billion dollars/yr in the United States (ref. l). In addition, valuable natural resources are being lost in terms of nonrecyclable strategic materials such as cobalt, chromium, and nickel. In the aeronautical gas turbine engine industry, which maintains an enviably high positive balance of international payments for the United States, hot section component maintenance costs in Iggo are expected to exceed two billion dollars/yr (ref. 2). Since a sizable fraction of the maintenance is thermal fatigue cracking related, there is a strong economic driver for better understanding of the failure processes of thermal fatigue. Emphasis is on developing more thermal fatigue resistant alloys, and utilizing these and existing materials as efficiently and effectively as their inherent durability characteristics permits. The very heart of current efforts directed toward low-cycle thermal fatigue is concentrating on being able to understand and to relate isothermal and thermal low-cycle fatigue. The task would be reasonably straightforward if the cyclic deformation mechanisms were independent of temperature, and the thermal cycling h_story did not alter the metallurgical state of the material either internally or at its exposed free surfaces. For the most part, however, none of these conditions exist and hence the problem becomes complicated. If there are mech- anlsms that are activated during thermal cycling and not during isothermal fatigue, then not only ls the problem complicated, It becomes physlcally Impos- sible to predict low-cycle thermal fatigue response solely from an isothermal basis. Correlations and engineering approximations have therefore become a necessary ingredient to the problem's solution. This has been the approach used In the prediction of low-cycle thermal fatigue In the past, and will ltkely remain as such for a number of years to come. Whlle most approaches proposed to-date are empirical, considerable qualitative insights have been achieved. Emphasis Is being placed now on making them more quantitative. lwo of the simplest and most common engineering approximations for pro- jectlng isothermal behavior into nonlsothermal cycling conditions are: (1) to assume that the isothermal fatigue resistance at the maximum temperature Is representative of the thermal cycling fatigue resistance, or (2) to assume that the lowest isothermal fatigue resistance within the range of temperatures of thermal cycling Is the one to use in estimating the thermal fatigue resistance. Other occaslonally assumed criterion are the use of an algebraic average or integrated equivalent temperature between the two extreme temperatures. None of these assumptions are based upon a sound understanding of the mechanisms of low-cycle thermal fatigue damage accumulation. An In-depth discussion of these and alternative approaches wlll be presented in a later section. First, however, It Is advisable to present a brief description of the thermal fatigue process. Fatigue In metals Is the consequence of repeated reversals of inelastic deformation. It Is now well established that solids must be able to be deformed by nonrecoverable inelastic deformation in order to be prone to pro- gresslve failure by the mechanism known as fatigue, lhls mechanism is a direct consequence of repeated cyclic inelastic deformation. When the deformation is highly localized and Is detectable only at the microscopic scale, many thousands or even millions of cycles must be applied to initiate a crack and prop- agate it to macroscopic proportions. If the cyclic inelastic deformation Is greater and distributed more homogeneously on the macroscopic scale, the llfe- time may be but a few cycles or as many as tens of thousands of cycles to failure. This Is the regime of low-cycle fatigue. While low-cycle fatigue can result from repeated large strains at constant temperature conditions, the technologically more important condition called thermal fatigue Is more often than not the source of the deformation that imparts low-cycle endurances In high temperature structures. In the case of thermal fatigue, the cyclic deformation is imposed as the result of the constrained differential thermal expansion within a solid caused by temperature gradients induced during alternate heating and cooling. Thermal fatigue is typically a low-cycle fatigue problem (approximately less than lO4 cycles to failure) owing principally to the low frequency of thermal cycling of most engineering structures. For example, aeronautical gas turbine engines experience one major thermal fatigue cycle per flight, i.e., start-up, Idle, acceleration, cruise, deceleration, Idle and shut-down. No more than a few flights per day are scheduled, so the rate of cycling is less than about lO00/yr. This is in the low cycle fatigue regime even If the engines are operated for lO yr or more. Mechanical components of engineering structures that operate at elevated temperatures undergo their major temperature change at the beginning and end of each usage. Rapid temperature changes produce high thermal gradients and thus high thermal stresses and strains, lhe cyclic loading conditions induced by temperature gradients are essentially deformation limited loadlngs. Hence, the laboratory studies of thermal fatigue (or more appropriately - thermal strain fatigue) are generally llmlted to straln-controlled low-cycle fatigue tests. Untll recently, the thermal fatigue resistance of materials was assessed by conducting isothermal low-cycle fatigue tests at the expected maximum temperature of the thermal fatigue cycle. Long time operation at the steady state elevated temperature of the structural component was simulated by imposi- tion of hold periods at the peak tensile or compressive strain within the Isothermal cycle. Thus long time exposure effects brought about by oxidation, hot corrosion, and creep could be assessed in the relatively simple laboratory simulation experiments. As a consequence, an extremely large Isothermal data base has grown over the years that supports a design philosophy that embodies the same slmpltsttc assumptions. As computer-controlled testing equipment and techniques have evolved, the capability to perform well-controlled, thermomechantcal fatigue* experiments, however, has begun to manifest Itself. Comparison of thermomechanlcal fatigue results with Isothermal results has sometimes revealed alarmlng discrepancies, particularly wherein thermomechanlcal fatigue resistance Is slgnlflcantly lower than would have been expected based upon isothermal fatigue resistance. A following section deals with results reported In the llterature. *Thermomechanlcal fatigue is the terminology adopted for mechanical fatigue that has a superimposed uniformly varying temperature cycle. The two cycles have the same period, but the temperature and the mechanical strain can be programmed to otherwise vary independently of one another. Thermal fatigue, on the other hand, Is associated wlth no external mechanical constraint; the stresses and strains are self imposed due to differential thermal expansion brought about by the temperature gradients. 3 NOMENCLAIURE ( axial strain (dimensionless) Ac total strain range elastic strain range ACe1 Inelastlc strain range ACln inelastic strain rate axlal stress i:l Aa stress range peak tensile stress in cycle frequency of cycle E modulus of elasticity AW1 tensile hysteresis energy Nf cycles to fatigue failure (crack initiation) 1 temperature Y exponents n m A coefficients