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.comFig. 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