The following provides remedies and preventive measures for mechanically-caused LCF damage. It is important that a specialist selects the suitable measures under consideration of factors such as operation and engine part types, in order to avoid a case of “disimprovement” (Volume 1, Chapter 3). The chapters that deal with individual components (Operating Loads and Part Behavior) contain further information. The topic of thermal fatigue is treated in a separate chapter. The measures can be divided into several larger categories (there is no claim to completeness):
Life span verification:
Procedure in case of acute damage:
Note: experience has shown that, unlike cracks under high-frequency vibrations, which usually cause fatigue in the HCF range, LCF cracks under low-frequency loads (e.g. startup/shutdown cycles) can be caught in periodic inspections before they cause part failure and catastrophic damage.
Figure "Damage accumulation and dwell times" (Ref.12.6.1-2): The easiest possibility for estimating the total damage from creep stress and dynamic loads is a linear superposition of the damages. The estimation is analogous to the Miner Rule for a linear damage accumulation of multi-stage stresses (Fig. "Dynamic fatigue life span estimations (Miner rule)"). It can technically be applied to all types of low-cycle problems.
It is especially important to ensure that the specimens used are representative of the material conditions in the part (Fig. "Lifespan verification by cyclic spin test ").
The various uses do not mean that the results always closely reflect reality. Dynamic damages and creep damages must occur simultaneously (left diagram). Centrifugal force-induced loads in rotor parts during the startup/shutdown cycle fulfill this demand fairly well. Dwell times during relief must be considered if they contribute to recovery processes. If thermal stress occurs due to the temperature changes, then the conditions for such a simple estimation are generally not met. The thermal stresses are usually phase-shifted relative to the stresses induced by centrifugal force. Therefore, the temperatures change under loads. In the case of thermal fatigue, relaxation processes cause the maximum stress to change (Fig. "Thermal fatigue promoting fatigue fractures"). If the maximum stresses are reached at very different temperatures, it can incite different creep mechanisms (Ill. 12.5.1-8). This would make the linear superposition completely unusable.
Another method of damage calculation is strain range partitioning (not shown), which takes into account plastic flow and creep (and relaxation).
Figure "Lifespan verification by cyclic spin test " (Refs.12.6.1-8, 12.6.1-12, and 12.6.1-22): LCF tests of specimens made from the titanium alloy Ti-6Al-4V yielded important realizations that make spectacular damages in large rotor parts (Examples 12.6.1-2 and 12.6.1-3) more understandable. Ref. 12.6.1-19 describes the material-specific sensitivity to compressive stress (e.g. Ti-6Al-4V, Waspaloy, etc.) or tensile stress (e.g. CrNi18/8 steels, IN100) during the dwell time. A dwell time sensitivity, in which fatigue and creep processes occur, was discovered in many materials.
The crack growth rate in Ti-6Al-4V can be several times greater with dwell times (test data: 5 minutes and 45 minutes) at the stress maximum than under dynamic loads without dwell times (test frequencies: 0.3-25 Hz). Interestingly, Ti-6Al-4V is sensitive to compressive stress (Ref. 12.6.1-19). In this case, the compressive stresses induced during LCF loading have an especially damaging effect during the “rest phase”.
The dwell time sensitivity must be determined for design-relevant data of the materials and, if available, must be considered when determining LCF life spans. Compressive stress-sensitivity and/or tensile stress-sensitivity must also be taken into account in this process.
12.6.1-1 P.König, A.Rossmann, “Ratgeber für Gasturbinen-Betreiber”, ASUE series, Vulkan-Verlag Essen ISBN 3-8027-2545-x, 1999.
12.6.1-2 H. Zenner, “Niedrig-Lastwechsel-Ermüdung (low cycle fatigue)”, VDI-Reports Nr. 268, 1976, pages 101-111.
12.6.1-3 W. Schütz, “Lebensdauer-Berechnung bei Beanspruchungen mit beliebiger Last-Zeit-Funktionen”, VDI-Reports Nr. 268, 1976, pages 113-125 + appendix, pages 127-137.
12.6.1-4 “NTSB Targets Turbine Cracks”, periodical “Aviation Week & Space Technology”, June 10, 1996, page 30.
12.6.1-5 “NTSB Investigates JT8D Engine Failure”, periodical “Aviation Week & Space Technology”, June 20, 1983, page 32.
12.6.1-6 “The NTSB urged the FAA”, periodical “Aviation Week & Space Technology”, March 2, 1998, page 21.
12.6.1-7 E.H. Phillips, “FAA, NTSB at Odds Over CF6 Fatigue Cracks”, periodical “Aviation Week & Space Technology”, April 29, 1996, page 36.
12.6.1-8 C.A. Stubbington, S.Pearson, “Effect of Dwell on the Growth of Fatigue Cracks in Ti-6Al-4V Alloy Bar”, periodical “Engineering Fracture Mechanics”, Pergamon Press Ltd. 1978, Vol 10, pages 723-756.
12.6.1-9 D.K. Wilkinson, “RB 211, The first Eighteen Months Operating Experience”, periodical “Tech Air”, November 1973, Vol 10, pages 723-756.
12.6.1-10.1 “Big fans”, und 12.6.1-10.2 “RB.211 investigation” periodical “Flight International”, 25 January , Number 3333, Volume 103, 1973, Vol 10.
12.6.1-11 S.W. Kandebo, “FAA Accelerates CF6 Inspections; Compressor Cracks Scrutinized”, periodical “Aviation Week & Space Technology”, August 28, 2000, page 57.
12.6.1-12 A.P. Woodfield, M.D. Gorman, R.R. Corderman, J.A. Stutliff, B. Yamrom, “Effect of Microstructure on Dwell Fatigue Behavior of Ti6242”, periodical “Titanium 95: Science and Technology”, pages 1116-1122.
12.6.1-13 NTSB Identification: LAX94IA018, Incident occurred OCT-19 1993.
12.6.1-14 D. Schütz, “Derzeitiger Stand der Lebensdauervorhersage für Bauteile”, VDI-Reports Nr. 268, 1976.
12.6.1-15 A.Rossmann, “Untersuchung von Schäden als Folge thermischer Beanspruchung”, contribution in J.Grosch “Schadenskunde im Maschinenbau”, Volume 308, from the series “Kontakt & Studium, Maschinenbau”, Expert Verlag, Volume 308, ISBN 3-8169-1202-8, 2. Edition1995, pages 162-187.
12.6.1-16 ASM Handbook, Metals Handbook, Ninth Edition, Volume 11, “Failure Analysis and Prevention”, Chapter “Fatigue Failures”, pages 102 and 112.
12.6.1-17 H. Huff, “Die zulässige Beanspruchung bei Ermüdungsbeanspruchung”, periodical “Materialwissenschaft und Werkstofftechnuk”, 32, 1-6, 2001.
12.6.1-18 E.H. Phillips, “Disk Crack Prompts CF6 Safety Alert”, periodical “Aviation Week & Space Technology”, September 4, 1995, page 31.
12.6.1-19 T. Goswami, “Low cycle fatigue-dwell effects and damage mechanisms”, periodical “International Journal of Fatigue”, 21, 1999, pages 55-76.
12.6.1-20 A.C. Rufin, “Extending the Fatigue Life of Aircraft Engine Components by Hole Cold Expansion Technology”, ASME Paper No, 92-GT-77 of the “37th International Gas Turbine and Aeroengine Congress and Exposition”,Cologne,Ger. June 1-4, 1992 and periodical “Journal of Engineering for Gas Turbines and Power, January 1993, Vol 115, pages 165-171.
12.6.1-21 T. Khaled, “An Investigation of Pore Cracking in Titanium Welds”, periodical “Journal 0f Materials Engineering and Performance”,Volume 3 (3), 1994, pages 419-433.
12.6.1-22 M.R. Bache, W.J. Evans, “Dwell Sensitive Fatigue Response of Titanium Alloys for Power Plant Applications”, Paper 2001-GT-424 of the “Int. Gas Turbine and Aeroengine Congress and Exhibition”, New Orleans, LA, June 4-9, 2001 and periodical “Transactions of the ASME”, Vol 125, January 2003, pages 241-245.