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What is Thermal Fatigue – Definition

Thermal fatigue is a specific type of fatigue failure mechanism that is induced by cyclic stresses (thermal expansion and contraction) from repetitive fluctuations in the temperature (heating and cooling) of equipment.

Thermal Fatigue

Thermal fatigue is a specific type of fatigue failure mechanism that is induced by cyclic stresses (thermal expansion and contraction) from repetitive fluctuations in the temperature (heating and cooling) of equipment. This type of fatigue is very important especially in power engineering, aeronautics and automotive engineering.

Thermal stresses arise in materials when they are heated or cooled. Thermal stresses effect the operation of facilities, both because of the large components subject to stress and because they are effected by the way in which the plant is operated. On cooling, residual tensile stresses are produced if the metal is prevented from moving (contracting) freely. Fatigue cracks can initiate and grow as cycling continues. Stress concentrations can be reduced through appropriate design changes that take thermal expansion and contraction into account. For example, expansion loops and bellows in elevated temperature piping and tubing systems take advantage of this principle. In nuclear power plants, heatup and cooldown rate limits are based upon the impact on the future fatigue life of the plant. The heatup and cooldown limits ensure that the plant’s fatigue life is equal to or greater than the plant’s operational life. Additionally, plant design modifications include for example heating up of the Emergency Core Cooling System (ECCS) water tanks or sumps in order to reduce the temperature difference between injected water and tha material of RPV.

In nuclear power plants, fundamental requirements during design and manufacturing for avoiding fatigue failure are different for different cases.

  • Pressurizer. Pressure in the primary circuit of PWRs is maintained by a pressurizer, a separate vessel that is connected to the primary circuit (hot leg) and partially filled with water which is heated to the saturation temperature (boiling point) for the desired pressure by submerged electrical heaters. For a pressurizer, the load variations are fairly low, but the cycle frequency is high. Therefore, a steel of high fatigue strength and of high ultimate tensile strength is desirable.
  • Reactor Pressure Vessel. The body of the reactor vessel is constructed of a high-quality low-alloy carbon steel, and all surfaces that come into contact with reactor coolant are clad with a minimum of about 3 to 10 mm of austenitic stainless steel in order to minimize corrosion. The reactor pressure vessel and piping, by contrast, are subjected to large load variations, but the cycle frequency is low; therefore, high ductility is the main requirement for the steel. Thermal sleeves are used in some cases, such as spray nozzles and surge lines, to minimize thermal stresses. Heatup and cooldown rate limits are based upon the impact on the future fatigue life of the plant. The heatup and cooldown limits ensure that the plant’s fatigue life is equal to or greater than the plant’s operational life. Additionally, plant design modifications include for example heating up of the Emergency Core Cooling System (ECCS) water tanks or sumps in order to reduce the temperature difference between injected water and tha material of RPV.
  • Primary Piping. Most mechanical fatigue failures in piping are a result of vibrations which are not uncommon occurrences. Virtually every piping system which contains a flowing fluid exhibits some degree of vibration. The cause of the vibration can differ. Pressure pulsations and movement from attached rotating equipment are amongst the most common causes of vibration in piping systems.
  • Steam Generators. Steam generators are heat exchangers used to convert feedwater into steam from heat produced in a nuclear reactor core. Each steam generator can contain anywhere from 3,000 to 16,000 tubes, each about  19mm diameter. If the steam generator’s feed-water supply fails for any reason, emergency measures must be taken quickly and this is done by a system for introducing cold water into the steam generator’s housing to keep the tube bundle and tube sheet from dangerously overheating. To avoid severe thermal shock, particularly to the tube sheet If the steam generator’s feed-water supply fails for any reason, the emergency feedwater system initiates its action and introduces cold water into the steam generator to keep the tube bundle and tube sheet from dangerously overheating. This causes significant stresses especially to the tube sheet.

Although the primary cause of the phenomenon of fatigue failure is not well known, it apparently arises from the initial formation of a small crack resulting from a defect or microscopic slip in the metal grains. The crack propagates slowly at first and then more rapidly when the local stress is increased due to a decrease in the load-bearing cross section. The metal then fractures. Fatigue failure can be initiated by microscopic cracks and notches, and even by grinding and machining marks on the surface; therefore, such defects must be avoided in materials subjected to cyclic stresses (or strains). Plant operations are performed in a controlled manner to mitigate the effects of cyclic stress. Heatup and cooldown limitations, pressure limitations, and pump operating curves are all used to minimize cyclic stress.

Special reference: U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 2 and 2. January 1993.

References:

Materials Science:

  1. U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
  2. U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 2 and 2. January 1993.
  3. William D. Callister, David G. Rethwisch. Materials Science and Engineering: An Introduction 9th Edition, Wiley; 9 edition (December 4, 2013), ISBN-13: 978-1118324578.
  4. Eberhart, Mark (2003). Why Things Break: Understanding the World by the Way It Comes Apart. Harmony. ISBN 978-1-4000-4760-4.
  5. Gaskell, David R. (1995). Introduction to the Thermodynamics of Materials (4th ed.). Taylor and Francis Publishing. ISBN 978-1-56032-992-3.
  6. González-Viñas, W. & Mancini, H.L. (2004). An Introduction to Materials Science. Princeton University Press. ISBN 978-0-691-07097-1.
  7. Ashby, Michael; Hugh Shercliff; David Cebon (2007). Materials: engineering, science, processing and design (1st ed.). Butterworth-Heinemann. ISBN 978-0-7506-8391-3.
  8. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.

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Fatigue

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