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596 Appendix G 70 80 6050 Ti-6-4 ΔS th (Predicted) ksi ΔS th (measured) ksi 40 30 20 10 70 80 60 50 40 30 20 10 0 0 FOD simulation specimens Notched tension specimens P&W: ballistic impact P&W: solenoid impact GEAE: solenoid impact GEAE: machined notch GEAE: machined notch residual stresses assumed present in ballistic and solenoid simulated FOD with compressive zone of 0.004 inches Figure G.40. Prediction of experimental versus predicted threshold stress using the WCN model. model applied to various sources of specimen data involving both FOD and notched specimens is shown in Figure G.40. This figure shows the good correlation of experiment and prediction. WCN Example In order to apply the WCN method, an example of its use should prove beneficial. First, assume an FOD event has occurred on the leading edge of a blade resulting in a notch with a depth of 0.3 mm (0.012 in). The notch has a root radius of 0.06 mm (0.0024 in.) and does not have a crack emanating from it. General formulas for k t can be looked up in handbooks, for example [19]. The k t for this particular notch is given in Equation (G.5): k t /b = 1+2b/ f /b (G.5) where f/b is given by Equation (G.6). f /b 1+0122 1 1+/b 5/2 (G.6) Appendix G 597 For the notch in this example, k t is approximately 11.85. This is a very sharp notch but k t s of this size have been seen in field damage. If this blade material has an endurance limit at 10 7 cycles of 650 MPa for R = 05, then the use of k t would predict that the maximum allowable stress should be 54.85 MPa. Using the WCN method, it is necessary to compute the threshold stress intensity factor for small cracks and correlate that to the threshold applied stress that can be applied to the blade and still remain in the limit of safe operation. The first step is to calculate the small crack parameter, a o . ∗ Equation (G.7) defines a o as a function of applied stress ratio: a o R = 1 K th R F e R 2 (G.7) where F = 1122 for a crack that goes through the thickness in a straight line (through crack) and 0.73 for a semi-elliptical crack that does not go through the thickness (thumb- nail crack), K th is the crack growth threshold for a given R, and e is the smooth bar endurance stress range for the cyclic life in question. For this example, we will assume an endurance limit of 10 7 cycles, an endurance stress of 650 MPa, a through crack, and a K th of 3MPa √ m all at a stress ratio of 0.5. Based on these assumptions, a o (0.5) is 2154m. The next step is to obtain the threshold stress that corresponds to the endurance limit based on these quantities. Hudak et al. [20], uses Equation (G.8) to determine S thL : S thL = K th √ 2 g n √ ba o +b t √ a o + √ b (G.8) In this equation, g n is a finite-thickness correction function that approaches 1.12 if the crack depth is much smaller than its length. Since the most interesting cases of FOD are those where cracks are small, it is conceivable that this approximation will be good in most cases. However, the function for g n can be found in various stress handbooks such as [19]. If g n is assumed to be 1.12, the equation above simplifies to (G.9): S thL = K th 112 1/2 √ a o + √ b (G.9) Since b and a o are independent of crack growth, the equation (G.9) results in a value of 68.8 MPa for S thL . The changes in the value of threshold stress for blade failure due to crack growth is due to the inclusion of small crack effects through the inclusion of notch size and to the incorporation of fracture mechanics crack growth thresholds into the stress prediction. A smaller notch with a sharper radius could have the same k t and thus ∗ See Chapter 4 for a discussion of the effective small crack threshold and the Kitagawa diagram. 598 Appendix G the same predicted fatigue endurance stress using the non-fracture mechanics approach. However, the change in notch size would further reduce the value of b from the example and the predicted endurance stress would increase accordingly if the WCN model were used. This makes physical sense because a very small notch with a k t of 40 should be less detrimental to blade life than a notch with the same k t that is three times as deep. In summary, it is suggested that FOD evaluation account for notch depth and fracture toughness of the material, rather than just fatigue strength and notch geometry. SUMMARY This appendix has presented the current state of the art in FOD simulation, modeling and life prediction. The most important part of experimental FOD simulation is the type of impact method and the stress state in the specimen. There are appropriate uses for each impact type and each specimen design. However, the most realistic and practical impact method is ballistic and the most accurate specimen design is one that captures the stress gradient in the airfoil of interest. In the area of analytical modeling, models have been developed to predict impact damage and residual stress due to impact. These models typically require very experienced users and can easily be abused to get the wrong answer. However, correlation with experimental data can be used to avoid these errors. Once residual stresses and damage are predicted, elastic–plastic finite element simulations can be completed and life prediction routines can be implemented. Life prediction routines typically result in a trade-off between accuracy and complexity, so the appropriate life prediction method should be chosen to meet the resolution needed by the analysis. REFERENCES 1. Best Practices for the Mitigation and Control of Foreign Object Damage–Induced High Cycle Fatigue in Gas Turbine Engine Compression System Airfoils, NATO AVT-094/RTG-027 Final Report, 2004. (http://www.rta.nato.int/Reports.asp) 2. Thompson, S.R., Ruschau, J.J., and Nicholas, T., “Influence of Residual Stresses on High Cycle Fatigue Strength of Ti-6Al-4V Subjected to Foreign Object Damage”, International Journal of Fatigue, 23, Supplement 1, 2001, pp. S405–S412. 3. Hudak, S.J. and Davidson, D.L., “Characterization of Service Induced FOD”, United States Air Force Technical Report, Improved High Cycle Fatigue Life Prediction, Appendix 5A, AFRL-ML-WP-TR-2001-4159, Wright-Patterson AFB, OH, January 2002. 4. Haake, F.K., Salivar, G.C., Hindle, E.H., Fischer, J.W., and Annis, C.G., “Threshold Fatigue Crack Growth Behaviour”, United States Air Force Technical Report, WRDC-TR-89-4085, Wright-Patterson AFB, OH, 1989. Appendix G 599 5. Chell, G;G., and Hudak, S.J., “Development and Application of Worst Case Notch (WCN) to FOD”, United States Air Force Technical Report, Improved High Cycle Fatigue Life Prediction, Appendix 5I, AFRL-ML-WP-TR-2001-4159, Wright-Patterson AFB, OH, January 2002. 6. Hamrick, J.L., “Effects of Foreign Object Damage from Small Hard Particles on the High- Cycle Fatigue Life of Ti-6Al-4V”, Ph.D. Dissertation, AFIT/DS/ENY/99-02, Air Force Institute of Technology, Wright-Patterson AFB, OH, September 1999. 7. Birkbeck, J.C., “Effects of FOD on the Fatigue Crack Initiation of Ballistically Impacted Ti-6Al-4V Simulated Engine Blades”, Ph.D. Thesis, School of Engineering, University of Dayton, Dayton, OH, August 2002. 8. Roder, O., Thompson, A.W., and Ritchie, R.O., “Simulation of Foreign Object Damage of Ti-6Al-4V Gas-Turbine Blades”, in Proceedings of the Third National Turbine Engine High Cycle Fatigue Conference, W.A. Stange and J. Henderson, eds, Universal Technology Corp., Dayton, OH, 1998, CD-ROM, Session 10, pp. 6–12. 9. Williams, D.P., Nowell, D., and Stewart, I.F., “The Effect and Assessment of Foreign Object Damage to Aero Engine Blades and Vanes”, Proceedings of the 5th National Turbine Fatigue High Cycle Fatigue Conference, Phoenix, AZ, 7–9 March 2000. 10. Weeks, C., Bastnagel, P., Cook, T., Daiuto, R., and Delp, J., “FOD Analytical Modeling – FOD Event Modeling”, United States Air Force Technical Report, Improved High Cycle Fatigue Life Prediction, Appendix 5E, AFRL-ML-WP-TR-2001-4159, Wright-Patterson AFB, OH, January 2002. 11. MSC/DYTRAN Version 4.0 User’s Manual, The MacNeal-Schwendler Corporation, 1997. 12. Tranter, P.H., Gould, P.J., and Harrison, G.F., “Laboratory Simulation and Finite Element Modeling of Aerofoil Impact Damage”, Proceedings of the 8th National Turbine Fatigue High Cycle Fatigue Conference, Monterey, CA, 14–16 April 2003. 13. LS-DYNA Users Manual, Livermore Software Technology Corporation, 2002. 14. Butler, A., Church, P., and Goldthorpe, B., “A Wide Ranging Constitutive Model for bcc Steels”, Jnl de Physique 4, C8-471, 1994. 15. Goldthorpe, B., “A Path Dependent Model for Ductile Fracture”, Jnl de Physique 7, C3-705, 1997. 16. Gallagher, J. et al., “Advanced High Cycle Fatigue (HCF) Life Assurance Methodologies”, Report # AFRL-ML-WP-TR-2005-4102, Air Force Research Laboratory, Wright-Patterson AFB, OH, July 2004. 17. Gallagher, J.P. et al., “Improved High Cycle Fatigue Life Prediction”, United States Air Force Technical Report, AFRL-ML-WP-TR-2001-4159, Wright-Patterson AFB, OH, January 2001. 18. The Behaviour of Short Fatigue Cracks, ed. K.J. Miller and E.R. de los Rios, Mechanical Engineering Publications Limited, London, 1986. 19. Tada, H., Paris, P., and Irwin, G., “The Stress Analysis of Cracks Handbook, 2nd Edition”, Del Research Corporation, 1985. 20. Hudak, S.J., Chell, G.G., Slavik, D., Nagy, A., and Feiger, J.H., “Influence of Notch Geometry on High Cycle Fatigue Threshold Stresses in Ti-6Al-4V”, Proceedings of the 6th National Turbine Fatigue High Cycle Fatigue Conference, Jacksonville, FL, 5–8 March 2001. Appendix H ∗ FOD in JSSG JSSG-2007 30 October 1998 DEPARTMENT OF DEFENSE JOINT SERVICE SPECIFICATION GUIDE (JSSG) ENGINES, AIRCRAFT, TURBINE 3.3.2 Ingestion capability (hazard resistance) 3.3.2.1 Bird ingestion The engine shall continue to operate and perform during and after the ingestion of birds as specified in Table VIII. 3.3.2.2 Foreign object damage (FOD) The engine shall meet the requirements of the specification for the design service life of 3.4.1.1 without repair after ingestion of foreign objects which produce damage equivalent to a stress concentration factor K t of _____ at the most critical locations of flow path components. 3.3.2.3 Ice ingestion The engine shall operate and perform in accordance with table IX, during and after ingestion of hailstones and sheet ice at the takeoff, cruise, and descent aircraft speeds. The engine shall not be damaged beyond field repair capability after ingesting the hailstones and ice. The excerpts in this Appendix are extracted from JSSG and are those that deal with tolerance of aircraft turbine engines to ingestion of foreign objects such as birds, sand, dust, ice, and other debris. They are extracted from JSSG document referenced in the title. 600 Appendix H 601 3.3.2.4 Sand and dust ingestion The engine shall meet all requirements of the specification during and after the sand and dust ingestion event specified herein. The engine shall ingest air containing sand and dust particles in a concentration of (a) mg sand/m 3 . The engine shall ingest the specified course and fine contaminant distribution for (b) and (c) hours, respectively. The engine shall oper- ate at intermediate thrust for TJ/TFs or maximum continuous power for TP/TSs with the specified concentration of sand and dust particles, with no greater than (d) percent loss in thrust or power, and (e) % gain in specific fuel consumption. Helicopter engines shall ingest the 0–80 micron (0–3.15 ×10 −3 in sand and dust of section 4.11.2.1.3 in a concentration of 53 mg/m 3 33 ×10 −6 lb/ft 3 air for 54 hours and inspection shall reveal no impending failure. A.3.3.2 Ingestion capability (hazard resistance). A.3.3.2.1 Bird ingestion. The engine shall continue to operate and perform during and after the ingestion of birds as specified in Table VIII. REQUIREMENT RATIONALE (A.3.3.2.1) Engines must be capable of ingesting birds encountered during missions without signif- icant power loss, deterioration, or safety implications. The total weapon system mission environment must be studied to examine the probability of bird strike occurrence, bird sizing criteria, flocking densities, mission routing, training, etc., to determine the design criteria for bird ingestion capability requirements for an engine. REQUIREMENT GUIDANCE (A.3.3.2.1) The following should be used to tailor Table VIII: In the event of specific air system bird strike criterion has not been established for the engine, the following birds vs. inlet area criteria should be used. The inlet area to be used should be the aircraft inlet or engine inlet whichever is smaller. The number of birds to be ingested should be based on inlet area as follows: one 100 gm (3.5 oz) bird per 300 cm 2 465in 2 of inlet area plus any fraction larger than 50% thereof, up to a maximum of 16 birds; one 1 kg (2.2 lb.) bird per 1500 cm 2 2325in 2 of inlet 602 Appendix H area plus any fraction larger than 50% thereof; one 2 kg (4.4 lb.) bird, regardless of the size of the inlet, provided the inlet is large enough to admit a 2 kg (4.4 lb.) bird. The 100 gm (3.5 oz) birds should be ingested at random intervals and be randomly dispersed over the inlet area. Birds weighing 1 kg (2.2 lb.) and larger should be directed at critical areas of the engine face. The bird velocity and engine power setting for each condition should be as described below: Birds weighing 100 gm (3.5 oz) (a maximum of 16 at a time) and birds weighing 1 kg (2.2 lb.) (one at a time) ingested at a bird velocity equal to the takeoff flight speed, with the engine at intermediate thrust for TJ/TFs or maximum power for TP/TSs. Birds weighing 100 gm (3.5 oz) (a maximum of 16 at a time) and birds weighing 1 kg (2.2 lb.) (one at a time) ingested at a bird velocity equal to the cruise flight speed with the engine at cruise power setting. Birds weighing 100 gm (3.5 oz) (a maximum of 16 at a time) and birds weighing 1 kg (2.2 lb.) (one at a time) ingested at a bird velocity equal to the descent flight speed with the engine at descent power. For aircraft that have a low level, high speed mission requirement: birds weighing 100 gm (3.5 oz) (a maximum of 16 at a time) and birds weighing 1 kg (2.2 lb.) (one at a time) ingested at a bird velocity equal to the aircraft maximum sea-level speed and the engine power setting required to achieve that speed. Birds weighing 2 kg (4.4 lb.) ingested at a bird velocity equal to the aircraft takeoff speed or low level operational airspeed, whichever is more severe, with engine power equal to that required for the flight condition. For 100 gm (3.5 oz) birds, the engine should sustain a performance of 95% or greater of the initial thrust or power, and all damage to the blades and vanes should be blendable (within repair limits) with flight line-type tooling. The 1 kg (2.2 lb.) bird ingestion may cause some damage; however, it should not result in immediate engine shutdown, and post-ingestion thrust or power levels should be 75% or greater of the initial thrust or power at the operating condition. Under condition e. above, no engine failure should occur which would result in damage to the aircraft or adjacent engines. No bird ingestion should prevent the engine from being safely shutdown. Performance recovery times will vary as a function of the bird size, number of birds, and size of the engine. The performance recovery time after ingestion of the 100 gm (3.5 oz) bird(s) should occur in 5 seconds or less after the final volley of birds has been ingested. The performance recovery time after ingestion of the 1 kg (2.2lb.) bird(s) should occur in 5 seconds or less for small engines, 5–10 seconds for moderate-sized turbofans or turbojets, and up to 5–15 seconds for large by-pass turbofans. Appendix H 603 Turboshaft engines should recover within 5 seconds after an ingestion event with no less than 95% of the power prior to the ingestion event, and without exceeding any engine control limits. Background: Bird strike durability is a necessary safety design criteria to be used in all engines. Analyses should be conducted to determine the sensitivities to blade design, blade to stator spacing design, control system operation, and stator design for structure performance, and engine control. Bird strike tolerance can be enhanced by providing large axial clearances between blade and stators at the front of the engine. REQUIREMENT LESSONS LEARNED (A.3.3.2.1) Since helicopter takeoff flight speed differs from fixed wing, the following was used for the T800: A bird of 50–100 grams (0.11–0.22 lb.) ingested at a bird velocity equal to Mach number of 0.1 and with the engine at maximum rated power set by rated Measured Gas Temperature (MGT). A bird of 50–100 grams (0.11–0.22 lb.) ingested at a bird velocity equal to Mach number of 0.3 with the engine at maximum continuous power set by rated MGT. A bird of 50–100 grams (0.11–0.22 lb.) ingested at a bird velocity equal to a Mach number of 0.2 with the engine at 25% of the maximum continuous rated power obtained in paragraph b. Results of findings for bird ingestion experience assembled by Aerospace Industry Association, “Bird Ingestion Experience for Aircraft Turbine Engines,” 1979, generally supports the FAA part 33.77 criteria. Similar military studies have indicated that the engine may receive over 60% of the total aircraft strikes, and most strikes occur below 1.8 km (6000 feet) at takeoff, landing, and low-level penetration speeds. A GAO/NSIAD-89-127 report, dated July 1989, states that from 1983 to 1987, military aircraft have collided with birds over 16,000 times. Many of these collisions caused only minor damage; however, the services lost six crew members, incurred $318 million in damages, and lost nine aircraft. During this period, the Air Force lost six aircraft, the Navy lost two aircraft, and the Army lost one aircraft. A review of nine military jet engines developed since the early 1970s showed that the Services lessened the military specification requirements in engine model specifications for the sizes and the numbers of medium birds used in testing engines. Recent studies (USAF/ASC & AIA) of bird ingestion data indicate that older specifica- tions did not accurately reflect the sizes and the number of birds actually ingested. Prior to the publication of this specification, engine military specifications required medium birds sizes for ingestion tests of 1.5 pounds for the Air Force and 2 pounds for the Navy. DOT Report DOT/FAA/CT-84/13 by Gary Frings, dated 9/84, states that bird ingestion is a rare but probable event. For every one million aircraft operating hours, 230 bird 604 Appendix H ingestions (of all weights) will occur, on average. An average bird weighs 26 ounces. Most likely weight of birds in the areas of runways is 11 ounces. A small percentage (< 3%) of ingestion events involved birds weighing 61 ounces. Ten percent of events involved ingestion of multiple birds into a single engine; 0.5% of events involved multiple engine ingestions; 5% of all bird ingestions resulted in engine failure. A.4.3.2.1 Bird ingestion The requirements of 3.3.2.1 shall be verified by: VERIFICATION RATIONALE (A.4.3.2.1) A test with a complete engine is necessary to ensure all interactive effects of a bird strike are properly accounted for in the engine design. Analysis and component test (fan and compressor rigs) have been successfully used to predict loading and deflection criteria leading up to full-scale engine tests. VERIFICATION GUIDANCE (A.4.3.2.1) Past verification methods have included analyses, demonstrations, and tests. The contractor should specify in the pretest data the critical target area for bird ingestion. Test target area is subject to approval by the Using Service. Background: A developmenttestprogram consisting ofanalysisand component testshouldbe established to provide confidence in the design prior to formal engine ingestion tests. The number of birds, bird sizes, engine RPMs, bird velocities, performance criteria, and amount of damage for the test engine should be based on the parameters of 3.3.2.1. Analysis should indicate the location for critical bird strikes at the front face of the engine and pass/fail criteria should be included in the test plan. The birds should be ingested in a random sequence and dispersed over theinlet areato simulatean encounterwith aflock. Synthetic“birds” havenot beenused by the military services to date, but their use should not be precluded in future programs if sufficient information and justification is presented to the Using Service. VERIFICATION LESSONS LEARNED (A.4.3.2.1) Severe out-of-balance conditions creating high vibrations, along with rotating to static structural contact, have been encountered following bird ingestion events. Appendix H 605 Table VIII. Bird ingestion Bird Size Number of Birds Bird Velocity Thrust/Power Setting Percent Thrust/ Power Retention Thrust/ Power Recovery Time Damage 100 gm (3.5 oz) Takeoff = Blendable 100 gm (3.5 oz) Cruise = Blendable 100 gm (3.5 oz) Low Level Hi-Speed = Blendable 100 gm (3.5 oz) Descent = Blendable 1 kg (2.2 lbs) Takeoff = Minor 1 kg (2.2 lbs) Cruise = Minor 1 kg (2.2 lbs) Low Level Hi-Speed = Minor 1 kg (2.2 lbs) Descent = Minor 2 kg (4.4 lbs) Takeoff or Low Level Hi-Speed = Contain Failure Each line of the table must be satisfied to comply with the requirements of 3.3.2.1. A.3.3.2.2 Foreign object damage (FOD). The engine shall meet the requirements of the specification for the design service life of 3.4.1.1 without repair after ingestion of foreign objects which produce damage equivalent to a stress concentration factor K t of _____ at the most critical locations of flow path components. REQUIREMENT RATIONALE (A.3.3.2.2) The engine inlet airflow velocities create conditions where loose foreign objects may be ingested into the engine resulting in gas path damage. The engine must tolerate the ingested objects up to a specified level. REQUIREMENT GUIDANCE (A.3.3.2.2) The following should be used to tailor the specification paragraph: A value of at least 3. Background: The stress concentration factor of 3 is similar to a notch in the fan or compressor blade or stator caused by impact damage from a small object. Typical foreign objects to be considered are nuts, bolts, rivets, rocks, aircraft parts, shell casings, and tools. Therefore, structures subject to this type of damage must be . prediction. A smaller notch with a sharper radius could have the same k t and thus ∗ See Chapter 4 for a discussion of the effective small crack threshold and the Kitagawa diagram. 598 Appendix G the same. shall operate and perform in accordance with table IX, during and after ingestion of hailstones and sheet ice at the takeoff, cruise, and descent aircraft speeds. The engine shall not be damaged. paragraph: A value of at least 3. Background: The stress concentration factor of 3 is similar to a notch in the fan or compressor blade or stator caused by impact damage from a small object. Typical foreign
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