fully constrained. The constraint of a part or material does not allow free expansion to occur. For example, piping attachments that are mechanically constrained will develop thermal fatigue cracks when thermally cycled The cyclic thermally induced stresses associated with frequent start-ups and shutdowns are frequently more severe than stresses of steady-state operations, and designers cannot al ways predict the use cycles of equipment that owner-operators impose Equipment sometimes is operated only during peak-demand periods; this imposes many start-ups and shutdowns, with attendant thermal stresses on pressure vessels and piping systems. When cracking occurs, it is in areas least able to accommodate severe cyclic loading, such areas are likely to be rigid or massive sections under conditions of high constraint or thermal stress Corrosion Concerns. In most cases, corrosion itself does not lead to structural failure, but it is the most expensive aspect of maintaining structural integrity. The effect of corrosion needs to be addressed by denoting the types of corrosion and where they can occur, inspection techniques to find the corrosion in situ, and repair or replacement methods to remove the offending material. There are many corrosion mechanisms such as stress-corrosion cracking, hydrogen embrittlement, and general corrosion that reduce the life expectancy of components. These life-limiting corrosion mechanisms need to be identified and considered in performing life assessments. These mechanisms are discussed in greater detail in this Volume and are addressed in the literature Improper Maintenance. The manufacturer of structures, equipments, and components often requires special maintenance as identified in handbooks, structural repair manuals, and technical orders(ref 20). It is imperative that these manuals be rigidly adhered to. It is easy to devise methods that may be shorter and easier than those discussed in the manual, but the manufacturer selected that particular procedure for sound reasons. It is dangerous to deviate from these methods, for the deviation itself may place undue stress or cause latent damage to components apart from those being examined. This high stress or latent damage can lead to premature failure of components, and that may result in undesirable consequences Proper maintenance and inspection of tanks, aircraft, and pressure vessels are mandatory. Extreme care during maintenance and the inspection following maintenance must be taken to ensure that proper procedures and techniques are rigorously followed References cited in this section 14. N1819U, NTSB-AAR-90-06, Aircraft Accident Report, United Airlines, Inc, Nov 1990 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 0. M.P. Kaplan and T.A. Wolff, Life Extension and Damage Tolerance of Aircraft, Fatigue and Fracture, Vol 19 ASM Handbook, p 557-565 22. H R. Millwater, S.H. Fitch, Y -T. Wu, D.S. Riha, M. P. Enright, G.R. Leverant, R.C. Mc Clung, C.J. Kuhlman, G.G. Chell, and Y. Lee, A Probabilistically-Based Damage Tolerance Analysis Computer Program for Hard Alpha Anomalies in Titanium Rotors, Paper 2000-GT-0421, IGTI, ASME Turbo Expo, 2000 23."Aircraft Structural Integrity and Failure Analysis of Structural Components, "Southwest Research Brochures 24. D.J. Benac and J. P. Pendley, "Integration of Design, Failure Analysis and Fleet Management to Prevent Aircraft Structural Failures, Asian Aircraft Conference( Singapore), 1991 W.D. Pilkey et al., Peterson's Stress Concentration Factors, 2nd ed, John Wiley Sons, 1997 26. A.F. Liu, High-Temperature Life Assessment, Fatigue and Fracture, Vol 19, ASM Handbook, ASM International, 1996, p 520-526 Role of the failure investigator The typical role of the failure investigator is that of a materials failure analyst looking at fracture surfaces or examining the microstructure for degradation, microstructural changes, or unusual crack morphologies. With the advent of life assessment methodologies the investigator's responsibilities have expanded to be an integral part of life assessment for structures. The flow diagram in Fig. 1 shows the tasks the investigator undertakes for evaluation of in-service failures and Thefileisdownloadedfromwww.bzfxw.com
fully constrained. The constraint of a part or material does not allow free expansion to occur. For example, piping attachments that are mechanically constrained will develop thermal fatigue cracks when thermally cycled. The cyclic thermally induced stresses associated with frequent start-ups and shutdowns are frequently more severe than stresses of steady-state operations, and designers cannot always predict the use cycles of equipment that owner-operators impose. Equipment sometimes is operated only during peak-demand periods; this imposes many start-ups and shutdowns, with attendant thermal stresses on pressure vessels and piping systems. When cracking occurs, it is in areas least able to accommodate severe cyclic loading; such areas are likely to be rigid or massive sections under conditions of high constraint or thermal stress. Corrosion Concerns. In most cases, corrosion itself does not lead to structural failure, but it is the most expensive aspect of maintaining structural integrity. The effect of corrosion needs to be addressed by denoting the types of corrosion and where they can occur, inspection techniques to find the corrosion in situ, and repair or replacement methods to remove the offending material. There are many corrosion mechanisms such as stress-corrosion cracking, hydrogen embrittlement, and general corrosion that reduce the life expectancy of components. These life-limiting corrosion mechanisms need to be identified and considered in performing life assessments. These mechanisms are discussed in greater detail in this Volume and are addressed in the literature. Improper Maintenance. The manufacturer of structures, equipments, and components often requires special maintenance as identified in handbooks, structural repair manuals, and technical orders (Ref 20). It is imperative that these manuals be rigidly adhered to. It is easy to devise methods that may be shorter and easier than those discussed in the manual, but the manufacturer selected that particular procedure for sound reasons. It is dangerous to deviate from these methods, for the deviation itself may place undue stress or cause latent damage to components apart from those being examined. This high stress or latent damage can lead to premature failure of components, and that may result in undesirable consequences. Proper maintenance and inspection of tanks, aircraft, and pressure vessels are mandatory. Extreme care during maintenance and the inspection following maintenance must be taken to ensure that proper procedures and techniques are rigorously followed. References cited in this section 14. N1819U, NTSB-AAR-90-06, Aircraft Accident Report, United Airlines, Inc., Nov 1990 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 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 22. H.R. Millwater, S.H. Fitch, Y.-T. Wu, D.S. Riha, M.P. Enright, G.R. Leverant, R.C. McClung, C.J. Kuhlman, G.G. Chell, and Y. Lee, “A Probabilistically-Based Damage Tolerance Analysis Computer Program for Hard Alpha Anomalies in Titanium Rotors,” Paper 2000-GT-0421, IGTI, ASME Turbo Expo, 2000 23. “Aircraft Structural Integrity and Failure Analysis of Structural Components,” Southwest Research Brochures 24. D.J. Benac and J.P. Pendley, “Integration of Design, Failure Analysis and Fleet Management to Prevent Aircraft Structural Failures,” Asian Aircraft Conference (Singapore), 1991 25. W.D. Pilkey et al., Peterson's Stress Concentration Factors, 2nd ed., John Wiley & Sons, 1997 26. A.F. Liu, High-Temperature Life Assessment, Fatigue and Fracture, Vol 19, ASM Handbook, ASM International, 1996, p 520–526 Role of the Failure Investigator The typical role of the failure investigator is that of a materials failure analyst looking at fracture surfaces or examining the microstructure for degradation, microstructural changes, or unusual crack morphologies. With the advent of life assessment methodologies the investigator's responsibilities have expanded to be an integral part of life assessment for structures. The flow diagram in Fig. 1 shows the tasks the investigator undertakes for evaluation of in-service failures and The file is downloaded from www.bzfxw.com
assessing the remaining life of aged structures. The failure analyst must be a part of the life assessment process to determine The location of origin The life-limiting failure mechanism The rate of propagation or time for degradation The actual service or operating conditions(stress, temperature, environment) The validation of life assessment model These responsibilities are discussed in the following sections Determining the Origin of the Failure. The first and foremost question the investigator must answer is where the crack initiated or where did the degradation start? In other words, where is the origin of the failure or location of degradation? A trained, patient, investigating engineer can identify the origin site through examination of the topographical and microscopic footprints. If the footprints point back to the origin, the investigator may subsequently find a dent, a pit, intergranular attack, a material defect, or a crack initiating from a corner, radius, or notch. If the origin is not identified, it may be difficult to identify the initiation event and hence the root cause. Furthermore, predicting the life of a structure must take into account the initiation event because some life assessment methodologies consider the time and mechanisn for initiation. If the initiating event is not accurately determined, then the life assessment results will not accurately assess the part or structure. For example, Fig. 8 shows a defect that was found in the radius of a gate valve stem(Ref 27). In the gate valve stem, the failure analyst determined that a preexisting forging defect was present at the radius. Because other gate valves were in service with similar gate valve stems, a question was raised by the power plant whether this and other gate valve systems could be used or if a costly replacement effort was needed. The investigator showed that no crack extension had occurred from the forging defect. Thus, with a periodic inspection of the gate valve stem, it was concluded that the risk of using the gate valves was low and immediate replacement of the stems was not necessary
assessing the remaining life of aged structures. The failure analyst must be a part of the life assessment process to determine: · The location of origin · The life-limiting failure mechanism · The rate of propagation or time for degradation · The actual service or operating conditions (stress, temperature, environment) · The validation of life assessment models These responsibilities are discussed in the following sections. Determining the Origin of the Failure. The first and foremost question the investigator must answer is where the crack initiated or where did the degradation start? In other words, where is the origin of the failure or location of degradation? A trained, patient, investigating engineer can identify the origin site through examination of the topographical and microscopic footprints. If the footprints point back to the origin, the investigator may subsequently find a dent, a pit, intergranular attack, a material defect, or a crack initiating from a corner, radius, or notch. If the origin is not identified, it may be difficult to identify the initiation event and hence the root cause. Furthermore, predicting the life of a structure must take into account the initiation event because some life assessment methodologies consider the time and mechanism for initiation. If the initiating event is not accurately determined, then the life assessment results will not accurately assess the part or structure. For example, Fig. 8 shows a defect that was found in the radius of a gate valve stem (Ref 27). In the gate valve stem, the failure analyst determined that a preexisting forging defect was present at the radius. Because other gate valves were in service with similar gate valve stems, a question was raised by the power plant whether this and other gate valve systems could be used or if a costly replacement effort was needed. The investigator showed that no crack extension had occurred from the forging defect. Thus, with a periodic inspection of the gate valve stem, it was concluded that the risk of using the gate valves was low and immediate replacement of the stems was not necessary
10m 0.5mm (b) 1 1.0mm Fig .8 Failure evaluation of a cracked gate valve stem determined that no subcritical crack growth had occurred from the forging defect. Therefore, similar gate valve stems could remain in service with a periodic inspection. (a) Cracked gate valve stem. Ix.(b) Forging defect in the radius 50x.(c) Scanning electron fractograph of the defect (1)and lab fracture(2). 15x. No subcritical crack growth had occurred from the forging defect. Thefileisdownloadedfromwww.bzfxw.com
Fig. 8 Failure evaluation of a cracked gate valve stem determined that no subcritical crack growth had occurred from the forging defect. Therefore, similar gate valve stems could remain in service with a periodic inspection. (a) Cracked gate valve stem. 1×. (b) Forging defect in the radius. 50×. (c) Scanning electron fractograph of the defect (1) and lab fracture (2). 15×. No subcritical crack growth had occurred from the forging defect. The file is downloaded from www.bzfxw.com
Often failure occurs not at one location, but at multiple locations. This complicates and compounds the life assessment approach. This applies to components such as rotating shafts, turbine blades, and pressure vessels to name a few. It also applies to complex structures such as bridges, aircraft, and buildings. For example, the effect of fatigue crack growth on the life of aircraft has been much studied in recent years. Since the accident involving Aloha Airlines Flight 243 in April 1988(Ref 13), the interest regarding multiple-site damage(MSD)has increased significantly. Additionally, much analytical and test work has been accomplished to study the effect of many small cracks and their linking together. Figure 9 is an example of multiple fatigue crack initiation that occurred in a lower wing skin because of abusive machining marks. Fractographic evaluation identified multiple crack fronts that linked together and formed one crack front ,社 Machine marks on Fracture surface … Pocket Poco! Lower wing skin Lab overload Origin oRigin 3 175in Orion Origin 3 View looking outboard Fig9 Fractographic evaluation of multiple-site fatigue damage in a lower wing skin. The evaluation determined that the damage was caused by abusive machining marks that accelerated the fatigue crack growth rate. (a) Location of cracks in the a lower wing skin pocket.(b) Machine marks on the pocket surface. (c)and(d) Fracture surface(e) Schematic of multiple origins, pop-ins, and 62 cm(2.45 in. )long total crack length Determining the Life-Limiting Mechanism. When conducting a failure investigation, it is important to identify the failure mechanism-how the component or structure failed, degraded, or malfunctioned. To perform a meaningful life assessment it is likewise important to know the failure mechanism or degradation process. The principal failure mechanisms are discussed in this Volume. Some more common failure mechanisms are brittle fracture, creep, low-or high-cycle fatigue, thermal fatigue, and corrosion. Embrittlement phenomena can occur, and metallurgical instabilities can also detrimentally affect the life of components; for example, carbide coarsening, g-phase formation, temper embrittlement, and so forth can facilitate rapid brittle fracture at low temperatures during transient conditions Combinations of these conditions can, however, cause failures if the environment modifies them As a rule, the failure mechanism is established by examination of the physical features observed in the microstructure, fracture surface, or changes in geometry. Each life-limiting failure mechanism has characteristic fingerprints that a trained failure investigator can identify and correlate to a particular failure mechanism. These physical features are then correlated to the operating conditions to determine the driver for the failure and the time frame that failure initiated and The failure analyst must be given the opportunity to investigate how something malfunctioned or failed and to determine how the usage or operating conditions affected the microstructure. The oversight of investigating the failure of a component can result in improper understanding of how something failed. What may appear as the obvious cause or potential cause for failure may not be the root cause, but rather a result. This in the long run is not only dangerous, but
Often failure occurs not at one location, but at multiple locations. This complicates and compounds the life assessment approach. This applies to components such as rotating shafts, turbine blades, and pressure vessels to name a few. It also applies to complex structures such as bridges, aircraft, and buildings. For example, the effect of fatigue crack growth on the life of aircraft has been much studied in recent years. Since the accident involving Aloha Airlines Flight 243 in April 1988 (Ref 13), the interest regarding multiple-site damage (MSD) has increased significantly. Additionally, much analytical and test work has been accomplished to study the effect of many small cracks and their linking together. Figure 9 is an example of multiple fatigue crack initiation that occurred in a lower wing skin because of abusive machining marks. Fractographic evaluation identified multiple crack fronts that linked together and formed one crack front. Fig. 9 Fractographic evaluation of multiple-site fatigue damage in a lower wing skin. The evaluation determined that the damage was caused by abusive machining marks that accelerated the fatigue crack growth rate. (a) Location of cracks in the a lower wing skin pocket. (b) Machine marks on the pocket surface. (c) and (d) Fracture surface. (e) Schematic of multiple origins, pop-ins, and 62 cm (2.45 in.) long total crack length Determining the Life-Limiting Mechanism. When conducting a failure investigation, it is important to identify the failure mechanism—how the component or structure failed, degraded, or malfunctioned. To perform a meaningful life assessment it is likewise important to know the failure mechanism or degradation process. The principal failure mechanisms are discussed in this Volume. Some more common failure mechanisms are brittle fracture, creep, low- or high-cycle fatigue, thermal fatigue, and corrosion. Embrittlement phenomena can occur, and metallurgical instabilities can also detrimentally affect the life of components; for example, carbide coarsening, σ-phase formation, temper embrittlement, and so forth can facilitate rapid brittle fracture at low temperatures during transient conditions. Combinations of these conditions can, however, cause failures if the environment modifies them. As a rule, the failure mechanism is established by examination of the physical features observed in the microstructure, fracture surface, or changes in geometry. Each life-limiting failure mechanism has characteristic fingerprints that a trained failure investigator can identify and correlate to a particular failure mechanism. These physical features are then correlated to the operating conditions to determine the driver for the failure and the time frame that failure initiated and propagated. The failure analyst must be given the opportunity to investigate how something malfunctioned or failed and to determine how the usage or operating conditions affected the microstructure. The oversight of investigating the failure of a component can result in improper understanding of how something failed. What may appear as the obvious cause or potential cause for failure may not be the root cause, but rather a result. This in the long run is not only dangerous, but
also very costly. For this reason, closer examination of broken, malfunctioned, or aged components is often needed. A very simple axiom should be followed: " Things are not al ways as they appear to be. Therefore, inaccurate determination of how something is assumed to have failed or malfunctioned will result in inaccurate life assessments Determining the Rate of Crack Growth or Time for Degradation. Once the origin of failure has been located or the life limiting mechanism has been established, another question related to life assessment is raised what was the time for the degradation phenomenon, crack initiation, or crack propagation? The failure analyst role is critical because often the investigator can establish the time for the life-limiting mechanism to occur based on fracture surface features and microstructures. Although this is not always the case, many time-dependent failure mechanisms, such as fatigue, creep and stress-corrosion cracking, demonstrate time, load, and environment fingerprints that aid the investigator in estimatin low fast a particular crack or degradation mechanism was acting. These features, such as fatigue striations, creep voids and corrosion species, can be used to date times of degradation Once the origin, life-limiting failure mechanism, and rate of crack growth or time for degradation have been determined the failure analyst may have enough information to perform a life assessment. The following case history illustrates how the fractographic fatigue features are used to determine when a crack initiated and how fast it may propagate and to predict when to inspect, replace, or continue to use a valve( Ref 27) Cracking occurred in two 25 mm(I in solenoid valve bodies reported to be 1100 kg(2500 lb), schedule 160 valves manufactured from type 316 stainless steel. The valves were in service for about ten years and were opened periodically about once a week Pressurized hot water at about 316C(600F) flowed through the open valve. The closed valve was at near-ambient temperature Circumferential cracks initiated at the bottom of the bore in which the seat was located Fig. 10a. The cracking extended about 100 to 110 around the base of the bore. Fractographic examination using a stereomicroscope (10 to 50x)and a scanning electron microscope determined the crack depth to be shallow-about a maximum depth of 1 mm(0.04 in measured along the inlet port. The fractographic evaluation determined the failure mechanism was fatigue. The transgranular cracks and striations in Fig. 10(c)are characteristic of fatigue. The fatigue striations were coarse and on the order of 5 x 10 mm/cycle(2 x 10 in /cycle). The reason for cracking was the thermal stresses that were generated when the valve was cycled from ambient to 316C (600F)and back to ambient outlet 10MM Fig. 10 Remaining life estimated on a failed solenoid valve by determining the total crack depth and rate of crack propagation. This information was correlated to the operating condition of the valve and the valve geometry, after which it was determined how much remaining life was left in the similar valve bodies.(a) Crack indicated by dye-penetrant inspection in bore of valve. Ix,(b)Crack relative to the inlet and outlet ports. 1. 25x.(c) Fatigue striation found on the fracture surface. 7000x During operation, one of the valves(valve A)was opened approximately three times per week, while the other valve (valve B)was opened approximately once per week. Thus, over the approximately 450 weeks of operation, the two valves may have experienced on the order of 1350 and 450 cycles, respectively. Based on the fracture surface striations and an average crack growth rate of 5 x 10-mm/cycle(2 x 10 in /cycle), the two cracks in valve body b, which were 0.4 and 0.3 mm(0.016 and 0.012 in )deep, would require 800 and 600 cycles, respectively, to grow to their depths. It is evident from this simple calculation that the computed growth cycles are of the same order as the estimated number of large thermal excursions experienced by the valve body. Furthermore, we can conclude that the cracks initiated very early in the service history of the valves Thefileisdownloadedfromwww.bzfxw.com
also very costly. For this reason, closer examination of broken, malfunctioned, or aged components is often needed. A very simple axiom should be followed: “Things are not always as they appear to be.” Therefore, inaccurate determination of how something is assumed to have failed or malfunctioned will result in inaccurate life assessments. Determining the Rate of Crack Growth or Time for Degradation. Once the origin of failure has been located or the lifelimiting mechanism has been established, another question related to life assessment is raised: what was the time for the degradation phenomenon, crack initiation, or crack propagation? The failure analyst role is critical because often the investigator can establish the time for the life-limiting mechanism to occur based on fracture surface features and microstructures. Although this is not always the case, many time-dependent failure mechanisms, such as fatigue, creep, and stress-corrosion cracking, demonstrate time, load, and environment fingerprints that aid the investigator in estimating how fast a particular crack or degradation mechanism was acting. These features, such as fatigue striations, creep voids, and corrosion species, can be used to date times of degradation. Once the origin, life-limiting failure mechanism, and rate of crack growth or time for degradation have been determined, the failure analyst may have enough information to perform a life assessment. The following case history illustrates how the fractographic fatigue features are used to determine when a crack initiated and how fast it may propagate and to predict when to inspect, replace, or continue to use a valve (Ref 27). Cracking occurred in two 25 mm (1 in.) solenoid valve bodies reported to be 1100 kg (2500 lb), schedule 160 valves manufactured from type 316 stainless steel. The valves were in service for about ten years and were opened periodically about once a week. Pressurized hot water at about 316 °C (600 °F) flowed through the open valve. The closed valve was at near-ambient temperature. Circumferential cracks initiated at the bottom of the bore in which the seat was located Fig. 10a. The cracking extended about 100° to 110° around the base of the bore. Fractographic examination using a stereomicroscope (10 to 50×) and a scanning electron microscope determined the crack depth to be shallow—about a maximum depth of 1 mm (0.04 in.) measured along the inlet port. The fractographic evaluation determined the failure mechanism was fatigue. The transgranular cracks and striations in Fig. 10(c) are characteristic of fatigue. The fatigue striations were coarse and on the order of 5 × 10-3 mm/cycle (2 × 10-5 in./cycle). The reason for cracking was the thermal stresses that were generated when the valve was cycled from ambient to 316 °C (600 °F) and back to ambient. Fig. 10 Remaining life estimated on a failed solenoid valve by determining the total crack depth and rate of crack propagation. This information was correlated to the operating condition of the valve and the valve geometry, after which it was determined how much remaining life was left in the similar valve bodies. (a) Crack indicated by dye-penetrant inspection in bore of valve. 1×. (b) Crack relative to the inlet and outlet ports. 1.25×. (c) Fatigue striation found on the fracture surface. 7000× During operation, one of the valves (valve A) was opened approximately three times per week, while the other valve (valve B) was opened approximately once per week. Thus, over the approximately 450 weeks of operation, the two valves may have experienced on the order of 1350 and 450 cycles, respectively. Based on the fracture surface striations and an average crack growth rate of 5 × 10-3 mm/cycle (2 × 10-5 in./cycle), the two cracks in valve body B, which were 0.4 and 0.3 mm (0.016 and 0.012 in.) deep, would require 800 and 600 cycles, respectively, to grow to their depths. It is evident from this simple calculation that the computed growth cycles are of the same order as the estimated number of large thermal excursions experienced by the valve body. Furthermore, we can conclude that the cracks initiated very early in the service history of the valves. The file is downloaded from www.bzfxw.com