References cited in this section 4. H P. Leighly, Jr, B L. Bramfitt, and S.J. Lawrence, RMS Titanic: A Metallurgical Problem, Pract. Failure Anal. voll(No.2), April2001,p10-13,33-37 5. R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, John Wiley Sons, 1976, p 230 6. S. Ross, Tacoma Narrows, 1940, Construction Disasters, Ist ed, McGraw-Hill, 1984, p 216-239 7. Y. Billah and R Scanlan, Resonance, Tacoma Narrows Bridge Failure and Undergraduate Physics, J. Physics, vols9(No.2),Feb1991,p1l8-123 8. E.R. Parker, Brittle Behavior of Engineering Structures, Wiley, 1957 9. S Atallah, U.S. History's Worst LNG Disaster, Firehouse, Jan 1979, p 29 10. T. Swift, Damage Tolerance Certification of Commercial Aircraft, Fatigue and fracture, Vol 19, ASM Handbook, ASM International, 1996, p 566-576 11. V.A. Hindas, F-111 Design Experience: Use of High Strength Steels, AIAA 2nd Aircraft Design and Operations Meeting 12. R. Viswanathan and J. Stringer, Failure Mechanisms of High Temperature Components in Power Plants, Trans ASME, Vol 122, July 2000 13. W.R. Hendricks, The Aloha Incident: A New Era for Aging Aircraft, Structural Integrity of Aging Aircraft, S N Altussi, et al., Ed, Springer-Verlag, 1991, p 153 14. N1819U, NTSB-AAR-90-06, Aircraft Accident Report, United Airlines, Inc, Nov 1990 15. Interim Guidelines: Evaluation, Repair, Modification and Design of Steel Moment Frames, FEMA-267, Federal Emergency Management Agency 16. R.W. Staehl, Combining Design and Corrosion for Predicting Life, Proc. Conf. Life Prediction of Corrodible Structures, Vol 1, NACE International, 1994, p 13 17. D.J. Benac and L.J. Goland, Failures of Pressure Vessels, Lesson 15, ASM Practical Failure Analysis Course, to be published 18. M F Kanninen and C.H. Popelar, Advanced fracture Mechanics, Oxford University Press, 1985 19. P F. Timmins, Operating Stress Maps for Failure Control, Fatigue and Fracture, Vol 19, ASM Handbook, ASM International, 1996, p 460 20 M P. Kaplan and T.A. Wolff, Life Extension and Damage Tolerance of Aircraft, Fatigue and fracture, Vol 19, ASM Handbook, p 557-565 21. D.J. Benac and D N. Hopkins, Investigation of Fatigue-Induced Socket-Welded Joint Failures for Small-Bor Piping Used in Power Plants, Pract. Failure AnaL., Vol 1(No 2), April 2001, p 71-82 Life-Limiting factors To assess the life of a structure, one first must understand the factors that limit life(Ref 20). It is the purpose of the following discussion to briefly discuss some of the aspects that control the durability and remaining life of structures Each structure or structural system has unique parameters of life expectancy. For example, the life of an aircraft is dependent on a number of variables, such as flight profiles, usage rate, external environment, material selection, and
References cited in this section 4. H.P. Leighly, Jr., B.L. Bramfitt, and S.J. Lawrence, RMS Titanic: A Metallurgical Problem, Pract. Failure Anal., Vol 1 (No. 2), April 2001, p 10–13, 33–37 5. R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, John Wiley & Sons, 1976, p 230 6. S. Ross, Tacoma Narrows, 1940, Construction Disasters, 1st ed., McGraw-Hill, 1984, p 216–239 7. Y. Billah and R. Scanlan, Resonance, Tacoma Narrows Bridge Failure and Undergraduate Physics, J. Physics, Vol 59 (No. 2), Feb 1991, p 118–123 8. E.R. Parker, Brittle Behavior of Engineering Structures, Wiley, 1957 9. S. Atallah, U.S. History's Worst LNG Disaster, Firehouse, Jan 1979, p 29 10. T. Swift, Damage Tolerance Certification of Commercial Aircraft, Fatigue and Fracture, Vol 19, ASM Handbook, ASM International, 1996, p 566–576 11. V.A. Hindas, F-111 Design Experience: Use of High Strength Steels, AIAA 2nd Aircraft Design and Operations Meeting 12. R. Viswanathan and J. Stringer, Failure Mechanisms of High Temperature Components in Power Plants, Trans. ASME, Vol 122, July 2000 13. W.R. Hendricks, The Aloha Incident: A New Era for Aging Aircraft, Structural Integrity of Aging Aircraft, S.N. Altussi, et al., Ed., Springer-Verlag, 1991, p 153 14. N1819U, NTSB-AAR-90-06, Aircraft Accident Report, United Airlines, Inc., Nov 1990 15. Interim Guidelines: Evaluation, Repair, Modification and Design of Steel Moment Frames, FEMA-267, Federal Emergency Management Agency 16. R.W. Staehl, Combining Design and Corrosion for Predicting Life, Proc. Conf. Life Prediction of Corrodible Structures, Vol 1, NACE International, 1994, p 138 17. D.J. Benac and L.J. Goland, Failures of Pressure Vessels, Lesson 15, ASM Practical Failure Analysis Course, to be published 18. M.F. Kanninen and C.H. Popelar, Advanced Fracture Mechanics, Oxford University Press, 1985 19. P.F. Timmins, Operating Stress Maps for Failure Control, Fatigue and Fracture, Vol 19, ASM Handbook, ASM International, 1996, p 460 20. M.P. Kaplan and T.A. Wolff, Life Extension and Damage Tolerance of Aircraft, Fatigue and Fracture, Vol 19, ASM Handbook, p 557–565 21. D.J. Benac and D.N. Hopkins, Investigation of Fatigue-Induced Socket-Welded Joint Failures for Small-Bore Piping Used in Power Plants, Pract. Failure Anal., Vol 1 (No. 2), April 2001, p 71–82 Life-Limiting Factors To assess the life of a structure, one first must understand the factors that limit life (Ref 20). It is the purpose of the following discussion to briefly discuss some of the aspects that control the durability and remaining life of structures. Each structure or structural system has unique parameters of life expectancy. For example, the life of an aircraft is dependent on a number of variables, such as flight profiles, usage rate, external environment, material selection, and
geometry. For fossil-fuel plant equipment that is exposed to elevated temperatures, life-limiting factors involve pressure, temperature, environment, fatigue cycles, and time at temperature. The following life-limiting factors are common most structures and should be considered in a failure analysis and a life assessment Material defects Fabrication practices Stress, stress concentration, and stress intensity Temperature Thermal and mechanical fatigue cycles Corrosion concerns Improper maintenance Material Defects. With the advent of strict quality-assurance controls and nondestructive inspection, systems manufacturing and material defects are not as common as in early years. Yet, they have and will be a potential concern in structural components. To determine if a defect is a cause for failure, it is important that the component or structure undergoes a comprehensive failure investigation to determine if such is the case. Defects greatly affect remaining life because the time for crack initiation has been reduced Defects are either material defects or mechanical defects. The material defect may manifest itself because of a metallurgical anomaly. This anomaly or metallurgical defect may occur as a result of faulty furnace control during heat treating, improper plating that may result in unwanted hydrogen in high-strength steel, processing problems(forging laps shuts, undesirable grain structure, etc. ) welding defects, and so forth. For example, a material defect, shown in Fig. 4 caused the historic failure of an F-111 aircraft in 1969. Another example of a preexisting material defect is failures of a titanium disk on United Flight 232, which failed near Sioux City, lowa, in July 1989(Ref 14 ) The Sioux City mishap was due to a manufacturing problem in a titanium fan disk, which had the presence of hard alpha case. As a result of the Sioux City incident, a probabilistic life assessment approach was taken to evaluating titanium disks(ref 22). It can be seen that these defects occur as a result of both manufacturing assembly and material processing A mechanical defect is often induced after the manufacturing process when the structure is in service. A mechanica defect such as a dent, gouge, impact, or deformation of the material can reduce component life. The result often is yielding or cracking of the structural component. If the mechanical defect is located at a high-stress location, the reduction in life can be significant and possibly catastrophic. For example, Fig. 6 shows damage at a fastener hole in a ving skin spar mechanically induced during the drilling of the hole. The failure analyst was able to identify the location of the damage and extent of fatigue crack growth, which was used to establish inspection intervals(ref 23) Thefileisdownloadedfromwww.bzfxw.com
geometry. For fossil-fuel plant equipment that is exposed to elevated temperatures, life-limiting factors involve pressure, temperature, environment, fatigue cycles, and time at temperature. The following life-limiting factors are common to most structures and should be considered in a failure analysis and a life assessment: · Material defects · Fabrication practices · Stress, stress concentration, and stress intensity · Temperature · Thermal and mechanical fatigue cycles · Corrosion concerns · Improper maintenance Material Defects. With the advent of strict quality-assurance controls and nondestructive inspection, systems, manufacturing and material defects are not as common as in early years. Yet, they have and will be a potential concern in structural components. To determine if a defect is a cause for failure, it is important that the component or structure undergoes a comprehensive failure investigation to determine if such is the case. Defects greatly affect remaining life because the time for crack initiation has been reduced. Defects are either material defects or mechanical defects. The material defect may manifest itself because of a metallurgical anomaly. This anomaly or metallurgical defect may occur as a result of faulty furnace control during heat treating, improper plating that may result in unwanted hydrogen in high-strength steel, processing problems (forging laps, shuts, undesirable grain structure, etc.), welding defects, and so forth. For example, a material defect, shown in Fig. 4, caused the historic failure of an F-111 aircraft in 1969. Another example of a preexisting material defect is failures of a titanium disk on United Flight 232, which failed near Sioux City, Iowa, in July 1989 (Ref 14). The Sioux City mishap was due to a manufacturing problem in a titanium fan disk, which had the presence of hard alpha case. As a result of the Sioux City incident, a probabilistic life assessment approach was taken to evaluating titanium disks (Ref 22). It can be seen that these defects occur as a result of both manufacturing assembly and material processing. A mechanical defect is often induced after the manufacturing process when the structure is in service. A mechanical defect such as a dent, gouge, impact, or deformation of the material can reduce component life. The result often is yielding or cracking of the structural component. If the mechanical defect is located at a high-stress location, the reduction in life can be significant and possibly catastrophic. For example, Fig. 6 shows damage at a fastener hole in a wing skin spar mechanically induced during the drilling of the hole. The failure analyst was able to identify the location of the damage and extent of fatigue crack growth, which was used to establish inspection intervals (Ref 23) The file is downloaded from www.bzfxw.com
damage Fatique 0.25 Critical crack size s020 0.15 Inspection interval Detectable crack size 0.10 0.05 0500100015002000250030003500 Flight time, h Fig. 6 Fractographic evaluation for life assessment purposes.(a) wing spar that had mechanically induced damage at a fastener hole (indicted by an arrow).(b) Plot showing how fracture information is used to establish initial and recurring inspection requirements Fabrication practices such as welding can affect the lifetime of a part. For example, in the fabrication of pressure vessels or pressure piping unfavorable results from welding, such as brittle cracking in the heat-affected zone(HAz), often result from the use of steels containing excessive amounts of residual elements that increase hardenability and consequently their susceptibility to cracking. Therefore, complete control of composition is of the utmost importance when welding is involved. In producing welds without embrittlement, it is necessary to follow the proper welding procedure. Selection of the proper filler metal is also important Sometimes imperfections are undetected and become evident during operations. For example, small cracks were not detected in a carbon steel ASTM A 105 steam drum nozzle that was arc gouged. Consequently, during a cold start-up, a 46 cm(18 in. )crack occurred. Figure 7 shows the cracked nozzle. Prior to the cold start-up, the steam drum had been in service for five years with no cracking problems. Investigation of the cracking problem determined that crack initiation occurred from the root of an arc-gouged notch. A metallurgical cross section through the arc-gouged notch revealed microcracks and a very hard layer (54 HRC) that were not removed after arc gouging the nozzle. The carbon steel base metal was 72 to 88 HRB. The crack extended during the cold start-up because of preexisting microcracks and an as- forged nozzle that had poor fracture toughness(Ref 17)
Fig. 6 Fractographic evaluation for life assessment purposes. (a) Wing spar that had mechanically induced damage at a fastener hole (indicted by an arrow). (b) Plot showing how fracture information is used to establish initial and recurring inspection requirements Fabrication practices such as welding can affect the lifetime of a part. For example, in the fabrication of pressure vessels or pressure piping unfavorable results from welding, such as brittle cracking in the heat-affected zone (HAZ), often result from the use of steels containing excessive amounts of residual elements that increase hardenability and consequently their susceptibility to cracking. Therefore, complete control of composition is of the utmost importance when welding is involved. In producing welds without embrittlement, it is necessary to follow the proper welding procedure. Selection of the proper filler metal is also important. Sometimes imperfections are undetected and become evident during operations. For example, small cracks were not detected in a carbon steel ASTM A 105 steam drum nozzle that was arc gouged. Consequently, during a cold start-up, a 46 cm (18 in.) crack occurred. Figure 7 shows the cracked nozzle. Prior to the cold start-up, the steam drum had been in service for five years with no cracking problems. Investigation of the cracking problem determined that crack initiation occurred from the root of an arc-gouged notch. A metallurgical cross section through the arc-gouged notch revealed microcracks and a very hard layer (54 HRC) that were not removed after arc gouging the nozzle. The carbon steel base metal was 72 to 88 HRB. The crack extended during the cold start-up because of preexisting microcracks and an asforged nozzle that had poor fracture toughness (Ref 17)
10m 00m 公 c Fig. 7 Effect of welding on the life of a carbon steel structure.(a) and(b)46 cm(18 in. long crack found in a carbon steel as-forged nozzle that was arc gouged. Failure occurred after five years in service during cold start-up procedure.(c) Micrograph showing a hardened layer(A)and the as-forged (acicular) microstructure (B). The black square shapes(C)are diamond pyramid hardness indentations rating and Nonoperating Environmental Effects. Probably the single most significant element affecting structural ice life and structural integrity is the actual usage or operating condition. Very early in the design and development phase, a design usage is identified. The design usage for an aircraft system considers design capability, operating environments, intended mission goals, types of weapons, and power-plant capabilities. Based on all considerations, the design usage is an attempt to truly identify the total environment as to how a structural component or system will be utilized in its expected service life. At that time, it is probably adequate in that it represents the intentions for how structure is going to be tested and eventually operated. However, the actual usage may be different. An example of this is unanticipated cracking that occurred in an aircraft structure at one structural control point location because the operational spectrum was 1 l times more severe than the actual design fatigue spectrum(Ref 24) The influence and degradation of material properties and therefore the reduction of life cycle is often caused by the strong influence of the operating and nonoperating environments. Life-limiting factors occur both while the structure or equipment is in service and when not in use. For example, while the aircraft is operational, the dynamic factors(fatigue flutter, and vibration) are the responsible parameters for determining the life aspects of selected components, but while the aircraft is sitting on the ground, corrosion behavior is normally the dominant driver. In a similar manner, process equipment subjected to elevated temperatures can suffer reduced the creep life, but also detrimental is equipment that is left stagnant with a corrosive medium in a pressure vessel. For pressure vessels and piping equipment, actual usage or operating conditions can even change from company to company or plant to plant. For example, the operation of a land based turbine can vary from plant-to-plant within the same company Thefileisdownloadedfromwww.bzfxw.com
Fig. 7 Effect of welding on the life of a carbon steel structure. (a) and (b) 46 cm (18 in.) long crack found in a carbon steel as-forged nozzle that was arc gouged. Failure occurred after five years in service during cold start-up procedure. (c) Micrograph showing a hardened layer (A) and the as-forged (acicular) microstructure (B). The black square shapes (C) are diamond pyramid hardness indentations. Operating and Nonoperating Environmental Effects. Probably the single most significant element affecting structural service life and structural integrity is the actual usage or operating condition. Very early in the design and development phase, a design usage is identified. The design usage for an aircraft system considers design capability, operating environments, intended mission goals, types of weapons, and power-plant capabilities. Based on all considerations, the design usage is an attempt to truly identify the total environment as to how a structural component or system will be utilized in its expected service life. At that time, it is probably adequate in that it represents the intentions for how the structure is going to be tested and eventually operated. However, the actual usage may be different. An example of this is unanticipated cracking that occurred in an aircraft structure at one structural control point location because the operational spectrum was 11 times more severe than the actual design fatigue spectrum (Ref 24). The influence and degradation of material properties and therefore the reduction of life cycle is often caused by the strong influence of the operating and nonoperating environments. Life-limiting factors occur both while the structure or equipment is in service and when not in use. For example, while the aircraft is operational, the dynamic factors (fatigue, flutter, and vibration) are the responsible parameters for determining the life aspects of selected components, but while the aircraft is sitting on the ground, corrosion behavior is normally the dominant driver. In a similar manner, process equipment subjected to elevated temperatures can suffer reduced the creep life, but also detrimental is equipment that is left stagnant with a corrosive medium in a pressure vessel. For pressure vessels and piping equipment, actual usage or operating conditions can even change from company to company or plant to plant. For example, the operation of a landbased turbine can vary from plant-to-plant within the same company. The file is downloaded from www.bzfxw.com
Stress, Stress Concentration, and Stress Intensity. In the earlier design days the principal design consideration was stress that is, the load to cause failure at a specific wall thickness or cross-sectional area. The criterion for failure could be yielding resulting in plastic deformation or failure or fracture due to exceeding the ultimate tensile strength of the failure mechanisms, stress-concentration effects and the stress intensity at a crack tip must be considered cK material. The stress on structural members is still critical, yet, with the increased understanding of crack growth and High-stress regions in a structural component are typically located around geometric details, such as a hole, fillet radius or notch. These local high-stress regions are described in terms of the remote or far-field stress of a component by multiplying the remote stress by a stress-concentration factor. Stress-concentration factors for various geometric details are derived from the theory of elasticity and/or obtained experimentally. Stress-concentration curves are available for different design geometries(Ref 25). These curves are useful when investigating a design and doing a life assessment Stress intensity is the controlling factor in subcritical crack growth propagation rates and the identification of the critical crack size for the onset of rapid overload(critical fracture). Stress and crack geometry are parameters that determine the stress intensity. The stress-intensity factor describes the stress distribution at a crack tip For a small edge crack in a thin flat plate subject to a uniform tensile stress, the stress-intensity factor can be determined from (Eq2) where KI is the stress-intensity factor, o is the applied stress, and a is the crack length. The subscript on the stress- intensity factor refers to the mode of crack loading. Equation 2 is simplified; often each geometry and crack will have a correction factor, B, to account for the crack shape, loading direction, and geometry. The failure investigator often provides the size and ratio of the crack length(c)to the crack depth(a), which is used to identify the correction factor, B For example, an edge crack normal to the free edge that is loaded in tension would have a p factor of 1. 12(Eq 3). For each crack and structure geometry, the approximate stress-intensity factor should be determined by(ref 18) (Eq3) Temperature Effects. Temperature affects structures because it can reduce the stress to failure, cause the material to be more brittle, or oxidize the surface. One of the first questions the failure investigator asks is: what was the temperature when the failure occurred, or what is the operating temperature? If the answer to this question is unknown, then the investigator deduces the temperature based on physical evidence and material properties Life of a metal component at elevated temperature, when subjected to either steady or fluctuating stress, is limited. In ontrast, at ambient temperatures and in the absence of a corrosive environment, the life of a component in steady-state load conditions may be unlimited, provided operating loads do not exceed the yield strength of the specific metal and wear is not serious. Stress produced at elevated temperature produces a condition of continuous strain called creep. By definition, creep is deformation as a function of time at constant load or stress Creep, after a period of time, may terminate in stress-rupture fracture, also known as creep rupture Most brittle fractures of steels occur because structural components are operating at temperatures below the ductile-brittle transition temperature or the nil-ductility transition temperature. Consequently, the structure is unable to maintain subcritical crack growth and failure occurs in a catastrophic manner. The ductile-to-brittle transition temperature can be above room temperature, putting at risk structures that operate below room temperature. This occurs in tanks such as molasses tanks, pressure vessels, ships such as the Liberty warships, and high-strength alloy steel structures Metallic structures oxidize when subjected to elevated temperatures. For example, carbon steel boiler tube used above 540C(1000F)is prone to oxidation Increase of oxidation resistance can occur with the addition of alloying elements such as chromium and molybdenum. a boiler tube for a 9Cr-1Mo material can have good oxidation resistance to 650C (1200F). Oxidation and high-temperature corrosion is controlled through the use of protective coatings. Elevated temperature life assessment concerns are discussed in detail in the article"Elevated -Temperature Life Assessment for Turbine Components, Piping, and Tubing in this Volume and in Ref 26 Thermal and Mechanical Fatigue Cycles. Fatigue fractures result from cyclic stressing, which progressively propagates a crack or cracks until the remaining section is no longer able to support the applied load. For example, pressure vessels and pressure piping are subject to high static stresses arising from the pressure of contained liquids or gases, to stresses resulting from misalignment of components, and to residual stresses induced during welding. The cyclical component ay be added mechanically--by vibration of associated equipment, pulsation from a compressor. It may also be added thermally, resulting in thermal fatigue if the component is cycled through a temperature range in service. Fatigue cracks ucleate from a stress riser such as a discontinuity or a notch, which produces triaxial stressing in the material. The stress riser may be macroscopic in size, such as a notch or discontinuity, or microscopic and not visible Mechanical conditions are not the only sources of cyclic loading, which can contribute to fatigue failure. Transient thermal gradients within a vessel can induce plastic strains; if these thermal gradients are applied repeatedly, the resulting yclic strain can induce failure. This process is known as thermal fatigue. Thermal fatigue is often considered a low-cycle failure mechanism that occurs due to operating conditions. Thermal fatigue can be defined as the gradual deterioration and eventual cracking of a material by alternating heating and cooling during which free thermal expansion is partially or
Stress, Stress Concentration, and Stress Intensity. In the earlier design days the principal design consideration was stress, that is, the load to cause failure at a specific wall thickness or cross-sectional area. The criterion for failure could be yielding resulting in plastic deformation or failure or fracture due to exceeding the ultimate tensile strength of the material. The stress on structural members is still critical, yet, with the increased understanding of crack growth and failure mechanisms, stress-concentration effects and the stress intensity at a crack tip must be considered. High-stress regions in a structural component are typically located around geometric details, such as a hole, fillet radius, or notch. These local high-stress regions are described in terms of the remote or far-field stress of a component by multiplying the remote stress by a stress-concentration factor. Stress-concentration factors for various geometric details are derived from the theory of elasticity and/or obtained experimentally. Stress-concentration curves are available for different design geometries (Ref 25). These curves are useful when investigating a design and doing a life assessment. Stress intensity is the controlling factor in subcritical crack growth propagation rates and the identification of the critical crack size for the onset of rapid overload (critical fracture). Stress and crack geometry are parameters that determine the stress intensity. The stress-intensity factor describes the stress distribution at a crack tip. For a small edge crack in a thin flat plate subject to a uniform tensile stress, the stress-intensity factor can be determined from: K a 1 = bs p (Eq 2) where KI is the stress-intensity factor, σ is the applied stress, and a is the crack length. The subscript on the stressintensity factor refers to the mode of crack loading. Equation 2 is simplified; often each geometry and crack will have a correction factor, β, to account for the crack shape, loading direction, and geometry. The failure investigator often provides the size and ratio of the crack length (c) to the crack depth (a), which is used to identify the correction factor, β. For example, an edge crack normal to the free edge that is loaded in tension would have a β factor of 1.12 (Eq 3). For each crack and structure geometry, the approximate stress-intensity factor should be determined by (Ref 18): 1 K a = 1.12s p (Eq 3) Temperature Effects. Temperature affects structures because it can reduce the stress to failure, cause the material to be more brittle, or oxidize the surface. One of the first questions the failure investigator asks is: what was the temperature when the failure occurred, or what is the operating temperature? If the answer to this question is unknown, then the investigator deduces the temperature based on physical evidence and material properties. Life of a metal component at elevated temperature, when subjected to either steady or fluctuating stress, is limited. In contrast, at ambient temperatures and in the absence of a corrosive environment, the life of a component in steady-state load conditions may be unlimited, provided operating loads do not exceed the yield strength of the specific metal and wear is not serious. Stress produced at elevated temperature produces a condition of continuous strain called creep. By definition, creep is deformation as a function of time at constant load or stress. Creep, after a period of time, may terminate in stress-rupture fracture, also known as creep rupture. Most brittle fractures of steels occur because structural components are operating at temperatures below the ductile-brittle transition temperature or the nil-ductility transition temperature. Consequently, the structure is unable to maintain subcritical crack growth and failure occurs in a catastrophic manner. The ductile-to-brittle transition temperature can be above room temperature, putting at risk structures that operate below room temperature. This occurs in tanks such as molasses tanks, pressure vessels, ships such as the Liberty warships, and high-strength alloy steel structures. Metallic structures oxidize when subjected to elevated temperatures. For example, carbon steel boiler tube used above 540 °C (1000 °F) is prone to oxidation. Increase of oxidation resistance can occur with the addition of alloying elements such as chromium and molybdenum. A boiler tube for a 9Cr-1Mo material can have good oxidation resistance to 650 °C (1200 °F). Oxidation and high-temperature corrosion is controlled through the use of protective coatings. Elevatedtemperature life assessment concerns are discussed in detail in the article “Elevated-Temperature Life Assessment for Turbine Components, Piping, and Tubing” in this Volume and in Ref 26. Thermal and Mechanical Fatigue Cycles. Fatigue fractures result from cyclic stressing, which progressively propagates a crack or cracks until the remaining section is no longer able to support the applied load. For example, pressure vessels and pressure piping are subject to high static stresses arising from the pressure of contained liquids or gases, to stresses resulting from misalignment of components, and to residual stresses induced during welding. The cyclical component may be added mechanically—by vibration of associated equipment, pulsation from a compressor. It may also be added thermally, resulting in thermal fatigue if the component is cycled through a temperature range in service. Fatigue cracks nucleate from a stress riser such as a discontinuity or a notch, which produces triaxial stressing in the material. The stress riser may be macroscopic in size, such as a notch or discontinuity, or microscopic and not visible. Mechanical conditions are not the only sources of cyclic loading, which can contribute to fatigue failure. Transient thermal gradients within a vessel can induce plastic strains; if these thermal gradients are applied repeatedly, the resulting cyclic strain can induce failure. This process is known as thermal fatigue. Thermal fatigue is often considered a low-cycle failure mechanism that occurs due to operating conditions. Thermal fatigue can be defined as the gradual deterioration and eventual cracking of a material by alternating heating and cooling during which free thermal expansion is partially or