《材料失效分析 Materials Failure Analysis》课程教学资源（参考书籍）Failure Analysis and Life Assessment of Structural Components and Equipment.pdf_P21-P25

It is also possible to use the fracture data approach to estimate the remaining life of the valve bodies. First, however, the end of the useful life of the valve body must be defined. a breach between the bore and the outer-pressure boundary would cause the valve to leak to the external environment and certainly end useful life. Approximately 22.2 mm(0.875 in )of additional crack growth would be required for the cracks to reach the outer surface of the body and create such a leak. a breach across the seat producing a leak path between the inlet and outlet ports, although less catastrophic than a leak to the outer surface. could also be considered to end the useful life of the valves The shortest distance between the bore and the outlet port in these valve bodies was 8.0 mm(0.313 in. ) Although, no cracking was present directly over the outlet port, if the conservative assumption is made that a crack initiates in this location on the very next cycle, 15, 560 cycles would be required at a growth rate of 5 x 10 mm/cycle (2 x 10 in /cycle) before the crack reached the outlet port. Thus, a conservative estimate of the minimum remaining life for these valve bodies would be 15,000 cycles to produce a leak across the seat from the inlet port to the outlet port. Growth of the existing cracks through to the outer surface would require almost three times as many cycles, that is, 45,000 cycles. Thus, it was concluded that significant life remains in these valve bodies although cracks were detected during the inspection process Determining the Operation History. The case history on the valve bodies demonstrates how the failure investigator can correlate physical features to the operational history. This is important because the component may often be operating in a different manner than the intended mode of operation. Thus, the investigator, using his investigative tools and examining the component, can sometimes determine, for example, that a component such as a turbine was operating much hotter than it was intended, or the stress condition on an aircraft structural member was higher than the design stress. If a life assessment were conducted using the expected operating temperature or intended design stress and not the actual operating temperature or stress, then the life assessment would inaccurately determine the remaining life. Therefore, the investigator needs to make a determination of how the physical features correlate to the intended design usage and the design usage to the actual operating conditions Validation of Life assessment Models. Another role the investigator serves is for validation of life assessment and methodologies models. Life assessment model validation often occurs using material coupon tests or full-scale testing of components. These tests control essential parameters of stress, environment, and cyclic conditions. These same parameters are input into a life assessment model, which estimates a component life. The validity of the model can be and at the end of the test Figure 1l shows the comparison between predicted crack growth and measured crack growth als demonstrated by making macroscopic and microscopic measurements of the crack lengths at predetermined time interva

It is also possible to use the fracture data approach to estimate the remaining life of the valve bodies. First, however, the end of the useful life of the valve body must be defined. A breach between the bore and the outer-pressure boundary would cause the valve to leak to the external environment and certainly end useful life. Approximately 22.2 mm (0.875 in.) of additional crack growth would be required for the cracks to reach the outer surface of the body and create such a leak. A breach across the seat producing a leak path between the inlet and outlet ports, although less catastrophic than a leak to the outer surface, could also be considered to end the useful life of the valves. The shortest distance between the bore and the outlet port in these valve bodies was 8.0 mm (0.313 in.). Although, no cracking was present directly over the outlet port, if the conservative assumption is made that a crack initiates in this location on the very next cycle, 15,560 cycles would be required at a growth rate of 5 × 10-3 mm/cycle (2 × 10-5 in./cycle) before the crack reached the outlet port. Thus, a conservative estimate of the minimum remaining life for these valve bodies would be 15,000 cycles to produce a leak across the seat from the inlet port to the outlet port. Growth of the existing cracks through to the outer surface would require almost three times as many cycles, that is, 45,000 cycles. Thus, it was concluded that significant life remains in these valve bodies although cracks were detected during the inspection process. Determining the Operation History. The case history on the valve bodies demonstrates how the failure investigator can correlate physical features to the operational history. This is important because the component may often be operating in a different manner than the intended mode of operation. Thus, the investigator, using his investigative tools and examining the component, can sometimes determine, for example, that a component such as a turbine was operating much hotter than it was intended, or the stress condition on an aircraft structural member was higher than the design stress. If a life assessment were conducted using the expected operating temperature or intended design stress and not the actual operating temperature or stress, then the life assessment would inaccurately determine the remaining life. Therefore, the investigator needs to make a determination of how the physical features correlate to the intended design usage and the design usage to the actual operating conditions. Validation of Life Assessment Models. Another role the investigator serves is for validation of life assessment and methodologies models. Life assessment model validation often occurs using material coupon tests or full-scale testing of components. These tests control essential parameters of stress, environment, and cyclic conditions. These same parameters are input into a life assessment model, which estimates a component life. The validity of the model can be demonstrated by making macroscopic and microscopic measurements of the crack lengths at predetermined time intervals and at the end of the test. Figure 11 shows the comparison between predicted crack growth and measured crack growth

increasing performance demands imposed by today's highly competitive economic climate, it is often necessary to extend the service life or expectancy. Thus, it is critical to understand how safe and reliable that equipment or structure might be over this additional time of service. Nondestructive evaluation is used throughout the life cycle and includes the nspection of new, aging, and in-service structures and equipment Many measurement techniques are employed in NDE. Those most widely used are visual, liquid penetrant, magnetic particle, eddy-current, ultrasonic, and radiographic testing. These techniques are discussed in detail in the article Nondestructive Evaluation and Life Assessment " in this Volume and in Nondestructive Evaluation and Quality Control, Vol 17 of the ASm Handbook. Nondestructive evaluation plays a large role in damage-tolerant approach employed by the aerospace and structures communities. It is assumed that the service-induced damage in a part will increase in time. For the case of fatigue measure of damage is crack size. However, the same concepts would apply to high-temperature creep, corrosion, wear,or some other degradation mechanism. In the damage-tolerant approach, the use of inspection allows a reduction of this conservatism of the safe-life approach. Figure 12 is an illustration of the damage-tolerant approach to life assessment. It is recognized that flaws will be present, and nde techniques are expected to remove those components from service that contain a flaw whose greater than the inspectable flaw. The expected life of the remaining components will now be eater. As indicated, the process could be repeated additional times, extending the life of remaining components F1 Time Fig. 12 Damage-tolerance approach to life assessment. Curves indicate progression of damage from different nucleating conditions, with broken segments representing regimes in which a perfect nDe technique would remove the component from service In addition to damage-tolerance requirements, nondestructive inspection of plant equipment, such as pressure vessels, tanks, and piping, is performed to assess fitness for service when exposed to corrosive media found in refineries, paper mills, and chemical plants Specific life assessment methodologies Fatigue/Damage-Tolerance Life Assessment. The lessons learned from the historic aircraft failures, such as the Comet, F 11 and the Aloha incident, have created the need to understand how to evaluate the life of structures subjected to fatigue loading. Life assessment of aircraft and also power-plant equipment subjected to fatigue loading largely stems from the for maintaining the structural safety of commercial transport, military aircraft, structures, and pressure vessels. ophy used development of the damage-tolerance philosophy based on fracture mechanics. Damage tolerance is the philosophy used The individuals tasked with the determination of the component life depend on the knowledge and results of the many who can supply the necessary information. Life assessment of fatigue-loaded structures requires considering some of the following issues Identification of fatigue critical structures Mission or profile of fatigue loading history Part geometries Damage-tolerant materials behavior Determination of inspection intervals Linear elastic and plastic fracture mechanics considerations Constant and variable loading on a structure

increasing performance demands imposed by today's highly competitive economic climate, it is often necessary to extend the service life or expectancy. Thus, it is critical to understand how safe and reliable that equipment or structure might be over this additional time of service. Nondestructive evaluation is used throughout the life cycle and includes the inspection of new, aging, and in-service structures and equipment. Many measurement techniques are employed in NDE. Those most widely used are visual, liquid penetrant, magnetic particle, eddy-current, ultrasonic, and radiographic testing. These techniques are discussed in detail in the article “Nondestructive Evaluation and Life Assessment” in this Volume and in Nondestructive Evaluation and Quality Control, Vol 17 of the ASM Handbook. Nondestructive evaluation plays a large role in damage-tolerant approach employed by the aerospace and structures communities. It is assumed that the service-induced damage in a part will increase in time. For the case of fatigue, the measure of damage is crack size. However, the same concepts would apply to high-temperature creep, corrosion, wear, or some other degradation mechanism. In the damage-tolerant approach, the use of inspection allows a reduction of this conservatism of the safe-life approach. Figure 12 is an illustration of the damage-tolerant approach to life assessment. It is recognized that flaws will be present, and NDE techniques are expected to remove those components from service that contain a flaw whose size is greater than the inspectable flaw. The expected life of the remaining components will now be greater. As indicated, the process could be repeated additional times, extending the life of remaining components. Fig. 12 Damage-tolerance approach to life assessment. Curves indicate progression of damage from different nucleating conditions, with broken segments representing regimes in which a perfect NDE technique would remove the component from service. In addition to damage-tolerance requirements, nondestructive inspection of plant equipment, such as pressure vessels, tanks, and piping, is performed to assess fitness for service when exposed to corrosive media found in refineries, paper mills, and chemical plants. Specific Life Assessment Methodologies Fatigue/Damage-Tolerance Life Assessment. The lessons learned from the historic aircraft failures, such as the Comet, F- 111 and the Aloha incident, have created the need to understand how to evaluate the life of structures subjected to fatigue loading. Life assessment of aircraft and also power-plant equipment subjected to fatigue loading largely stems 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 individuals tasked with the determination of the component life depend on the knowledge and results of the many who can supply the necessary information. Life assessment of fatigue-loaded structures requires considering some of the following issues: · Identification of fatigue critical structures · Mission or profile of fatigue loading history · Part geometries · Damage-tolerant materials behavior · Determination of inspection intervals · Linear elastic and plastic fracture mechanics considerations · Constant and variable loading on a structure

The retardation effects of fatigue crack growth rate Corrosion effects for crack initiation and propagation Effects of stress concentration and notches The size, shape, and number of fatigue crack fronts Validation of prediction models using test data and field failures For more detailed discussions on these fatigue-related topics, see the article"Fatigue Life Assessment "in this Volume and Ref 2 8 The failure investigators role in the fatigue life assessment is so critical because he or she often is the one who can provide information on whether multiple cracks occurred, how fast the crack was growing, and was any environment present to cause cracking or even reduce the crack rate due to corrosion buildup in the crack. Using investigative tools the investigator can often identify the crack length and depth, which is used in estimating g factors The following case history demonstrates how the investigator contributes to the fatigue life assessment process by correlating the load profile to the rate of crack propagation(Ref 29 ). During testing, a thrust reverser aluminum fitting with a swaged bearing cracked, Fig. 13. The thrust reverser typically experiences a maximum load condition when deployed and a ground idle condition. Fractographic evaluation determined that fatigue crack initiation started in the bore of the hole. Further evaluation, using the scanning electron microscope, identified a fatigue pattern of narrow and wide striation spacing Fig. 13d that correlated to the load profile(Fig. 13e). Fatigue-striation measurements were made to estimate the total number of fatigue cycles for crack extension. When the cycles for crack extension were correlated to the total number of test cycles and predicted life cycle, it was concluded that an atypical stress condition was present that caused premature crack initiation and propagation Origin (b) Maximum reverse trust Take-off Ground 40660780900 (e) Fig 13 Correlation of fatigue-striation patterns with the load history to estimate when a crack started in relation to the actual or predicted life of a part (a) Cracked fitting.(b) and (c) Crack origin location.(d)Two distinct fatigue-striation patterns that were correlated to the load profile shown in(e) Elevated -Temperature Life Assessment. Besides fatigue loading, elevated-temperature exposure is one of the leading causes of reduced life of structure components. Gas turbine blades, steam lines, heater, boiler, and superheater tubes are subjected to elevated temperatures that can cause degradation, deformation, bulging, cracking, or bursting. Thus, it is possible for the life expectancy to be reduced. For example, combustion turbine hot section rotating blades are fabricated from nickel-base superalloys and operate at high temperatures and aggressive environments. Both metallurgical and mechanical property degradation of the blade material occurs during service, which can limit the useful service life of blades. It is important to assess the condition of blades periodically and remaining life estimated to provide guidelines for replacement or reconditioning of the blades. Similarly, steel piping and tubing subjected to elevated temperature and stress suffer a reduction in life because of unplanned temperature excursions or higher operating stresses Thefileisdownloadedfromwww.bzfxw.com

· The retardation effects of fatigue crack growth rate · Corrosion effects for crack initiation and propagation · Effects of stress concentration and notches · The size, shape, and number of fatigue crack fronts · Validation of prediction models using test data and field failures For more detailed discussions on these fatigue-related topics, see the article “Fatigue Life Assessment” in this Volume and Ref 28. The failure investigator's role in the fatigue life assessment is so critical because he or she often is the one who can provide information on whether multiple cracks occurred, how fast the crack was growing, and was any environment present to cause cracking or even reduce the crack rate due to corrosion buildup in the crack. Using investigative tools, the investigator can often identify the crack length and depth, which is used in estimating β factors. The following case history demonstrates how the investigator contributes to the fatigue life assessment process by correlating the load profile to the rate of crack propagation (Ref 29). During testing, a thrust reverser aluminum fitting with a swaged bearing cracked, Fig. 13. The thrust reverser typically experiences a maximum load condition when deployed and a ground idle condition. Fractographic evaluation determined that fatigue crack initiation started in the bore of the hole. Further evaluation, using the scanning electron microscope, identified a fatigue pattern of narrow and wide striation spacing Fig. 13d that correlated to the load profile (Fig. 13e). Fatigue-striation measurements were made to estimate the total number of fatigue cycles for crack extension. When the cycles for crack extension were correlated to the total number of test cycles and predicted life cycle, it was concluded that an atypical stress condition was present that caused premature crack initiation and propagation. Fig. 13 Correlation of fatigue-striation patterns with the load history to estimate when a crack started in relation to the actual or predicted life of a part. (a) Cracked fitting. (b) and (c) Crack origin location. (d) Two distinct fatigue-striation patterns that were correlated to the load profile shown in (e) Elevated-Temperature Life Assessment. Besides fatigue loading, elevated-temperature exposure is one of the leading causes of reduced life of structure components. Gas turbine blades, steam lines, heater, boiler, and superheater tubes are subjected to elevated temperatures that can cause degradation, deformation, bulging, cracking, or bursting. Thus, it is possible for the life expectancy to be reduced. For example, combustion turbine hot section rotating blades are fabricated from nickel-base superalloys and operate at high temperatures and aggressive environments. Both metallurgical and mechanical property degradation of the blade material occurs during service, which can limit the useful service life of blades. It is important to assess the condition of blades periodically and remaining life estimated to provide guidelines for replacement or reconditioning of the blades. Similarly, steel piping and tubing subjected to elevated temperature and stress suffer a reduction in life because of unplanned temperature excursions or higher operating stresses. The file is downloaded from www.bzfxw.com

Therefore, it is often necessary to conduct blade, piping and tube life assessments. The advancements in elevated- temperature life assessment have made it possible stimate remaining life in critical components used for power generation. Traditionally, microstructure-based and hardness-based techniques have been used to assess the condition of blades and tubes. In the past, they have been used mainly to estimate the "equivalent temperatures"of the tubes, which are then utilized in conjunction with standard rupture data to calculate the fractional creep life consumed. In recent years, analytical models correlating physical features such as cavities and scale are used to predict life. Relationships have also been established between the degree of creep cavitation, as measured in replicas, and the creep life fraction consumed Furthermore, verification of the equivalent temperature in piping and tubing are based on scale thickness. Typical life assessment techniques for elevated-temperature exposure include Life-fraction rule Parameter-based assessments Thermal mechanical fatigue Coating evaluations Hardness testing Microstructural evaluations Creep cavitation damage assessment Oxide-scale-based life prediction High-temperature crack growth methods Greater detail of the elevated-temperature life assessment methods is discussed in the article"Elevated -Temperature Life Assessment for Turbine Components, Piping, and Tubing" in this Volume and in Ref 26 The role of the investigator is vital in determining the life-limiting elevated-temperature failure mechanisms such as creep, thermal fatigue, or embrittling phenomenon such as carbide coarsening. For example, in refineries and power plants, superheaters and heater tubes require remaining life assessments. Examination of the microstructure and correlating that to operating temperature and stress-rupture data, the life assessment engineer can estimate the remaining life on a component Elevated-temperature life assessment involves performing in situ replication of the microstructure, scale analysis, and sectioning of tube to perform stress-rupture testing. This remaining life analysis allows possible life extension of superheater or heater tubes past the previously considered retirement age. Accelerated stress-rupture tests allows for calculation of the actual operating stress and temperature. In addition, the maximum heater operating temperature can be calculated to obtain a safe run life until the desired tube replacement at a planned turnaround Figure 14 shows a stress rupture curve used for evaluation of turbine blades and heater tubes. Often the life -fraction rule is used to determine the remaining life(see the article"Elevated-Temperature Life Assessment for Turbine Components, Piping, and Tubing" in this Volume) Dove this stress Average rupture strength ess rupure tost datn Minimum rupture strength

Therefore, it is often necessary to conduct blade, piping and tube life assessments. The advancements in elevatedtemperature life assessment have made it possible to estimate remaining life in critical components used for power generation. Traditionally, microstructure-based and hardness-based techniques have been used to assess the condition of blades and tubes. In the past, they have been used mainly to estimate the “equivalent temperatures” of the tubes, which are then utilized in conjunction with standard rupture data to calculate the fractional creep life consumed. In recent years, analytical models correlating physical features such as cavities and scale are used to predict life. Relationships have also been established between the degree of creep cavitation, as measured in replicas, and the creep life fraction consumed. Furthermore, verification of the equivalent temperature in piping and tubing are based on scale thickness. Typical life assessment techniques for elevated-temperature exposure include: · Life-fraction rule · Parameter-based assessments · Thermal mechanical fatigue · Coating evaluations · Hardness testing · Microstructural evaluations · Creep cavitation damage assessment · Oxide-scale-based life prediction · High-temperature crack growth methods Greater detail of the elevated-temperature life assessment methods is discussed in the article “Elevated-Temperature Life Assessment for Turbine Components, Piping, and Tubing” in this Volume and in Ref 26. The role of the investigator is vital in determining the life-limiting elevated-temperature failure mechanisms such as creep, thermal fatigue, or embrittling phenomenon such as carbide coarsening. For example, in refineries and power plants, superheaters and heater tubes require remaining life assessments. Examination of the microstructure and correlating that to operating temperature and stress-rupture data, the life assessment engineer can estimate the remaining life on a component. Elevated-temperature life assessment involves performing in situ replication of the microstructure, scale analysis, and sectioning of tube to perform stress-rupture testing. This remaining life analysis allows possible life extension of superheater or heater tubes past the previously considered retirement age. Accelerated stress-rupture tests allows for calculation of the actual operating stress and temperature. In addition, the maximum heater operating temperature can be calculated to obtain a safe run life until the desired tube replacement at a planned turnaround. Figure 14 shows a stressrupture curve used for evaluation of turbine blades and heater tubes. Often the life-fraction rule is used to determine the remaining life (see the article “Elevated-Temperature Life Assessment for Turbine Components, Piping, and Tubing” in this Volume)