ANALYSIS OF LOW CYCLE FATIGUE PROPERTIES OF SINGLE CRYSTAL NICKEL-BASE TURBINE BLADE SUPERALLOYS

Transcription

1 ANALYSIS OF LOW CYCLE FATIGUE PROPERTIES OF SINGLE CRYSTAL NICKEL-BASE TURBINE BLADE SUPERALLOYS By EVELYN M. OROZCO-SMITH A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 6

2 Copyright 6 by Evelyn M. Orozco-Smith

3 To my loving parents, Alvaro E. and Elizabeth Orozco, for always believing in me and to my husband, Andrew P. Smith, for always being there for me.

4 ACKNOWLEDGMENTS The author is thankful for the guidance given by Dr. Nagaraj Arakere and Dr. Gregory Swanson at the NASA Marshall Space Flight Center. The author also gratefully acknowledges the NASA Graduate Student Research Fellowship for its financial and technical support. iv

5 TABLE OF CONTENTS page ACKNOWLEDGMENTS... iv LIST OF TABLES... vi LIST OF FIGURES... vii ABSTRACT... ix CHAPTER INTRODUCTION... MATERIAL SUMMARY... Elastic Modulus...4 Tensile Properties...5 Creep Properties...5 FAILURE CRITERIA...7 Fatigue Failure Theories Used in Isotropic Metals...9 Application of Failure Criteria to Uniaxial LCF Test Data... 4 LCF TEST DATA ANALYSIS...8 PWA49 Data at F in Air...8 PWA49 Data at Room Temperature (75 F) in High Pressure Hydrogen...8 PWA49 Data at 4 F and 6 F in High Pressure Hydrogen... SC LCF Data at 8 F in Air CONCLUSION...4 REFERENCES...4 BIOGRAPHICAL SKETCH...4 v

6 LIST OF TABLES Table page - Direction cosines of material (x, y, z) and specimen (x, y, z ) coordinate systems Direction cosines for example Strain controlled LCF test data for PWA49 at F for four specimen orientations Maximum values of shear stress and shear strain on the slip systems and normal stress and strain values on the same planes PWA49 LCF high pressure hydrogen (5 psi) data at ambient temperature PWA49 LCF data measured in high pressure hydrogen (5 psi) at 4 F PWA49 LCF data measured in high pressure hydrogen (5 psi) at 6 F LCF data for single crystal Ni-base superalloy SC at 8 F in air....9 vi

7 LIST OF FIGURES Figure page - Primary (close pack) and secondary (non-close pack) slip directions on the octahedral planes for a FCC crystal [6] Cube slip planes and slip directions for an FCC crystal [6] Material (x, y, z) and specimen (x, y, z ) coordinate systems Strain range vs. cycles to failure for LCF test data (PWA49 at F) [ max + ε n ] vs. N... ε 4- + n + no E vs. N n (+ k max y ) vs. N... ε max 4-5 ( ) vs. N Shear stress amplitude [ max ] vs. N LCF data for PWA49 at room temperature in 5 psi high pressure hydrogen: strain amplitude vs. cycles to failure Shear stress amplitude ( max ) vs. cycles to failure for PWA49 at room temperature in 5 psi hydrogen LCF data for PWA49 at 4 F in 5 psi high pressure hydrogen: strain amplitude vs. cycles to failure LCF data for PWA49 at 6 F in 5 psi high pressure hydrogen: strain amplitude vs. cycles to failure... vii

8 4- Shear stress amplitude ( max ) vs. cycles to failure for PWA49 at 4 F in 5 psi hydrogen Shear stress amplitude ( max ) vs. cycles to failure for PWA49 at 6 F in 5 psi hydrogen LCF data for SC at 8 F in air: strain amplitude vs. cycles to failure Shear stress amplitude ( max ) vs. cycles to failure for SC at 8 F in air...8 viii

9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science ANALYSIS OF LOW CYCLE FATIGUE PROPERTIES OF SINGLE CRYSTAL NICKEL-BASE TURBINE BLADE SUPERALLOYS By Evelyn M. Orozco-Smith August 6 Chair: N. K. Arakere Major Department: Mechanical and Aerospace Engineering The superior creep, stress rupture, melt resistance, and thermomechanical fatigue capabilities of single-crystal Ni-base superalloys PWA 48/49 and PWA 484 over polycrystalline alloys make them excellent choices for aerospace structures. Both alloys are used in the NASA SSME Alternate Turbopump design, a liquid hydrogen fueled rocket engine. The failure modes of single crystal turbine blades are complicated and difficult to predict due to material orthotropy and variations in crystal orientations. The objective of this thesis is to perform a detailed analysis of experimentally determined low cycle fatigue (LCF) data for a single crystal material with different specimen orientations in order to determine the most effective parameter in predicting fatigue failure. This study will help in developing a methodical approach to designing damage tolerant Nibase single crystal superalloy blades (as well as other components made of this material) with increased fatigue and temperature capability and lay a foundation for a mechanistic based life prediction system. ix

10 CHAPTER INTRODUCTION In the aerospace industry turbine engine components, such as vanes and blades, are exposed to severe environments consisting of high operating temperatures, corrosive environments, high mean stresses, and high cyclic stresses while maintaining long component lifetimes. The consequence of structural failure is expensive and hazardous. Because directionally solidified (DS) columnar-grained and single crystal superalloys have the highest elevated-temperature capabilities of any superalloys, they are widely used for these structures. Understanding how single crystal materials behave and predicting how they fatigue and crack is important because of their widespread use in the commercial, military, and space propulsion industries [, ]. Single crystal materials are used extensively in applications where the prediction of fatigue life is crucial and their anisotropic nature hampers this prediction. Single-crystal materials are different from polycrystalline alloys in that they have highly orthotropic properties, making the orientation of the crystal lattice relative to the part geometry a main factor in the analysis. In turbine blades the low modulus orientation is solidified parallel to the material growth direction to acquire better thermal fatigue and creeprupture resistance [, 4]. There are computer codes that can calculate stress intensity factors for a given stress field and fatigue life for isotropic materials; however, assessing a reasonable fatigue life for orthotropic materials requires that material testing data be altered to the isotropic conditions. The ability to apply damage tolerant concepts to

11 single crystal structure design and to lay a foundation for a mechanistic based life prediction system is critical [5]. The objective of this thesis is to present a detailed analysis of experimentally determined low cycle fatigue (LCF) properties for different specimen orientations. Because mechanical and fatigue properties of single crystal materials are highly dependent on crystal orientation [, 6-], LCF properties for different specimen orientations are analyzed in this paper. Fatigue failure parameters are investigated for LCF data of single crystal material based on the shear stresses, normal stresses, and strain amplitudes on the possible slip systems for a face-centered cubic (FCC) crystal. The LCF data is analyzed for PWA49/48 at F in air; for PWA49/48 at 75 F, 4 F, and 6 F in high pressure hydrogen; and for SC (Ni-6.8 Al-.8 Mo-6) at 8 F in air [, 8]. Ultimately, a fatigue life equation is developed based on a powerlaw curve fit of the failure parameter to the LCF test data.

12 CHAPTER MATERIAL SUMMARY Single crystal nickel-base superalloys provide superior creep, stress rupture, melt resistance and thermomechanical fatigue capabilities over their polycrystalline counterparts [, 5-6]. Nickel based single-crystal superalloys are precipitation strengthened, cast monograin superalloys based on the Ni-Cr-Al system. The microstructure consists of approximately 6% by volume of precipitates in a matrix. The precipitate, is based on the intermetallic compound Ni Al, is the strengthening phase in nickel-base superalloys, and is a face centered cubic (FCC) structure. The base,, is comprised of nickel with cobalt, chromium, tungsten and tantalum in solution [5]. Single crystal superalloys have highly orthotropic material properties that vary significantly with direction relative to the crystal lattice [5, ]. Currently the most widely used single crystal turbine blade superalloys are PWA 48/49, PWA 484, CMSX-4 and Rene N-4. These alloys play an important role in commercial, military and space propulsion systems. PWA49, which is identical to PWA48 except with tighter chemical constituent control, is currently being used in the NASA SSME alternate turbopump, a liquid hydrogen fueled rocket engine. Single-crystal materials differ significantly from polycrystalline alloys in that they have highly orthotropic properties, making the position of the crystal lattice relative to the part geometry a significant factor in the overall analysis. Directional solidification is used to produce a single crystal turbine blade with the <> low modulus orientation parallel to the growth direction, which imparts good thermal fatigue and creep-rupture

13 4 resistance [, 5-6]. The secondary direction normal to the growth direction is random if a grain selector is used to form the single crystal. If seeds are used to generate the single crystal both the primary and secondary directions can be selected. However, in most turbine blade castings, grain selectors are used to produce the desired <> growth direction. In this case, the secondary orientations of the single crystal components are determined but not controlled. Initially, control of the secondary orientation was not considered necessary [7]. However, recent reviews of space shuttle main engine (SSME) turbine blade lifetime data has indicated that secondary orientation has a significant impact on high cycle fatigue resistance [,8]. The mechanical and fatigue properties of single crystals is a strong function of the test specimen crystal orientation [,, 5-8, ]. Some of the properties and the effect of orientation on those properties, which are used for design purposes, are discussed below. Elastic Modulus For single crystal superalloys, the elastic or Young s modulus (E) can be expressed as a function of orientation over the standard stereographic triangle by Equation (.) [9]: E - S [(S S ) S 44 ][cos φ(sin φ - sin θ cos φ cos θ)] (.) where θ is the angle between the growth direction and <> and φ is the angle between the <> - <> boundary of the triangle. The terms S, S and S 44 are the elastic compliances. Since the <> orientation exhibits the lowest room temperature modulus, any deviation of the crystal from the <> orientation results in an increase in the modulus. The <> orientation exhibits the highest modulus and the modulus of the <> orientation is intermediate to that of the <> and <> directions.

14 5 Tensile Properties The tensile properties of superalloys are primarily controlled by the composition and the size of the precipitates [, ]. Single crystal superalloys with the <> orientation deform by octahedral slip on the close packed {} planes and exhibit yield strengths similar to those of the conventionally cast, equiaxed, polycrystalline superalloys. Lower yield strengths and greater ductilities are reported for samples with <> orientations. The <> oriented samples exhibit the highest strengths but have the lowest ductilities at all test temperatures. Single crystals with high modulus orientations (i.e., <> and <>) can exhibit lower strengths as a result of their deforming on {} cube planes which have a lower critical resolved shear stress. Tensile failure typically occurs in planar bands due to concentration of slip that is characteristic of -strengthened alloys. The planar, inhomogeneous nature of slip results in concentrated strains and ultimately slip plane failure with the formation of macroscopic crystallographic facets on the fracture surface of tensile samples that appear brittle. At test temperatures above 9 C, deformation becomes more homogeneous and the facets become less pronounced. In addition to being a function of orientation, the yield strength of single crystals is also a function of the type of loading []. The tensile and the compressive yield stresses are not equal. Creep Properties In general, the creep properties of single crystal alloys are anisotropic, depending on both orientation and precipitate size and morphology. In addition, the test temperature has an effect on the orientation anisotropy and the dependence of creep strength on precipitate size [, 4].

15 6 At intermediate temperatures (75 C - 85 C), the creep behavior of Ni-base single crystal superalloys is extremely sensitive to crystal orientation and precipitate size [6, 7]. For a size in the range of.5 to.5µm, the highest creep strength is observed in samples oriented near <>. Samples with orientations near the <> - <> boundary exhibited extremely short creep lives.

16 CHAPTER FAILURE CRITERIA This chapter depicts the development of the formulas that govern single crystal fatigue theory by using failure parameters of polycrystalline materials. The development requires an understanding of the behavior of the single crystal material. Slip in metal crystals often occurs on planes of high atomic density in closely packed directions. The four octahedral planes corresponding to the high-density planes in the FCC crystal are shown in Fig. - [6]. Each octahedral plane has six slip directions associated with it. Three of these are termed easy-slip or primary slip directions and the other three are secondary slip directions. Thus there are primary and secondary slip directions associated with the four octahedral planes [6]. In addition, there are six possible slip directions in the three cube planes, as shown in Fig. -. Deformation mechanisms operative in high fraction nickel-base superalloys such as PWA 48/49 and SC with FCC crystal structure are divided into three temperature regions [5]. In the low temperature regime (6 C to 47 C, 79 F to 8 F) the principal deformation mechanism is by ()/<> slip; and hence fractures produced at these temperatures exhibit () facets. Above 47 C (8 F) thermally activated cube cross slip is observed which is manifested by an increasing yield strength up to 87 C (6 F) and a proportionate increase in () dislocations that have cross slipped to () planes. Thus nickel-based FCC single crystal superalloys slip primarily on the octahedral and cube planes in specific slip directions. 7

17 8 Plane Primary:,, Secondary:, 4, 5 6 Plane Primary: 4, 5, 6 Secondary: 6, 7, Plane Primary: 7, 8, 9 Secondary: 9,, Plane 4 Primary:,, Secondary:,, Figure -. Primary (close pack) and secondary (non-close pack) slip directions on the octahedral planes for a FCC crystal [6]. Plane Plane Plane 9 Figure -. Cube slip planes and slip directions for an FCC crystal [6].

18 9 Fatigue Failure Theories Used in Isotropic Metals Four fatigue failure theories used for polycrystalline material subjected to multiaxial states of fatigue stress were considered towards identifying fatigue failure criteria for single crystal material. Since turbine blades are subjected to large mean stresses from the centrifugal stress field, any fatigue failure criteria chosen must have the ability to account for high mean stress effects. Kandil et al. [5] presented a shear and normal strain based model, shown in Equation (.), based on the critical plane approach which postulates that cracks nucleate and grow on certain planes and that the normal strains to those planes assist in the fatigue crack growth process. In Equation (.) max is the max shear strain on the critical plane, ε n the normal strain on the same plane, S is a constant, and N is the cycles to initiation. + S ε n f ( N) (.) max Socie et al. [6] presented a modified version of this theory, shown in Equation (.), to include mean stress effects. Here the maximum shear strain amplitude ( ) is modified by the normal strain amplitude ( ε) and the mean stress normal to the maximum shear strain amplitude ( no ). ε n + + E no f ( N) (.) Fatemi and Socie [7] have presented an alternate shear based model for multiaxial mean-stress loading that exhibits substantial out-of-phase hardening, shown in Equation (.). This model indicates that no shear direction crack growth occurs if there is no shear alternation.

19 n (+ k max y ) f ( N) (.) Smith et al. [8] proposed a uniaxial parameter to account for mean stress effects which was modified for multiaxial loading, shown in Equation (.4), by Banantine and Socie [9]. Here the maximum principal strain amplitude is modified by the maximum stress in the direction of maximum principal strain amplitude that occurs over one cycle. ε ( max ) f ( N) (.4) Two other parameters were also investigated: the maximum shear stress amplitude, max, and the maximum shear strain amplitude, ε max on the slip systems. These parameters seemed like good candidates since deformation mechanisms in single crystals are controlled by the propagation of dislocation driven by shear. Application of Failure Criteria to Uniaxial LCF Test Data The polycrystalline failure parameters described by Equations (.) through (.4) will be applied for single crystal uniaxial strain controlled LCF test data. Transformation of the stress and strain tensors between the material and specimen coordinate systems (Fig. -) is necessary for implementing the failure theories outlined. The direction cosines between the (x, y, z) and (x, y, z ) coordinate axes are given in Table -.

20 x <> y <> z <> x y z Figure -. Material (x, y, z) and specimen (x, y, z ) coordinate systems. Table -. Direction cosines of material (x, y, z) and specimen (x, y, z ) coordinate systems. x y z x` y` z` The components of stresses and strains in the (x, y, z ) system in terms of the (x, y, z) system is given by Equations (.5) and (.6) [] { } [ ]{ } { } [ ]{ } ε ε ε Q Q ' ' ; (.5) { } [ ] { } [ ] { } { } [ ] { } [ ] { } ε ε ε ε ε Q Q Q Q ; (.6) where { } { } { } { } xy zx yz z y x xy zx yz z y x xy zx yz z y x xy zx yz z y x and ε ε ε ε ε ε ε ε ;, (.7)

21 [ ] ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( Q (.8) and [ ] ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ε Q (.9) The transformation matrix [Q] is orthogonal and hence [Q] - [Q] T [Q ]. The generalized Hooke s law for a homogeneous anisotropic body in Cartesian coordinates (x, y, z) is given by Equation (.) []. { } [ ]{ } ε ij a (.) where [a ij ] is the matrix of 6 elastic coefficients, of which only are independent, since [a ij ] [a ji ]. The elastic properties of FCC crystals exhibit cubic symmetry, also described as cubic syngony. Materials with cubic symmetry have three independent elastic constants derived from the elastic modulus, E xx and E yy, shear modulus, G yz, and Poisson ratio, ν yx and ν xy. Therefore, Equation (.) reduces to Equation (.).

22 a a a a a a a a a a ij (.) a44 a 44 a44 [ ] where the elastic constants are a, a44, a Exx G yz ν E yx xx ν E xy yy (.) The elastic constants in the generalized Hooke s law of an anisotropic body, [a ij ], vary with the direction of the coordinate axes. In the case of an isotropic body the constants are invariant in any orthogonal coordinate system. The elastic constant matrix [a ij ] in the (x, y, z ) coordinate system that relates {ε } and { } is given by the transformation Equation (.) []. 6 6 T [ a ij ] [ Q] [ aij ][ Q] ( i, j,,...,6) m n a mn Q mi Q nj (.) Shear stresses in the slip systems, shown in Figures - and -, are denoted by,. The shear stresses on the 4 octahedral slip systems are shown in Equation (.4) [6].

23 4 yz zx xy zz yy xx yz zx xy zz yy xx, (.4) The shear stresses on the six cube slip systems are shown in Equation (.5) [6]. yz zx xy zz yy xx (.5) Engineering shear strains on the slip systems are calculated using similar kinematic relations. As an example a uniaxial test specimen is loaded in the [] direction (chosen as the x axis in Fig. -) under strain control. The applied strain for the specimen is.9 %. The material properties are E xx.54e-7 psi, G yz.57e-7 psi, and ν yx.49. The problem is to calculate the stresses and strains in the material coordinate system and the shear stresses on the slip systems. The x axis is aligned along the [] direction and the y axis is chosen to lie in the xz plane. This yields the direction cosines shown in Table -.

24 5 Table -. Direction cosines for example. x y z x` y` z` The stress-strain relationship in the specimen coordinate system is given by Equation (.6) { ε } [ ]{ } ij a (.6) The [a ij ] matrix is calculated using Equation (.) and is shown as Equation (.7). (All of the elements in [a ij ] have units of psi -.) [ ].55E- 8 a ij E E E- 9.57E E E E E- 8.57E E- 8.E E E E- 8.45E E E - 7 (.7) The uniaxial stress, x, is the only nonzero stress in the specimen coordinate system and is show in Equation (.8). ε x.9 x 4.776E5 psi (.8) a.55e 8 Use of Equation (.) yields the result for {ε } shown in Equation (.9). ε x E5. ε y E - 4 ε E - ε a ij (.9) yz.785e E - zx xy 6.45E - z { } [ ]

25 6 The stresses and strains in the material coordinate system can be calculated using Equation (.6) as shown in Equation (.)..59E E E E E E E - 5.7E - 5.7E -.49E -.49E -.49E - xy zx yz z y x xy zx yz z y x ε ε ε, (.) The shear stresses on the slip planes are calculated using Equations (.4) and (.5) as shown in Equation (.)..5E + 5.5E + 5.5E + 5.5E E E E + 4.5E E E + 4.5E E E E E E E ,, (.) The engineering shear strains on the slip planes are shown in Equation (.).

26 , E E E E E E E E E -, - 8.8E - 8.8E - 8.8E - 8.8E - 8.8E - 8.8E (.) The normal stresses and strains on the principal and secondary octahedral planes are shown in Equation (.)....., E E E E E E E n n n n n n n n ε ε ε ε (.) The normal stresses and strains on the cube slip planes are simply the normal stresses and strains in the material coordinate system along (), (), and () axes. This procedure computes the normal stresses, shear stresses, and strains in the material coordinate system for uniaxial test specimens loaded in strain control in different orientations.

27 CHAPTER 4 LCF TEST DATA ANALYSIS This chapter illustrates the application of the four theories introduced in Equations (.) through (.4) in Chapter as well as max, and ε max to measured fatigue data for PWA49 and SC specimens. Initially, all of the theories are applied to straincontrolled LCF data for PWA49 in air at F. The theories are then reduced to one that shows good correlation. This is then applied to various sets of measured straincontrolled LCF data to see how they compare for PWA49 specimens in air at room temperature, for PWA49 specimens in high-pressure hydrogen (5 psi) at 4 F and 6 F,and for SC specimens in air at 8 F []. PWA49 Data at F in Air Strain controlled LCF tests conducted at F in air for PWA48/49 uniaxial smooth specimens for four different orientations is shown in Table 4-. The four specimen orientations are <>, <>, <>, and <>. Figure 4- shows the plot of strain range vs. cycles to failure. A wide scatter is observed in the data with poor correlation for a power law fit. The first step towards applying the failure criteria discussed earlier is to compute the shear stresses, normal stresses, and strains on all slip systems for each data point for maximum and minimum test strain values, as outlined in the example problem. The maximum shear stress and strain for each data point for minimum and maximum test strain values is selected from the values corresponding to the slip systems. The maximum normal stress and strain value on the planes, where the shear stress is maximum, is also calculated. These values are tabulated in Table 4-. 8

28 9 Both the maximum shear stress and maximum shear strain occur on the same slip system for the four different configurations examined. For the <> and <> configurations the maximum shear stress and strain occur on the secondary slip system ( 4, 4 and 5, 5 respectively). For the <> and <> configurations maximum shear stress and strain occur on the cube slip system ( 5, 5 and 9, 9 respectively). Using Table 4- the composite failure parameters highlighted in Equations (-4) can be calculated and plotted as a function of cycles to failure. Figures 4- through 4-5 show that the four parameters based on polycrystalline fatigue failure parameters, Equations (.)-(.4), do not correlate well with the test data. This may be due to the insensitivity of these parameters to the critical slip systems. The parameter that gives the best correlation is a power law fit to the maximum shear stress amplitude [ max ] shown in Fig The parameter max is appealing to use for its simplicity; its power law curve fit is shown in Equation (4.). max 97,758 N (4.) Since the deformation mechanisms in single crystals are controlled by the propagation of dislocations driven by shear, the max might indeed be a good fatigue failure parameter to use.

29 Power Law Curve Fit (R.469): ε.8 N <> <> <> <> Cycles to Failure Figure 4-. Strain range vs. cycles to failure for LCF test data (PWA49 at F).

30 Power Law Curve Fit (R.): [ max + ε n ].49 N <>. <> <>.5 <>..5 Cycles to Failure Figure 4-. [ max + ε n ] vs. N

31 Power Law Curve Fit (R ε.9): + n + no E N -. <> <> <> <> Figure 4-. Cycles to Failure ε + n + no E vs. N

32 Power Law Curve Fit (R.8): n (+ k max y ).4 N <>. <>.5 <> <>..5 Figure 4.4. Cycles to Failure n (+ k max y ) vs. N

33 4 5 5 Power Law Curve Fit (R.89): ε ( max ) 4.6 N -.9 <> <> <> <> 5 5 Cycles to Failure Figure 4-5. ε ( max ) vs. N

34 5 Power Law Curve Fit (R.674): 97,758 N <> <> 5 <> <> 5 5 Cycles to Failure Figure 4-6. Shear stress amplitude [ max ] vs. N

35 6 Table 4-. Strain controlled LCF test data for PWA49 at F for four specimen orientations. Cycles Specimen Max Min R Strain to Orientation Test Strain Test Strain Ratio Range Failure <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> <> <>

36 Table 4-. Maximum values of shear stress and shear strain on the slip systems and normal stress and strain values on the same planes. Specimen Orientation max min / ε max ε min ε/ max min max min <> E E+5 6.8E E E E E+5.96E+4.6E+5 8.9E+4.9E E+4 59 max E E-4.E E E E E max E E+4 8.7E+4 6.7E+4 6.7E E E E E E <>.4-7.6E-.5.5E- -.E-.5.5E+5 -.E+5.5E+5.59E+5-7.8E+4.7E E+5.77E+4.49E+5.5E+5.96E+4.5E+5 84 max E E E E+5.6E E+5 6 max E+5.E E E E+4-5.E+4.7E+5.8E+4 -.7E+4 7.5E E E-4.5E E E+5 7.7E E E+5.E+5.5E+5.6E+5 7.8E+4 8.E <> E+5.6E+5.E+5.E E E E+5 7.4E+5 8.6E E max E+4 8.E+4 6.5E+4 6.5E max E+4 8.E+4 6.5E+4 6.5E <>.5 6.5E E E+5 5.8E+5.7E E+5 67 max E+5.E+5.7E+5.7E+5 75 max 5..E E E E+4.E E+5 The following definitions apply max Max shear strain of slip systems for max specimen test strain value min Max shear strain of slip systems for min specimen test strain value max Max shear stress of slip systems for max specimen test strain value min Max shear stress of slip systems for min specimen test strain value psi psi psi psi psi psi Cycles to Failure 7

37 8 PWA49 Data at Room Temperature (75 F) in High Pressure Hydrogen Turbine blades in the Space Shuttle Main Engine (SSME) Alternate High Pressure Fuel Turbopump (AHPFTP) are made of PWA49 single crystal material [, 8, ]. The blades are subjected to high-pressure hydrogen. From a fatigue crack nucleation perspective, the effects of high-pressure hydrogen are most detrimental at room temperature and are less pronounced at higher temperatures [5, ]. The interaction between the effects of environment, temperature and stress intensity determines which point-source defect species (carbides, eutectics, and micropores) initiates a crystallographic or noncrystallographic fatigue crack [7] in PWA48/49. At room temperature (6 C), in high-pressure hydrogen, the eutectic / initiates fatigue cracks by an interlaminar (between and ) failure mechanism, resulting in noncrystallographic fracture [5, ]. In room temperature air, carbides typically initiate crystallographic fracture. Fatigue cracks frequently nucleate at microporosities when tested in air at moderate temperature (above 47 C). Figure 4-7 shows the strain amplitude vs. cycles to failure LCF data for PWA49 at room temperature (6 C, 75 F) in 5 psi hydrogen, for three different specimen orientations. Testing was performed under strain control. The data in Fig. 4-7 shows a fairly wide scatter. Table 4- shows the LCF data and other fatigue damage parameters evaluated on the slip planes. Figure 4-8 shows a plot of [ max ] vs. cycles to failure with the power law curve fit showing a poor correlation. The presence of high-pressure hydrogen at room temperature activates the eutectic and causes noncrystallographic fracture, as explained earlier. This type of noncrystallographic fracture is not captured well by an analysis of shear stresses on slip planes. A failure parameter that can model

38 9 the interlaminar failure mechanism between the and structures would likely provide better results.. Strain Amplitude (%) <> <> <> Cycles to Failure Figure 4-7. LCF data for PWA49 at room temperature in 5 psi high pressure hydrogen: strain amplitude vs. cycles to failure.

39 Power Law Curve Fit (R.46): 8,49 N E+5.6E+5.4E+5 <> <> <> Pow er Law Fit Max Shear Stress Amplitude.E+5.E+5 8.E+4 6.E+4 4.E+4.E+4.E Cycles to Failure Figure 4-8. Shear stress amplitude ( max ) vs. cycles to failure for PWA49 at room temperature in 5 psi hydrogen.

40 Table 4-. PWA49 LCF high pressure hydrogen (5 psi) data at ambient temperature. Max Min Strain Strain Specimen Orientation <> max 5 <> max 7 <> max 5 ε max ε min Strain Ratio R ε min /ε max Strain Range max ε max max (psi) max (psi) Cycles to Failure ,85 8, ,85 8, ,88 44, ,88 44, ,88 44, ,88 44, ,9 8, ,9 8, , 6, , 6, ,5 7, ,5 7, ,5 7, ,5 7, ,4, ,4, ,8 55, ,8 55, ,8 55, ,87 66, ,87 66, ,87 66, ,94 77, ,94 77, ,94 77,

41 PWA49 Data at 4 F and 6 F in High Pressure Hydrogen At higher temperatures hydrogen does not activate the eutectic failure mechanism, and under these conditions max is a good failure parameter for modeling LCF data. Figures 4-9 and 4- show the strain amplitude vs. cycles to failure for PWA49 in high-pressure hydrogen (5 psi) at 4 F and 6 F, respectively. There are only three data points at 4 F and four at 6 F because of the difficulty and expense in performing fatigue tests under these conditions. These tests were conducted at the NASA MSFC. Figures 4- and 4- show the plots of [ max ] vs. cycles to failure for 4 F and 6 F temperatures, respectively. The power law curve fits are seen to have a good correlation because the resulting fractures are crystallographic in nature at these high temperatures. Tables 4-4 and 4-5 show the LCF data and the fatigue parameters Strain Amplitude (%) <> <> Cycles to Failure Figure 4-9. LCF data for PWA49 at 4 F in 5 psi high pressure hydrogen: strain amplitude vs. cycles to failure.

42 .5 <> <> Strain Amplitude (%) Cycles to Failure Figure 4-. LCF data for PWA49 at 6 F in 5 psi high pressure hydrogen: strain amplitude vs. cycles to failure.

43 4.6E+5.4E+5 Power Law Curve Fit (R^.66):,56 N -. <> <> Pow er Law Fit.E+5 Max Shear Stress Amplitude.E+5 8.E+4 6.E+4 4.E+4.E+4.E Cycles to Failure Figure 4-. Shear stress amplitude ( max ) vs. cycles to failure for PWA49 at 4 F in 5 psi hydrogen.

44 5.6E+5.4E+5 Power Law Curve Fit (R^.965): 8,4 N -.4 <> <> Pow er Law Fit.E+5 Max Shear Stress Amplitude.E+5 8.E+4 6.E+4 4.E+4.E+4.E Cycles to Failure Figure 4-. Shear stress amplitude ( max ) vs. cycles to failure for PWA49 at 6 F in 5 psi hydrogen.

45 6 Table 4-4. PWA49 LCF data measured in high pressure hydrogen (5 psi) at 4 F. Max Min Strain Cycles Specimen Strain Strain Strain Ratio Orientation Range max ε max max max to (psi) (psi) ε max ε min ε min /ε max Failure <> max ,4,5 7 <> ,9 66,9 5 max ,5 8,8 Table 4-5. PWA49 LCF data measured in high pressure hydrogen (5 psi) at 6 F. Max Min Strain Cycles Specimen Strain Strain Strain Ratio Orientation Range max ε max max max to (psi) (psi) ε max ε min ε min /ε max Failure <> ,555 96,4 max ,45 74,6 <> , 67,9 4 max ,597 74,57 95 SC LCF Data at 8 F in Air Figure 4- shows the strain amplitude vs. cycles to failure LCF data for SC at 8 F in air for 5 different specimen orientations: <>, <>, <>, <>, and <> [7]. A wide amount of scatter is seen in the plot. Figure 4-4 shows [ max ] vs. cycles to failure plot with an excellent correlation for a power law fit. Table 4-6 shows the LCF data and the fatigue parameters.

46 7 Strain Amplitude (%) <> <> <> <> <>. 5 5 Cycles to Failure Figure 4-. LCF data for SC at 8 F in air: strain amplitude vs. cycles to failure.

47 8 8.E+4 7.E+4 6.E+4 Power Law Curve Fit (R^.79):,75 N <> <> <> <> <> Pow er Law Fit Max Shear Stress Amplitude 5.E+4 4.E+4.E+4.E+4.E+4.E Cycles to Failure Figure 4-4. Shear stress amplitude ( max ) vs. cycles to failure for SC at 8 F in air.

48 9 Table 4-6. LCF data for single crystal Ni-base superalloy SC at 8 F in air. Cycles Specimen Strain Orientation Range max ε max max max to (psi) (psi) Failure..7E-.E- 5.95E+4.59E <>..7E-.E- 5.95E+4.59E E- 8.E E+4.7E+5 68 max E- 7.E E+4 8.8E E- 6.E-.56E E+4 65 [] <> max 5 <> max 7 <> max 9.8.4E E- 6.8E+4.859E E E- 6.8E+4.859E E E- 5.45E+4.77E E- 6.49E E E E E- 4.76E+4 8.5E E E- 4.76E+4 8.5E E E-.88E+4 7.4E E-.454E- 5.94E E E-.454E- 5.94E E E-.846E- 4.8E+4 8.6E E-.769E-.467E E E-.769E-.467E E E-.769E-.467E E E-.99E-.98E+4 5.7E E-.777E-.597E E E- 5.87E- 5.77E+4.88E E- 5.87E- 5.77E+4.88E E- 5.87E- 5.77E+4.88E E E- 5.94E E E- 4.E E E E- 4.E E E E-.597E- 4.98E E E-.8E-.467E E E E E+4.494E E E E+4.494E E E E+4.494E E E E+4.57E+5 88 <> 5 max.8.98e E E+4.5E

49 CHAPTER 5 CONCLUSION The purpose of this study was to find a parameter that best fits the experimental data for single crystal materials PWA48/49 and SC at various temperatures, environmental conditions, and specimen orientations. Several fatigue failure criteria, based on the normal stresses, shear stresses, and strains on the 4 octahedral and six cube slip systems for a FCC crystal, are evaluated for strain controlled uniaxial LCF data. The maximum shear stress amplitude max on the slip systems was found to be an effective fatigue failure parameter, based on the curve fit between max and cycles to failure. The parameter [ max ] did not characterize the room temperature LCF data in high-pressure hydrogen well because of the eutectic failure mechanism activated by hydrogen at room temperature. LCF data in high-pressure hydrogen at 4 F and 6 F was characterized well by the max failure parameter. Since deformation mechanisms in single crystals are controlled by the propagation of dislocations driven by shear, max might indeed be a good fatigue failure parameter to use. This parameter must be verified further for a wider range of R-values and specimen orientations as well as at different temperatures and environmental conditions. 4

50 REFERENCES. S. E. Cunningham, D. P. DeLuca, and F. K. Haake, Crack Growth and Life Prediction in Single-Crystal Nickel Superalloys, Materials Directorate, Wright Laboratory, FR59, Vol., February B. J. Peters, C. M. Biondo, and D. P. DeLuca, Investigation of Advanced Processed Single-Crystal Turbine Blade Alloys, George C. Marshall Space Flight Center, NASA, FR47, December J. Moroso, Effect of Secondary Crystal Orientation on Fatigue Crack Growth in Single Crystal Nickel Turbine Blade Superalloys, M.S. Thesis, Mechanical Engineering Department, University of Florida, Gainesville, May B. A. Cowels, High Cycle Fatigue in Aircraft Gas Turbines: An Industry Perspective, International Journal of Fracture, Vol. 8, pp. 47-6, D. Deluca and C. Annis, Fatigue in Single Crystal Nickel Superalloys, Office of Naval Research, Department of the Navy, FR8, August D. C. Stouffer and L. T. Dame, Inelastic Deformation of Metals: Models, Mechanical Properties, and Metallurgy, John Wiley & Sons, New York, M. Gell and D.N. Duhl, The Development of Single Crystal Superalloy Turbine Blades, Processing and Properties of Advanced High-Temperature Materials, Eds. S.M. Allen, R.M. Pelloux, and R. Widmer, ASM, Metals Park, Ohio, pp. 4, N. K. Arakere and G. Swanson, Effect of Crystal Orientation on Fatigue Failure of Single Crystal Nickel Base Turbine Blade Superalloys, ASME Journal of Engineering of Gas Turbines and Power, Vol. 4, Issue, pp. 6-76, January. 9. M. McLean, Mechanical Behavior: Superalloys, Directionally Solidified Materials for High Temperature Service, The Metals Society, London, pp. 5, 98.. B. H. Kear and B. J. Piearcey, Tensile and Creep Properties of Single Crystals of the Nickel-Base Superalloy Mar-M, Trans. AIME, 9, pp. 9,

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52 BIOGRAPHICAL SKETCH Evelyn Orozco-Smith was born in Hialeah, Florida, in 974. She attended the University of Florida in Gainesville, Florida, where she received a Bachelor of Science in aerospace engineering in 997. She worked for Pratt & Whitney in the structures group creating and analyzing finite element models of the Space Shuttle Main Engine (SSME) High Pressure Fuel Turbo Pump, which at the time were under final approval review for production. In 999 she enrolled at the University of Florida to pursue a Master of Science from the Mechanical Engineering Department under the direction of Dr. Nagaraj K. Arakere on a project funded by NASA. She now works at Kennedy Space Center as a systems engineer processing the Main Propulsion System and the SSME for the Space Shuttle Program. 4