necessary, then, to pay more attention to predicting and ensuring performance Predicting and ensuring performance is fundamentally a part of the design process for buildings, power plants, aircraft, refineries, and ships( Ref 16) For any given design, the mission and the intended use are established Predicting the performance and design life of a component depends on defining what life or performance is required for a given duration while the component operates generally in combinations of mechanical and chemical environments. Defining performance may involve defining end points such as: acceptable length of propagating cracks, maximum depth of propagating pits, acceptable remaining thickness of corroding pipes, maximum number of fatigue cycles or extent of cumulative damage, maximum number of plugged tubes, maximum number of failed circuits, maximum leakage, or appearance of a maximum area or number of rust spots. Defining such end points is a critical part of predicting life since prediction defines when these end points will be reached and therefore when"failure occurs Defining failure is also related to what is meant by the"design life. For example, for the aerospace industry, an airplane may be designed for 8000 flight hours and analyzed for two lifetimes or 16,000 flight hours. For the power industry, the design life of components is sometimes taken as 40 years. This means that the equipment is expected to perform satisfactorily at its rated output for 40 years. This is not to say that some maintenance is not necessary. However, to assert to a customer that a component has a 40 year design life it is necessary to develop bases for such a claim. Such bases are usually provided by analyses and by accelerated testing in the laboratory and with prototype and model testing As part of the life assessment process, it is important to understand how a structural component-whether a pressure vessel, shaft, or structural member-is designed in order to understand how it may fail and to perform meaningful life assessment. For example, the first step in the design of any pressure vessel is to select the proper design code based on its intended use. For example, a pressure vessel may be a power or heating boiler, a nuclear reactor chamber, a chemical process chamber, a hydrostatic test chamber used to test underwater equipment, or a pressure vessel for human occupancy. Once the intended use is identified, the appropriate design code can be selected. For example, pressure vessels use codes provided by many organizations and certifying agencies, such as the American Society of Mechanical Engineers(ASME), the American Bureau of Shipping(ABS), and European agencies have pressure vessel design codes Strict adherence to these codes for the design, fabrication, testing, and quality control and assurance allows the finished pressure vessel to be certified by the appropriate authorizing agency( ref 17) One of the first incentives to develop a pressure vessel code occurred after the Boston Molasses tank incident in 1919 when the tank failed by overstress, consequently releasing more than 2 million gallons of molasses and resulting in the loss of life and property(Ref 5 ). Even after that catastrophic failure and understanding the nature of the failure, another molasses tank failure occurred in New Jersey in 1973. Figure 2 shows the destruction caused by the molasses tank incident. These molasses tank incidences demonstrate how important it is to prevent failures, and it underscores that good designs consider the operating conditions and limitations of materials of construction Thefileisdownloadedfromwww.bzfxw.com
necessary, then, to pay more attention to predicting and ensuring performance. Predicting and ensuring performance is fundamentally a part of the design process for buildings, power plants, aircraft, refineries, and ships (Ref 16). For any given design, the mission and the intended use are established. Predicting the performance and design life of a component depends on defining what life or performance is required for a given duration while the component operates generally in combinations of mechanical and chemical environments. Defining performance may involve defining end points such as: acceptable length of propagating cracks, maximum depth of propagating pits, acceptable remaining thickness of corroding pipes, maximum number of fatigue cycles or extent of cumulative damage, maximum number of plugged tubes, maximum number of failed circuits, maximum leakage, or appearance of a maximum area or number of rust spots. Defining such end points is a critical part of predicting life since prediction defines when these end points will be reached and therefore when “failure” occurs. Defining failure is also related to what is meant by the “design life.” For example, for the aerospace industry, an airplane may be designed for 8000 flight hours and analyzed for two lifetimes or 16,000 flight hours. For the power industry, the design life of components is sometimes taken as 40 years. This means that the equipment is expected to perform satisfactorily at its rated output for 40 years. This is not to say that some maintenance is not necessary. However, to assert to a customer that a component has a 40 year design life, it is necessary to develop bases for such a claim. Such bases are usually provided by analyses and by accelerated testing in the laboratory and with prototype and model testing. As part of the life assessment process, it is important to understand how a structural component—whether a pressure vessel, shaft, or structural member—is designed in order to understand how it may fail and to perform meaningful life assessment. For example, the first step in the design of any pressure vessel is to select the proper design code based on its intended use. For example, a pressure vessel may be a power or heating boiler, a nuclear reactor chamber, a chemical process chamber, a hydrostatic test chamber used to test underwater equipment, or a pressure vessel for human occupancy. Once the intended use is identified, the appropriate design code can be selected. For example, pressure vessels use codes provided by many organizations and certifying agencies, such as the American Society of Mechanical Engineers (ASME), the American Bureau of Shipping (ABS), and European agencies have pressure vessel design codes. Strict adherence to these codes for the design, fabrication, testing, and quality control and assurance allows the finished pressure vessel to be certified by the appropriate authorizing agency (Ref 17). One of the first incentives to develop a pressure vessel code occurred after the Boston Molasses tank incident in 1919 when the tank failed by overstress, consequently releasing more than 2 million gallons of molasses and resulting in the loss of life and property (Ref 5). Even after that catastrophic failure and understanding the nature of the failure, another molasses tank failure occurred in New Jersey in 1973. Figure 2 shows the destruction caused by the molasses tank incident. These molasses tank incidences demonstrate how important it is to prevent failures, and it underscores that good designs consider the operating conditions and limitations of materials of construction. The file is downloaded from www.bzfxw.com
Fig 2 Failed molasses tank, which fractured suddenly in New Jersey in March 1973. This catastrophic and sudden brittle fracture resulted in the release of the molasses in the tank similar to the boston molasses tank disaster in 1919 The next step in the design process is to identify the design parameters, such as configuration, design pressure, and so forth, Table 2 presents an example of a design parameter list applicable for a chemical process chamber(Ref 17). These design parameters are the same parameters considered when conducting a pressure vessel failure investigation and life Table 2 Pressure vessel design parameters Required design code) Penetration and location requirements Basic chamber configuration(Cylindrical or Contents and/or process within the pressure vessel spherical; flat, spherical, or elliptical end details, Internal volume capacity Estimated operational pressure and temperature cycle history (number of cycles at what pressures and temperatures over the Minimum inside diameter Piping, external and internal attachment requirements Minimum inside length Test chamber surroundings(enclosed in building or exposed to Chamber orientation(for cylindrical chambers, Test chamber physical geographical location longitudinal axis vertical or horizontal Support configuration(saddle supports, bottom Vessel special material requirements(vessel material other than cylindrical skirt, legs, etc Maximum internal operating pressure Vessel protection requirements(painted surfaces, stainless steel overlays at seals, cathodic protection, etc Maximum external operating pressure(vacuum,Fabrication requirements Design operating temperature range Material selection (a) AsMe Boiler and Pressure Vessel Code, Section VIl, Div. 1, 2, or 3; ABS; other Given the design parameters, the proper material(s)is selected for the structural component. Safety and economy are often the governing factors when selecting a material for pressure vessels. The material is selected based on its mechanical, corrosion, creep, toughness, and thermal properties as applicable. If necessary, the appropriate weld material is selected based on the chosen base material. Material is assigned an allowable stress value based on its ultimate and yield strengths and operating temperature range. This allowable stress value is then used in design equations or compared to results obtained from detailed analyses The design process then proceeds with the determination of the sizes and/or thickness of the various components. The design process is completed with the creation of the engineering and fabrication drawings. These drawings should include the dimensional information, but also specify materials, weld identification, weld procedures, and required weld inspections. Other helpful information to include on the drawings is basic parameters such as design pressure, design temperature range, design code, and other information deemed necessary for the particular structural component Structural Design Approaches. The criteria of failure are determined by the strength of materials, fracture toughness, creep resistance, fatigue behavior, and the corrosion resistance of materials. These are briefly discussed in this section Strength of Materials. In the strength-of-materials design approach one typically has a specific structural geometry (assumed to be defect free) for which the load-carrying capacity must be determined. To accomplish this, a calculation is first made to determine the relation between the load and the maximum stress that exists in the structure. The maximum stress so determined is then compared with the strength of the material. An acceptable design is achieved when the maximum stress is less than the strength of the material, suitably reduced by a factor of safety It can be assumed that failure will not occur unless omax exceeds the yield strength of the material, oY. To ensure this, a factor of safety (S) can be introduced to account for material variability and/or unanticipated greater service loading. The strength-of-materials approach is a good approach for materials with no defects and simple structures. Figure 3 shows the strength-of-materials approach and the engineering design regime based on a factor of safety
Fig. 2 Failed molasses tank, which fractured suddenly in New Jersey in March 1973. This catastrophic and sudden brittle fracture resulted in the release of the molasses in the tank similar to the Boston Molasses tank disaster in 1919. The next step in the design process is to identify the design parameters, such as configuration, design pressure, and so forth. Table 2 presents an example of a design parameter list applicable for a chemical process chamber (Ref 17). These design parameters are the same parameters considered when conducting a pressure vessel failure investigation and life assessment. Table 2 Pressure vessel design parameters Required design code(a) Penetration and location requirements Basic chamber configuration. (Cylindrical or spherical; flat, spherical, or elliptical end details; etc.) Contents and/or process within the pressure vessel Internal volume capacity Estimated operational pressure and temperature cycle history (number of cycles at what pressures and temperatures over the vessel's lifetime) Minimum inside diameter Piping, external and internal attachment requirements Minimum inside length Test chamber surroundings (enclosed in building or exposed to elements) Chamber orientation (for cylindrical chambers, longitudinal axis vertical or horizontal) Test chamber physical geographical location Support configuration (saddle supports, bottom cylindrical skirt, legs, etc.) Vessel special material requirements (vessel material other than carbon steel, internal cladding, etc.) Maximum internal operating pressure Vessel protection requirements (painted surfaces, stainless steel overlays at seals, cathodic protection, etc.) Maximum external operating pressure (vacuum, etc.) Fabrication requirements Design operating temperature range Material selection (a) ASME Boiler and Pressure Vessel Code, Section VII, Div. 1, 2, or 3; ABS; other Given the design parameters, the proper material(s) is selected for the structural component. Safety and economy are often the governing factors when selecting a material for pressure vessels. The material is selected based on its mechanical, corrosion, creep, toughness, and thermal properties as applicable. If necessary, the appropriate weld material is selected based on the chosen base material. Material is assigned an allowable stress value based on its ultimate and yield strengths and operating temperature range. This allowable stress value is then used in design equations or compared to results obtained from detailed analyses. The design process then proceeds with the determination of the sizes and/or thickness of the various components. The design process is completed with the creation of the engineering and fabrication drawings. These drawings should include the dimensional information, but also specify materials, weld identification, weld procedures, and required weld inspections. Other helpful information to include on the drawings is basic parameters such as design pressure, design temperature range, design code, and other information deemed necessary for the particular structural component. Structural Design Approaches. The criteria of failure are determined by the strength of materials, fracture toughness, creep resistance, fatigue behavior, and the corrosion resistance of materials. These are briefly discussed in this section. Strength of Materials. In the strength-of-materials design approach one typically has a specific structural geometry (assumed to be defect free) for which the load-carrying capacity must be determined. To accomplish this, a calculation is first made to determine the relation between the load and the maximum stress that exists in the structure. The maximum stress so determined is then compared with the strength of the material. An acceptable design is achieved when the maximum stress is less than the strength of the material, suitably reduced by a factor of safety. It can be assumed that failure will not occur unless σmax exceeds the yield strength of the material, σY. To ensure this, a factor of safety (S) can be introduced to account for material variability and/or unanticipated greater service loading. The strength-of-materials approach is a good approach for materials with no defects and simple structures. Figure 3 shows the strength-of-materials approach and the engineering design regime based on a factor of safety
Elastic-plastie Linear elastic fracture mechanics fracture mechanics behavior 、 regime 51/s Engineering design Strength-of-materials behavior 1/s Stress ratio(omaxoy) Fig.3A general plot of the ratios of the toughness and stress showing the relationship between linear elastic fracture mechanics and strength of materials as it relates to fracture and structural integrity(ref 18) Linear Elastic Fracture Mechanics. Brittle fractures. similar to those mentioned in Table 1. are avoided using a linear elastic fracture mechanics design approach. This approach considers that the structure, instead of being defect-free, contains a crack(Ref 18). The governing structural mechanics parameter when a crack is present, at least in the linear approach, is an entity called the stress-intensity factor. This parameter, which is conventionally given the symbol K,can be determined from a mathematical analysis similar to that used to obtain the stresses in an uncracked component. For a relatively small crack in a simple structure, an analysis of the flawed structure beam would give to a reasonable approximation K=1.12√ma (Eq1) where a is the depth of the bracket and omax is the stress that would occur at the crack location in the absence of the crack The basic relation in fracture mechanics is one that equates K to a critical value. This critical value is often taken as a property of the material called the plane-strain fracture toughness, conventionally denoted as Kle. When equality is achieved between K and Klc, the crack is presumed to grow in an uncontrollable manner. Hence, the structure can be designed to be safe from fracture by ensuring that K is less than Kle. The liberty warship fractures are a classic example of a structural failure caused by Klc exceeding K and uncontrolled crack growth The essential difference from the strength of materials approach is that the fracture mechanics approach explicitly introduces a new physical parameter: the size of a(real or postulated)cracklike flaw. In fracture mechanics the size of a crack is the dominant structural parameter. It is the specification of this parameter that distinguishes fracture mechanics from conventional failure analyses The generalization of the basis for engineering structural-integrity assessments that fracture mechanics provides is portrayed in terms of the failure boundary shown in Fig. 3. Clearly, fracture mechanics considerations do not eliminate the traditional approach. Structures using reasonably tough materials(high Klc)and having only small cracks (low K)will lie in the strength of materials regime. Conversely, if the material is brittle(low Klc)and strong(high oY), the presence of even a small crack is likely to trigger fracture. The fracture mechanics assessment is then the crucial one The special circumstances that would be called into play in the upper right-hand corner of Fig. 3 are worth noting. In this regime, a cracked structure would experience large-scale plastic deformation prior to crack extension. Additional information is provided in the article Failure Assessment Diagrams"in this Volume and in Ref 19 Damage Tolerance Approach Life assessment of aircraft and power-plant equipment stems largely from the development of the damage-tolerance philosophy based on fracture mechanics. Damage tolerance is the philosophy used for maintaining the structural safety of commercial transport, military aircraft, structures, and pressure vessels. The use of fracture mechanics and damage tolerance has evolved into the design program for structures that are damage tolerant, that is, designed to operate with manufacturing and in-service-induced defects(Ref 20) Thefileisdownloadedfromwww.bzfxw.com
Fig. 3 A general plot of the ratios of the toughness and stress showing the relationship between linear elastic fracture mechanics and strength of materials as it relates to fracture and structural integrity (Ref 18) Linear Elastic Fracture Mechanics. Brittle fractures, similar to those mentioned in Table 1, are avoided using a linear elastic fracture mechanics design approach. This approach considers that the structure, instead of being defect-free, contains a crack (Ref 18). The governing structural mechanics parameter when a crack is present, at least in the linear approach, is an entity called the stress-intensity factor. This parameter, which is conventionally given the symbol K, can be determined from a mathematical analysis similar to that used to obtain the stresses in an uncracked component. For a relatively small crack in a simple structure, an analysis of the flawed structure beam would give to a reasonable approximation: max K a = 1.12s p (Eq 1) where α is the depth of the bracket and σmax is the stress that would occur at the crack location in the absence of the crack. The basic relation in fracture mechanics is one that equates K to a critical value. This critical value is often taken as a property of the material called the plane-strain fracture toughness, conventionally denoted as KIc. When equality is achieved between K and KIc, the crack is presumed to grow in an uncontrollable manner. Hence, the structure can be designed to be safe from fracture by ensuring that K is less than KIc. The Liberty warship fractures are a classic example of a structural failure caused by KIc exceeding K and uncontrolled crack growth. The essential difference from the strength of materials approach is that the fracture mechanics approach explicitly introduces a new physical parameter: the size of a (real or postulated) cracklike flaw. In fracture mechanics the size of a crack is the dominant structural parameter. It is the specification of this parameter that distinguishes fracture mechanics from conventional failure analyses. The generalization of the basis for engineering structural-integrity assessments that fracture mechanics provides is portrayed in terms of the failure boundary shown in Fig. 3. Clearly, fracture mechanics considerations do not eliminate the traditional approach. Structures using reasonably tough materials (high KIc) and having only small cracks (low K) will lie in the strength of materials regime. Conversely, if the material is brittle (low KIc) and strong (high σY), the presence of even a small crack is likely to trigger fracture. The fracture mechanics assessment is then the crucial one. The special circumstances that would be called into play in the upper right-hand corner of Fig. 3 are worth noting. In this regime, a cracked structure would experience large-scale plastic deformation prior to crack extension. Additional information is provided in the article “Failure Assessment Diagrams” in this Volume and in Ref 19. Damage Tolerance Approach. Life assessment of aircraft and power-plant equipment stems largely from the development of the damage-tolerance philosophy based on fracture mechanics. Damage tolerance is the philosophy used for maintaining the structural safety of commercial transport, military aircraft, structures, and pressure vessels. The use of fracture mechanics and damage tolerance has evolved into the design program for structures that are damage tolerant, that is, designed to operate with manufacturing and in-service-induced defects (Ref 20). The file is downloaded from www.bzfxw.com
Damage-tolerance evaluation has been interpreted in the past as a means to allow continued safe operation in the presence of known cracking. This interpretation is incorrect. No regulations allow the strength of the structure to be knowingly degraded below ultimate strength(1.5 x limit). The damage-tolerance evaluation is merely a means of providing an inspection program for a structure that is not expected to crack under normal circumstances, but may crack in service due to inadvertent circumstances. If cracks are found in primary structure, they must be repaired. The only allowable exception is through an engineering evaluation, which must show that the strength of the structure will never be degraded below ultimate strength operations or in-service conditions After many major fatigue failures in the 1950s on both military and commercial aircraft, the most notable of which were the DeHavilland Comet failures in early 1954, the U.S. Air Force (USAF) initiated the Aircraft Structural Integrity Program(ASIP)in 1958(Ref 10). The fatigue methodology adopted in the AsIP was the reliability approach, which became known as the"safe-life method This safe-life approach, used in the development of USaF aircraft in the 1960s involved analysis and testing to four times the anticipated service life. On the commercial scene, another philosophy, " fail safety, was introduced in the early 1960s, and a choice between safe -life and fail-safe methods was allowed by commercial airworthiness requirements. However, it was found that the safe-life method did not prevent fatigue cracking o thin the service life, even though the aircraft were tested to four lifetimes to support one service life(i.e, scatter factor of 4 ) One notable example is the F-lll aircraft 94, which crashed in 1969(Ref 11). The F-11l aircraft had a safe- life of 4000 flight hours. However, a material defect caused the F-lll aircraft, which used high-strength steel (ultimate tensile strength of 1655 to 1793 MPa, or 240 to 260 ksi, toughness of about 66 MPa Vm, or 60 ksi vin )for the wing box(Fig 4). The defect was not observed during inspection, and a fatigue crack initiated and grew for only about 0.38 mm(0.015 in. ) The aircraft was flown for 107 flights safely, at which time catastrophic failure occurred, causing the destruction of the aircraft stayed with wing stayed with airplane Wing pivot fitting Area of anomaly (b 0.905in. Brittle fracture 0.236 in deer 0.282 in thick 0.015 in. fatigue crack growth Material anomaly
Damage-tolerance evaluation has been interpreted in the past as a means to allow continued safe operation in the presence of known cracking. This interpretation is incorrect. No regulations allow the strength of the structure to be knowingly degraded below ultimate strength (1.5 × limit). The damage-tolerance evaluation is merely a means of providing an inspection program for a structure that is not expected to crack under normal circumstances, but may crack in service due to inadvertent circumstances. If cracks are found in primary structure, they must be repaired. The only allowable exception is through an engineering evaluation, which must show that the strength of the structure will never be degraded below ultimate strength operations or in-service conditions. After many major fatigue failures in the 1950s on both military and commercial aircraft, the most notable of which were the DeHavilland Comet failures in early 1954, the U.S. Air Force (USAF) initiated the Aircraft Structural Integrity Program (ASIP) in 1958 (Ref 10). The fatigue methodology adopted in the ASIP was the reliability approach, which became known as the “safe-life” method. This safe-life approach, used in the development of USAF aircraft in the 1960s, involved analysis and testing to four times the anticipated service life. On the commercial scene, another philosophy, “fail safety,” was introduced in the early 1960s, and a choice between safe-life and fail-safe methods was allowed by commercial airworthiness requirements. However, it was found that the safe-life method did not prevent fatigue cracking within the service life, even though the aircraft were tested to four lifetimes to support one service life (i.e., scatter factor of 4). One notable example is the F-111 aircraft 94, which crashed in 1969 (Ref 11). The F-111 aircraft had a safe-life of 4000 flight hours. However, a material defect caused the F-111 aircraft, which used high-strength steel (ultimate tensile strength of 1655 to 1793 MPa, or 240 to 260 ksi, toughness of about 66 MPa m , or 60 ksi in ) for the wing box (Fig. 4). The defect was not observed during inspection, and a fatigue crack initiated and grew for only about 0.38 mm (0.015 in.). The aircraft was flown for 107 flights safely, at which time catastrophic failure occurred, causing the destruction of the aircraft
Fig. 4 Fatigue cracking in an aircraft wing fitting for the f-lll Aircraft 94 that crashed in 1969.(a)and (b)Location of the left wing-pivot box fitting. The 22 mm(0.91 in material defect was not observed during inspection, and a fatigue crack initiated and grew for only about 0.38 mm(0.015 in. before unstable brittle fracture occurred The leak-before-break design approach is prevalent in pressure-containing equipment such as pressure vessels and piping used in the nuclear and fossil-fuel power-generation plants, refineries, and chemical plants. Failure analysis and life assessment of pressure-containing systems is essential. Although leak-before-break failures are not catastrophic, they are costly and can affect plant operations. Therefore, analyses are often performed to predict when the next internal or external inspection should be performed. Typical life-limiting mechanisms include stress-corrosion cracking, fatigue, and thermal fatigue. Welded structures that could initiate a crack are often susceptible to these mechanisms The leak-before-break concept generally refers to a pressure-contaminant system failure in which a part-through wall crack extends to become a through-wall crack, thus allowing fluid to escape. If no further crack extension occurs, then the loss of the fluid is detected and no further crack growth occurs. Alternatively, when a through-wall crack propagates along the wall, a catastrophic event can occur(Ref 18) Pow plant piping materials that are ductile, such as stainless steel and nickel-base alloys, often leak before break. Figure 5 shows small-bore, socket-welded piping that will initiate fatigue cracks at either the toe of the weld or the root of the weld. These ductile socket-welded pipes leak before catastrophic failure occurs(Ref 21) Axial leg clal weld toe Socket weld Fig. 5 Stainless steel piping such as small-bore piping is designed to leak before break. a fatigue crack either initiates at the toe or the root of the weld(a) Typical socket fitting with a fillet weld. (b) Micrograph of a cross section through a socket-welded joint showing fatigue crack that initiated from the weld root and extended through the weld.(c) Micrograph of through the pipe axia weld toe crack showing a fatigue-induced crack that extended Elevated-Temperature Concerns For elevated-temperature equipment and structures subjected to steady-state or cyclic stresses, the principal design considerations are creep control, oxidation prevention through the use of oxidation-resistant materials or coatings, and selection of materials that have good stress-rupture and creep properties. The criteria for failure is(1)to not go below a minimum stress-rupture strength for a given operating stress and temperature and(2)to not operate above a certain temperature that alters the microstructure or oxidizes the material Corrosion Allowances. Designs are configured such that the operating or loading stresses can be minimized for safe operations. It is necessary to consider the effects that an environment will have on the material; it is just as important as considering structural loads on a component. It is important that environments are known and controlled in such a way that corrosion is minimized on all surfaces. This means that designs consider effects of crevices, galvanic couples, flows stresses, and temperatures to ensure that all the surfaces of materials will be minimally degraded within the design life (Ref 16) A common design approach for pressure vessels and tanks to deal with corrosion is to provide a"corrosion allowance, which takes the form of additional thickness based on available information on rates of general corrosion over the design life. For example, a carbon steel vessel designed for 25 years of service in sulfuric acid at a corrosion rate of 5 mils/yr (0.005 in. yr) would have a corrosion allowance of 125 mils. However, such allowances cannot deal with stress-corrosion cracking, pitting, intergranular cracking, or effects of long-range cells. Use of a corrosion allowance can be disastrously misleading since its use suggests that all corrosion problems have been solved. It should be pointed out that exceeding the corrosion allowance does not necessarily mean the vessel would fail or is unsuitable for service. It is an indication that the vessel should be evaluated for continued service Thefileisdownloadedfromwww.bzfxw.com
Fig. 4 Fatigue cracking in an aircraft wing fitting for the F-111 Aircraft 94 that crashed in 1969. (a) and (b) Location of the left wing-pivot box fitting. The 22 mm (0.91 in.) material defect was not observed during inspection, and a fatigue crack initiated and grew for only about 0.38 mm (0.015 in.) before unstable brittle fracture occurred. The leak-before-break design approach is prevalent in pressure-containing equipment such as pressure vessels and piping used in the nuclear and fossil-fuel power-generation plants, refineries, and chemical plants. Failure analysis and life assessment of pressure-containing systems is essential. Although leak-before-break failures are not catastrophic, they are costly and can affect plant operations. Therefore, analyses are often performed to predict when the next internal or external inspection should be performed. Typical life-limiting mechanisms include stress-corrosion cracking, fatigue, and thermal fatigue. Welded structures that could initiate a crack are often susceptible to these mechanisms. The leak-before-break concept generally refers to a pressure-contaminant system failure in which a part-through wall crack extends to become a through-wall crack, thus allowing fluid to escape. If no further crack extension occurs, then the loss of the fluid is detected and no further crack growth occurs. Alternatively, when a through-wall crack propagates along the wall, a catastrophic event can occur (Ref 18). Power-plant piping materials that are ductile, such as stainless steel and nickel-base alloys, often leak before break. Figure 5 shows small-bore, socket-welded piping that will initiate fatigue cracks at either the toe of the weld or the root of the weld. These ductile socket-welded pipes leak before catastrophic failure occurs (Ref 21). Fig. 5 Stainless steel piping such as small-bore piping is designed to leak before break. A fatigue crack either initiates at the toe or the root of the weld. (a) Typical socket fitting with a fillet weld. (b) Micrograph of a cross section through a socket-welded joint showing fatigue crack that initiated from the weld root and extended through the weld. (c) Micrograph of axial weld toe crack showing a fatigue-induced crack that extended through the pipe wall Elevated-Temperature Concerns. For elevated-temperature equipment and structures subjected to steady-state or cyclic stresses, the principal design considerations are creep control, oxidation prevention through the use of oxidation-resistant materials or coatings, and selection of materials that have good stress-rupture and creep properties. The criteria for failure is (1) to not go below a minimum stress-rupture strength for a given operating stress and temperature and (2) to not operate above a certain temperature that alters the microstructure or oxidizes the material. Corrosion Allowances. Designs are configured such that the operating or loading stresses can be minimized for safe operations. It is necessary to consider the effects that an environment will have on the material; it is just as important as considering structural loads on a component. It is important that environments are known and controlled in such a way that corrosion is minimized on all surfaces. This means that designs consider effects of crevices, galvanic couples, flows, stresses, and temperatures to ensure that all the surfaces of materials will be minimally degraded within the design life (Ref 16). A common design approach for pressure vessels and tanks to deal with corrosion is to provide a “corrosion allowance,” which takes the form of additional thickness based on available information on rates of general corrosion over the design life. For example, a carbon steel vessel designed for 25 years of service in sulfuric acid at a corrosion rate of 5 mils/yr (0.005 in./yr) would have a corrosion allowance of 125 mils. However, such allowances cannot deal with stress-corrosion cracking, pitting, intergranular cracking, or effects of long-range cells. Use of a corrosion allowance can be disastrously misleading since its use suggests that all corrosion problems have been solved. It should be pointed out that exceeding the corrosion allowance does not necessarily mean the vessel would fail or is unsuitable for service. It is an indication that the vessel should be evaluated for continued service. The file is downloaded from www.bzfxw.com