Although often used as quality-control tools, several nondestructive tests are useful in failure investigation and analysis hagnetic-particle inspection of ferrous metals, liquid-penetrant inspection, ultrasonic inspection, and sometimes eddy current inspection. All these tests are used to detect surface cracks and discontinuities. Radiography is used mainly for internal examination. A photographic record of the results of nondestructive inspection is a necessary part of record keeping in the investigation Magnetic-particle inspection utilizes magnetic fields to locate surface and subsurface discontinuities in ferromagnetic materials. When the material or part to be tested is magnetized, discontinuities that generally lie transverse to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part This leakage field, and therefore the presence of the discontinuity, is detected by means of fine ferromagnetic particles applied over the surface, some of which are gathered and held by the leakage field. The magnetically held collection of particles forms an outline of the discontinuity and indicates its size, shape, and extent. Frequently, a fluorescent material is combined with the particles so that discontinuities can be detected visually under ultraviolet light. This method reveals surface cracks that are not visible to the naked eye Liquid-penetrant inspection is used to detect surface flaws in materials. It is used mainly, but not exclusively, with nonmagnetic materials, on which magnetic-particle inspection cannot be used. This technique involves the spreading of a liquid penetrant on the sample. Liquid penetrants can seep into small cracks and flaws(as fine as 1 um)in the surface of the sample by capillary action. The excess liquid is wiped from the surface, and a developer is applied that causes the liquid to be drawn from the cracks or flaws that are open at the surface. The liquid itself is usually a very bright color or contains fluorescent particles that, under ultraviolet light, cause discontinuities in the material to stand out The main advantages of the liquid-penetrant method are its ability to be used on nonmagnetic materials, its low cost, its portability, and the ease with which results can be interpreted. The principal limitations of the liquid-penetrant method Discontinuities must be open to the surface Testpieces must be cleaned before and after testing because the liquid penetrant may corrode the metal Surface films may prevent detection of discontinuities Penetrant may be a source of contamination that masks results in subsequent chemical analysis of fracture surfaces The process is generally not suited to inspection of low-density powder-metallurgy parts or other porous materials Ultrasonic inspection methods depend on sound waves of very high frequency being transmitted through metal and reflected at any boundary such as a metal/air boundary at the surface of the metal, or a metal/crack boundary at discontinuity within the part or component. High-frequency sound waves can detect small irregularities, but they are easily absorbed, particularly by coarse-grained materials The application of ultrasonic testing is limited in failure analysis because accurate interpretations depend on reference standards to isolate the variables. In some instances, ultrasonic testing has proved to be a useful tool in failure analysis, particularly in the investigation of large castings and forgings. Cracks, laminations, shrinkage cavities, bursts, flakes, pores, disbonds, and other discontinuities that produce reflective interfaces can be easily detected. Inclusions and other nhomogeneities can also be detected by causing partial reflection or scattering of the ultrasonic waves or by producing some other detectable effect on the ultrasonic waves The disadvantages of ultrasonic inspection include Manual operation requires careful attention by experienced technicians Extensive technical knowledge is required for the development of inspection procedures Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect Discontinuities that are present in a shallow layer immediately beneath the surface may not be detectable Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being Reference standards are needed, both for calibrating the equipment and for characterizing flaws Radiography uses x-rays or gamma rays, which are directed through the sample to a photographic film. After the film has been developed, it can be examined by placing it in front of a light source. The intensity of the light passing through the film will be proportional to the density of the sample and the path length of the radiation. Thus, lighter areas on the plate correspond to the denser areas of the sample, whereas darker areas indicate a crack or defect running in the direction of the incident beam The main advantages of radiography are its ability to detect internal discontinuities and to provide permanent photographic records. However, certain types of flaws are difficult to detect by radiography. Laminar defects, such as cracks, present problems unless they are essentially parallel to the radiation beam. Tight, meandering cracks in thick Thefileisdownloadedfromwww.bzfxw.com
Although often used as quality-control tools, several nondestructive tests are useful in failure investigation and analysis: magnetic-particle inspection of ferrous metals, liquid-penetrant inspection, ultrasonic inspection, and sometimes eddycurrent inspection. All these tests are used to detect surface cracks and discontinuities. Radiography is used mainly for internal examination. A photographic record of the results of nondestructive inspection is a necessary part of record keeping in the investigation. Magnetic-particle inspection utilizes magnetic fields to locate surface and subsurface discontinuities in ferromagnetic materials. When the material or part to be tested is magnetized, discontinuities that generally lie transverse to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part. This leakage field, and therefore the presence of the discontinuity, is detected by means of fine ferromagnetic particles applied over the surface, some of which are gathered and held by the leakage field. The magnetically held collection of particles forms an outline of the discontinuity and indicates its size, shape, and extent. Frequently, a fluorescent material is combined with the particles so that discontinuities can be detected visually under ultraviolet light. This method reveals surface cracks that are not visible to the naked eye. Liquid-penetrant inspection is used to detect surface flaws in materials. It is used mainly, but not exclusively, with nonmagnetic materials, on which magnetic-particle inspection cannot be used. This technique involves the spreading of a liquid penetrant on the sample. Liquid penetrants can seep into small cracks and flaws (as fine as 1 μm) in the surface of the sample by capillary action. The excess liquid is wiped from the surface, and a developer is applied that causes the liquid to be drawn from the cracks or flaws that are open at the surface. The liquid itself is usually a very bright color or contains fluorescent particles that, under ultraviolet light, cause discontinuities in the material to stand out. The main advantages of the liquid-penetrant method are its ability to be used on nonmagnetic materials, its low cost, its portability, and the ease with which results can be interpreted. The principal limitations of the liquid-penetrant method are: · Discontinuities must be open to the surface. · Testpieces must be cleaned before and after testing because the liquid penetrant may corrode the metal. · Surface films may prevent detection of discontinuities. · Penetrant may be a source of contamination that masks results in subsequent chemical analysis of fracture surfaces. · The process is generally not suited to inspection of low-density powder-metallurgy parts or other porous materials. Ultrasonic inspection methods depend on sound waves of very high frequency being transmitted through metal and reflected at any boundary such as a metal/air boundary at the surface of the metal, or a metal/crack boundary at a discontinuity within the part or component. High-frequency sound waves can detect small irregularities, but they are easily absorbed, particularly by coarse-grained materials. The application of ultrasonic testing is limited in failure analysis because accurate interpretations depend on reference standards to isolate the variables. In some instances, ultrasonic testing has proved to be a useful tool in failure analysis, particularly in the investigation of large castings and forgings. Cracks, laminations, shrinkage cavities, bursts, flakes, pores, disbonds, and other discontinuities that produce reflective interfaces can be easily detected. Inclusions and other inhomogeneities can also be detected by causing partial reflection or scattering of the ultrasonic waves or by producing some other detectable effect on the ultrasonic waves. The disadvantages of ultrasonic inspection include: · Manual operation requires careful attention by experienced technicians. · Extensive technical knowledge is required for the development of inspection procedures. · Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect. · Discontinuities that are present in a shallow layer immediately beneath the surface may not be detectable. · Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected. · Reference standards are needed, both for calibrating the equipment and for characterizing flaws. Radiography uses x-rays or gamma rays, which are directed through the sample to a photographic film. After the film has been developed, it can be examined by placing it in front of a light source. The intensity of the light passing through the film will be proportional to the density of the sample and the path length of the radiation. Thus, lighter areas on the plate correspond to the denser areas of the sample, whereas darker areas indicate a crack or defect running in the direction of the incident beam. The main advantages of radiography are its ability to detect internal discontinuities and to provide permanent photographic records. However, certain types of flaws are difficult to detect by radiography. Laminar defects, such as cracks, present problems unless they are essentially parallel to the radiation beam. Tight, meandering cracks in thick The file is downloaded from www.bzfxw.com
sections usually cannot be detected even when properly oriented. Minute discontinuities, such as inclusions in wrought material, flakes, microporosity, and microfissures, cannot be detected unless they are sufficiently segregated to yield a detectable gross effect. Laminations normally are not detectable by radiography because of their unfavorable orientation usually parallel to the surface. Laminations seldom yield differences in absorption that enable laminated areas to be distinguished from lamination-free areas ldy-current inspection can be used on all materials that conduct electricity. If a coil conducting an alternating current is placed around or near the surface of the sample, it will set up eddy currents within the material by electromagnetic induction. These eddy currents affect the impedance in the exciting coil or any other pickup coil that is nearby. Cracks or flaws within the sample will cause distortions in the eddy current, which in turn cause distortion in the impedance of the coil. The resulting change in impedance can be detected by attaching the appropriate electrical circuits and a meter. Flaws or cracks will show up as some deflection or fluctuation on the meter The advantages of electromagnetic inspection are Both surface and subsurface defects are detectable No special operator skills are required The process is adaptable to continuous monitoring The process may be substantially automated and is capable of high speeds No probe contact is needed Limitations of electromagnetic inspection include Depth of penetration is shalle Materials to be inspected must be electrically conductive Indications are influenced by more than one variable Reference standards are Residual Stress Analysis. X-ray diffraction is the most common method for direct, nondestructive measurement of residual(internal)stresses in metals. Stresses are determined by measuring the submicroscopic distortion of crystalline lattice structures by tensile or compressive residual stresses. However, it should be pointed out that measurement of residual stresses near fractures or cracks may be erroneous because the residual stresses have already been relieved by the fracture and cracks. Testing of undamaged similar, or exemplar, parts is frequently used as the only alternative in order to understand the residual stress system in the failed part prior to failure. More information is in the article "X-Ray Diffraction Residual Stress Measurement in Failure Analysis" in this Volume Acoustic-emission inspection detects and analyzes minute acoustic-emission signals generated by discontinuities in materials under applied stress. Proper analysis of these signals can provide information concerning the location and structural significance of the detected discontinuities Some of the significant applications of acoustic-emission inspection are Continuous surveillance of pressure vessels and nuclear primary-pressure boundaries for the detection and location of active flaws Detection of incipient fatigue fracture in aircraft structures Monitoring of both fusion and resistance weldments during welding and cooling Determination of the onset of stress-corrosion cracking(SCC)and hydrogen damage in susceptible structures Use as a study tool for the investigation of fracture mechanisms and of behavior of materials Periodic inspection of tanks and aerial-device booms made of composite materials This type of data may be useful background information in a failure analysis, or the technique might be used in evaluation of stress effects. Sources of acoustic emission that generate stress waves in material include local dynamic movements uch as the initiation and propagation of cracks, twinning, slip, sudden reorientation of grain boundaries, and bubble formation during boiling. This energy may originate from stored elastic energy, as in crack propagation, or from stored chemical-free energy, as in phase transformation Experimental stress analysis can be done by several methods, all of which may be valuable in determining machine loads and component stresses that can cause failures. Stress coating can be used effectively for locating small areas of high strains, determining the directions of the principal strains, and measuring the approximate magnitude of tensile and compressive strains. Gages can then be placed at the high-strain areas and in the principal-strain directions to measure the strain accurately on gage lengths 0.5 to 150 mm(0.02 to 6 in. ) Although there are many mechanical, optical, and electrical devices capable of accurate strain measurements, the bonded electrical-resistance strain gage has become the standard tool for general laboratory and field use
sections usually cannot be detected even when properly oriented. Minute discontinuities, such as inclusions in wrought material, flakes, microporosity, and microfissures, cannot be detected unless they are sufficiently segregated to yield a detectable gross effect. Laminations normally are not detectable by radiography because of their unfavorable orientation, usually parallel to the surface. Laminations seldom yield differences in absorption that enable laminated areas to be distinguished from lamination-free areas. Eddy-current inspection can be used on all materials that conduct electricity. If a coil conducting an alternating current is placed around or near the surface of the sample, it will set up eddy currents within the material by electromagnetic induction. These eddy currents affect the impedance in the exciting coil or any other pickup coil that is nearby. Cracks or flaws within the sample will cause distortions in the eddy current, which in turn cause distortion in the impedance of the coil. The resulting change in impedance can be detected by attaching the appropriate electrical circuits and a meter. Flaws or cracks will show up as some deflection or fluctuation on the meter. The advantages of electromagnetic inspection are: · Both surface and subsurface defects are detectable. · No special operator skills are required. · The process is adaptable to continuous monitoring. · The process may be substantially automated and is capable of high speeds. · No probe contact is needed. Limitations of electromagnetic inspection include: · Depth of penetration is shallow. · Materials to be inspected must be electrically conductive. · Indications are influenced by more than one variable. · Reference standards are required. Residual Stress Analysis. X-ray diffraction is the most common method for direct, nondestructive measurement of residual (internal) stresses in metals. Stresses are determined by measuring the submicroscopic distortion of crystalline lattice structures by tensile or compressive residual stresses. However, it should be pointed out that measurement of residual stresses near fractures or cracks may be erroneous because the residual stresses have already been relieved by the fracture and cracks. Testing of undamaged similar, or exemplar, parts is frequently used as the only alternative in order to understand the residual stress system in the failed part prior to failure. More information is in the article “X-Ray Diffraction Residual Stress Measurement in Failure Analysis” in this Volume. Acoustic-emission inspection detects and analyzes minute acoustic-emission signals generated by discontinuities in materials under applied stress. Proper analysis of these signals can provide information concerning the location and structural significance of the detected discontinuities. Some of the significant applications of acoustic-emission inspection are: · Continuous surveillance of pressure vessels and nuclear primary-pressure boundaries for the detection and location of active flaws · Detection of incipient fatigue fracture in aircraft structures · Monitoring of both fusion and resistance weldments during welding and cooling · Determination of the onset of stress-corrosion cracking (SCC) and hydrogen damage in susceptible structures · Use as a study tool for the investigation of fracture mechanisms and of behavior of materials · Periodic inspection of tanks and aerial-device booms made of composite materials This type of data may be useful background information in a failure analysis, or the technique might be used in evaluation of stress effects. Sources of acoustic emission that generate stress waves in material include local dynamic movements, such as the initiation and propagation of cracks, twinning, slip, sudden reorientation of grain boundaries, and bubble formation during boiling. This energy may originate from stored elastic energy, as in crack propagation, or from stored chemical-free energy, as in phase transformation. Experimental stress analysis can be done by several methods, all of which may be valuable in determining machine loads and component stresses that can cause failures. Stress coating can be used effectively for locating small areas of high strains, determining the directions of the principal strains, and measuring the approximate magnitude of tensile and compressive strains. Gages can then be placed at the high-strain areas and in the principal-strain directions to measure the strain accurately on gage lengths 0.5 to 150 mm (0.02 to 6 in.). Although there are many mechanical, optical, and electrical devices capable of accurate strain measurements, the bonded electrical-resistance strain gage has become the standard tool for general laboratory and field use
Photoelastic coatings have also been used for laboratory stress measurements. For this technique, a birefringent coating of controlled thickness is bonded to the testpiece with a reflective cement. Optical analysis is similar to conventiona analysis but requires special equipment. The analysis may be recorded on color film with single-frame or movie camera Practices in Failure Analysis FI ractures Establishing the origin of a fracture is essential in failure analysis, and the location of the origin determines which measures oh ould be taken to pare ent ve ret tihe dif thio ratred the fract e-surtae fe tharescterci tis cthat sh wa crack branching, and river patterns. Features that help identify the crack origin include concentric fibrous marks, radial marks, and beach marks. By a study of these features, crack progress can be traced back to the point of origin, and then it can be ascertained whether the crack was initiated by an inclusion, a porous region, a segregated phase, a corrosion pit,a machined notch, a forging lap, a nick, a mar, or another type of discontinuity, or was simply the result of overloading However, time employed in ascertaining all the circumstances of a failure is extremely important. When a broken component is received for examination, the investigator is sometimes inclined to prepare specimens immediately without devising an investigative procedure. To proceed without forethought may destroy important evidence and waste time. Some of the questions that should be raised concerning the nature, history, functions, and properties of the fractured part, and the manner in which it interacts with other parts, are Loading. Were the nature, rate, and magnitude of the applied load correctly anticipated in the design of the part? Were repeated or cyclic loadings involved? What was the direction of the principal stress relative to the shape of the part? Were residual stresses present to an undesirable degree? Material. Was the recommended alloy used? Were its mechanical properties at the level expected? Were surface or internal discontinuities present that could have contributed to failure? Did the microstructure conform to that prescribed? Shape. Did the part comply with all pertinent dimensional requirements of the specification? Did the part have sufficient section thickness to prevent local overloading? Were fillets formed with sufficiently large radii? Were there adequate clearances between interacting parts? Were any of the contours deformed during service? Was there evidence of mechanical surface damage? Environment. Was the part exposed to a corrosive environment or to excessively high or low temperatures? Was interaction( for example, galvanic)between the material of the part and that of adjacent componen? Was there the surface of the part suitably protected? Were the properties of the part altered by the exposure Examination of a fracture begins with visual scrutiny, which establishes Whether there is gross evidence of mechanical abuse Whether there are indications of excessive corrosion Whether the part is deformed Whether there are obvious secondary fractures Whether the origin of the crack can be readily identified Whether the direction of crack propagation can be easily recognized Often it is helpful to have an undamaged part of the same design as the fractured part available during this portion of the examination. The findings of this scrutiny will permit many deductions to be drawn concerning the service conditions existing at and prior to the time of fracture. These findings can then be extended by an examination of the fracture surface at low magnification with a stereomicroscope and then at high magnification by electron microscopy, metallography (occasionally ), or some combination of these examination techniques. a survey at low magnification is important for identification of those areas that need further inspection at high magnification. The salient features are recorded in fractographs of appropriate magnification for report purposes and for future reference should subsequent handling or sectioning destroy evidence needed for failure analysis Fracture of a part in service is often intimately associated with the type of environment to which the part was exposed Active chemical environments include water salt air. salt water. acid solutions. alkaline solutions. molten metal. and even solid metal. Thermal environments that affect metal properties and fracture include exposure to low(cryogenic, for example) and high temperatures Thefileisdownloadedfromwww.bzfxw.com
Photoelastic coatings have also been used for laboratory stress measurements. For this technique, a birefringent coating of controlled thickness is bonded to the testpiece with a reflective cement. Optical analysis is similar to conventional analysis but requires special equipment. The analysis may be recorded on color film with single-frame or movie camera. Practices in Failure Analysis Fractures Establishing the origin of a fracture is essential in failure analysis, and the location of the origin determines which measures should be taken to prevent a repetition of the fracture. The fracture-surface characteristics that show the direction of crack propagation (and conversely, the direction toward the origin) include features such as chevron marks, crack branching, and river patterns. Features that help identify the crack origin include concentric fibrous marks, radial marks, and beach marks. By a study of these features, crack progress can be traced back to the point of origin, and then it can be ascertained whether the crack was initiated by an inclusion, a porous region, a segregated phase, a corrosion pit, a machined notch, a forging lap, a nick, a mar, or another type of discontinuity, or was simply the result of overloading. However, time employed in ascertaining all the circumstances of a failure is extremely important. When a broken component is received for examination, the investigator is sometimes inclined to prepare specimens immediately without devising an investigative procedure. To proceed without forethought may destroy important evidence and waste time. Some of the questions that should be raised concerning the nature, history, functions, and properties of the fractured part, and the manner in which it interacts with other parts, are: · Loading. Were the nature, rate, and magnitude of the applied load correctly anticipated in the design of the part? Were repeated or cyclic loadings involved? What was the direction of the principal stress relative to the shape of the part? Were residual stresses present to an undesirable degree? · Material. Was the recommended alloy used? Were its mechanical properties at the level expected? Were surface or internal discontinuities present that could have contributed to failure? Did the microstructure conform to that prescribed? · Shape. Did the part comply with all pertinent dimensional requirements of the specification? Did the part have sufficient section thickness to prevent local overloading? Were fillets formed with sufficiently large radii? Were there adequate clearances between interacting parts? Were any of the contours deformed during service? Was there evidence of mechanical surface damage? · Environment. Was the part exposed to a corrosive environment or to excessively high or low temperatures? Was the surface of the part suitably protected? Were the properties of the part altered by the exposure? Was there interaction (for example, galvanic) between the material of the part and that of adjacent components? Examination of a fracture begins with visual scrutiny, which establishes: · Whether there is gross evidence of mechanical abuse · Whether there are indications of excessive corrosion · Whether the part is deformed · Whether there are obvious secondary fractures · Whether the origin of the crack can be readily identified · Whether the direction of crack propagation can be easily recognized Often it is helpful to have an undamaged part of the same design as the fractured part available during this portion of the examination. The findings of this scrutiny will permit many deductions to be drawn concerning the service conditions existing at and prior to the time of fracture. These findings can then be extended by an examination of the fracture surface at low magnification with a stereomicroscope and then at high magnification by electron microscopy, metallography (occasionally), or some combination of these examination techniques. A survey at low magnification is important for identification of those areas that need further inspection at high magnification. The salient features are recorded in fractographs of appropriate magnification for report purposes and for future reference should subsequent handling or sectioning destroy evidence needed for failure analysis. Fracture of a part in service is often intimately associated with the type of environment to which the part was exposed. Active chemical environments include water, salt air, salt water, acid solutions, alkaline solutions, molten metal, and even solid metal. Thermal environments that affect metal properties and fracture include exposure to low (cryogenic, for example) and high temperatures. The file is downloaded from www.bzfxw.com
Selection and Preservation of fracture surfaces The proper selection, preservation, and cleaning of fracture surfaces is vital to prevent important evidence from being destroyed or obscured. Surfaces of fractures may suffer either mechanical or chemical damage. Mechanical damage may arise from several sources, including the striking of the surface of the fracture by other objects. This can occur during actual fracture in service or when removing or transporting a fractured part for analysis Usually, the surface of a fracture can be protected during shipment by a cloth or cotton covering, but this may remove some loosely adhering material that might contain the primary clue to the cause of the fracture. Touching or rubbing the surface of a fracture with the fingers should definitely be avoided. Also, no attempt should be made to fit together the ections of a fractured part by placing them in contact. This generally accomplishes nothing and almost always causes damage to the fracture surface. The use of corrosion-inhibiting paper to package samples should be considered Chemical(corrosion) damage to a fracture specimen can be prevented in several ways. For instance, because the of the fracture, many laboratories prefer not to use corrosion-preventive coatings on a fracture specimen. When posite c identification of foreign material present on a fracture surface may be important in the overall determination of the cause it is best to dry the fracture specimen, preferably using a jet of dry, compressed air(which will also blow extraneous foreign material from the surface), and then to place it in a dessicator or pack it with a suitable dessicant. However, clean, fresh fracture surfaces should be coated when they cannot be protected from the elements. After failure of large structures several days may be required to remove critical specimens, and so coating the fracture surfaces would be the prudent Cleaning of fractured surfaces should be avoided in general, but must be done for SEM examination and often to reveal acroscopic fractographic features. Cleaning should proceed in stages using the least aggressive procedure first, then proceeding to more aggressive procedures if needed. Washing the fracture surface with water should especially avoided. However, specimens contaminated with seawater or with fire-extinguishing fluids require thorough washing, usually with water, followed by a rinse with acetone or alcohol before storage in a dessicator or coating with a dessicant Sometimes cleaning may also be required for removal of obliterating debris and dirt, or to prepare the fracture surface for SEM examination. Other acceptable cleaning procedures include use of a dry-air blast or of a soft-hair artist's brush treating with inorganic solvents, either by immersion or by jet; treating with mild acid or alkaline solutions(depending the metal)that will attack deposits but to which the base metal is essentially inert; ultrasonic cleaning; and application and stripping of plastic replicas Cleaning with cellulose acetate tape is one of the most widely used methods, particularly when the surface of a fracture has been affected by corrosion. A strip of acetate about 0. 1 mm(0.005 in thick and of suitable size is softened by immersion in acetone and placed on the fracture surface. The initial strip is backed by a piece of unsoftened acetate, and then the replica is pressed hard onto the surface of the fracture using a finger. The drying time will depend on the extent to which the replicating material was softened, and this in turn will be governed by the texture of the surface of the fracture. Drying times of not less than 15 to 30 min are recommended. The dry replica is lifted from the fracture, using a scalpel or tweezers. The replicating procedure must be repeated several times if the fracture is badly contaminated. When a clean and uncontaminated replica is obtained, the process is complete. An advantage of this method is that the debris removed from the fracture is preserved for any subsequent examination that may be necessary for identification by x-ray or electron diffraction techniques. To be complete, the analyst should filter solvents used for cleaning to recapture insoluble particulates Sectioning. Because examination tools, including hardness testers and optical and electron microscopes, are limited as to the size of specimen they can accept, it is often necessary to remove from a failed component a fracture-containing portion or section that is of a size convenient to handle and examine. This is a destructive process and may spoil evidence n potential litigation cases Before cutting or sectioning, the fracture area should be carefully protected. All cutting should be done so that surfaces of fractures and areas adjacent to them are not damaged or altered; this includes keeping the fracture surface dry, whenever possible. For large parts, the common method of removing specimens is by flame cutting. Cutting must be done at a sufficient distance from the fracture site so that the microstructure of the metal underlying the surface of the fracture is not altered by the heat of the flame, and so that none of the molten metal from flame cutting is deposited on the surface of the fracture Heat from any source can affect metal properties and microstructures during cutting. Therefore, dry abrasive cutoff wheels should never be used near critical surfaces that will be examined microscopically. Therefore, sectioning should be performed with jewelers'saws, precision diamond-edged, thin cutoff wheels, hacksaws, band saws, or soft abrasive cutoff wheels flooded with water-based soluble oil solution to keep metal surfaces cool and corrosion-free. Dry cutting with an air-driven abrasive disk may also be used with care to remove small specimens from large parts if kept cool, along with coating the fracture surface for protection fracture surfaces for examination and study These cracks may provide more information than the primary facile se. Secondary Cracks. When the primary fracture has been damaged or corroded to such a degree that most of the information relevant to the cause of the failure is obliterated, it is desirable to open any secondary cracks to expose their
Selection and Preservation of Fracture Surfaces The proper selection, preservation, and cleaning of fracture surfaces is vital to prevent important evidence from being destroyed or obscured. Surfaces of fractures may suffer either mechanical or chemical damage. Mechanical damage may arise from several sources, including the striking of the surface of the fracture by other objects. This can occur during actual fracture in service or when removing or transporting a fractured part for analysis. Usually, the surface of a fracture can be protected during shipment by a cloth or cotton covering, but this may remove some loosely adhering material that might contain the primary clue to the cause of the fracture. Touching or rubbing the surface of a fracture with the fingers should definitely be avoided. Also, no attempt should be made to fit together the sections of a fractured part by placing them in contact. This generally accomplishes nothing and almost always causes damage to the fracture surface. The use of corrosion-inhibiting paper to package samples should be considered. Chemical (corrosion) damage to a fracture specimen can be prevented in several ways. For instance, because the identification of foreign material present on a fracture surface may be important in the overall determination of the cause of the fracture, many laboratories prefer not to use corrosion-preventive coatings on a fracture specimen. When possible, it is best to dry the fracture specimen, preferably using a jet of dry, compressed air (which will also blow extraneous foreign material from the surface), and then to place it in a dessicator or pack it with a suitable dessicant. However, clean, fresh fracture surfaces should be coated when they cannot be protected from the elements. After failure of large structures several days may be required to remove critical specimens, and so coating the fracture surfaces would be the prudent decision. Cleaning of fractured surfaces should be avoided in general, but must be done for SEM examination and often to reveal macroscopic fractographic features. Cleaning should proceed in stages using the least aggressive procedure first, then proceeding to more aggressive procedures if needed. Washing the fracture surface with water should especially be avoided. However, specimens contaminated with seawater or with fire-extinguishing fluids require thorough washing, usually with water, followed by a rinse with acetone or alcohol before storage in a dessicator or coating with a dessicant. Sometimes cleaning may also be required for removal of obliterating debris and dirt, or to prepare the fracture surface for SEM examination. Other acceptable cleaning procedures include use of a dry-air blast or of a soft-hair artist's brush; treating with inorganic solvents, either by immersion or by jet; treating with mild acid or alkaline solutions (depending on the metal) that will attack deposits but to which the base metal is essentially inert; ultrasonic cleaning; and application and stripping of plastic replicas. Cleaning with cellulose acetate tape is one of the most widely used methods, particularly when the surface of a fracture has been affected by corrosion. A strip of acetate about 0.1 mm (0.005 in.) thick and of suitable size is softened by immersion in acetone and placed on the fracture surface. The initial strip is backed by a piece of unsoftened acetate, and then the replica is pressed hard onto the surface of the fracture using a finger. The drying time will depend on the extent to which the replicating material was softened, and this in turn will be governed by the texture of the surface of the fracture. Drying times of not less than 15 to 30 min are recommended. The dry replica is lifted from the fracture, using a scalpel or tweezers. The replicating procedure must be repeated several times if the fracture is badly contaminated. When a clean and uncontaminated replica is obtained, the process is complete. An advantage of this method is that the debris removed from the fracture is preserved for any subsequent examination that may be necessary for identification by x-ray or electron diffraction techniques. To be complete, the analyst should filter solvents used for cleaning to recapture insoluble particulates. Sectioning. Because examination tools, including hardness testers and optical and electron microscopes, are limited as to the size of specimen they can accept, it is often necessary to remove from a failed component a fracture-containing portion or section that is of a size convenient to handle and examine. This is a destructive process and may spoil evidence in potential litigation cases. Before cutting or sectioning, the fracture area should be carefully protected. All cutting should be done so that surfaces of fractures and areas adjacent to them are not damaged or altered; this includes keeping the fracture surface dry, whenever possible. For large parts, the common method of removing specimens is by flame cutting. Cutting must be done at a sufficient distance from the fracture site so that the microstructure of the metal underlying the surface of the fracture is not altered by the heat of the flame, and so that none of the molten metal from flame cutting is deposited on the surface of the fracture. Heat from any source can affect metal properties and microstructures during cutting. Therefore, dry abrasive cutoff wheels should never be used near critical surfaces that will be examined microscopically. Therefore, sectioning should be performed with jewelers' saws, precision diamond-edged, thin cutoff wheels, hacksaws, band saws, or soft abrasive cutoff wheels flooded with water-based soluble oil solution to keep metal surfaces cool and corrosion-free. Dry cutting with an air-driven abrasive disk may also be used with care to remove small specimens from large parts if kept cool, along with coating the fracture surface for protection. Secondary Cracks. When the primary fracture has been damaged or corroded to such a degree that most of the information relevant to the cause of the failure is obliterated, it is desirable to open any secondary cracks to expose their fracture surfaces for examination and study. These cracks may provide more information than the primary fracture
In opening cracks for examination, care must be exercised to prevent damage, primarily mechanical, to the surface of the fracture. This can usually be accomplished if opening is done in such a way that the two surfaces of the fracture are moved in opposite directions, normal to the fracture plane. Generally, a saw cut can be made from the back of the fractured part to a point near the tip of the crack, using extreme care to avoid actually reaching the tip of the crack. This saw cut will reduce the amount of solid metal that must be broken. The final breaking of the specimen can be done in By clamping the two sides of the fractured part in a tensile-testing machine, if the shape permits, and pulling By placing the specimen in a vise and bending one half away from the other by striking it with a hammer in a manner that will avoid damage to the surfaces of the crack By gripping the halves of the fracture in pliers or vise grips and bending or pulling them apart Cooling the part with liquid nitrogen often reduces the force and plastic deformation necessary to fracture the part Fortunately, there is little confusion during subsequent examination as to which part of the fracture surface was obtained in opening the crack. It is recommended that both the crack separation and the visible crack length be measured prior to opening. The analyst may have to use dye penetrant or other nondestructive evaluation technique to actually see the length of a tightly closed crack. Often, the amount of strain that occurred in the specimen can be determined from a measurement of the separation between the adjacent halves of a fracture. This should be done before preparation for opening a secondary crack has begun. The lengths of cracks may also be important for analyses of fatigue fractures or for consideration for the application of fracture mechanics Macroscopic Examination of Fracture Surfaces One very important part of any failure analysis is the macroscopic examination of fracture surfaces. Performed at magnifications from 1 to 50 or 100x, it may be conducted by the unaided eye, a hand lens or magnifier, a low-power stereoscopic microscope, or a SEM. Macroscopic photography of up to 20x magnification requires a high-quality camera and special lenses; alternatively, a large magnifying glass may be used to enlarge a specific area in the photo, such as a crack or other small detail. A metallographic microscope with macroobjectives and lights may be used for somewhat higher magnifications. However, depth of field becomes extremely limited with light optics. For much greater depth of field, a SEM may be used for low-magnification photography as well as higher-magnification work. Stereo or three dimensional photographs may also be made to reveal the topographic features of a fracture or other surface Frequently, a specimen may be too large or too heavy for the stage of the metallograph or the chamber of a SEM, and cutting or sectioning the specimen may be difficult or not allowed because of legal limitations at the time. In these instances, excellent results can be achieved by examining and-where appropriate-photographing replicas made by the method for cleaning fractures(see discussion under "Cleaning"). These replicas can be coated with a thin layer(about 20 nm, or 2 x 10 m, thick)of vacuum-deposited gold or aluminum to improve their reflectivity, or they may be shadowed at an angle to increase the contrast of fine detail. The replicas may be examined by incident-light or transmitted-light microscopy. Because they are electrically conductive, the coated replicas may also be examined with a SEM The amount of information that can be obtained from examination of a fracture surface at low-power magnification is extensive. A careful scan of the exterior surface of the part in the area adjacent to the fracture is required to determine whether specific stress raisers are present of a type that could have initiated the fracture. If any marks possess sha reentrant angles, they constitute sites of stress concentration, a frequent cause of crack initiation. In this situation, obvious remedy is more careful handling procedures and better inspection. Tool marks are another source of stress concentration. A fillet that has too small a radius, even though the surface of the fillet may be an excellent example of high-grade machining, is a recognized initiation site for fatigue cracks. Sharp-bottomed tool marks can initiate fatigue fractures even though the general contour of the area has a generous radius The shape, size, and cross section of a specimen or structural component can have a large effect on both the macroscopic and the microscopic appearance of the fracture surface, especially when pronounced stress raisers are present. Holes, corners, notches, machining marks, and, most of all, preexisting cracks actively influence fracture appearance Pronounced stress raisers are more likely to be contained in a large part than in a small part, because large parts have greater volume and surface area The orientation of the fracture surfaces must be consistent with the proposed mode of failure and the known loads on the failed part. Failure in monotonic tension produces a flat(square) fracture normal (perpendicular)to the maximum tensile stress and frequently a slant(shear) fracture at about 45. This 45 slant fracture is often called a"shear lip. Many fractures are flat at the center, but surrounded by a picture frame of slant fracture. An example of this behavior is to be found in the familiar cup-and-cone fracture of a round tensile test bar Thefileisdownloadedfromwww.bzfxw.com
In opening cracks for examination, care must be exercised to prevent damage, primarily mechanical, to the surface of the fracture. This can usually be accomplished if opening is done in such a way that the two surfaces of the fracture are moved in opposite directions, normal to the fracture plane. Generally, a saw cut can be made from the back of the fractured part to a point near the tip of the crack, using extreme care to avoid actually reaching the tip of the crack. This saw cut will reduce the amount of solid metal that must be broken. The final breaking of the specimen can be done in several ways: · By clamping the two sides of the fractured part in a tensile-testing machine, if the shape permits, and pulling · By placing the specimen in a vise and bending one half away from the other by striking it with a hammer in a manner that will avoid damage to the surfaces of the crack · By gripping the halves of the fracture in pliers or vise grips and bending or pulling them apart Cooling the part with liquid nitrogen often reduces the force and plastic deformation necessary to fracture the part. Fortunately, there is little confusion during subsequent examination as to which part of the fracture surface was obtained in opening the crack. It is recommended that both the crack separation and the visible crack length be measured prior to opening. The analyst may have to use dye penetrant or other nondestructive evaluation technique to actually see the length of a tightly closed crack. Often, the amount of strain that occurred in the specimen can be determined from a measurement of the separation between the adjacent halves of a fracture. This should be done before preparation for opening a secondary crack has begun. The lengths of cracks may also be important for analyses of fatigue fractures or for consideration for the application of fracture mechanics. Macroscopic Examination of Fracture Surfaces One very important part of any failure analysis is the macroscopic examination of fracture surfaces. Performed at magnifications from 1 to 50 or 100×, it may be conducted by the unaided eye, a hand lens or magnifier, a low-power stereoscopic microscope, or a SEM. Macroscopic photography of up to 20× magnification requires a high-quality camera and special lenses; alternatively, a large magnifying glass may be used to enlarge a specific area in the photo, such as a crack or other small detail. A metallographic microscope with macroobjectives and lights may be used for somewhat higher magnifications. However, depth of field becomes extremely limited with light optics. For much greater depth of field, a SEM may be used for low-magnification photography as well as higher-magnification work. Stereo or threedimensional photographs may also be made to reveal the topographic features of a fracture or other surface. Frequently, a specimen may be too large or too heavy for the stage of the metallograph or the chamber of a SEM, and cutting or sectioning the specimen may be difficult or not allowed because of legal limitations at the time. In these instances, excellent results can be achieved by examining and—where appropriate—photographing replicas made by the method for cleaning fractures (see discussion under “Cleaning”). These replicas can be coated with a thin layer (about 20 nm, or 2 × 10-8 m, thick) of vacuum-deposited gold or aluminum to improve their reflectivity, or they may be shadowed at an angle to increase the contrast of fine detail. The replicas may be examined by incident-light or transmitted-light microscopy. Because they are electrically conductive, the coated replicas may also be examined with a SEM. The amount of information that can be obtained from examination of a fracture surface at low-power magnification is extensive. A careful scan of the exterior surface of the part in the area adjacent to the fracture is required to determine whether specific stress raisers are present of a type that could have initiated the fracture. If any marks possess sharp reentrant angles, they constitute sites of stress concentration, a frequent cause of crack initiation. In this situation, the obvious remedy is more careful handling procedures and better inspection. Tool marks are another source of stress concentration. A fillet that has too small a radius, even though the surface of the fillet may be an excellent example of high-grade machining, is a recognized initiation site for fatigue cracks. Sharp-bottomed tool marks can initiate fatigue fractures even though the general contour of the area has a generous radius. The shape, size, and cross section of a specimen or structural component can have a large effect on both the macroscopic and the microscopic appearance of the fracture surface, especially when pronounced stress raisers are present. Holes, corners, notches, machining marks, and, most of all, preexisting cracks actively influence fracture appearance. Pronounced stress raisers are more likely to be contained in a large part than in a small part, because large parts have greater volume and surface area. The orientation of the fracture surfaces must be consistent with the proposed mode of failure and the known loads on the failed part. Failure in monotonic tension produces a flat (square) fracture normal (perpendicular) to the maximum tensile stress and frequently a slant (shear) fracture at about 45°. This 45° slant fracture is often called a “shear lip.” Many fractures are flat at the center, but surrounded by a “picture frame” of slant fracture. An example of this behavior is to be found in the familiar cup-and-cone fracture of a round tensile test bar. The file is downloaded from www.bzfxw.com