Failure analysis and life assessment of structural components and Equipment Introduction LIFE ASSESSMENT of structural components is used to avoid catastrophic failures and to maintain safe and reliable functioning of equipment. The articles in the"Structural Life Assessment Methods"Section in this Volume are written to provide an overview of the prevalent life assessment methodologies for structural components. Because the failure analyst is often asked questions concerning remaining life, fitness-for-service inspection intervals, and reliability of structural components and equipment, it is necessary that the failure analyst be aware of life assessment methodologies to address the questions and concerns of the industry he or the serves. Life assessment method advances and changes in technologies for structural components and equipment will require the investigator to adapt to the need of the industry. Furthermore, the failure investigator role has expanded from providing accurate identification of life-limiting failure mechanisms and degradation phenomena to also providing the time for degradation or damage, and crack growth rate to be used in life assessment estimates. Thus, the failure investigator's input is essential for meaningful life assessment of structural components. This article provides an overview of the structural design process, the failure analysis process, the failure investigator's role, and how failure analysis of structural components integrates into determination of remaining life, fitness-for-service, and other life assessment concerns. The topics discussed in this article include Industry perspectives on failure and life assessment of components Structural design philosophies Life-limiting factors The role of the failure analyst in life assessment The role of nondestructive inspection Fatigue life assessment Elevated-temperature life assessment Fitness-for-service life assessment Probabilistic and deterministic approaches Industry Perspectives on Failure and life Assessment of Components As noted previously, life assessment of structural components is a means to avoid catastrophic failures and to maintain safe and reliable functioning of equipment. Catastrophic failures of structural components occur rather infrequently, but when they do, they take a heavy toll on human lives in addition to the cost of repairs, replacement power, and litigation costs. In 1982, the National Bureau of Standards commissioned a study to determine the direct and indirect cost of fracture in the United States. It was estimated that $120 billion are spent annually to cover direct costs and costs associated with fracture-related accidents(Ref 1). The estimates took into account the necessity of overdesigning to prevent failure, added inspections, repairs, and replacement of degraded materials. Needless to say, the costs and stakes for failure are high. In addition, the cost savings are reat if failure can be mitigated or prevented For the failure investigator, a failure is often defined as the rupture, fracture, or cracking of a structural member The industrial definition of failure is often quite different from the textbook definition. A component, in practice, is deemed to have failed when it can no longer perform its intended function safely, reliably, and economically. Any one of these criteria can constitute failure. For example, a steam turbine blade whose tip has eroded affects turbine efficiency and hence affects the economics of operation adversely. The blade should therefore be replaced even though it can continue to operate. Component failures are thus defined in terms of functional"rather than"structural failures. Replacement of parts can be based on economic considerations, reliability, and material properties. In the discipline of life assessment, equipment and structures are evaluated
Failure Analysis and Life Assessment of Structural Components and Equipment Introduction LIFE ASSESSMENT of structural components is used to avoid catastrophic failures and to maintain safe and reliable functioning of equipment. The articles in the “Structural Life Assessment Methods” Section in this Volume are written to provide an overview of the prevalent life assessment methodologies for structural components. Because the failure analyst is often asked questions concerning remaining life, fitness-for-service, inspection intervals, and reliability of structural components and equipment, it is necessary that the failure analyst be aware of life assessment methodologies to address the questions and concerns of the industry he or she serves. Life assessment method advances and changes in technologies for structural components and equipment will require the investigator to adapt to the need of the industry. Furthermore, the failure investigator role has expanded from providing accurate identification of life-limiting failure mechanisms and degradation phenomena to also providing the time for degradation or damage, and crack growth rate to be used in life assessment estimates. Thus, the failure investigator's input is essential for meaningful life assessment of structural components. This article provides an overview of the structural design process, the failure analysis process, the failure investigator's role, and how failure analysis of structural components integrates into determination of remaining life, fitness-for-service, and other life assessment concerns. The topics discussed in this article include: · Industry perspectives on failure and life assessment of components · Structural design philosophies · Life-limiting factors · The role of the failure analyst in life assessment · The role of nondestructive inspection · Fatigue life assessment · Elevated-temperature life assessment · Fitness-for-service life assessment · Probabilistic and deterministic approaches Industry Perspectives on Failure and Life Assessment of Components As noted previously, life assessment of structural components is a means to avoid catastrophic failures and to maintain safe and reliable functioning of equipment. Catastrophic failures of structural components occur rather infrequently, but when they do, they take a heavy toll on human lives in addition to the cost of repairs, replacement power, and litigation costs. In 1982, the National Bureau of Standards commissioned a study to determine the direct and indirect cost of fracture in the United States. It was estimated that $120 billion are spent annually to cover direct costs and costs associated with fracture-related accidents (Ref 1). The estimates took into account the necessity of overdesigning to prevent failure, added inspections, repairs, and replacement of degraded materials. Needless to say, the costs and stakes for failure are high. In addition, the cost savings are great if failure can be mitigated or prevented. For the failure investigator, a failure is often defined as the rupture, fracture, or cracking of a structural member. The industrial definition of failure is often quite different from the textbook definition. A component, in practice, is deemed to have failed when it can no longer perform its intended function safely, reliably, and economically. Any one of these criteria can constitute failure. For example, a steam turbine blade whose tip has eroded affects turbine efficiency and hence affects the economics of operation adversely. The blade should therefore be replaced even though it can continue to operate. Component failures are thus defined in terms of “functional” rather than “structural” failures. Replacement of parts can be based on economic considerations, reliability, and material properties. In the discipline of life assessment, equipment and structures are evaluated
to determine if they are suitable, reliable, and economical for continual service. If deemed unreliable, the equipment or structure may be repaired, refurbished, or replaced any component the failure criteria need to be defined and established. Failure does not always involve acture or rupture. Progressive damage of structure and components under operating conditions leads to exhaustion of life, thus leading to failure. Damage may be defined as a"progressive and cumulative change acting to degrade the structural performance of the load-bearing component or components which make up the plant"(Ref 2). Life may be defined as the"period during which a component can perform its intended function safely, reliably, and economically"(Ref 3). With some modifications, the definitions used by Viswanathan and Dooley for fossil-fuel power steam plant components(Ref 3)can be used to define failure and life of components; that is, component life is expended when Design life has elapsed Calculations predict life exhaustion Service time has reached some arbitrarily chosen fraction of calculated or experimental failure life Previous failure statistics indicate high probability of failure Frequency of repair renders continued operation uneconomical Nondestructive inspection reveals cracking Surface degradation from corrosion, including coating degradation, is excessive Grain-boundary attack and/or pitting by oxidation/hot corrosion, is excessive Foreign object damage Destructive sampling and testing indicate life exhaustion Excessive deformation has occurred due to creep, causing distortion and unfavorable changes in Sudden and complete fracture occurs References cited in this section 1. J.J. Duga et al, The Economic Effects of fracture in the United States, Report to the National Bureau of Standards. Battelle Columbus Laboratories. March 1983 2. L.F. Coffin, Damage Evaluation and Life Prediction for High-Temperature Gas Turbine Materials 2382-3, Electric Power Research Institute, April 1986,p 1. 1-1/e Moterials,EPRI AP-4477,Project Proc. Conf on Life Prediction for High Temperature Gas Turbin 3. R. Viswanathan andR. B Dooley, Creep Life Assessment Techniques for Fossil Power Plant Boiler Pressure Parts, Proc. Conf on Life Prediction for High Temperature Gas Turbine Materials, EPRI AP 4477, Project 2382-3, Electric Power Research Institute, April 1986, p 2. 1-2.28 Structural Design Philosophies Historic Failures. It is often stated that history repeats itself. Yet, when it comes to structural components and equipment, structural designers, original equipment manufacturers (OEMs), and users do not want a repeat of history. The consequences and costs of fractured, cracked, corroded, and malfunctioned equipment are unwanted. Through the years, history has demonstrated that failures occur; history has also shown that the engineering communities have responded to prevent failure from occurring again. Table 1(Ref 4, 5, 6,7,8,9,10, 11, 12, 13, 14, 15)identifies some of the historic structural failures that have occurred in the 20th century. These historic failures as well as other failures have revolutionized design philosophies, inspection techniques and practices, material development, and material processing and controls and have redefined the criteria for failure. Furthermore, the pursuit of understanding how and why these ailures occurred have resulted in the development of structural-integrity programs, enhanced analytical modeling and prediction techniques, accurate life assessment methods, and a fortified commitment to avoid the recurrence of these failures through improved designs. The examples cited in Table I were serious and often tragic failures that had a great impact on structural designs and life assessment developments. However, not all failures or malfunctions of equipment is as pivotal in history as those mentioned in Table 1. Yet, it is emphasized that any failure, no matter how seemingly asignificant, should be investigated and the findings used to improve the design and increase the life and reliability of at component or equipment Thefileisdownloadedfromwww.bzfxw.com
to determine if they are suitable, reliable, and economical for continual service. If deemed unreliable, the equipment or structure may be repaired, refurbished, or replaced. In any component the failure criteria need to be defined and established. Failure does not always involve fracture or rupture. Progressive damage of structure and components under operating conditions leads to exhaustion of life, thus leading to failure. Damage may be defined as a “progressive and cumulative change acting to degrade the structural performance of the load-bearing component or components which make up the plant” (Ref 2). Life may be defined as the “period during which a component can perform its intended function safely, reliably, and economically” (Ref 3). With some modifications, the definitions used by Viswanathan and Dooley for fossil-fuel power steam plant components (Ref 3) can be used to define failure and life of components; that is, component life is expended when: · Design life has elapsed. · Calculations predict life exhaustion. · Service time has reached some arbitrarily chosen fraction of calculated or experimental failure life. · Previous failure statistics indicate high probability of failure. · Frequency of repair renders continued operation uneconomical. · Nondestructive inspection reveals cracking. · Surface degradation from corrosion, including coating degradation, is excessive. · Grain-boundary attack and/or pitting by oxidation/hot corrosion, is excessive. · Foreign object damage is severe. · Destructive sampling and testing indicate life exhaustion. · Excessive deformation has occurred due to creep, causing distortion and unfavorable changes in clearances. · Sudden and complete fracture occurs. References cited in this section 1. J.J. Duga et al., The Economic Effects of Fracture in the United States, Report to the National Bureau of Standards, Battelle Columbus Laboratories, March 1983 2. L.F. Coffin, Damage Evaluation and Life Prediction for High-Temperature Gas Turbine Materials, Proc. Conf. on Life Prediction for High Temperature Gas Turbine Materials, EPRI AP-4477, Project 2382-3, Electric Power Research Institute, April 1986, p 1.1–1.17 3. R. Viswanathan and R.B. Dooley, Creep Life Assessment Techniques for Fossil Power Plant Boiler Pressure Parts, Proc. Conf. on Life Prediction for High Temperature Gas Turbine Materials, EPRI AP- 4477, Project 2382-3, Electric Power Research Institute, April 1986, p 2.1–2.28 Structural Design Philosophies Historic Failures. It is often stated that history repeats itself. Yet, when it comes to structural components and equipment, structural designers, original equipment manufacturers (OEMs), and users do not want a repeat of history. The consequences and costs of fractured, cracked, corroded, and malfunctioned equipment are unwanted. Through the years, history has demonstrated that failures occur; history has also shown that the engineering communities have responded to prevent failure from occurring again. Table 1 (Ref 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15) identifies some of the historic structural failures that have occurred in the 20th century. These historic failures as well as other failures have revolutionized design philosophies, inspection techniques and practices, material development, and material processing and controls and have redefined the criteria for failure. Furthermore, the pursuit of understanding how and why these failures occurred have resulted in the development of structural-integrity programs, enhanced analytical modeling and prediction techniques, accurate life assessment methods, and a fortified commitment to avoid the recurrence of these failures through improved designs. The examples cited in Table 1 were serious and often tragic failures that had a great impact on structural designs and life assessment developments. However, not all failures or malfunctions of equipment is as pivotal in history as those mentioned in Table 1. Yet, it is emphasized that any failure, no matter how seemingly insignificant, should be investigated and the findings used to improve the design and increase the life and reliability of that component or equipment. The file is downloaded from www.bzfxw.com
Table 1 Historic failures and their impact on life assessment concerns Failure I Year Reason for failu Life assessment developments Titanic(Ref 4) 1912 Ship hits iceberg and watertight Improvement in steel grades compartments rupture Safety procedures established for lifeboats Warning systems established for Molasses Tank Failures 1919, Brittle fracture of the tank as a result of poor Design codes for storage tanks (Ref 5) 1973 ductility and higher loads Consideration give to causes for Tacoma Bridge Failure 1940 Aerodynamic instability and failure caused Sophisticated analytical models (Ref 6, 7 by wind vortices and bridge desi developed for resonance Bridge design changed to account for World War II Liberty 1942- 1289 of the 4694 warships suffered brittle Selection of increased toughness ships (ref 8) 1952 fracture or structure failure at the welded mater Improved fabrication practices Development of fracture mechanics Liquefied natural gas 1944 Failure and explosion of an LNG pressure Selection and development of (LNG) storage tank vessel due to a possible welding defect and materials with improved toughness at (Ref 9) improperly heat treated material resulting in the service temperature of-160C( obsequent fatigue crack growth 250°F) Comet aircraft failures 1950s Fatigue crack initiation in pressurized skins Development of the fatigue"safe-life" (Ref 10) due to high gross stresses and stress concentration effects from geometric features Evaluation of the effects of geometry Evaluation of the effects of stiffeners on stress distribution Establishment of aircraft structural gram(ASIP)in 1958 F-111 Aircraft No 94 1969 Fatigue failure due to material defect in Improved inspection techniques wing pivot fitting(Ref high-strength steel l1) damage-tolerant design philosophy relopment of materials with Seam-welded high- 1986- Cavitation and creep voids in welds Development of elevated-temperature energy piping failures 2000 resulting in catastrophic high-energy rupture life assessment techniques for (Ref 12 cavitation and creep failure Aloha Incident, Boeing 1988 Accelerated corrosion and multiple fatigue Improved aircraft maintenance and 737(Ref13) crack-initiation sites in riveted fuselage skin inspection procedures Life assessment methods developed Sioux City Incident 1989 Hard alpha case present in titanium fan disk Increased process controls on (Ref 14) resulted in fatigue crack initiation and processing of titanium ingots catastrophic failure
Table 1 Historic failures and their impact on life assessment concerns Failure Year Reason for failure Life assessment developments Titanic (Ref 4) 1912 Ship hits iceberg and watertight compartments rupture. Improvement in steel grades Safety procedures established for lifeboats Warning systems established for icebergs Molasses Tank Failures (Ref 5) 1919, 1973 Brittle fracture of the tank as a result of poor ductility and higher loads Design codes for storage tanks developed Consideration given to causes for brittle fracture Tacoma Bridge Failure (Ref 6, 7 1940 Aerodynamic instability and failure caused by wind vortices and bridge design Sophisticated analytical models developed for resonance Bridge design changed to account for aerodynamic conditions World War II Liberty ships (Ref 8) 1942– 1952 1289 of the 4694 warships suffered brittle fracture or structure failure at the welded steel joints. Selection of increased toughness material Improved fabrication practices Development of fracture mechanics Liquefied natural gas (LNG) storage tank (Ref 9) 1944 Failure and explosion of an LNG pressure vessel due to a possible welding defect and improperly heat treated material resulting in subsequent fatigue crack growth Selection and development of materials with improved toughness at the service temperature of -160 °C (- 250 °F) Comet aircraft failures (Ref 10) 1950s Fatigue crack initiation in pressurized skins due to high gross stresses and stress concentration effects from geometric features Development of the fatigue “safe-life” approach Evaluation of the effects of geometry and notches on fatigue behavior Evaluation of the effects of stiffeners on stress distribution Establishment of aircraft structural integrity program (ASIP) in 1958 F-111 Aircraft No. 94 wing pivot fitting (Ref 11) 1969 Fatigue failure due to material defect in high-strength steel Improved inspection techniques Change from fatigue “safe-life” to damage-tolerant design philosophy Development of materials with improved toughness Seam-welded highenergy piping failures (Ref 12) 1986– 2000 Cavitation and creep voids in welds resulting in catastrophic high-energy rupture Development of elevated-temperature life assessment techniques for cavitation and creep failure Aloha Incident, Boeing 737 (Ref 13) 1988 Accelerated corrosion and multiple fatigue crack-initiation sites in riveted fuselage skin Improved aircraft maintenance and inspection procedures Life assessment methods developed for multiple-site damage (MSD) Sioux City Incident (Ref 14) 1989 Hard alpha case present in titanium fan disk resulted in fatigue crack initiation and catastrophic failure. Increased process controls on processing of titanium ingots
Failure Year Reason for failure Life assessment developments Development of probabilistic design and assessment using dedicated computer rograms for titanium disks Earthquakes in Kobe 1994, Failure occurred in I-beams and columns Development of earthquake resistant City, Japan, and 1995 due to joint configuration and welding structures Northridge. California practices that resulted in low ductility of the (Ref 15) Improved joint designs and weldi practices for structural steels ding Improved controls on steel Overview of the Design Process. Because of failures similar to those in Table 1, predicting performance and assessing the remaining life with greater confidence becomes increasingly important as costs for manufacturers and operators need to be reduced. Furthermore, the cost of failure is progressively greater as systems become more complex, downtime costs increase, and liability for failure increases. a brief discussion follows on the design process because it is important for failure investigators and life assessment engineers to understand some of the design issues. Each structure has unique design requirements, but all structures are designed using some basic design principles. Figure 1 illustrates the relationship among the design phase, testing, systematic failure analysis, and life assessment of components Thefileisdownloadedfromwww.bzfxw.com
Failure Year Reason for failure Life assessment developments Development of probabilistic design approach and analytical life assessment using dedicated computer programs for titanium disks Earthquakes in Kobe City, Japan, and Northridge, California (Ref 15) 1994, 1995 Failure occurred in I-beams and columns due to joint configuration and welding practices that resulted in low ductility of the steel. Development of earthquake resistant structures Improved joint designs and welding practices for structural steels Improved controls on steel manufacture Overview of the Design Process. Because of failures similar to those in Table 1, predicting performance and assessing the remaining life with greater confidence becomes increasingly important as costs for manufacturers and operators need to be reduced. Furthermore, the cost of failure is progressively greater as systems become more complex, downtime costs increase, and liability for failure increases. A brief discussion follows on the design process because it is important for failure investigators and life assessment engineers to understand some of the design issues. Each structure has unique design requirements, but all structures are designed using some basic design principles. Figure 1 illustrates the relationship among the design phase, testing, systematic failure analysis, and life assessment of components. The file is downloaded from www.bzfxw.com
Materials and full-scale testing Design criteria Design analysis Material selection and life predictions Manufacturing and fabrication Structural component in-service In-service failure Structural aging or malfunction fit service concern Nondestructive Field perating inspections examinations conditions Mechanical Microscopic Fracture Laboratory Deposit and and physical examinations aminations surface chemical property test analysIs analysis Materials characterized Failure mechanism identified Environmental factors established ComponentFracture Engineering StructuralIOperations testing mechanics analyses analysis evaluation Root cause determined Fit for service Not fit for service Corrective Identify next Repair or Remove Replace with nspection interval I restrict service from service new structure ig. 1 Flow diagram showing the relationship between the design phase and the investigative tasks for in-service failure structural aging and fitness-for-service of structural components One alternative for avoiding failures used in the past was to overdesign and to operate overconservatively. The economic nalties for both are increasingly significant; however, the economic penalties for failures are significant as well. It is
Fig. 1 Flow diagram showing the relationship between the design phase and the investigative tasks for in-service failure, structural aging, and fitness-for-service of structural components One alternative for avoiding failures used in the past was to overdesign and to operate overconservatively. The economic penalties for both are increasingly significant; however, the economic penalties for failures are significant as well. It is