It may be that there is no true product failure. In fact, one important question that may well be asked is: Did a failure really occur? It is possible to have an undesirable event that involves fracture, wear, deformation, or corrosion but that is not really a component failure. For example, discovering a fatigue crack in a 40-year-old structural component, in many cases, may be less of a surprise than finding one that is free from such cracks For these and similar reasons, it is good practice to avoid the use of terms such as failed part, at least until th investigation has revealed strong evidence that a failure has indeed occurred. The terms subject part, subject component, or physical evidence are preferred. It is also good to have an appreciation for what a failure is not The objectives of failure analysis can vary, as suggested by some of the different types of objectives listed in Table 1. Early in every investigation, those with an interest should determine exactly what their objective is or, have no legal obligation to do so, they still, parties have a genuine desire to prevent recurrences,even and timing considerations usually determine the scope of the investigation. Aside from the cost of the investigation itself, a sure answer that comes after repeat failures may be less valuable than a reasonably sound answer before repeat failures Table 1 Objectives in failure analysis Types of objective Possible precipitating situation Product life cycle Product development Demands of the market Prototyping I Product improvement i Warranty costs L Ongoing Assignment of eparations for financial/physical damage or bodily injury or After subject sponsibility death Prevention of recurrence Any After subject event However, despite these various ways of defining objectives and conclusions in failure analysis, it is still fundamentally worthwhile for the knowledge gained. Obviously, success is more satisfying than failure, but experienced practitioners of failure analysis have learned the value of taking the time to extract lessons about both the technical and people-related causes of the failure. When the challenge is to keep learning, every investigation adds to our competence. Often, problems cannot be solved from the same level of understanding in which they are created Thus, failure analysis may often require a keen and inquisitive outlook, which very good and satisfying way to keep learning on a technical, professional, and personal level for an entire career The Failure Analysis Process: An Overview Debbie Aliya, Aliya Analytical Scope and planning The scope of a failure analysis depends on the depth and complexity of the project. Many failure analysts have experienced being told to find the root cause of a particular failure in half an hour! Usually, this is impossible or leads to superficial results. The scope of the investigation must also be targeted toward finding the real root (physical or human) cause of the failure. Root-cause failure analysis has generated much attention in recent years,and it is a good development when the term root cause is given to mean the particular physical or human effect that precipitated or assisted in a failure. Sometimes the term root-cause analysis is misconstrued to mean figuring out whether a component meets a specification. If it does not meet specification, then the lack of conformity to the specification becomes a convenient"root cause. This use of the term is not only superficial but also invalid. Any approach that does not attempt to link the particular physical effect with the particular lack of conformity and its direct expected consequences is not a valid failure analysis approach During the planning stages on the possible scope of an investigation, it is sometimes useful to focus attention on the potential complexity of the problem and on when the physical cause may have occurred. Various categories of complexity are listed in Table 2. These categories of complexity are not exclusive. All failure analysis work must be founded on the physical causes. Sometimes one is asked to determine a cause based on the verbal Thefileisdownloadedfromwww.bzfxw.com
It may be that there is no true product failure. In fact, one important question that may well be asked is: “Did a failure really occur?” It is possible to have an undesirable event that involves fracture, wear, deformation, or corrosion but that is not really a component failure. For example, discovering a fatigue crack in a 40-year-old structural component, in many cases, may be less of a surprise than finding one that is free from such cracks. For these and similar reasons, it is good practice to avoid the use of terms such as failed part, at least until the investigation has revealed strong evidence that a failure has indeed occurred. The terms subject part, subject component, or physical evidence are preferred. It is also good to have an appreciation for what a failure is not. The objectives of failure analysis can vary, as suggested by some of the different types of objectives listed in Table 1. Early in every investigation, those with an interest should determine exactly what their objective is or, more likely, what their objectives are. If the parties have a genuine desire to prevent recurrences, even if they have no legal obligation to do so, they still need to decide how far they want to go toward this goal. Economic and timing considerations usually determine the scope of the investigation. Aside from the cost of the investigation itself, a sure answer that comes after repeat failures may be less valuable than a reasonably sound answer before repeat failures. Table 1 Objectives in failure analysis Types of objective Possible precipitating situation Product life cycle Product development Demands of the market Prototyping Product improvement Warranty costs Ongoing Assignment of responsibility Reparations for financial/physical damage or bodily injury or death After subject event Prevention of recurrence Any After subject event However, despite these various ways of defining objectives and conclusions in failure analysis, it is still fundamentally worthwhile for the knowledge gained. Obviously, success is more satisfying than failure, but experienced practitioners of failure analysis have learned the value of taking the time to extract lessons about both the technical and people-related causes of the failure. When the challenge is to keep learning, every investigation adds to our competence. Often, problems cannot be solved from the same level of understanding in which they are created. Thus, failure analysis may often require a keen and inquisitive outlook, which is a very good and satisfying way to keep learning on a technical, professional, and personal level for an entire career. The Failure Analysis Process: An Overview Debbie Aliya, Aliya Analytical Scope and Planning The scope of a failure analysis depends on the depth and complexity of the project. Many failure analysts have experienced being told to find the root cause of a particular failure in half an hour! Usually, this is impossible or leads to superficial results. The scope of the investigation must also be targeted toward finding the real root (physical or human) cause of the failure. Root-cause failure analysis has generated much attention in recent years, and it is a good development when the term root cause is given to mean the particular physical or human effect that precipitated or assisted in a failure. Sometimes the term root-cause analysis is misconstrued to mean figuring out whether a component meets a specification. If it does not meet specification, then the lack of conformity to the specification becomes a convenient “root cause.” This use of the term is not only superficial but also invalid. Any approach that does not attempt to link the particular physical effect with the particular lack of conformity and its direct expected consequences is not a valid failure analysis approach. During the planning stages on the possible scope of an investigation, it is sometimes useful to focus attention on the potential complexity of the problem and on when the physical cause may have occurred. Various categories of complexity are listed in Table 2. These categories of complexity are not exclusive. All failure analysis work must be founded on the physical causes. Sometimes one is asked to determine a cause based on the verbal The file is downloaded from www.bzfxw.com
description of a component. In some limited cases, this may be all that is possible, if the part is gone. However, the value of the failure analysis is limited in those cases. Also note that a failure problem is defined in a broader context going from top to bottom of Table 2. This is particularly important for failure analysis within a manufacturing organization. Today, many companies manufacture components or assemblies for other companies. The individuals at the contract manufacturing companies may not have any way of learning the importance of specified or unspecified requirements of the components that they are manufacturing until they have the opportunity to learn as a result of a failure investigation. Progressive companies have been taking advantage of their human resources for years by providing more training and education, because they know that it generally has a positive effect on the profitability of the company Table 2 Complexity of investigation Depth and Example Allowed effects of findings Part is found to be of the wrong Part gets rehardened. Maybe inspection cause(s)or hardness frequency is increased Individual Heat treat department employee does Heat treatment department employees are sent people causes not understand importance of to a basic metallurgy and testing class. hardness and does not check part Latent causes or Nobody tells furnace operator about Design and supervisory personnel are given factors the importance of hardness testing. It custom training by a competent materials does not matter that she fabricated engineer on the subject of how to specify test results, until the pyrometer fails, hardness test protocol. Maybe an investigation causing incorrect temperature into the heat treat furnace uniformity is exposure during austenitization. undertaken Culture-based Hardness runs on low end of Marketing people are given additional training root cause(s) specified range for years but never so that they do not give unreasonable causes a problem until someone, expectations of product performance.(It is due to economic downturn, decides recognized that an optimistic outlook is part of to use item for much longer than sales and marketing. The question is, where does the original expected service life one draw the line as to what is reasonable and unreasonable. Marketing people are given adequate resources to follow up with customers It is also important to keep in mind when root cause(s) could have been introduced. Table 3 lists some examples. It is often difficult to assign a failure to a single phase of problem creation. Many times, there are complex interactions. Only an integrated design approach will create robust processes and thus robust parts The designer must communicate with personnel from the sales department, as well as customers, maintenance personnel, manufacturing personnel, and material suppliers. Input from failure analysts who evaluate broken, worn,corroded, or deformed parts from durability testing is an important source of information that is often neglected by designers Table 3 Physical causes and time of occurrence Physical Example Likely effects of findings Alternative effect(s)of findings causes Raw material Excessive microsegregation Material supplier is Additional investigation is manufacturing causes inability to achieve blamed performed on how to improve post heat treat mechanical heat treat line to make process more robust. Design phase Designer does not know Heat treater is blamed for Designer reviews accuracy of hardness test poor process control specifications, and broadens and specifies a that is specification if that is too narrow to achieve acceptable based on
description of a component. In some limited cases, this may be all that is possible, if the part is gone. However, the value of the failure analysis is limited in those cases. Also note that a failure problem is defined in a broader context going from top to bottom of Table 2. This is particularly important for failure analysis within a manufacturing organization. Today, many companies manufacture components or assemblies for other companies. The individuals at the contract manufacturing companies may not have any way of learning the importance of specified or unspecified requirements of the components that they are manufacturing until they have the opportunity to learn as a result of a failure investigation. Progressive companies have been taking advantage of their human resources for years by providing more training and education, because they know that it generally has a positive effect on the profitability of the company. Table 2 Complexity of investigation Depth and complexity of investigation Example Allowed effects of findings Physical cause(s) or factor(s) Part is found to be of the wrong hardness. Part gets rehardened. Maybe inspection frequency is increased. Individual people causes Heat treat department employee does not understand importance of hardness and does not check part. Heat treatment department employees are sent to a basic metallurgy and testing class. Latent causes or factors Nobody tells furnace operator about the importance of hardness testing. It does not matter that she fabricated test results, until the pyrometer fails, causing incorrect temperature exposure during austenitization. Design and supervisory personnel are given custom training by a competent materials engineer on the subject of how to specify hardness test protocol. Maybe an investigation into the heat treat furnace uniformity is undertaken. Culture-based root cause(s) Hardness runs on low end of specified range for years but never causes a problem until someone, due to economic downturn, decides to use item for much longer than the original expected service life. Marketing people are given additional training so that they do not give unreasonable expectations of product performance. (It is recognized that an optimistic outlook is part of sales and marketing. The question is, where does one draw the line as to what is reasonable and unreasonable.) Marketing people are given adequate resources to follow up with customers. It is also important to keep in mind when root cause(s) could have been introduced. Table 3 lists some examples. It is often difficult to assign a failure to a single phase of problem creation. Many times, there are complex interactions. Only an integrated design approach will create robust processes and thus robust parts. The designer must communicate with personnel from the sales department, as well as customers, maintenance personnel, manufacturing personnel, and material suppliers. Input from failure analysts who evaluate broken, worn, corroded, or deformed parts from durability testing is an important source of information that is often neglected by designers. Table 3 Physical causes and time of occurrence Physical causes Example Likely effects of findings Alternative effect(s) of findings Raw material manufacturing Excessive microsegregation causes inability to achieve post heat treat mechanical properties. Material supplier is blamed. Additional investigation is performed on how to improve heat treat line, to make process more robust. Design phase Designer does not know accuracy of hardness test and specifies a range that is too narrow to achieve. Heat treater is blamed for poor process control. Designer reviews specifications, and broadens specification if that is acceptable, based on
performance criteria, or modifies design as required. Manufacturing Part loader malfunctions, Heat treater is blamed for Product design manager is hase and a small fraction of the poor process control inspired to perform detailed load does not experience failure mode and effects proper thermal cycle, but analysis(FMEA) in presence none of the bad ones are in of mechanical. materials the test group manufacturing, and maintenance personnel, and Service phase Part is subject to Hardness data can be Product design manager unexpected and easy to misinterpret ired to perform detailed undetected heat, which leading to assignment of FMEA in presence of changes the hardness the same likely cause mechanical. materials listed above. if manufacturing, and microstructure analysis is maintenance personnel, and not included customers Avoiding Errors. The failure analyst needs to be aware that sorting out the causes of failures can cause economic and noneconomic(e.g, psychological) consequences to particular individuals or companies who are implicated for carelessness, negligence, simple ignorance, or other errors or omissions. Thus, it is important to avoid mistakes, as they could cause as much harm as, or more harm than, the original failure Analytical mistakes may be technical in nature, such as an incorrect measurement of a mechanical prope Analytical mistakes may also be subtle. An example may be not questioning a suspicious hardness composition data point. Another example error in judgment of the significance of something that normally a minor detail. If this causes one to overlook things that bear close scrutiny, an incorrect conclusion may be drawn. Making sure that all relevant details are examined can help point to a clear conclusion and is a key to competent failure analysis work In situations that involve loss of life, human injury, or large economic damage, professional analysts should be very careful to do work only within their areas of competence. It is important to know the limits of one 's own knowledge and to know when help is needed. In fact, input from people from many areas will probably be involved in all but the most basic physical-cause investigations. If the failure involves complex interactions of latent factors, an interdisciplinary approach is generally required to prepare prevention strategy recommendations Fear of overlooking important details is probably the biggest reason that many experienced analysts refuse to perform failure analysis work unless they are given the time and budget to do a complete investigation. It is very easy to draw the wrong conclusions if one does not consider the "big picture" from multiple angles. A broad view is more likely to lead to a coherent conclusion or set of conclusions. To emphasize this important point again, a failure analysis must include an evaluation of the consistency of results from different tests or analytical methods. A single test result does not constitute a legitimate foundation for a failure analysis. Other common pitfalls in failure investigations suggested by Dennies(Ref 3)include Jumping to conclusions Not understanding the problem Not understanding how the failed system is supposed to operate Not considering all possible failure causes Tearing system apart without a developed plan We need to tear it apart as soon as possible Failing to follow through Not asking for help Thinking it is so easy to do Destroying evidence due to lack of plar Failure analysis is a profession that is rarely perfected in a given individual, and even experienced practitioners should remain aware of these potential pitfalls. It is also important to understand that failure analysis is not(ref 3) Thefileisdownloadedfromwww.bzfxw.com
performance criteria, or modifies design as required. Manufacturing phase Part loader malfunctions, and a small fraction of the load does not experience proper thermal cycle, but none of the bad ones are in the test group Heat treater is blamed for poor process control. Product design manager is inspired to perform detailed failure mode and effects analysis (FMEA) in presence of mechanical, materials, manufacturing, and maintenance personnel, and customers. Service phase Part is subject to unexpected and undetected heat, which changes the hardness Hardness data can be easy to misinterpret, leading to assignment of the same likely cause listed above, if microstructure analysis is not included. Product design manager is inspired to perform detailed FMEA in presence of mechanical, materials, manufacturing, and maintenance personnel, and customers. Avoiding Errors. The failure analyst needs to be aware that sorting out the causes of failures can cause economic and noneconomic (e.g., psychological) consequences to particular individuals or companies who are implicated for carelessness, negligence, simple ignorance, or other errors or omissions. Thus, it is important to avoid mistakes, as they could cause as much harm as, or more harm than, the original failure. Analytical mistakes may be technical in nature, such as an incorrect measurement of a mechanical property. Analytical mistakes may also be subtle. An example may be not questioning a suspicious hardness or composition data point. Another example is an error in judgment of the significance of something that is normally a minor detail. If this causes one to overlook things that bear close scrutiny, an incorrect conclusion may be drawn. Making sure that all relevant details are examined can help point to a clear conclusion and is a key to competent failure analysis work. In situations that involve loss of life, human injury, or large economic damage, professional analysts should be very careful to do work only within their areas of competence. It is important to know the limits of one's own knowledge and to know when help is needed. In fact, input from people from many areas will probably be involved in all but the most basic physical-cause investigations. If the failure involves complex interactions of latent factors, an interdisciplinary approach is generally required to prepare prevention strategy recommendations. Fear of overlooking important details is probably the biggest reason that many experienced analysts refuse to perform failure analysis work unless they are given the time and budget to do a complete investigation. It is very easy to draw the wrong conclusions if one does not consider the “big picture” from multiple angles. A broad view is more likely to lead to a coherent conclusion or set of conclusions. To emphasize this important point again, a failure analysis must include an evaluation of the consistency of results from different tests or analytical methods. A single test result does not constitute a legitimate foundation for a failure analysis. Other common pitfalls in failure investigations suggested by Dennies (Ref 3) include: · Jumping to conclusions · Not understanding the problem · Not understanding how the failed system is supposed to operate · Not considering all possible failure causes · Tearing system apart without a developed plan: “We need to tear it apart as soon as possible.” · Failing to follow through · Not asking for help · Thinking it is so easy to do · Destroying evidence due to lack of planning Failure analysis is a profession that is rarely perfected in a given individual, and even experienced practitioners should remain aware of these potential pitfalls. It is also important to understand that failure analysis is not (Ref 3): The file is downloaded from www.bzfxw.com
Give me your best guess Not identifying root cause(s) Reworking or repairing: It will be quicker to fix it Swapping parts: Remove and replace mentality Ignoring the problem: Wait until the problem goes away We were going to change it later anyway Band-aid fixes: Return to the supplier; scrap; another system, supplier, or inventory · Never happens:“Oops Reference cited in this section 3. D P. Dennies, Boeing Co., private communication Planning and Preparation It should be clear that the objectives and scope should be defined and understood early in every investigation. If resources for a complete and detailed investigation leading to a high degree of technical certainty are not available, the investigator is encouraged to clarify for himself or herself, as well as the others involved, what is oped to be determined after following a particular protocol. This clarification process should be done before any destructive testing Even a very limited investigation is by no means useless. Often, the people involved have two or three failure scenarios in mind. It is often possible for the trained analyst to rule out some of these scenarios with a small amount of work. A case in point is when an automotive repair shop owner wanted to know if employee negligence had caused a premature fracture in an externally threaded fastener. The fracture occurred two months after the repair job. The fracture was found by a different auto repair shop. There were no indications of progressive cracking. The people from the second repair shop accused the first shop of gross negligence. In this situation, it might be reasonable to suggest that something other than the first mechanic's negligence caused the fracture. Even without revealing the whole story, the information provided with a simple fractographic evaluation was useful to those involved Guidelines on the preparation of a protocol for a failure analysis may vary. For a part investigation, it may be a simple checklist(Fig. 1)that is included in a client report. In larger investigations, other methods(Table 4)may be considered to help plan and identify priorities. Each has advantages, drawbacks, and limitations in any given situation. When planning the actual step-by-step activities of the investigation, one should keep in mind that the degree of comprehensiveness necessary will be determined to a large degree not only by what the involved parties want to know but also by how strong their desire is to know it. In practice, the strength of the desire is measured in practical terms by the budget and timing considerations
· “Give me your best guess” · Not identifying root cause(s) · Reworking or repairing: “It will be quicker to fix it.” · Swapping parts: Remove and replace mentality · Ignoring the problem: “Wait until the problem goes away.” · “We were going to change it later anyway.” · Band-aid fixes: Return to the supplier; scrap; another system, supplier, or inventory · Never happens: “Oops” Reference cited in this section 3. D.P. Dennies, Boeing Co., private communication Planning and Preparation It should be clear that the objectives and scope should be defined and understood early in every investigation. If resources for a complete and detailed investigation leading to a high degree of technical certainty are not available, the investigator is encouraged to clarify for himself or herself, as well as the others involved, what is hoped to be determined after following a particular protocol. This clarification process should be done before any destructive testing. Even a very limited investigation is by no means useless. Often, the people involved have two or three failure scenarios in mind. It is often possible for the trained analyst to rule out some of these scenarios with a small amount of work. A case in point is when an automotive repair shop owner wanted to know if employee negligence had caused a premature fracture in an externally threaded fastener. The fracture occurred two months after the repair job. The fracture was found by a different auto repair shop. There were no indications of progressive cracking. The people from the second repair shop accused the first shop of gross negligence. In this situation, it might be reasonable to suggest that something other than the first mechanic's negligence caused the fracture. Even without revealing the whole story, the information provided with a simple fractographic evaluation was useful to those involved. Guidelines on the preparation of a protocol for a failure analysis may vary. For a part investigation, it may be a simple checklist (Fig. 1) that is included in a client report. In larger investigations, other methods (Table 4) may be considered to help plan and identify priorities. Each has advantages, drawbacks, and limitations in any given situation. When planning the actual step-by-step activities of the investigation, one should keep in mind that the degree of comprehensiveness necessary will be determined to a large degree not only by what the involved parties want to know but also by how strong their desire is to know it. In practice, the strength of the desire is measured in practical terms by the budget and timing considerations
Factor/Variable Observations/Comments Chemistry Conforms to AISI 1030 Microstructure Proeutectoid ferrite and pearlite as expected in a hypoeutectoid annealed/normalized plain carbon steel Grain Size Variable: coarse(1 to 2 ASTM grain size)in the heavy thickness and fine (3 to 5 grain size)in the thin web. Knoop Hardness web( Thin section):260,266,252,245,233(HK300 Avg=251 HK300(20HRC) Heavy section:275,248,309,241,273,301(HK300) Avg= 275 HK300(25HRC) Inclusions/Segregation Appears most probably The EDS spectra show presence of Sulfur(S), Phosphorus(P), Silicon(Si), Calcium(Ca) and other elements(Fe and Mn are expected). Some of the phosphorus nay have been contributed by the laboratory cleaning procedure Forging Defects No gross forging defects such as laps or folds were detected. Overheating/Burning Possible overheating causing grain growth and grain boundary precipitation/formation of sulfides/phosphides/oxides and other non-metallics Residual Stress/Tightening Unknown. Not expected to be significant Overload Operating Loads The concave side of brace is always under tension magnitude of forces unknown Hydrogen Damage Appears possible but secondary to grain growth due to overheating Hard spots Higher Knoop reading near cracked edge. Fracture Appearance The initiation region shows fine dimpled structure which is interpreted to be due to the presence of grain boundary precipitates formed during forging processing. The fracture way from the initiation area is a mixture of ductile and intergranular modes; and has a faceted appearance. Further into the core, the fracture mode is predominantly of the cleavage (less ductile or brittle, transgranular) type Fatig No fatigue striations were detected. It is believed the crack front traveled rapidly. Corrosion in Service Rust only. No evidence of any other form of corrosion detected Thefileisdownloadedfromwww.bzfxw.com
The file is downloaded from www.bzfxw.com