Practices in Failure analysis Introduction ANALYZiNg the inevitable failures that occur during testing. manufacturing. and service is an essential ngineering process for continual improvement in product reliability. This article describes the general procedures, techniques, and precautions employed in the investigation and analysis of metallurgical failures that occur in service. The stages of investigation are discussed. and the various features of the more common causes of failure characteristics are described for fracture. corrosion and wear failures The mechanisms of fracture corrosion, and wear failures are explained in more detail in other Sections of this volume Practices in Failure Analysis Stages of a Failure Analysis Although the sequence is subject to variation depending on the nature of the failure and the availability of physical evidence or background information, there are stages that are common to all successful failure analyses. The combination of these stages comprises the total investigation and analysis. The following list includes many of the commonly used stages. The sequence in which these stages are used is not necessarily critical, and not all of the stages will or can be used in every failure analysis. However, a key principle is to not unnecessarily disrupt conditions that may require closer examination at a later date. Moreover, an additional constraint is to follow the Federal Rules of Evidence during investigation of a failure that may be destined for The stages discussed in this article begin first with the preliminary steps of information gathering such Collection of background data and selection of samples Preliminary examination of the failed part(visual examination and record keeping) Nondestructive testing These preliminary steps may then be followed by assessment of the damage and conditions leading to failure investigated. In an analysis of a fracture, the following steps are described and/or wear conditions are being These stages may differ depending on whether fracture, corrosion, Selection, identification, preservation, and/or cleaning of critical specimens Macroscopic examination and analysis (fracture surfaces, secondary cracks, and other surface nomen Microscopic examination and analysis of fracture surfaces Stress analysis to determine the actual stress state of the failed component Fracture mechanic Determination of the fracture mode Following these topics on the analysis of fractures, separate sections also briefly describe factors and methods in the analysis of corrosion and wear failures In addition, investigations of a failure may utilize various techniques to characterize the condition of material These include Metallography or microstructural analy Mechanical testing
Practices in Failure Analysis Introduction ANALYZING the inevitable failures that occur during testing, manufacturing, and service is an essential engineering process for continual improvement in product reliability. This article describes the general procedures, techniques, and precautions employed in the investigation and analysis of metallurgical failures that occur in service. The stages of investigation are discussed, and the various features of the more common causes of failure characteristics are described for fracture, corrosion, and wear failures. The mechanisms of fracture, corrosion, and wear failures are explained in more detail in other Sections of this Volume. Practices in Failure Analysis Stages of a Failure Analysis Although the sequence is subject to variation depending on the nature of the failure and the availability of physical evidence or background information, there are stages that are common to all successful failure analyses. The combination of these stages comprises the total investigation and analysis. The following list includes many of the commonly used stages. The sequence in which these stages are used is not necessarily critical, and not all of the stages will or can be used in every failure analysis. However, a key principle is to not unnecessarily disrupt conditions that may require closer examination at a later date. Moreover, an additional constraint is to follow the Federal Rules of Evidence during investigation of a failure that may be destined for court. The stages discussed in this article begin first with the preliminary steps of information gathering such as: · Collection of background data and selection of samples · Preliminary examination of the failed part (visual examination and record keeping) · Nondestructive testing These preliminary steps may then be followed by assessment of the damage and conditions leading to failure. These stages may differ depending on whether fracture, corrosion, and/or wear conditions are being investigated. In an analysis of a fracture, the following steps are described: · Selection, identification, preservation, and/or cleaning of critical specimens · Macroscopic examination and analysis (fracture surfaces, secondary cracks, and other surface phenomena) · Microscopic examination and analysis of fracture surfaces · Stress analysis to determine the actual stress state of the failed component · Fracture mechanics · Determination of the fracture mode Following these topics on the analysis of fractures, separate sections also briefly describe factors and methods in the analysis of corrosion and wear failures. In addition, investigations of a failure may utilize various techniques to characterize the condition of material. These include: · Metallography or microstructural analysis · Mechanical testing
Chemical analyses(bulk, local, surface corrosion products, and deposits or coatings) Testing under simulated service conditions Finally, the investigation concludes with a synthesis and interpretation of results. This step may actually require reiteration of previous steps or the introduction of new steps. Similar to design, failure analysis can be an iterative process of discovery and reexamination. Failure analysis can also be a multidisciplinary process and that may require consulting with experts in other disciplines throughout the investigation. Once all information has been assembled, then the final step is to synthesize all the evidence and formulate conclusions. This requires writing a report with follow-up recommendations on preventing future failures. The goal of every failure analyst is to determine not only the failure mechanism but also the root cause, which may be related to misuse, poor maintenance practices, or improper application, or related to the material properties, design,or manufacture of the product In cases that involve personal injury or will most likely involve legal pursuit of compensation from another company, care must be taken in preserving the scene and physical evidence. Accidental or deliberate the evidence, even though they may not have caused the original failure e to the person or company destroying destruction of evidence can result in diverting the legal liability of a failure to the Practices in Failure Analysi Collection of Background Data and Samples The failure investigation should include gaining an acquaintance with all pertinent details relating to the failure, collecting the available information regarding the design, manufacture, processing, and service histories of the failed component or structure, and reconstructing, insofar as possible, the sequence of events leading to the failure. Collection of background data on the manufacturing and fabricating history of a component should begin with obtaining specifications and drawings and should encompass all the design aspects of the failed part as well as all manufacturing and fabrication details---machining, welding, heat treating, coating, quality-control records, and pertinent purchase specifications Additional information on upfront planning of investigations is also described in the article"Organization of a Failure Investigation"in this volume Collecting Data and Samples On-Site Investigation. In the investigation of failures, it is also often desirable for the analyst to visit the scene, but for the nalysis of some components it may be impractical or impossible for the failure analyst to visit the failure site. Under hese circumstances, data and samples may be collected at the site by field engineers or by other personnel under the direction of the failure analyst. A field failure report sheet or checklist can be used to ensure that all pertinent information regarding the failure is recorded There also are situations where it is essential to perform failure analyses on the site. While it is recommended that examination be done in a laboratory, the requirements for on-site testing may involve the use of portable laboratories with metallographic equipment for grinding, mechanical polishing, and etching. Small specimens can be cut from a part on the site for preparation, examination, and photography immediately or upon return to a fully equipped laboratory Photography is, of course, essential; it should be performed by the analyst or perhaps a professional photographer in the case of a large-scale accident scene. Other considerations for on-site examination at an accident scene are also discussed in more detail in the article"Modeling and accident Reconstruction in this volume It is also frequently desirable to make acetate tape replicas or room-temperature-vulcanized(rTv) rubber replicas of fracture surfaces or of wear patterns of large parts during an on-site failure analysis. Several replicas should be made of the fracture-origin region using acetate tape softened in acetone, dried, then carefully stripped from the fracture surface Upon return to the laboratory, the replicas may be gold coated and examined with a scanning electron microscope(sEM Foreign particles removed from the fracture surface also may be analyzed The RTV rubber replica can be applied over a rather large area with less chance of missing a critical spot. A combination of acetate tape and rtV rubber replicas can assure the investigator of better coverage of the area in question. room- temperature-vulcanized rubber does not provide the sensitivity of an acetate replica, and a setup time of several hours is required. However, the added area can be very important in an investigation Thefileisdownloadedfromwww.bzfxw.com
· Chemical analyses (bulk, local, surface corrosion products, and deposits or coatings). · Testing under simulated service conditions Finally, the investigation concludes with a synthesis and interpretation of results. This step may actually require reiteration of previous steps or the introduction of new steps. Similar to design, failure analysis can be an iterative process of discovery and reexamination. Failure analysis can also be a multidisciplinary process and that may require consulting with experts in other disciplines throughout the investigation. Once all information has been assembled, then the final step is to synthesize all the evidence and formulate conclusions. This requires writing a report with follow-up recommendations on preventing future failures. The goal of every failure analyst is to determine not only the failure mechanism but also the root cause, which may be related to misuse, poor maintenance practices, or improper application, or related to the material properties, design, or manufacture of the product. In cases that involve personal injury or will most likely involve legal pursuit of compensation from another company, care must be taken in preserving the scene and physical evidence. Accidental or deliberate destruction of evidence can result in diverting the legal liability of a failure to the person or company destroying the evidence, even though they may not have caused the original failure. Practices in Failure Analysis Collection of Background Data and Samples The failure investigation should include gaining an acquaintance with all pertinent details relating to the failure, collecting the available information regarding the design, manufacture, processing, and service histories of the failed component or structure, and reconstructing, insofar as possible, the sequence of events leading to the failure. Collection of background data on the manufacturing and fabricating history of a component should begin with obtaining specifications and drawings and should encompass all the design aspects of the failed part as well as all manufacturing and fabrication details—machining, welding, heat treating, coating, quality-control records, and pertinent purchase specifications. Additional information on upfront planning of investigations is also described in the article “Organization of a Failure Investigation” in this Volume. Collecting Data and Samples On-Site Investigation. In the investigation of failures, it is also often desirable for the analyst to visit the scene, but for the analysis of some components it may be impractical or impossible for the failure analyst to visit the failure site. Under these circumstances, data and samples may be collected at the site by field engineers or by other personnel under the direction of the failure analyst. A field failure report sheet or checklist can be used to ensure that all pertinent information regarding the failure is recorded. There also are situations where it is essential to perform failure analyses on the site. While it is recommended that examination be done in a laboratory, the requirements for on-site testing may involve the use of portable laboratories with metallographic equipment for grinding, mechanical polishing, and etching. Small specimens can be cut from a part on the site for preparation, examination, and photography immediately or upon return to a fully equipped laboratory. Photography is, of course, essential; it should be performed by the analyst or perhaps a professional photographer in the case of a large-scale accident scene. Other considerations for on-site examination at an accident scene are also discussed in more detail in the article “Modeling and Accident Reconstruction” in this Volume. It is also frequently desirable to make acetate tape replicas or room-temperature-vulcanized (RTV) rubber replicas of fracture surfaces or of wear patterns of large parts during an on-site failure analysis. Several replicas should be made of the fracture-origin region using acetate tape softened in acetone, dried, then carefully stripped from the fracture surface. Upon return to the laboratory, the replicas may be gold coated and examined with a scanning electron microscope (SEM). Foreign particles removed from the fracture surface also may be analyzed. The RTV rubber replica can be applied over a rather large area with less chance of missing a critical spot. A combination of acetate tape and RTV rubber replicas can assure the investigator of better coverage of the area in question. Roomtemperature-vulcanized rubber does not provide the sensitivity of an acetate replica, and a setup time of several hours is required. However, the added area can be very important in an investigation. The file is downloaded from www.bzfxw.com
Hardness testing with a portable hardness testing instrument also may be performed during on-site failure analysis Several different types of testers are available and in general are either electronic or mechanical in principle. Obviously small size and light weight are advantages in portable testers The major components of the portable laboratory may include A custom-made machine, plus auxiliary materials, for grinding and polishing small, mounted or unmounted metal A right-angle head, electric drill motor with attachments and materials for grinding and polishing selected spots on large parts or assemblies. It is also used for driving the grinding and polishing machine described in the previous item a portable microscope, with camera attachment and film for use in photographing metallographic specimens Equipment and materials for mounting and etching specimens a handheld single-lens reflex 35-mm camera, with macrolenses and film a pocket-size magnifier, and a ruler or scale A hacksaw and blades for cutting specimens Portable hardness tester Acetate tape, acetone, and containers RTV rubber for replicas Service History. The availability of a complete service history depends on how detailed and thorough the record keeping was prior to the failure. a complete service record greatly simplifies the assignment of the failure analyst. In collecting service histories, special attention should be given to environmental details such as normal and abnormal loading. accidental overloads, cyclic loads, temperature variations, temperature gradients, and operation in a corrosive environment. In most instances, however, complete service records are not available, forcing the analyst to work from fragmentary service information. When service data are sparse, the analyst must, to the best of his or her ability, deduce the service conditions. Much depends on the analyst's skill and judgment, because a misleading deduction can be more harmful than the absence of information Photographic Records. Photographs of the failed component or structure are oftentimes critical to an accurate analysis. A detail that appears almost inconsequential in a preliminary investigation may later be found to have serious consequences thus, a complete, detailed photographic record of the scene and failed component can be essential Photographs should be of professional quality, but this is not always possible. For the analyst who does his own photography, a single-lens reflex 35-mm or larger camera with a macrolens, extension bellows, and battery-flash unit is capable of producing excellent results. It may be desirable to supplement the 35-mm equipment with an instant camera and close-up lenses. Techniques and lighting are discussed in more detail in the article"Photography in Failure Analysis in this volume When accurate color rendition is required, the subject should be photographed with a color chart, which should be sent to the photographic studio for use as a guide in developing and printing. Some indication of size, such as a scale, coin, hand and so forth, should be included in the photograph Samples should be selected judiciously before starting the examination, especially if the investigation is to be lengthy or involved. As with photographs, the analyst is responsible for ensuring that the samples will be suitable for the intended purpose and that they adequately represent the characteristics of the failure. It is advisable to look for additional evidenc of damage beyond that which is immediately apparent. For failures involving large structures or key machinery, there is often a financially urgent need to remove the damaged structure or repair the machine to return to production. This is a valid reason to move evidence, but a reasonable attempt must be made to allow other parties, who may become involved in a potential legal case, to inspect the site. All concerned parties then can agree on the critical samples and the best way to remove them. If all parties are not available, care must be taken not to damage or alter critical elements to avoid spoiling evidence Guidelines governing sample collection are covered in ASTM E 620, E860, E 1020, and especially E 678. It is also recommended that samples be taken from other parts of the failed equipment as they may display supportive damage It is often necessary to compare failed components with similar components that did not fail to determine whether the failure was brought about by service conditions or was the result of an error in manufacture. For example, if a boiler tube fails and overheating is suspected to be the cause, and if investigation reveals a spheroidized structure in the boiler tube at the failure site (which may be indicative of overheating in service), then comparison with an unexposed tube will determine if the tubes were supplied in the spheroidized condition As another example, in the case of a bolt failure it is desirable to examine the nuts and other associated parts that may have contributed to the failures. Also, in failures involving corrosion, stress-corrosion, or corrosion fatigue, a sample of the fluid that has been in contact with the metal, or of any deposits that have formed, will often be required for analysis Abnormal Conditions and Wreckage Analysis. In addition to developing a history of the failed part it is also advisable to determine if any abnormal conditions prevailed. Determine also whether events-such as an accident--occurred in
Hardness testing with a portable hardness testing instrument also may be performed during on-site failure analysis. Several different types of testers are available and in general are either electronic or mechanical in principle. Obviously, small size and light weight are advantages in portable testers. The major components of the portable laboratory may include: · A custom-made machine, plus auxiliary materials, for grinding and polishing small, mounted or unmounted metal specimens · A right-angle head, electric drill motor with attachments and materials for grinding and polishing selected spots on large parts or assemblies. It is also used for driving the grinding and polishing machine described in the previous item · A portable microscope, with camera attachment and film for use in photographing metallographic specimens · Equipment and materials for mounting and etching specimens · A handheld single-lens reflex 35-mm camera, with macrolenses and film · A pocket-size magnifier, and a ruler or scale · A hacksaw and blades for cutting specimens · Portable hardness tester · Acetate tape, acetone, and containers · RTV rubber for replicas Service History. The availability of a complete service history depends on how detailed and thorough the record keeping was prior to the failure. A complete service record greatly simplifies the assignment of the failure analyst. In collecting service histories, special attention should be given to environmental details such as normal and abnormal loading, accidental overloads, cyclic loads, temperature variations, temperature gradients, and operation in a corrosive environment. In most instances, however, complete service records are not available, forcing the analyst to work from fragmentary service information. When service data are sparse, the analyst must, to the best of his or her ability, deduce the service conditions. Much depends on the analyst's skill and judgment, because a misleading deduction can be more harmful than the absence of information. Photographic Records. Photographs of the failed component or structure are oftentimes critical to an accurate analysis. A detail that appears almost inconsequential in a preliminary investigation may later be found to have serious consequences; thus, a complete, detailed photographic record of the scene and failed component can be essential. Photographs should be of professional quality, but this is not always possible. For the analyst who does his own photography, a single-lens reflex 35-mm or larger camera with a macrolens, extension bellows, and battery-flash unit is capable of producing excellent results. It may be desirable to supplement the 35-mm equipment with an instant camera and close-up lenses. Techniques and lighting are discussed in more detail in the article “Photography in Failure Analysis” in this Volume. When accurate color rendition is required, the subject should be photographed with a color chart, which should be sent to the photographic studio for use as a guide in developing and printing. Some indication of size, such as a scale, coin, hand, and so forth, should be included in the photograph. Samples should be selected judiciously before starting the examination, especially if the investigation is to be lengthy or involved. As with photographs, the analyst is responsible for ensuring that the samples will be suitable for the intended purpose and that they adequately represent the characteristics of the failure. It is advisable to look for additional evidence of damage beyond that which is immediately apparent. For failures involving large structures or key machinery, there is often a financially urgent need to remove the damaged structure or repair the machine to return to production. This is a valid reason to move evidence, but a reasonable attempt must be made to allow other parties, who may become involved in a potential legal case, to inspect the site. All concerned parties then can agree on the critical samples and the best way to remove them. If all parties are not available, care must be taken not to damage or alter critical elements to avoid spoiling evidence. Guidelines governing sample collection are covered in ASTM E 620, E 860, E 1020, and especially E 678. It is also recommended that samples be taken from other parts of the failed equipment as they may display supportive damage. It is often necessary to compare failed components with similar components that did not fail to determine whether the failure was brought about by service conditions or was the result of an error in manufacture. For example, if a boiler tube fails and overheating is suspected to be the cause, and if investigation reveals a spheroidized structure in the boiler tube at the failure site (which may be indicative of overheating in service), then comparison with an unexposed tube will determine if the tubes were supplied in the spheroidized condition. As another example, in the case of a bolt failure it is desirable to examine the nuts and other associated parts that may have contributed to the failures. Also, in failures involving corrosion, stress-corrosion, or corrosion fatigue, a sample of the fluid that has been in contact with the metal, or of any deposits that have formed, will often be required for analysis. Abnormal Conditions and Wreckage Analysis. In addition to developing a history of the failed part it is also advisable to determine if any abnormal conditions prevailed. Determine also whether events—such as an accident—occurred in
service that may have initiated the failure, or if any recent repairs or overhauls had been carried out and why. In addition, it is also necessary to inquire whether or not the failure was an isolated example or if others have occurred, either in the component under consideration or in another of a similar design. In the routine examination of a brittle fracture, it is important to know if, at the time of the accident or failure, the prevailing temperature was low, and/or if some measure of shock loading was involved. When dealing with failures of crankshafts or other shafts, it is generally desirable to ascertain the conditions of the bearings and whether any misalignment existed, either within the machine concerned or between the driving and driven components In an analysis where multiple components and structures are involved, it is essential that the position of each piece be documented before any of the pieces are touched or moved Such recording usually requires extensive photography, the preparation of suitable sketches, and the taking and tabulation of appropriate measurements of the pieces Next, it may be necessary to take an inventory to determine if all of the pieces or fragments are present at the site of the accident. For example, an investigation of an aircraft accident involves the development of a considerable inventory including listing the number of engines, flaps, landing gear, and the various parts of the fuselage and wings. It is essential to establish whether all the primary parts of the aircraft were aboard at the time that it crashed Providing an inventory although painstaking, is often invaluable. An experienced investigator determined the cause of a complex aircraft accident when he observed that a portion of one wing tip was missing from the main impact site. This fragment was subsequently located several miles back on the flight path of the aircraft. The fragment provided evidence of a fatigue failure and was the first component separated from the aircraft, thus accounting for the crash The most common problem encountered in examining wreckage involves the establishment of the sequence of fractures to determine the origin of the initial failure. Usually, the direction of crack growth can be detected from marks on a fracture surface, such as V-shaped chevron marks. The typical sequence of fractures is shown in Fig. 1(b), where A and B represent fractures that intersect at about 90. Here the sequence of fractures is clearly discernible from crack branching Obviously, fracture A must have occurred prior to fracture b because the presence of fracture a served to arrest cracking at fracture B. This method of sequencing is called the T-junction procedure and is an important technique in wreckage PROPAG森ToN LOCATION OF CRACK。RGN P Subsequent fracture. A fracture. B Fig. 1 General features to locate origin from crack paths (a)branching and(b) sequencing of cracking by the T-junction procedure, where fracture a precedes and arrests fracture B Provided the fragments are not permitted to contact each other, it is also helpful to carefully fit together the fragments of broken components which, when assembled and photographed, may indicate the sequence in which fractures occurred Figure 2 shows a lug that was part of a pin-joint assembly; failure occurred when the pin broke out of the lug. With the broken pieces of the lug fitted together, it is apparent from the deformation that fracture a must have preceded fractures B Thefileisdownloadedfromwww.bzfxw.com
service that may have initiated the failure, or if any recent repairs or overhauls had been carried out and why. In addition, it is also necessary to inquire whether or not the failure was an isolated example or if others have occurred, either in the component under consideration or in another of a similar design. In the routine examination of a brittle fracture, it is important to know if, at the time of the accident or failure, the prevailing temperature was low, and/or if some measure of shock loading was involved. When dealing with failures of crankshafts or other shafts, it is generally desirable to ascertain the conditions of the bearings and whether any misalignment existed, either within the machine concerned or between the driving and driven components. In an analysis where multiple components and structures are involved, it is essential that the position of each piece be documented before any of the pieces are touched or moved. Such recording usually requires extensive photography, the preparation of suitable sketches, and the taking and tabulation of appropriate measurements of the pieces. Next, it may be necessary to take an inventory to determine if all of the pieces or fragments are present at the site of the accident. For example, an investigation of an aircraft accident involves the development of a considerable inventory, including listing the number of engines, flaps, landing gear, and the various parts of the fuselage and wings. It is essential to establish whether all the primary parts of the aircraft were aboard at the time that it crashed. Providing an inventory, although painstaking, is often invaluable. An experienced investigator determined the cause of a complex aircraft accident when he observed that a portion of one wing tip was missing from the main impact site. This fragment was subsequently located several miles back on the flight path of the aircraft. The fragment provided evidence of a fatigue failure and was the first component separated from the aircraft, thus accounting for the crash. The most common problem encountered in examining wreckage involves the establishment of the sequence of fractures to determine the origin of the initial failure. Usually, the direction of crack growth can be detected from marks on a fracture surface, such as V-shaped chevron marks. The typical sequence of fractures is shown in Fig. 1(b), where A and B represent fractures that intersect at about 90°. Here the sequence of fractures is clearly discernible from crack branching. Obviously, fracture A must have occurred prior to fracture B because the presence of fracture A served to arrest cracking at fracture B. This method of sequencing is called the T-junction procedure and is an important technique in wreckage analysis. Fig. 1 General features to locate origin from crack paths (a) branching and (b) sequencing of cracking by the T-junction procedure, where fracture A precedes and arrests fracture B Provided the fragments are not permitted to contact each other, it is also helpful to carefully fit together the fragments of broken components which, when assembled and photographed, may indicate the sequence in which fractures occurred. Figure 2 shows a lug that was part of a pin-joint assembly; failure occurred when the pin broke out of the lug. With the broken pieces of the lug fitted together, it is apparent from the deformation that fracture A must have preceded fractures B The file is downloaded from www.bzfxw.com
and C. However, parts will not fit well together because of plastic deformation that occurred before or during the fracture process. B Fig. 2 Fractured lug, part of a pin-joint assembly, showing sequence of fracture Fracture a preceded fractures B and c Preliminary Examination The failed part, including all its fragments, should be subjected to a thorough visual examination before any cleaning is undertaken. Often, soils and debris found on the part provide useful evidence in establishing the cause of failure or in determining a sequence of events leading to the failure. For example, traces of paint or corrosion found on a portion of a fracture surface may provide evidence that the crack was present in the surface for some time before complete fracture occurred. Such evidence should be recorded photographical Visual Inspection. The preliminary examination should begin with unaided visual inspection. The unaided eye has exceptional depth of focus, the ability to examine large areas rapidly and to detect changes of color and texture. Some of these advantages are lost when any optical or electron-optical device is used. Particular attention should be given to the urfaces of fractures and to the paths of cracks. The significance of any indications of abnormal conditions or abuse in service should be observed and assessed, and a general assessment of the basic design and workmanship of the part should also be made. Each important feature, including dimensions, should be recorded, either in writing or by sketches or photographs It cannot be emphasized too strongly that the examination should be performed as carefully as possible, because clues to the cause of breakdown often are present, but may be missed if the observer is not vigilant. Inspection of the topographic features of the failed component should start with an unaided visual examination and proceed to higher and higher magnification. A magnifying glass followed by a low-power microscope is an invaluable aid in detection of small details Examination and Photography of the Damaged/Failed Part or Sample. The next step should be preliminary examination and general photography of the entire part and damaged or failed regions. Where fractures are involved, the entire fractured part, including broken pieces, should be examined and photographed to record their size and condition and to show how the fracture is related to the components. This should be followed by careful examination of the fracture. The examination should begin with the use of direct lighting and proceed at various angles of oblique lighting to delineate and emphasize fracture characteristics. This should also assist in determining which areas of the fracture are of prime interest and which magnifications will be possible(for a given picture size) to bring out fine details. When this evaluation has been completed, it is appropriate to proceed with photography of the fracture, recording what each photograph shows, its magnification, and how it relates to the other photographs. For information on photographic equipment, materials, and techniques, see the article Photography in Failure Analysis" in this Volume and the article entitled" Photography of Fractured Parts and Fracture Surfaces, on pages 78 to 90 in Fractography, Volume 12 of the Metals Handbook, 9th edition(now Volume 12 of the ASM Handbook) Nondestructive Inspection
and C. However, parts will not fit well together because of plastic deformation that occurred before or during the fracture process. Fig. 2 Fractured lug, part of a pin-joint assembly, showing sequence of fracture. Fracture A preceded fractures B and C. Preliminary Examination The failed part, including all its fragments, should be subjected to a thorough visual examination before any cleaning is undertaken. Often, soils and debris found on the part provide useful evidence in establishing the cause of failure or in determining a sequence of events leading to the failure. For example, traces of paint or corrosion found on a portion of a fracture surface may provide evidence that the crack was present in the surface for some time before complete fracture occurred. Such evidence should be recorded photographically. Visual Inspection. The preliminary examination should begin with unaided visual inspection. The unaided eye has exceptional depth of focus, the ability to examine large areas rapidly and to detect changes of color and texture. Some of these advantages are lost when any optical or electron-optical device is used. Particular attention should be given to the surfaces of fractures and to the paths of cracks. The significance of any indications of abnormal conditions or abuse in service should be observed and assessed, and a general assessment of the basic design and workmanship of the part should also be made. Each important feature, including dimensions, should be recorded, either in writing or by sketches or photographs. It cannot be emphasized too strongly that the examination should be performed as carefully as possible, because clues to the cause of breakdown often are present, but may be missed if the observer is not vigilant. Inspection of the topographic features of the failed component should start with an unaided visual examination and proceed to higher and higher magnification. A magnifying glass followed by a low-power microscope is an invaluable aid in detection of small details of the failed part. Examination and Photography of the Damaged/Failed Part or Sample. The next step should be preliminary examination and general photography of the entire part and damaged or failed regions. Where fractures are involved, the entire fractured part, including broken pieces, should be examined and photographed to record their size and condition and to show how the fracture is related to the components. This should be followed by careful examination of the fracture. The examination should begin with the use of direct lighting and proceed at various angles of oblique lighting to delineate and emphasize fracture characteristics. This should also assist in determining which areas of the fracture are of prime interest and which magnifications will be possible (for a given picture size) to bring out fine details. When this evaluation has been completed, it is appropriate to proceed with photography of the fracture, recording what each photograph shows, its magnification, and how it relates to the other photographs. For information on photographic equipment, materials, and techniques, see the article “Photography in Failure Analysis” in this Volume and the article entitled “Photography of Fractured Parts and Fracture Surfaces,” on pages 78 to 90 in Fractography, Volume 12 of the Metals Handbook, 9th edition (now Volume 12 of the ASM Handbook). Nondestructive Inspection