- FATIGUE TESTING Halford, Lerch, and McGaw Section 8-B: Fatigue, CreepFatigue, and Thermomechanical Fatigue Life Testing of AUoys Gary R. Halfod and Bradley A. Lerch Glenn Research Center at Lewis Field National Aeronautics and Space Administration Cleveland, Ohio and Michael A. McGaw McGaw Technology, Inc. Lakewood, Ohio ASMFatigue Testing l8grh 01/31/00 4:36 PM - FATIGUE TESTING Halford, Lerch, and McGaw D. Laboratory Documentation of Set Ups, Procedures, Calibrations, and Maintenance E. Conducting a Fatigue Test XI. REFERENCES 3 ASMFatigue Testing 18grh 01/31/00 4:34 PM - FATIGUE TESTING Halford, Lerch, and McGaw OUTLINE I. INTRODUCTION TO FATIGUE CRACK INITIATION LIFE TESTING II. PROCESS OF FATIGUE CRACK INITIATION AND EARLY G R O m m. FATIGUE TESTING MACHINES A. Fatigue Loading Modes B. Classifications of Fatigue Testing Machines 1. Axial (Direct-Stress) Fatigue Testing Machines i. Electromechanica E systems ii. Sentohydraulic closed-loop systems 2. Bending Fatigue Machines i. Cantilever beam machines ii. Rotating beam machines C. Regimes of Operation 1. High-Cycle Fatigue 2. Low-Cycle Fatigue IV. ANCILLARY EQUIPMENT & SPECIMENS A. Load Measurement, Application, and Control 1 Hydraulic Cylinder 2. Load Cell 3. Servo-valve B. Gripping Systems C. Extensometry & Strain Measuring Devices 1. Axial Extensometers 2. Diametral Extensometers D. Heating Systems E. Environmental Chambers F. Representation of Material and Specimen Configurations G. Specimen Machining and Surface Preparation 1. Cylindrical Specimens 2. Flat Sheet and Plate Specimens H. Alignment Considerations I. Graphic Recorders ASMFatigue Testing l8grh 01131100 3::; PM2:Zs. !" : 2 - FATIGUE TESTING Hatford. Lerch, and McGaw V. ELECTRONIC TEST CONTROLS A. Load Frames: Analog and Digital Controls B. Comparison: Analog and Digital Controllers C. Furnace Controls D. Test Program Development E. Single Purpose Software F. General Purpose Software G. Custom Application Software H. Data Acquisition Requirements I. Data Analysis VI. BASELINE ISOTHERMAL FATIGUE TESTING A. Testing Regime B. Calibration and Standard Test Procedures C. Generating Fatigue Crack Initiation Data D. Criteria for Defining Fatigue Life E. Information to be Documented for Baseline Fatigue Tests F. Example Crack Initiation Fatigue Life Curves VII. TESTING FOR EFFECTS OF VARIABLES ON FATIGUE RESISTANCE A. Pre-Existing Variables 1. Bulk Property and Surface Related Effects 2. Geometric Effects B. Concurrent Variables 1. Bulk Property and Surface Related Effects 2. Active Loading Related Effects i. Mean stresses ii. Multiaxiality (see Section , this Volume) iii. Cumulativef atigue damQge iv. Temperature related eflects VIII. CREEP-FATIGUE INTERACTION A. Background B. Creep-Fatigue Testing Ix. THERMOMECHANICAL FATIGUE A. Background B. TMF Testing C. TMF Life Modeling X. REFERENCES XI. FIGURE CAPTIONS 3 ASMFatigue Testing l8grh 01131100 3:2 1 IJW: 3f" 1: '' .! - FATIGUE TESTING Halford, Lercli, and McGaw Section 8-B: Fatigue, Creep-Fatigue, and Thermomechanical Fatigue Testing of AlIoys I. INTRODUCTION TO FATIGUE CRACK INITIATION LIFE TESTING The fatigue crack initiation resistance of an alloy is determined by conducting a series of tests over a range of values of stress amplitude or strain range. The observed number of cycks ta failure is plotted sgainst the stress amplitude cr s t x hr ange to obtain a fatigue curve. The fatigue properties quoted for an alloy are typically the constants used in the equation(s) that describe the fatigue curve. Fatigue lives of interest may be as low as ld or higher than 10' cycles. Because of the enormous scatter associated with fatigue, dozens of tests may be needed to confidently establish a fatigue curve, and the cost may run into several thousands of dollars. To further establish the effects on fatigue life of the test temperature, environment, alloy condition, mean stress effects, creep-fatigue effects, thermomechanical cycling, etc. requires an extraordinarily large and usually very costly test matrix. The total effort required to establish the fatigue resistance of an alloy should not be taken lightly. Fatigue crack initiation tests are conducted on relatively small and presumed to be initially crack-free, samples of an alloy that are intended to be representative of the alloy's metallurgical and physical condition. Generally, samples are smooth and have uniformIy polished surfaces within the test section. Some may have intentionally machined notches of well-controlled geometry, but the surface at the root of the notch is usually not polished. The purpose of polishing is to attain a reproducible surface finish. This is to elinninate surface finish as an uncontrolled variable. Representative test specimen geometries will be discussed later. Test specimens are cyclically loaded until macroscopically observable cracks initiate and eventually grow to failure. Normally, the fatigue failure life of a specimen is defned as the number of cycles to separation of the specimen into two pieces. Alternative defitions are becoming more common, particularly for low-cycle fatigue testing, wherein some prescribed indication of impending failure due to cracking is adopted. Specific criteria will be described later. As a rule, cracks that develop during testing are not measured nor are the test parameters ASMFatigue Testing 18grh 0 1 3 1 2 1 I " 4 - FATIGUE TESTING Halford. Lerch, and McGaw intentionally altered owing to the presence of cracking. The topic of fatigue crack propagation testing of alloys is discussed in Sections 8-D, -E, & -H (?). Microscopic size fatigue cracks tend to nucleate quite early in cyclic life (in fust 1 to 10%)i n the high-strain, plasticity-dominated, low-cycle life regime. In this regime, cyclic plasticity is widespread throughout the specimen test section, and the range of plastic strain is used as a measure of the severity of fatigue "loading". On the other hand, cracks begin to appear quite late in cyclic life (90 to 99%)i n the very low strain, elastically dominated, high-cyclic life regime. There is a gradual transition between these two extremes of behavior for intermediate strain ranges and cyclic lifetimes. In the high-cyclic life regime, the cyclic behavior at the macroscopic, phenomenological level is usually considered by design engineers to be linearly elastic and thermodynamically reversible. It is important to recognize, however, that the micro-mechanisms of fatigue crack nucleation and growth in metals and alloys are linked directly to the occurrence of reversed cyclic plasticity. Fatigue will not occur with out it. Even fatigue cracking that occurs in the range of a billion cycles to failure or more must involve reversed plasticity. It is also important to recognize that the fatigue process is a progressively degenerative one. For any given condition of cyclic loading that eventually leads to a fatigue failure, there is some, albeit minute, permanent change from one cycle to the next. While the macroscopic behavior may appear to be linear, reversible, elastic, etc., at the microstructural level, irreversible, non-linear, inelastic deformations occur in highly localized regions that accumulate until macroscopically observable cracking occurs. Although the results of crack initiation tests conducted on small specimens do not precisely establish the fatigue life of a large part, such tests do provide useful information on the intrinsic fatigue crack initiation behavior of a metal or alloy. As a result, such data can be utilized to develop engineering design criteria to prevent initiation of fatigue cracks in structural components. The use of small-specimen fatigue test data are the basis of fatigue design codes for pressure vessels, piping components, nuclear reactors, turbine blades, wheels, and shafts, complex welded, riveted, or bolted structures, automotive and off-highway equipment, exotic aerospace components, and even soldered joints of lead-less electronic chips. All alloys and metals in structural elements are susceptible to fatigue crack initiation if the structure is subjected to sufficiently large and numerous amplitudes of cyclic loading. ASMFatigue Testing llgrh 01131100 .?:?I P:vT?:.l.: :';.! 5 - FATIGUE TESTING Halford. Lerch, and McGaw Following a brief description of the phenomena of crack initiation and early growth, this section examines specimen design and preparation, as well as the apparatus used in crack initiation testing. Variables that influence the resistance of alloys to fatigue crack initiation, such as the effect of mean and residual stress, stress concentrations, stress amplitude, and surface properties, are briefly reviewed. The initial portion of this section deals with fatigue testing of alloys in the regime wherein the isothermal temperature of testing is below the range wherein behavior is significantly influenced by timedependent mechanisms such as creep, oxidation, and metallurgical transformations. The testing procedures, instnunentation, and hardware must be altered ti, accommodate Creep-Fatigue (C-F) Testing md Theme Mechanical Fatigue (TMF)T esting, and these items will be addressed as required throughout this section. For information on the planning and design of fatigue test matrices and statistical {?I. analysis of the test results, see the article "FatigueD ata Analysis" in this Volume 11. PROCESS OF FATIGUE CRACK INITIATION AND EARLY GROWTI3 Fatigue crack initiation and early growth requires cyclic inelastic deformation. For alloys and metals tested at sub-creep temperatures the non-linear inelastic behavior is invariably plasticity, i.e., the slip associated with dislocation motion along the most densely packed crystallographic planes aligned favorably with the maximum resolved shear stress. In low-cycle fatigue testing, the cyclic plasticity is widely spread throughout the gage portion of the specimen and is readily measured with commercially available strain measuring devices. In this regime the cyclic stresses will be near or above the conventional offset yield strength of the alloy. Cyclic strain hardening or softening typically also occurs. On the other hand, at very long cyclic lives cyclic plasticity is still present, although certainly not detectable with conventional strain measurement techniques. Reversed crystallographic slip is highly localized within a few of the most favorably oriented grains or near highly localized stress concentrations. Stress-strain response appears to be totally elastic in this life regime. Because - FATIGUE TESTING Halford, Lerch, and McGaw of the extreme localization at the smallest cyclic stresses and strains and hence Iongest lives, the tendency is for only one major crack to initiate and grow to failure in this regime. In the high strain regime, corresponding to low-cycle fatigue lives, there is a tendency for the material to develop multiple crack initiations and early growth followed by eventual link-up of independent cracks into a single fatal fatigue crack. The transition between Iow-cycIe fatigue and high-cycle fatigue is essentially a gradual one with mechanisms varying more in degree than in kind. The region between low- and high-cycle fatigue is referred to as intermediate- cycle fatigue. With few exceptions, such as rolling contact fatigue and influences of mechanical or metallurgical surface treatments, cracks initiate at a free surface. Usually the surface is the external surface of the specimen, although it could be an internal surface associated with a void or a de-bonded internal particle. Cyclic plasticity is less constrained at a free surface because of the fewer nearest neighbors and hence fewer atomic bonds available to inhibit dislocation motion. Dislocations also exit and disappear at free surfaces, leaving one atomic-sized step for each dislocation that exists on a particular slip plane. Typically, more than one slip plane is involved. Any given slip plane experiences non-reversed slip, i.e., the amount of slip in the slip direction of the plane during one direction of loading is not recovered in the opposite direction when the direction of loading is reversed. Rather, the overall deformation is recovered, but some of it may be on parallel slip planes. The active parallel slip planes are separated by numerous atomic distances and form what are known as a slip bands. Within a band the to-and-fro slip is not uniform, resulting in considerable disarray beneath the surface and outcroppings that are highly irregular. These are referred to as persistent slip bands, i.e. those deeper than several microns below the free surface. Persistent slip bands remain active throughout the bulk of the cyclic life. As the number of applied fatigue cycles of cyclic plasticity increase, the severity of the irregularity increases until such time as the outcroppings form extrusionlintrusion pairs within the slip bands. Intrusions are the nuclei or formative stages of atomic-sized fatigue cracks known as Stage I cracks (defmed as cracking along the crystallographic slip plane), The intrusion grows slowly with continued cycling. Once the depth of the intrusion is great enough, the surrounding material perceives it as a crack that exerts its own highly localized stress-strain 7 ASMFatigue Testing I8grh 01131100 3:L 1 PMZ::". !'!.! FATIGUE TESTING - Halford, Lerch, and McGaw field. At this stage of the evolving fatigue process the nucleated crack's stress-strain fieId, which superimposes itself on the applied stress-strain field, becomes the dominant field. The cracking response changes accordingly and the global crack direction turn to become perpendicular to the maximum principal stress direction immediately in front of the crack. This signals the onset of Stage 11 fatigue cracking which generally prevails until fatigue failure occurs. Inspection of a fatigue fracture surface with the naked eye generally reveals primarily Stage I1 cracking as Stage I cracks are seldom greater than a grain size or two in depth. Cracks may also start at the location of surface irregularities due to grain boundaries, chemical attack, an& casting Gr mcmg imperfections. Nevertheless, cyclic plasticity is always a necessary ingredient for the nucleation process. Although the scenario described above is simplified, it provides phenomenological insight into the gradual, progressive nature of the fatigue process that are useful in understanding cyclic testing in the low-, intermediate-, and high-cycle fatigue regimes. There are no sharp demarcations between the three regions when described by the number of cycIes to failure. In fact, the distinction is better founded in terms of the magnitude of the range of cyclic plastic strain than in terms of number of cycles, because of the overwhelming influexlce of the plasticity. As an example, high-ductility, low-strength metals such as copper behave m a low-cycle fatigue manner even at a lo6 cycles to failure, because the cyclic strain range may be half plastic and half elastic even at this life level. By contrast, a lowductility, high strength hardened ball-bearing steel exhibits high-cycle type fatigue behavior at cyclic lives of only I O ~ owing to the minuscule amount of plasticity that is overwhelmed by the large elastic component of cyclic strain. Numerous types of testing machines have been developed for fatigue crack initiation testing. Fatigue testing machines covered in this Section deal with nominally uniaxial normal stress applications. Multiaxial fatigue loading, including torsional and contact fatigue (rolling - FATIGUE TESTING Halford. Lerch, and McGaw elements, gears, impact, etc.) are covere&i n other Sections. Most fatigue testing machines have been developed to a high degree and are marketed commercially to laboratories for conducting a wide variety of fatigue testing. Machines have been developed for various modes of loading which, in turn, dictate the configuration of the test specimen. Considerable variation of specimen geometry can be accommodated by each of the modes as discussed in the following paragraph. A. Fatigue Loading Modes Three basic modes of loading are used; a) direct axial loading, b) plane bending, and c) rotating beam. Specimens for direct axial stress machines may have a wide range of cross- sectional geometries (solid or hollow), and have a uniform gage Iength with axial cross- sections that are round, elliptical, square, rectangular, or thin sheet. Non-uniform cross-section specimens include sharp notches and low-stress concentration, hourglass-shaped specimens for diametral strain control. Bending specimens may have cross-sections of uniform width and thickness for 3- or 4-point loading, or tapered cross-sections (designed for constant stress gong the length) for cantilevered plane bending or rotating beam testing. All bending specimens could be machined with stress concentrations in the form of notches. Examples of standardized fatigue test specimens will be presented later. B. Classifications of Fatigue Testing Machines In addition to the loading mode, fatigue-testing machines are fuaher classified by their basic drive mechanism and by the test parameter to be controlled. The basic drive system is most often electrical. An electric motor directly drives rotating beam testing machines. An eccentric cam attached to a drive motor deflects the cantilevered end of a plane bending fatigue specimen. Eccentric cams coupled with flexure- plate, parallel-motion pivoted lever arms can also drive axially loaded specimens in a direct stress machine. Direct stress machines can also be modified with fixturing to perform plane bending fatigue tests, either in 3-point or 4-point bending. The rotary motion of an eccentric on 9 ASMFatigue Testing l8grh 01/31/00 3:: 1 Phcl2:l: !':.'.