Fig. 14 Replication and microstructural evaluation combined with stress-rupture testing as performed on turbine blades and heater tubes to estimate the remaining life (a) Land based turbine()Heater tubes(c)Typical stress-rupture curve for 9Cr-1Mo material showing that the stress-rupture test results were still above the minimum rupture strength. Thus, the tubes were considered usable until the next inspection and testing Another type of elevated-temperature life assessment is evaluating structures and components that have been heat damaged by fire, lack of lubricant, or excessive temperature exposure. This often requires hardness testing microstructural evaluations, or heat-damage discoloration assessment. API 579(American Petroleum Institute) provides some guidance on evaluating structures exposed to fire damage(Ref 30) During operation of rotating equipment that requires lubrication to reduce heating and the coefficient of friction, overheating can occur if the lubricant is lost or degraded. Therefore, a life assessment and reliability question is raised whether the equipment is fit for service. As an example, a rotating journal in a power-generator plant had lost lubricating oil, resulting in the overheating of the journal ( Fig. 15). The loss of lubricant caused the temperatures to be hot enough to heat the journal above the critical transformation temperature. Consequently, a very brittle hard layer of martensite formed. Hardness testing showed that the surface could be ground to remove the martensite layer and still be above the minimum diameter of the journal, but hardness testing also showed a slight drop in hardness below the minimum diameter, indicating tempered material. A torsional fatigue analysis that considered load events and reduction in strer was performed and showed that the as-ground journal was acceptable and fit for service Thefileisdownloadedfromwww.bzfxw.com
Fig. 14 Replication and microstructural evaluation combined with stress-rupture testing as performed on turbine blades and heater tubes to estimate the remaining life. (a) Landbased turbine. (b) Heater tubes. (c) Typical stress-rupture curve for 9Cr-1Mo material showing that the stress-rupture test results were still above the minimum rupture strength. Thus, the tubes were considered usable until the next inspection and testing. Another type of elevated-temperature life assessment is evaluating structures and components that have been heat damaged by fire, lack of lubricant, or excessive temperature exposure. This often requires hardness testing, microstructural evaluations, or heat-damage discoloration assessment. API 579 (American Petroleum Institute) provides some guidance on evaluating structures exposed to fire damage (Ref 30). During operation of rotating equipment that requires lubrication to reduce heating and the coefficient of friction, overheating can occur if the lubricant is lost or degraded. Therefore, a life assessment and reliability question is raised whether the equipment is fit for service. As an example, a rotating journal in a power-generator plant had lost lubricating oil, resulting in the overheating of the journal (Fig. 15). The loss of lubricant caused the temperatures to be hot enough to heat the journal above the critical transformation temperature. Consequently, a very brittle hard layer of martensite formed. Hardness testing showed that the surface could be ground to remove the martensite layer and still be above the minimum diameter of the journal, but hardness testing also showed a slight drop in hardness below the minimum diameter, indicating tempered material. A torsional fatigue analysis that considered load events and reduction in strength was performed and showed that the as-ground journal was acceptable and fit for service. The file is downloaded from www.bzfxw.com
Heat Damage Diameter, mm 343 334 329 326 450 Martensite formed 400 Heat damaged 350 300 Before heat damaged Minimum specimen Tempered material Original diameter Minimum diameter 0 138013.5013.3513.3013.2513.1513.00129512.9012.8512.80 Diameter. in Fig. 15 Life assessment of rotating equipment damaged by exposure to excessive temperatures or loss of lubricant.(a) Journal that overheated because of the loss of lubricating oil.(b) Hardness test results of the heat-damaged region indicated the formation of a hard martensite layer and a tempered structure. After removal of the martensite, fatigue analysis verified that the journal could be put back in service. Fitness for Service. When conducting investigations and determining how a component or structure failed or may fail, the users often want to determine if other equipment is fit for service(FFS). Many analytical methodologies are employed
Fig. 15 Life assessment of rotating equipment damaged by exposure to excessive temperatures or loss of lubricant. (a) Journal that overheated because of the loss of lubricating oil. (b) Hardness test results of the heat-damaged region indicated the formation of a hard martensite layer and a tempered structure. After removal of the martensite, fatigue analysis verified that the journal could be put back in service. Fitness for Service. When conducting investigations and determining how a component or structure failed or may fail, the users often want to determine if other equipment is fit for service (FFS). Many analytical methodologies are employed
and being developed to determine how fit a structure or equipment is for continued service. Historically, the asMe and API design codes for new pressure vessels do not address the fact that equipment degrades while in service and that structural deficiencies due to degradation from the original fabrication may be found during subsequent investigations Due to this concern in the refinery and chemical industries, API 579, Fitness-for-Service, was developed by a joint technical community to ensure structural integrity of pressurized equipment(Ref 30). Results of an FFS assessment can be used to make run-repair-replace decisions to help ensure that pressurized equipment containing detected flaws can continue to operate safely. One such analytical tool developed and mentioned in API 579 is the failure assessment diagram(FAD), which is discussed in the following paragraphs. The failure analyst may or may not be directly involved in structural life assessments using these analytical tools During life assessment, often there is a need for optimized modeling of fracture critical structural components and connections using elastic-plastic fracture mechanics. Such applications can require sophisticated analytical techniques that require time and/or resources beyond those available to the designer or analyst. One of the first engineering tools to ddress this dilemma was the Welding Institute Dimensionless CTOD Design Curve and was included in the first edition of the British Standards Institution(BSI) fitness-for-purpose guidance PD 6493(Ref 31). The engineering tool receiving attention currently is the FAD. This approach has been used, primarily, in the electric power industry both in Great Britain as the R6 criteria(Ref 32)and the United States as both the failure assessment diagram(Ref 33)and the deformation plasticity failure assessment diagram(DPFAD)(Ref 34 ) Both the R6 and dPFAD approaches utilize the j- integral criteria for fracture driving force and resistance (i. e, toughness) The United Kingdom Control Electricity Geometry Board(CEGB) first proposed the failure analysis diagram shown in Fig. 16(a). The approach addressed postyield fracture using an interpolation formula between the two cases of linear elastic fracture and plastic collapse. If a point describing the state of a component or structure falls below the r6 curve then the structure is considered safe. a point falling on or above the ro curve is considered failure. Figure 16(b) shows a typical FAD curve for steels(Ref 35). Kr is the fracture ratio, which is the ratio of the crack driving force(including residual stress)to the material toughness. Lr or Sr is the collapse ratio, which is the ratio of the applied stress at design load to the applied stress at plastic collapse. For more details, see the article Failure Assessment Diagrams" in this Volume Failure Unacceptable region (inside the Lr cut-of) Cut-off for steels with a yield plateau Cut-olf for ASTM A 508 Cut-off for C-Mn steels Cut-off for tainless steels aw=02 020406081012 40608101214161820 Collapse ratio (s) Fig. 16 Failure assessment diagrams used to evaluate the elastic-plastic behavior of tructures to estimate safe operation.(a) The CegB r6 curve. If a point describing the state of a component or structure falls below the r6 curve then the structure is considered safe. a point falling on or above the r6 curve is considered failure.(b)The failure assessment diagram from API 579 for steel(Ref 35). K is the fracture ratio, which is the ratio of the crack driving force (including residual stress) to the material toughness Lr is the collapse ratio, which is the ratio of the applied stress at design load to the applied stress at plastic collapse Similar to the CegB r6 curve, points beneath the curve are considered safe and points on or above the curve are considered unacceptable. Probabilistic Analyses. Life assessment often involves statistical calculations to quantify the probability of failure. This probability of failure is used to make decisions on the risk of continuing to operate a piece of equipment Application of probabilistic analysis methods for life assessment of structural components is becoming more prevalent Thefileisdownloadedfromwww.bzfxw.com
and being developed to determine how fit a structure or equipment is for continued service. Historically, the ASME and API design codes for new pressure vessels do not address the fact that equipment degrades while in service and that structural deficiencies due to degradation from the original fabrication may be found during subsequent investigations. Due to this concern in the refinery and chemical industries, API 579, “Fitness-for-Service,” was developed by a joint technical community to ensure structural integrity of pressurized equipment (Ref 30). Results of an FFS assessment can be used to make run-repair-replace decisions to help ensure that pressurized equipment containing detected flaws can continue to operate safely. One such analytical tool developed and mentioned in API 579 is the failure assessment diagram (FAD), which is discussed in the following paragraphs. The failure analyst may or may not be directly involved in structural life assessments using these analytical tools. During life assessment, often there is a need for optimized modeling of fracture critical structural components and connections using elastic-plastic fracture mechanics. Such applications can require sophisticated analytical techniques that require time and/or resources beyond those available to the designer or analyst. One of the first engineering tools to address this dilemma was the Welding Institute Dimensionless CTOD Design Curve and was included in the first edition of the British Standards Institution (BSI) fitness-for-purpose guidance PD 6493 (Ref 31). The engineering tool receiving attention currently is the FAD. This approach has been used, primarily, in the electric power industry both in Great Britain as the R6 criteria (Ref 32) and the United States as both the failure assessment diagram (Ref 33) and the deformation plasticity failure assessment diagram (DPFAD) (Ref 34). Both the R6 and DPFAD approaches utilize the Jintegral criteria for fracture driving force and resistance (i.e., toughness). The United Kingdom Control Electricity Geometry Board (CEGB) first proposed the failure analysis diagram shown in Fig. 16(a). The approach addressed postyield fracture using an interpolation formula between the two cases of linear elastic fracture and plastic collapse. If a point describing the state of a component or structure falls below the R6 curve then the structure is considered safe. A point falling on or above the R6 curve is considered failure. Figure 16(b) shows a typical FAD curve for steels (Ref 35). Kr is the fracture ratio, which is the ratio of the crack driving force (including residual stress) to the material toughness. Lr or Sr is the collapse ratio, which is the ratio of the applied stress at design load to the applied stress at plastic collapse. For more details, see the article “Failure Assessment Diagrams” in this Volume. Fig. 16 Failure assessment diagrams used to evaluate the elastic-plastic behavior of structures to estimate safe operation. (a) The CEGB R6 curve. If a point describing the state of a component or structure falls below the R6 curve then the structure is considered safe. A point falling on or above the R6 curve is considered failure. (b) The failure assessment diagram from API 579 for steel (Ref 35). Kf is the fracture ratio, which is the ratio of the crack driving force (including residual stress) to the material toughness. Lr is the collapse ratio, which is the ratio of the applied stress at design load to the applied stress at plastic collapse. Similar to the CEGB R6 curve, points beneath the curve are considered safe and points on or above the curve are considered unacceptable. Probabilistic Analyses. Life assessment often involves statistical calculations to quantify the probability of failure. This probability of failure is used to make decisions on the risk of continuing to operate a piece of equipment. Application of probabilistic analysis methods for life assessment of structural components is becoming more prevalent as: The file is downloaded from www.bzfxw.com
Failure models continue to be enhanced for structural life prediction. Engineers increasingly realize that significant uncertainties and variations in loads, material properties geometrical, and other parameters occur in real world structures The need increases to quantify the probability of failure of structural components and to design for appropriate levels of safety Computational resources become less of an impediment through enhancements in computational algorithms and computer efficiency Engineers recognize that factor-of-safety approaches may not give the desired reliability or may lead to overdesigned structures with significant penalties The need for a rational consistent methodology on which to base important decisions such as to inspect, replace or repair a structure is realized The modern era of probabilistic structural design and analysis started after the Second World War. In 1947, a paper entitled, "The Safety of Structures, " appeared in the Transactions of the American Society of Civil Engineers. This historical paper, written by A M. Freudenthal, suggested that rational methods of developing safety factors for engineering structures should give due consideration to observed statistical distributions of the design factors An illustration of the probabilistic approach is shown in Fig. 17(a). In this methodology, uncertainties are explicitly modeled with probability density functions and the probability of failure is quantified. Several solutions methods exist to compute the probability of failure. One such method, Monte Carlo sampling, is shown schematically in Fig. 17(b). A more detailed discussion is found in the article"Analysis Methods for Probabilistic Life Assessment in this Volume
· Failure models continue to be enhanced for structural life prediction. · Engineers increasingly realize that significant uncertainties and variations in loads, material properties, geometrical, and other parameters occur in real world structures. · The need increases to quantify the probability of failure of structural components and to design for appropriate levels of safety. · Computational resources become less of an impediment through enhancements in computational algorithms and computer efficiency. · Engineers recognize that factor-of-safety approaches may not give the desired reliability or may lead to overdesigned structures with significant penalties. · The need for a rational consistent methodology on which to base important decisions such as to inspect, replace, or repair a structure is realized. The modern era of probabilistic structural design and analysis started after the Second World War. In 1947, a paper entitled, “The Safety of Structures,” appeared in the Transactions of the American Society of Civil Engineers. This historical paper, written by A.M. Freudenthal, suggested that rational methods of developing safety factors for engineering structures should give due consideration to observed statistical distributions of the design factors. An illustration of the probabilistic approach is shown in Fig. 17(a). In this methodology, uncertainties are explicitly modeled with probability density functions and the probability of failure is quantified. Several solutions methods exist to compute the probability of failure. One such method, Monte Carlo sampling, is shown schematically in Fig. 17(b). A more detailed discussion is found in the article “Analysis Methods for Probabilistic Life Assessment” in this Volume
Model input uncertainties Validated physical model Reliability function Material property Response and Material ilure model properties Using uniform deterministic model number, generate of the system a realization for Define random according to variables(type, its distribution moments) Set no of ng random Compute system experiments variable values probability of failure evaluate the performance of No successful experiments Start experiments the system Total no of experiments Generate a Examine system uniformly distributed performance random number for fail or not fail? each random variable Fig. 17(a) The probabilistic approach and(b) the monte Carlo flow diagram used for a turbine blade Thefileisdownloadedfromwww.bzfxw.com
Fig. 17 (a) The probabilistic approach and (b) the Monte Carlo flow diagram used for a turbine blade The file is downloaded from www.bzfxw.com