High Cycle Fatigue: A Mechanics of Materials Perspective part 34 pot

316 Effects of Damage on HCF Properties P Q –b –a a b Figure 6.53. Contact geometry for flat pad with radius. b is the semi-width of the overall contact and a is the semi-width of the central flat part. 0.3 0.5 0.4 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.6 0.7 0.8 0.9 1 Q /P = 0.1 Q /P = 0.3 Q /P = 0.5 Q /P = 0.7 Q /P = 0.9 Slip zone friction coefficient, f n Hertzian case, a /b = 0 Almost sharp punch, a /b = 0.99 Mean friction coefficient, f m Figure 6.54. Numerical results for different contact geometrics, a/b =002 045 07 09 0 99 [65]. These numerical findings are consistent with those of the previously cited references where larger local values of COF are deduced from combined experimental and numerical findings. These higher values are known to affect the local contact stresses which, in turn, can have a significant impact on crack initiation criteria and a lesser impact on crack growth behavior away from the local contact stress field. Thus, the value of COF becomes an important aspect in the analysis of fretting-fatigue conditions and, unfortunately, is not an experimentally measurable quantity. Fretting Fatigue 317 6.14.1. Average versus local coefficient of friction The average COF, , is commonly obtained experimentally by increasing the shear load in a contact region until the bodies in contact just start to slide with respect to each other. Of greater importance, however, for modeling purposes, is the local value of m in the slip region in stick–slip contacts. The average value and local slip zone value have been shown to be different. The slip zone coefficient of friction,  s , is obtained from the average value as follows [27]  s =+ 2    s  0 B √ 1−B +sin −1 √ 1−Bd s (6.31) where B = Q/ s P  0 is the initial value of COF, and  is the average value. Equa- tion (6.31) ignores the effect of the bulk load on the contact tractions and assumes that the friction coefficient in the stick zone does not change. Araújo and Nowell [43] later reported a change in the equation due to a slight error that they describe as affecting the final form of the equation but the values are little changed. The revised equation has the form  s =+ 4Q P   2 −cot  s − s  − 2    s  s − Q P tan  s  (6.32) where  s =sin −1  Q  s P s =cos −1  Q  s P (6.33) 6.15. SUMMARY COMMENTS Three main factors have to be addressed in a robust life prediction scheme for fretting fatigue: 1. The initiation life has to be treated separately from the propagation life. While no definition can be given for the flaw size that separates initiation from propagation, many instances arise where the crack propagation results are insensitive to the actual value used. The use of the value of a 0 from the El Haddad short crack formulation appears to be a reasonable approach, but a 0 takes on different values depending on stress ratio, R, in many cases. It is also somewhat dependent on the geometry of the body into which the crack propagates, but generally only for long cracks. 2. Point stresses in the presence of steep stress gradients cannot be used as part of a realistic criterion for crack initiation. Instead, they must be modified as in notch 318 Effects of Damage on HCF Properties fatigue in order to be able to consolidate them with smooth bar fatigue data. In fretting fatigue we are normally dealing with stress fields that are the equivalent of those encountered at the tip of a very sharp notch. Multiaxial stress states can be an important consideration when establishing initiation criteria. 3. The stresses necessary to initiate a crack in fretting fatigue in the contact region may be altered by the presence of damage from the fretting process itself, by changes in COF, , by changes in the surface roughness, or through local plasticity and/or material removal at the surface. These considerations should be taken into account in model development. In all of the above, the application to HCF as opposed to LCF can be addressed in some cases by treating the problem as one where the threshold for crack propagation is the governing criterion, whether or not a real crack has initiated. In such cases, it would have to be shown whether or not an initiated fretting-fatigue crack would continue to propagate based on the combination of a local stress field from the contact conditions and a far-field stress field from the loaded component into which the crack is propagating. There have been many successes using fracture-mechanics-based criteria for HCF thresholds (and LCF lives) in fretting fatigue based on the use of a friction coefficient that is appropriate for the particular experimental configuration. For HCF thresholds, it is assumed, further, that the threshold crack growth value in a gradient stress field is the same as that obtained under uniform stress (see Chapter 8 for a discussion of factors that affect crack growth threshold values). For an extensive coverage of fretting fatigue including background, experiments and experimental results, and analytical methods including life prediction, the reader is referred to the review article by Farris et al. [66]. REFERENCES 1. Vingsbo, O. and Söderberg, D., “On Fretting Maps”, Wear, 126, 1988, pp. 131–147. 2. Hutson, A.L., Nicholas, T., and Goodman, R., “Fretting Fatigue of Ti-6Al-4V Under Flat-on-Flat Contact”, Int. J. Fatigue, 21, 1999, pp. 663–669. 3. Araújo, J.A. and Nowell, D., “Analysis of Pad Size Effects in Fretting Fatigue Using Short Crack Arrest Methodologies”, Int. J. Fatigue, 21, 1999, pp. 947–956. 4. Tomlinson, G.A., Thorpe, P.L., and Gough, H.J., “An Investigation of the Fretting Corrosion of Closely Fitting Surfaces”, Proc. Inst. Mech. Engrs, London, 141, 1939, pp. 223–237. 5. Conner, B.P. and Nicholas, T., “Using a Dovetail Fixture to Study Fretting Fatigue and Fretting Palliatives”, ASME Jour. Eng. Mat. Tech., 128, 2006, pp.133–141. 6. Gean, M.C., “Finite Element Analysis of the Mechanics of Blade Disk Contacts”, M.S. Thesis, School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN, May 2004. 7. Golden, P.J. and Nicholas, T., “The Effect of Angle on Dovetail Fretting Experiments in Ti- 6Al-4V”, Fatigue Fract. Engng Mater. Struct., 28, 2005, pp.1169–1175. Fretting Fatigue 319 8. Hutson, A.L., Nicholas, T., Olson, S.E., and Ashbaugh, N. E. “Effect of Sample Thickness on Local Contact Behavior in a Flat-on-Flat Fretting Fatigue Apparatus”, Int. J. Fatigue, 23, 2001, pp. S445–S453. 9. Tada, H., Paris, P.C., and Irwin, G.R., Stress Analysis of Cracks Handbook, Del Research Corp., Hellertown, PA, 1973. 10. Eden, E.M., Rose, W.N., and Cunningham, F.L., “The Endurance of Metals – Experiments on Rotating Beams at University College”, Proc. Inst. Mech. Engrs., London, 4, 1911, pp. 839–974. 11. Tomlinson, G.A., “The Rusting of Steel Surfaces in Contact”, Proc. Roy. Society (London), A115, 1927, pp. 472–483. 12. Forrest, P.G., Fatigue of Metals, Pergamon Press, Oxford, 1962 (U.S.A. Edition distributed by Addison-Wesley Publishing Co., Reading, MA). 13. Poon, C. and Hoeppner, D.W., “The Effect of Environment on the Mechanism of Fretting Fatigue”, Wear, 52, 1979, pp. 175–191. 14. Schütz, W., “A History of Fatigue”, Engng Fract. Mech., 54, 1996, pp. 263–300. 15. Giannakopoulos, A.E., Venkatesh, T.A., Lindley, T.C., and Suresh, S., "The Role of Adhesion in Contact Fatigue”, Acta Materialia, 47, 1999, pp. 4653–4664. 16. Taylor, D.E., “Environmental Fretting Fatigue”, Fretting Fatigue, ESIS 18, R.B. Waterhouse and T.C. Lindley, eds, Mechanical Engineering Publications, London, 1994, pp. 375–389. 17. Conner, B.P., Hutson, A.L., and Chambon, L., “Observations of Fretting Fatigue Micro-Damage of Ti-6Al-4V”, Wear, 255, 2003, pp. 259–268. 18. Faanes, S. and Fernando, U.S., “Life Prediction in Fretting Fatigue using Fracture Mechanics”, Fretting Fatigue, ESIS 18, R.B. Waterhouse and T.C. Lindley, eds, Mechanical Engineering Publications, London, 1994, pp. 149–159. 19. Waterhouse, R.B., “Fretting Fatigue”, Int. Mater. Rev., 37, 1992, pp. 77–97. 20. Hills, D.A., Nowell, D., and O’Connor, J.J., “On the Mechanics of Fretting Fatigue”, Wear, 125, 1988, pp. 129–146. 21. Szolwinski, M. and Farris, T., “Mechanics of Fretting Fatigue Crack Formation”, Wear, 198, 1996, pp. 93–107. 22. Golden, P.J. and Grandt, A.F. Jr., “Fracture Mechanics Based Fretting Fatigue Life Predictions in Ti-6Al-4V”, Eng. Fract. Mech., 71, 2004, pp. 2229–2243. 23. Mutoh, Y. and Xu, J Q., “Fracture Mechanics Approach to Fretting Fatigue and Problems to be Solved”, Tribology International, 36, 2003, pp. 99–107. 24. Hills, D.A., Nowell, D., and Sackfield, A., Mechanics of Elastic Contacts, Butterworth- Heinemann, Oxford, 1993. 25. Mindlin, R.D., “Compliance of Elastic Bodies in Contact”, ASME J. Appl. Mech., 16, 1949, pp. 259–268. 26. Golden, P.J. and Calcaterra, J.R., “A Fracture Mechanics Life Prediction Methodology Applied to Dovetail Fretting”, to be published in Tribology Int. Lyon presentation. 27. Hills, D.A. and Nowell, D., Mechanics of Fretting Fatigue, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1994. 28. Dobromirski, J.M., “Variables of Fretting Processes: Are There 50 of Them?”, Fretting Fatigue, ESIS 18, R.B. Waterhouse and T.C. Lindley, eds, Mechanical Engineering Publications, Lon- don, 1994, pp. 60–66. 29. Hutson, A.L., Neslen, C., and Nicholas, T., “Characterization of Fretting Fatigue Crack Initia- tion Processes in Ti-6Al-4V”, Tribology International, 36, 2003, pp 133–143. 30. Ciavarella, M., Hills, D.A., and Monno, G., “The Influence of Rounded Edges on Indentation by a Flat Punch”, Proc. Inst. Mech. Engrs., J. Mech. Eng. Sci., 212C, 1998, pp. 319–328. 320 Effects of Damage on HCF Properties 31. Giannakopoulos, A.E., Lindley, T.C., Suresh, S., and Chenut, C., "Similarities of Stress Con- centrations in Contact at Round Punches and Fatigue at Notches: Implications to Fretting Fatigue Crack Initiation”, Fatigue Fract. Eng. Mater. Struct, 23, 2000, pp. 562–571. 32. Johnson, K.L., Contact Mechanics, Cambridge University Press, Cambridge, 1985. 33. Murthy, H., Farris, T.N., and Slavik, D.C., “Fretting Fatigue of Ti-6A1-4V Subjected to Blade/Disk Contact Loading”, Developments in Fracture Mechanics for the New Century50th Anniversary of Japan Society of Materials Science, Osaka, Japan, May 2001, pp. 41–48. 34. McVeigh, P.A., Harish, G., Farris, T.N., and Szolwinski, M.P., “Modeling Contact Conditions in Nominally Flat Contacts for Application to Fretting Fatigue of Turbine Engine Components”, Int. J. Fatigue, 21, 1999, pp. S157–S165. 35. Lindley, T.C., “Fretting Fatigue in Engineering Alloys”, Int. J. Fatigue, 19, Supp. No. 1, 1997, pp. S-39–S49. 36. Ruiz, C., Boddington, P.H.B., and Chen, K.C., “An Investigation of Fatigue and Fretting in a Dovetail Joint”, Expt. Mechanics, 24, 1984, pp. 208–217. 37. Magaziner, R., Jin, O., and Mall, S., “Slip Regime Explanation of Observed Size Effects in Fretting”, Wear, 257, 2004, pp. 190–197. 38. Naboulsi, S. and Mall, S., “Fretting Fatigue Crack Initiation Behavior Using Process Volume Approach and Finite Element Analysis”, Tribology International, 36, 2003, pp. 121–131. 39. Lykins, C.D., Mall, S., and Jain, V.K., “A Shear Stress-Based Parameter for Fretting Fatigue Crack Initiation”, Fatigue Fract. Eng. Mater. Struct., 24, 2001, pp. 461–473. 40. Findley, W.N., “Fatigue of Metals Under Combinations of Stresses”, Trans. ASME, 79, 1957, pp. 1337–1348. 41. Namjoshi, S., Mall, S., Jain, V., and Jin, O., “Fretting Fatigue Crack Initiation Mechanism in Ti-6Al-4V”, Fatigue Fract. Eng. Mater. Struct., 25, 2002, pp. 955–964. 42. Neu, R., Pape, J., and Swalla, D.R., “Methodologies for Linking Nucleation and Propagation Approaches for Predicting Life Under Fretting Fatigue”, Fretting Fatigue; Current Technology and Practices, ASTM STP 1367, D.W. Hoeppner, V. Chandrasekaran, and C.B. Elliot, eds, American Society for Testing and Materials, West Conshohocken, PA, 2000, pp. 369–388. 43. Araújo, J.A. and Nowell, D., “The Effect of Rapidly Varying Contact Stress Fields on Fretting Fatigue”, Int. J. Fatigue, 24, 2002, pp. 763–775. 44. Fatemi, A. and Socie, D.F., “A Critical Plane Approach to Multiaxial Fatigue Damage Including Out of Phase Loading”, Fatigue Fract. Eng. Mater. Struct, 11, 1988, pp. 149–165. 45. Nowell, D. and Dini, D., “Stress Gradient Effects in Fretting Fatigue”, Tribology International, 36, 2003, pp. 71–78. 46. Nix, K.J. and Lindley, T.C., “The Application of Fracture Mechanics to Fretting Fatigue”, Fatigue Fract. Eng. Mater. Struct., 8, 1985, pp. 143–160. 47. Nicholas, T., Hutson, A., John, R., and Olson, S., “A Fracture Mechanics Methodology Assess- ment for Fretting Fatigue”, Int. J. Fatigue, 25, 2003, pp. 1069–1077. 48. Naboulsi, S. and Nicholas, T., “Limitations of the Coulomb Friction Assumption in Fretting Fatigue Analysis”, Int. J. Solids Struct., 40, 2003, pp. 6497–6512. 49. Chan, K.S., Davidson, D.L., Lee, Y-D., and Hudak, S.J., Jr., “A Fracture Mechanics Approach to High Cycle Fretting Fatigue Based on The Worst Case Fret Concept: Part I – Model Development”, Int. J. Fract., 112, 2001, pp. 299–330. 50. McVeigh, P.A. and Farris, T.N., “Analysis of Surface Stresses and Stress Intensity Factors Present During Fretting Fatigue”, Proceedings of the 40 th AIAA/ASME/ASCE/ASC Structures, Structural Dynamics, and Materials Conf., vol. 2, St Louis, MO, April 1999, pp. 1188–1196 (CD-ROM). See also: McVeigh, P.A., “Analysis of Fretting Fatigue in Aircraft Structures: Fretting Fatigue 321 Stresses, Stress Intensity Factors, and Life Predictions”, Ph.D. Thesis, School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN, 1999. 51. Beghini, M., Bertini, L., and Fontanari, V., “Weight Function for an Inclined Edge Crack in a Semi-plane", Int. J. Fract., 99, 1999, pp. 281–292. 52. Jin, O., Mall, S., and Sahan, O., “Fretting Fatigue Behavior of Ti-6Al-4V at Elevated Temper- ature”, Int. J. Fatigue, 27, 2005, pp. 395–401. 53. Golden, P.J., Hutson, A.L., Bartha, B., and Nicholas, T., “A Comparison of Fretting Fatigue Test Fixtures”, presented at ICEM 12 – 12th International Conference on Experimental Mechanics, Bari, Italy, 29 August–2 September 2004. Conference proceedings only. 54. Jin, O. and Mall, S., “Effects of Independent Pad Displacement on Fretting Fatigue Behavior of Ti-6Al-4V”, Wear, 253, 2002, pp. 585–596. 55. Naboulsi, S. and Mall, S., “Investigation of High Cycle and Low Cycle Fatigue Interaction on Fretting Behavior”, Int. J. of Mechanical Sciences, 44, 2002, pp. 1625–1645. 56. McVeigh, P.A., Harish, G., Farris, T.N., and Szolwinski, M.P., “Modeling Contact Conditions in Nominally Flat Contacts for Application to Fretting Fatigue of Turbine Engine Components”, Int. J. Fatigue, 21, 1999, pp. S157–S165. 57. Swalla, D.R. and Neu, R.W., “Influence of Coefficient of Friction on Fretting Fatigue Crack Nucleation Prediction”, Tribology International, 34, 2001, pp. 493–503. 58. Goh, C H., Wallace, J.M., Neu, R.W., and McDowell, D.L., “Polycrystal Plasticity Simulations of Fretting Fatigue”, Int. J. Fatigue, 23, 2001, pp. S423–S435. 59. Farris, T.N., Murthy, H., and Matlik, J.F., “Fretting Fatigue”, Comprehensive Structural Integrity Fracture of Materials from Nano to Macro; Volume 4: Cyclic Loading and Fatigue, R.O. Ritchie and Y. Murakami, eds, Elsevier, 2003, pp 281–326. 60. Hutson, A.L., Niinomi, M., Nicholas, T., and Eylon, D., “Effect of Various Surface Conditions on Fretting Fatigue Behaviour of Ti-6Al-4V”, Int. J. Fatigue, 24, 2002, pp. 1223–1234. 61. Namjoshi, S.A., Jain, V.K., and Mall, S., “Effects of Shot-Peening on Fretting Fatigue Behav- ior of Ti-6Al-4V”, ASME Journal of Engineering Materials and Technology, 124, 2002, pp. 222–228. 62. Nicholas, T., Hutson, A.L., Olson, S., and Ashbaugh, N., “In Search of a Parameter for Fretting Fatigue”, Advances in Fracture Research, Proceedings of ICF-10, K. Ravi-Chandar, B.L. Karihaloo, T. Kishi, R.O. Ritchie, A.T. Yokobori Jr., and T. Yokobori, eds, Paper # 0809, CD ROM, Elsevier, 2001. 63. Hutson, A.L., John, R., and Nicholas, T., “Fretting Fatigue Crack Analysis in Ti-6Al-4V”, Int. J. Fatigue, 27, No. 10–12 (Special Issue on Fatigue Damage of Structural Materials), 2005, pp. 1582–1589. 64. Hills, D.A., Nowell, D., and Sackfield, A., “Surface Fatigue Consideration in Fretting”, Inter- face Dynamics, Proceedings of the 14th Leeds-Lyon Symposium on Tribology, D. Dowson, C.M. Taylor, M. Godet, and D. Berthe, eds, Elsevier, Amsterdam, 1988, pp. 35–50. 65. Dini, D. and Nowell, D, “Prediction of the Slip Zone Friction Coefficient in Flat and Rounded Contact”, Wear, 254, 2003, pp. 364–369. 66. Farris, T.N., Murthy, H., and Matlik, J.F., “Fretting Fatigue”, Comprehensive Structural Integrity Fracture of Materials from Nano to Macro; Volume 4: Cyclic Loading and Fatigue R.O. Ritchie and Y. Murakami, eds, Elsevier, 2003, pp. 281–326. Chapter 7 Foreign Object Damage 7.1. INTRODUCTION Foreign object damage, commonly referred to as FOD, involves the damage caused by objects ingested into turbine engines. These events typically take place upon takeoff and landing. Ingested objects, which involve a range of soft and hard materials of all sizes and shapes, cause damage when they strike rotating blades, static vanes, or other parts of the engine where they cause a reduction in strength of the component. The damage is typically in the form of a notch covering a wide range of notch depths, radii, and possible cracking at the root of the notch. The problem is most severe when the extent of the damage causes instantaneous failure of a component. Such events are difficult to prevent although methods such as the use of screens have been unsuccessfully attempted at times in the past. Debris on runways can be ingested into engines and is taken care of by routine inspection of runways and taxiways and removal of such debris. Nonetheless, objects are commonly ingested and can cause damage in the form of nicks, dents, tears, and so forth. Damage, if not severe, can cause a reduction of the static strength of a component which is handled in design by the use of some type of knockdown factor to account for FOD in regions where such events are expected or known to take place from prior experience. A greater concern arises when the component that is damaged is also undergoing some type of forced or resonant vibration that can lead to HCF in the damaged area. The problem of FOD is somewhat related to fatigue of notches, discussed in Chapter 5, but is much more complicated because it involves additional aspects that are discussed later. The severity of a notch, from a geometric point of view, is characterized by a stress concentration factor, k t , which, under FOD, can vary significantly in service. Different types of damages are illustrated later in this chapter as well as in Appendix G involving a wide variation in geometry and corresponding values of k t . Robust engineering methods are needed to predict the influence of these geometric variables on HCF threshold stresses. It is the HCF failure and the HCF strength of damaged materials that is the main subject of this chapter dealing with the HCF properties of materials and components that are subjected to FOD. Both real and simulated FOD are discussed, the latter based on the assumption that damage from an impacting object can be simulated by other impact events. Low velocity and quasi-static impact events can produce damage that appears to be geometrically similar to that obtained under ballistic impact. How representative these events are in reproducing actual FOD is an important consideration in the study of FOD. A further concern is the nature of the ingested object that causes the damage in 322 Foreign Object Damage 323 engines and is rarely identified in total after the event. Only the damaged component is left behind to study the nature of FOD. In turbine engines, FOD is caused most by the ingestion of birds, sand, dust, ice, and other debris. This general problem is addressed by the US Department of Defense through a specification guide that is applicable to all of the Services – Army, Navy, Air Force, Marines, and Coast Guard. The specification guide, however, does not deal with methods for designing against FOD or conducting analysis of damaged hardware. Rather, it provides guidance for the testing of engines to qualify them for tolerance to ingested objects of specified materials and sizes. Often, in the introduction of a new engine, the objects that the engine may see in service are left up to the discretion of the organization or service that builds the engine for specific applications and missions. This highly empirical approach to FOD, detailed in a Joint Service Specification Guide (JSSG), is summarized in Appendix H. Although the guide, in general, deals with the types of debris noted above, it does not specifically deal with large sand particles or small stones which have been found to be both common and very damaging to airfoils in gas turbine engines. This type of debris will hereafter be referred to as hard particles. FOD can also be caused by soft body impact such as due to birds. A common misconception is that ice acts as a hard body impactor. Depending on temperature, ice can often appear to display the characteristics of hard body impact, but the majority of time ice should be thought of as a soft body impactor. In addition to objects ingested into an engine that cause what we term FOD, there are instances where the debris causing damage comes from an internal failure. For example, tips of blades have been known to break off, usually due to some type of resonant fatigue condition. Such an object can then proceed further down the engine and cause impact damage on another component. Objects that are produced from internal failures and subsequently impact some other engine component are considered to cause domestic object damage (DOD). Although this section deals with the general subject of FOD, there is no attempt to distinguish between FOD and DOD because both involve a dynamic impact event. Thus, the terminology FOD will be used to describe all impact events irrespective of the origin of the impacting object. In this section, some observations and approaches to FOD from small hard objects are presented. While no “standard” object for engine qualification exists, work is continuing to define objects that might be impacted into blades that will cause the same level and geometry of damage as is experienced in operational engines. For example, in one case, a steel cube whose dimensions are approximately 3 mm on a side has been impacted sideways against a blade and has been shown to cause damage that closely represents what is seen in field hardware (see Appendix G). The damage, however, is specific to one engine on one aircraft that performs certain missions and is probably not applicable to all blades in all engines. In what follows, damage from a variety of sources, and methods for predicting such damage and the resultant debit in HCF capability of a material, is discussed. 324 Effects of Damage on HCF Properties The author is fortunate to have a contribution to this book in the form of Appendix G, which supplements this section on FOD with detailed discussions of experimental simula- tions of impact events, a discussion of the field events from which the impact conditions were deduced, and details of analytical and numerical simulation methods. In this section, we will concentrate on the nature of the FOD event and the resultant damage and relevant parameters related to the damage and resulting debits to fatigue limit strength. 7.2. FIELD EXPERIENCE AND OBSERVATIONS Foreign object damage is of great concern for both military and commercial engines. Field experience has shown that FOD results in nicks and dents and reduces life expectancy of those components exposed to vibratory loading. Most of the FOD experienced in the field comes from sand ingestion with depths of about 0.076 mm (0.003") or less (see Appendix G). Figure 7.1 illustrates engine blades with several types of FOD indicated by the circles. Due to these types of events, supplementary FOD inspections and guidelines (a) (b) (c) (d) Figure 7.1. In-service FOD: (a) 1st stage fan rotor blade; (b) compressor rotor blade; (c) close-up of (a); (d) close-up of (b). Foreign Object Damage 325 have been implemented into engine maintenance. The guidelines provide instructions on serviceability and repair limits for damaged engine blades, which are based on the depth of the damage. For numerous engines, field FOD inspection is conducted before and after each flight by the maintenance crew chief, in order to prevent catastrophic in-flight failure [1]. Written requirements in the form of technical orders (TO) for both flight-line and engine shop maintenance provide direction on the execution of the FOD inspection as well as serviceability limits for each rotor blade and stator vane in the engine. Limits for FOD found during a flight-line inspection are generally specified. The serviceability limits determine whether the damage needs to be serviced or not. If the damage depth is below the limit noted in the TO, it is recorded in the maintenance log. If the damage depth exceeds the serviceability limit, it is then blended out by filing the blade. In addition, if the damage depth exceeds the repairable limit, the engine is pulled out of the aircraft and sent into the engine shop for replacement. If the FOD depth found on the 1st stage fan rotor blade exceeds the serviceability limit, the 2nd and 3rd stage rotor blades must also be inspected using a borescope. As an example, the current serviceability limit for the first stage fan rotor blade on one Air Force fighter engine is 0.76 mm (0.030"). If the damage depth exceeds the maximum repairable limit of 3.8 mm (0.120"), the engine is pulled out of the aircraft for blade replacement. In addition, the entire compressor section must be borescoped to determine if there has been further engine FOD. Once blade replacement is accomplished, the engine is thoroughly re-inspected before returning the engine to operation [1]. 7.3. REPAIR BY BLENDING There is no solid technical ground on which blend and repair limits have been established. Rather, it is an experience-based approach that works well if the only stresses that a blade sees are those due to centrifugal loading. The LCF stresses due to engine speed variations and the corresponding cycle counts are all taken into account with adequate safety factors. Two aspects of FOD and the resulting HCF capability are not well taken care of in a blending operation. Little consideration is given to the possibility of a resonant vibration occurring on the blended blade geometry, especially if the blended area becomes the hot spot for the vibratory stresses. More important, possible damage may exist beyond the depth of the nick that is blended out. In addition to the remaining damage, the area beyond the blend may retain residual tensile stress from the FOD event. Characterization of residual stress fields and microstructural damage from FOD events is certainly not a mature technical area at this point in time. There have been instances where blending of FOD indents has exposed a deeper region that contains residual tensile stresses. These residual stresses, combined with centrifugal and vibratory stresses in the region, have led to HCF failures in the “repaired” region. . typically take place upon takeoff and landing. Ingested objects, which involve a range of soft and hard materials of all sizes and shapes, cause damage when they strike rotating blades, static vanes,. the crack propagation results are insensitive to the actual value used. The use of the value of a 0 from the El Haddad short crack formulation appears to be a reasonable approach, but a 0 takes. HCF failure and the HCF strength of damaged materials that is the main subject of this chapter dealing with the HCF properties of materials and components that are subjected to FOD. Both real and