Fig. 14 Deformation from threads that left impressions on the inside surface of a hole as the members sheared with respect to each other. These few examples illustrate that understanding and interpretation of distortion may be important in a wide range of failure investigations. These examples are by no means unique or unusual, and many readers may no doubt have experienced similar situations. These examples also show the need for a thorough investigation to ensure that the true root cause is identified. For example, when a component is found that does not meet specification, it is most tempting and may seem most expedient for the investigator to simply blame the deficient component. Similarly, if the cause of a failure is not immediately apparent, it may be easiest to blame the operator, driver, or pilot. However, if the true root cause is overlooked due to a superficial investigation, then it is possible that the accident, fire, and so forth will occur again, with unnecessary loss of life or property Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Deformation by design Distortion and deformation are not al ways indications of failure. It is hopefully well recognized that any component experiencing a stress will also exhibit an associated elastic distortion. In most designs, stress levels are such that this distortion is minimal and may not even be noticed. However, some components are expected to experience significant elastic and sometimes even plastic distortion during their service lives Springs of all types are an obvious example of a cyclic elastic distortion by design. Many of the examples earlier in this article have dealt with failures of springs through development of unwanted plastic distortion Springs can also be susceptible to fatigue failures. See the article"Fatigue Failures"in this Volume. Threaded fasteners are another type of component in which understanding of the elastic distortion is important In the design of bolted connections the elastic stretch of the threaded fastener is large relative to the compression of the joint members. With the compliance of the fastener being significantly greater than the joint, the cyclic stress amplitude on the fastener is minimized. It is generally desired to maintain as high a clamp load as possible without introducing plastic deformation into either the fastener or the joint components Failure analysis of bolted connections requires consideration of the initial clamp load as well as factors that may cause a reduction in the clamp load in service. Reference 9 is a useful resource for those analyzing the failure of a bolted connection With respect to plastic distortion, a major consideration in the design of any automobile is how it will perform under various accident scenarios. The structure of the vehicle is sacrificed to protect the occupants. The vehicle components must possess a combination of strength and the capability to deform and absorb the energy of the ollision, thereby reducing the forces and energy experienced by the occupants. Sometimes, and not surprisingly, the deformation process leads to fracture. Some observers may decry the deformed appearance of the automobile after an accident, saying"they don' t make them like they used to. "More sophisticated analysis will consider the vehicle dynamics to calculate the forces of the accident This, in turn, allows consideration of the stresses experienced by the vehicle components, and whether or not the deformation observed is that which would be expected based on design analysis and actual crash testing. For further details, see the article Modeling and Accident Reconstruction"in this Volume as well as Sae Standards and Federal Motor Vehicle Safety Standards Armor provides another example of a type of engineered product that deforms in service without bein considered"failed. The armor succeeds when it is able to resist the bullet or projectile. While some armors are high strength and distortion will not be observed, others absorb the energy of the projectile by deformation Again, success or failure is determined by comparison with design expectations Reference cited in this section 9. J.H. Bickford, Introduction to the Design and Behaviour of Bolted Joints, 3rd ed, Marcel Dekker, 1995 Thefileisdownloadedfromwww.bzfxw.com
Fig. 14 Deformation from threads that left impressions on the inside surface of a hole as the members sheared with respect to each other. These few examples illustrate that understanding and interpretation of distortion may be important in a wide range of failure investigations. These examples are by no means unique or unusual, and many readers may no doubt have experienced similar situations. These examples also show the need for a thorough investigation to ensure that the true root cause is identified. For example, when a component is found that does not meet specification, it is most tempting and may seem most expedient for the investigator to simply blame the deficient component. Similarly, if the cause of a failure is not immediately apparent, it may be easiest to blame the operator, driver, or pilot. However, if the true root cause is overlooked due to a superficial investigation, then it is possible that the accident, fire, and so forth will occur again, with unnecessary loss of life or property. Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A. Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Deformation by Design Distortion and deformation are not always indications of failure. It is hopefully well recognized that any component experiencing a stress will also exhibit an associated elastic distortion. In most designs, stress levels are such that this distortion is minimal and may not even be noticed. However, some components are expected to experience significant elastic and sometimes even plastic distortion during their service lives. Springs of all types are an obvious example of a cyclic elastic distortion by design. Many of the examples earlier in this article have dealt with failures of springs through development of unwanted plastic distortion. Springs can also be susceptible to fatigue failures. (See the article “Fatigue Failures” in this Volume.) Threaded fasteners are another type of component in which understanding of the elastic distortion is important. In the design of bolted connections, the elastic stretch of the threaded fastener is large relative to the compression of the joint members. With the compliance of the fastener being significantly greater than the joint, the cyclic stress amplitude on the fastener is minimized. It is generally desired to maintain as high a clamp load as possible without introducing plastic deformation into either the fastener or the joint components. Failure analysis of bolted connections requires consideration of the initial clamp load as well as factors that may cause a reduction in the clamp load in service. Reference 9 is a useful resource for those analyzing the failure of a bolted connection. With respect to plastic distortion, a major consideration in the design of any automobile is how it will perform under various accident scenarios. The structure of the vehicle is sacrificed to protect the occupants. The vehicle components must possess a combination of strength and the capability to deform and absorb the energy of the collision, thereby reducing the forces and energy experienced by the occupants. Sometimes, and not surprisingly, the deformation process leads to fracture. Some observers may decry the deformed appearance of the automobile after an accident, saying “they don't make them like they used to.” More sophisticated analysis will consider the vehicle dynamics to calculate the forces of the accident. This, in turn, allows consideration of the stresses experienced by the vehicle components, and whether or not the deformation observed is that which would be expected based on design analysis and actual crash testing. For further details, see the article “Modeling and Accident Reconstruction” in this Volume as well as SAE Standards and Federal Motor Vehicle Safety Standards. Armor provides another example of a type of engineered product that deforms in service without being considered “failed.” The armor succeeds when it is able to resist the bullet or projectile. While some armors are high strength and distortion will not be observed, others absorb the energy of the projectile by deformation. Again, success or failure is determined by comparison with design expectations. Reference cited in this section 9. J.H. Bickford, Introduction to the Design and Behaviour of Bolted Joints, 3rd ed., Marcel Dekker, 1995 The file is downloaded from www.bzfxw.com
Analysis of Distortion and Deformation Revised by Roch ]. Shipley and David A Moore, Packer Engineering and william Dobson, Binary Engineering Associates, Inc Acknowledgment This article was revised from previous editions in Failure Analysis and Prevention, Volume 10, Metals Handbook 8th edition(1975)and Volume 11, ASM Handbook, 9th edition(1986) Analysis of Distortion and Deformation Revised by Roch ]. Shipley and David A Moore, Packer Engineering and william Dobson, Binary Engineering Associates, Inc. References 1. J.W. Jones, Limit Analysis, Mach Des., Vol 45 (No 23), 20 Sept 1973, p 146-151 2. D. Goldner, Plastic Bending in Tubular Beams, Mach Des., Vol 45(No 24), Oct 1973, p 152-155 3. D.J. Ulpi, Understanding How Components Fail, 2nd ed, ASM International, 1999, p 16-19 4. J.A. Collins, Failure of materials in Mechanical Design, 2nd ed, Wiley and Sons, 1993 5. A.J. Lena, Precipitation Reactions in Iron-Base Alloys, in Precipitation from Solid Solution, American Society for Metals, 1959. p 224-327 6. G. Totten, M. Howes, and T Inoue, Ed, Handbook of Residual Stress and Deformation of Steel, ASM International. 2002 7. F. McClintock and A. Argon, Mechanical Behavior of Material, Chapter 12, Residual Stress, Addison- Wesley,1966,p420-434 8. C.R. Preschmann and R.I. Stephens, Inelastic Cyclic Buckling, Exp. Mech., Vol 12(No 9), Sept 1972, p 426428 9. J.H. Bickford, Introduction to the Design and Behaviour of Bolted Joints, 3rd ed, Marcel Dekker, 1995
Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A. Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Acknowledgment This article was revised from previous editions in Failure Analysis and Prevention, Volume 10, Metals Handbook 8th edition (1975) and Volume 11, ASM Handbook, 9th edition (1986). Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A. Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. References 1. J.W. Jones, Limit Analysis, Mach. Des., Vol 45 (No. 23), 20 Sept 1973, p 146–151 2. D. Goldner, Plastic Bending in Tubular Beams, Mach. Des., Vol 45 (No. 24), 4 Oct 1973, p 152–155 3. D.J. Wulpi, Understanding How Components Fail, 2nd ed., ASM International, 1999, p 16–19 4. J.A. Collins, Failure of Materials in Mechanical Design, 2nd ed., Wiley and Sons, 1993 5. A.J. Lena, Precipitation Reactions in Iron-Base Alloys, in Precipitation from Solid Solution, American Society for Metals, 1959. p 224–327 6. G. Totten, M. Howes, and T. Inoue, Ed., Handbook of Residual Stress and Deformation of Steel, ASM International, 2002 7. F. McClintock and A. Argon, Mechanical Behavior of Material, Chapter 12, Residual Stress, AddisonWesley, 1966, p 420–434 8. C.R. Preschmann and R.I. Stephens, Inelastic Cyclic Buckling, Exp. Mech., Vol 12 (No. 9), Sept 1972, p 426–428 9. J.H. Bickford, Introduction to the Design and Behaviour of Bolted Joints, 3rd ed., Marcel Dekker, 1995
Failures of shafts Revised by donald J ulpi, Metallurgical Consultant Introduction A SHAFT is a metal bar---usually cylindrical in shape and solid, but sometimes hollow--that is used to support rotating components or to transmit power or motion by rotary or axial movement. Even fasteners, such as bolts or studs, can be considered to be stationary shafts, usually with tensile forces, but sometimes combined with bending and/or torsional forces. In addition to failures in shafts this article will discuss failures in connectin rods, which translate rotary motion to linear motion(and conversely) and in piston rods, which translate the action of fluid power to linear motion Shafts operate under a broad range of service conditions, including dust-laden or corrosive atmospheres and temperatures that vary from extremely low, as in arctic or cryogenic environments, to extremely high, as in gas turbines. In addition, shafts may be subjected to a variety of loads-in general, tension, torsion, compression bending, or combinations of these Shafts are also sometimes subjected to vibratory stresses Apart from wear by bearings, which can be a major contributor to shaft failure(see the section"Wear"in this article), the most common cause of shaft failure is metal fatigue. Fatigue is a weakest link phenomenon; hence, failures start at the most vulnerable point in a dynamically stressed area-typically a stress raiser, which may be mechanical, metallurgical, or sometimes a combination of the two. Mechanical stress raisers include such features as small fillets, sharp corners, grooves, splines, keyways, nicks, and press or shrink fits. Shafts often break at edges of press-fitted or shrink-fitted members, where high degrees of stress concentration exist. Such stress concentration effectively reduces fatigue resistance, especially when coupled with fretting. Metallurgical stress raisers may be quench cracks, corrosion pits, gross nonmetallic inclusions, brittle second-phase particles weld defects or arc strikes Occasionally, brittle fractures are encountered, particularly in low-temperature environments or as a result of impact or a rapidly applied overload. Brittle fracture may thus be attributable to inappropriate choice of material because of incomplete knowledge of operating conditions and environment or failure to recognize their significance, but it may also be the result of abuse or misuse of the product under service conditions for which it was not intended Surface treatments can cause hydrogen to be dissolved in high-strength steels and may cause shafts to become embrittled even at room temperature. Electroplating, for instance, has caused failures of high-strength steel shafts. Baking treatments applied immediately after plating are used to ensure removal of hydrogen Ductile fracture of shafts is usually caused by accidental overload and is relatively rare in normal operation Creep, a form of distortion at elevated temperatures, can lead to stress rupture and can also cause shafts having close tolerances to fail because of excessive changes in critical dimensions Failures of shafts Revised by Donald J. Ulpi, Metallurgical Consultant Fracture origins Fractures of shafts originate at points of stress concentration either inherent in design or introduced during fabrication or operation. Design features that concentrate stress include ends of keyways, edges of press-fitted members, fillets at shoulders, and edges of oil holes. Stress concentrators produced during fabrication include grinding damage, machining marks or nicks, and quench cracks resulting from heat treating operations Frequently, stress concentrators are introduced during hot or cold forming of shafts; these include surface discontinuities, such as laps, seams, pits and forging laps, and internal imperfections, such as bursts. Internal Thefileisdownloadedfromwww.bzfxw.com
Failures of Shafts Revised by Donald J. Wulpi, Metallurgical Consultant Introduction A SHAFT is a metal bar—usually cylindrical in shape and solid, but sometimes hollow—that is used to support rotating components or to transmit power or motion by rotary or axial movement. Even fasteners, such as bolts or studs, can be considered to be stationary shafts, usually with tensile forces, but sometimes combined with bending and/or torsional forces. In addition to failures in shafts, this article will discuss failures in connecting rods, which translate rotary motion to linear motion (and conversely) and in piston rods, which translate the action of fluid power to linear motion. Shafts operate under a broad range of service conditions, including dust-laden or corrosive atmospheres and temperatures that vary from extremely low, as in arctic or cryogenic environments, to extremely high, as in gas turbines. In addition, shafts may be subjected to a variety of loads—in general, tension, torsion, compression, bending, or combinations of these. Shafts are also sometimes subjected to vibratory stresses. Apart from wear by bearings, which can be a major contributor to shaft failure (see the section “Wear” in this article), the most common cause of shaft failure is metal fatigue. Fatigue is a weakest link phenomenon; hence, failures start at the most vulnerable point in a dynamically stressed area—typically a stress raiser, which may be mechanical, metallurgical, or sometimes a combination of the two. Mechanical stress raisers include such features as small fillets, sharp corners, grooves, splines, keyways, nicks, and press or shrink fits. Shafts often break at edges of press-fitted or shrink-fitted members, where high degrees of stress concentration exist. Such stress concentration effectively reduces fatigue resistance, especially when coupled with fretting. Metallurgical stress raisers may be quench cracks, corrosion pits, gross nonmetallic inclusions, brittle second-phase particles, weld defects, or arc strikes. Occasionally, brittle fractures are encountered, particularly in low-temperature environments or as a result of impact or a rapidly applied overload. Brittle fracture may thus be attributable to inappropriate choice of material because of incomplete knowledge of operating conditions and environment or failure to recognize their significance, but it may also be the result of abuse or misuse of the product under service conditions for which it was not intended. Surface treatments can cause hydrogen to be dissolved in high-strength steels and may cause shafts to become embrittled even at room temperature. Electroplating, for instance, has caused failures of high-strength steel shafts. Baking treatments applied immediately after plating are used to ensure removal of hydrogen. Ductile fracture of shafts is usually caused by accidental overload and is relatively rare in normal operation. Creep, a form of distortion at elevated temperatures, can lead to stress rupture and can also cause shafts having close tolerances to fail because of excessive changes in critical dimensions. Failures of Shafts Revised by Donald J. Wulpi, Metallurgical Consultant Fracture Origins Fractures of shafts originate at points of stress concentration either inherent in design or introduced during fabrication or operation. Design features that concentrate stress include ends of keyways, edges of press-fitted members, fillets at shoulders, and edges of oil holes. Stress concentrators produced during fabrication include grinding damage, machining marks or nicks, and quench cracks resulting from heat treating operations. Frequently, stress concentrators are introduced during hot or cold forming of shafts; these include surface discontinuities, such as laps, seams, pits and forging laps, and internal imperfections, such as bursts. Internal The file is downloaded from www.bzfxw.com
stress concentrators can also be introduced during solidification of ingots from which forged shafts are made Generally, these stress concentrators are internal discontinuities, such as pipe, segregation, porosity, shrinkage, and nonmetallic inclusions Fractures also result from bearing misalignment that is either introduced at assembly or caused by deflection of supporting members in service, from mismatch of mating parts, and from careless handling in which the shaft is nicked, gouged, or scratched To a lesser degree, shafts can fracture from misapplication of material. Such fractures result from use of materials having high ductile-to-brittle transition temperatures, low resistance to hydrogen embrittlement temper embrittlement, or caustic embrittlement, or chemical compositions or mechanical properties other than those specified. In some instances, fractures may originate in regions of partial or total decarburization or excessive carburization, where mechanical properties are different because of variations in chemical composition Failures of shafts Revised by Donald J. ulpi, Metallurgical Consultant Examination of failed shafts As with any failure, examination of a failed shaft should include gathering as much background information as possible about the shaft. This information should include design parameters, operating environment, manufacturing procedures, and service history. Detailed knowledge of these factors can often be helpful in guiding the direction of failure investigation and corrective action Design Parameters. The failure analyst should have copies of the detail and assembly drawings as well as the material and testing specifications that involve the shaft. Potential stress raisers or points of stress concentration, such as splines, keyways, cross holes, and changes in shaft diameter, should be noted. The type of material, mechanical properties, heat treatment, test locations, nondestructive examination used, and other processing requirements should also be noted. Special processing or finishing treatments, such as shot peening, fillet rolling, burnishing, plating, metal spraying, and painting, can influence performance, and the analyst should be aware of such treatments Mechanical Conditions. How a shaft is supported or assembled in its working mechanism and the relationship between the failed part and its associated members can be valuable information. The number and location of bearings or supports, as well as how their alignment may be affected by deflections or distortions that can occur as a result of mechanical loads, shock, vibrations, or thermal gradients, should be considered The method of connecting the driving or driven member to the shaft, such as press fitting, welding, or use of a threaded connection, a set screw, or a keyway, can influence failure. It is also important if power is transmitted to or taken from the shaft by gears, splines, belts, chains, or torque converters Inspection records may provide the inspection history of the part and indicate any questionable areas humber Manufacturing records may indicate when the part was made and the material supplier, heat, or lot number Service History. Checking the service records of an assembly should reveal when the parts were installed, serviced, overhauled, and inspected. These records should also show whether service or maintenance operations were conducted in accordance with the manufacturer's recommendations. Often, talking with operators or maintenance personnel may reveal pertinent unrecorded information Initial Examination. Ideally, the entire assembly that was involved in the failure should be made available for examination. However, in the real world this is not al ways possible. Samples of oil, grease, and loose debris should be carefully removed from all components, identified, and stored for future reference. The components can then be cleaned with a solvent that will not remove or obliterate any rust, oxidation, burnishing marks, or other pertinent evidence Surfaces of all areas that may have been involved in or may have contributed to the failure should be examined noting scuff marks, burnished areas, abnormal surface blemishes, and wear. These marks should be associated with some abnormal service condition, if possible. In addition, the failed part should be examined to establish
stress concentrators can also be introduced during solidification of ingots from which forged shafts are made. Generally, these stress concentrators are internal discontinuities, such as pipe, segregation, porosity, shrinkage, and nonmetallic inclusions. Fractures also result from bearing misalignment that is either introduced at assembly or caused by deflection of supporting members in service, from mismatch of mating parts, and from careless handling in which the shaft is nicked, gouged, or scratched. To a lesser degree, shafts can fracture from misapplication of material. Such fractures result from use of materials having high ductile-to-brittle transition temperatures, low resistance to hydrogen embrittlement, temper embrittlement, or caustic embrittlement, or chemical compositions or mechanical properties other than those specified. In some instances, fractures may originate in regions of partial or total decarburization or excessive carburization, where mechanical properties are different because of variations in chemical composition. Failures of Shafts Revised by Donald J. Wulpi, Metallurgical Consultant Examination of Failed Shafts As with any failure, examination of a failed shaft should include gathering as much background information as possible about the shaft. This information should include design parameters, operating environment, manufacturing procedures, and service history. Detailed knowledge of these factors can often be helpful in guiding the direction of failure investigation and corrective action. Design Parameters. The failure analyst should have copies of the detail and assembly drawings as well as the material and testing specifications that involve the shaft. Potential stress raisers or points of stress concentration, such as splines, keyways, cross holes, and changes in shaft diameter, should be noted. The type of material, mechanical properties, heat treatment, test locations, nondestructive examination used, and other processing requirements should also be noted. Special processing or finishing treatments, such as shot peening, fillet rolling, burnishing, plating, metal spraying, and painting, can influence performance, and the analyst should be aware of such treatments. Mechanical Conditions. How a shaft is supported or assembled in its working mechanism and the relationship between the failed part and its associated members can be valuable information. The number and location of bearings or supports, as well as how their alignment may be affected by deflections or distortions that can occur as a result of mechanical loads, shock, vibrations, or thermal gradients, should be considered. The method of connecting the driving or driven member to the shaft, such as press fitting, welding, or use of a threaded connection, a set screw, or a keyway, can influence failure. It is also important if power is transmitted to or taken from the shaft by gears, splines, belts, chains, or torque converters. Manufacturing records may indicate when the part was made and the material supplier, heat, or lot number. Inspection records may provide the inspection history of the part and indicate any questionable areas. Service History. Checking the service records of an assembly should reveal when the parts were installed, serviced, overhauled, and inspected. These records should also show whether service or maintenance operations were conducted in accordance with the manufacturer's recommendations. Often, talking with operators or maintenance personnel may reveal pertinent unrecorded information. Initial Examination. Ideally, the entire assembly that was involved in the failure should be made available for examination. However, in the real world this is not always possible. Samples of oil, grease, and loose debris should be carefully removed from all components, identified, and stored for future reference. The components can then be cleaned with a solvent that will not remove or obliterate any rust, oxidation, burnishing marks, or other pertinent evidence. Surfaces of all areas that may have been involved in or may have contributed to the failure should be examined, noting scuff marks, burnished areas, abnormal surface blemishes, and wear. These marks should be associated with some abnormal service condition, if possible. In addition, the failed part should be examined to establish
the general area or location of failure, noting the proximity to any possible stress concentrations found when examining the part drawings Fracture surfaces should be examined visually to determine if there are indications of one or more fracture mechanisms and if there is an apparent crack origin. Surfaces of the component adjacent to the fracture surface should be examined for secondary cracks, pits, or imperfections. Photographs should be taken to record the condition of the pertinent parts before physical evidence is destroyed by subsequent examinations Nondestructive methods of inspection, such as ultrasonic inspection, can sometimes provide useful information Such methods may reveal other cracks that have not progressed to rupture, these cracks may have other cracks or fracture surfaces that are not as badly damaged as the primary fracture and that can be diagnosed more Also, some machines and structures may have other shafts similar to the one that failed these shafts may have the same service history as the failed and examination may reveal cracks that can provide usefu information. Whenever possible, the analyst should also compare shafts that are from machines in service but ave not failed Macroscopic Examination(from less than actual size to approximately 50x). Many characteristic marks on fracture surfaces, though visually identifiable, can be better distinguished with macroscopic examination. A magnifying glass or binocular microscope can be used to study the unique vestigial marks left on tensile fracture surfaces in the form of fibrous, radial, and shear-lip zones from which the relative amounts of ductility and toughness possessed by the metal can be appraised. In the study of fatigue fractures, macroscopic examination of such features as beach marks. ratchet marks. fast-fracture zones and crack-initiation sites if present, may yield information relative to the kinds and magnitude of the stresses that caused failure Microscopic Examination(50 to 2000x )Metallographic sections taken through fractures are used to classify fracture paths(transgranular or intergranular), to establish the mode of fracture(shear or cleavage), and to locate and identify crack-initiation sites. Plating before mounting can be used to preserve the edge or edges of the fracture surface. In addition, metallographic examination can reveal microstructure near the fracture surface, the grain size of the material, and the presence of undue segregation, inclusions, alloy concentrations, brittle grain-boundary phases, decarburization, and fabricating imperfections Scanning Electron Microscopy(SEM)and Transmission Electron Microscopy (TEM) Examination(10 to More Than 20,000x). The optical metallograph is limited for fracture studies by its restricted depth of field(the scanning electron microscope has a depth of field about 300 times that of the optical microscope). Thus, the electron microscope is better suited for fractographic work. With it, fractures may be classified by fracture path, fracture mechanism, and fracture features. There are two fracture paths: transgranular(or transcrystalline) and intergranular(or intercrystalline) The fracture mechanisms and related microscopic features of transgranular fracture are microvoid coalescence (dimples, associated with ductile fracture), tearing(tear ridges), cleavage(river patterns, feather marks, Wallner lines, and cleavage tongues, all associated with brittle fracture), and fatigue(striations and tire tracks) The fracture mechanism of intergranular fracture is grain-boundary separation. The causes may be the presence of grain-boundary phases, alloy-depleted boundaries, and environmental or mechanical factors such as stress- corrosion cracking, hydrogen damage, heat damage, and triaxial stress states Mechanical testing and chemical analysis occasionally pinpoint the cause of a failure as wrong material improper heat treatment, or in-service changes in properties. Hardness testing and spectroscopic analysis should be performed as a matter of course. Impact tests, tensile tests, and other special mechanical tests may be performed if manufacturing procedures are in doubt or if other paths of investigation are not fruitful Failures of shafts Revised by Donald J. wuli, Metallurgical Consultant Stress Systems Acting on Shafts Thefileisdownloadedfromwww.bzfxw.com
the general area or location of failure, noting the proximity to any possible stress concentrations found when examining the part drawings. Fracture surfaces should be examined visually to determine if there are indications of one or more fracture mechanisms and if there is an apparent crack origin. Surfaces of the component adjacent to the fracture surface should be examined for secondary cracks, pits, or imperfections. Photographs should be taken to record the condition of the pertinent parts before physical evidence is destroyed by subsequent examinations. Nondestructive methods of inspection, such as ultrasonic inspection, can sometimes provide useful information. Such methods may reveal other cracks that have not progressed to rupture; these cracks may have other cracks or fracture surfaces that are not as badly damaged as the primary fracture and that can be diagnosed more readily. Also, some machines and structures may have other shafts similar to the one that failed; these shafts may have the same service history as the failed shaft, and examination may reveal cracks that can provide useful information. Whenever possible, the analyst should also compare shafts that are from machines in service but have not failed. Macroscopic Examination (from less than actual size to approximately 50×). Many characteristic marks on fracture surfaces, though visually identifiable, can be better distinguished with macroscopic examination. A magnifying glass or binocular microscope can be used to study the unique vestigial marks left on tensilefracture surfaces in the form of fibrous, radial, and shear-lip zones from which the relative amounts of ductility and toughness possessed by the metal can be appraised. In the study of fatigue fractures, macroscopic examination of such features as beach marks, ratchet marks, fast-fracture zones, and crack-initiation sites, if present, may yield information relative to the kinds and magnitude of the stresses that caused failure. Microscopic Examination (50 to 2000×.) Metallographic sections taken through fractures are used to classify fracture paths (transgranular or intergranular), to establish the mode of fracture (shear or cleavage), and to locate and identify crack-initiation sites. Plating before mounting can be used to preserve the edge or edges of the fracture surface. In addition, metallographic examination can reveal microstructure near the fracture surface, the grain size of the material, and the presence of undue segregation, inclusions, alloy concentrations, brittle grain-boundary phases, decarburization, and fabricating imperfections. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) Examination (10 to More Than 20,000×). The optical metallograph is limited for fracture studies by its restricted depth of field (the scanning electron microscope has a depth of field about 300 times that of the optical microscope). Thus, the electron microscope is better suited for fractographic work. With it, fractures may be classified by fracture path, fracture mechanism, and fracture features. There are two fracture paths: transgranular (or transcrystalline) and intergranular (or intercrystalline). The fracture mechanisms and related microscopic features of transgranular fracture are microvoid coalescence (dimples, associated with ductile fracture), tearing (tear ridges), cleavage (river patterns, feather marks, Wallner lines, and cleavage tongues, all associated with brittle fracture), and fatigue (striations and tire tracks). The fracture mechanism of intergranular fracture is grain-boundary separation. The causes may be the presence of grain-boundary phases, alloy-depleted boundaries, and environmental or mechanical factors such as stresscorrosion cracking, hydrogen damage, heat damage, and triaxial stress states. Mechanical testing and chemical analysis occasionally pinpoint the cause of a failure as wrong material, improper heat treatment, or in-service changes in properties. Hardness testing and spectroscopic analysis should be performed as a matter of course. Impact tests, tensile tests, and other special mechanical tests may be performed if manufacturing procedures are in doubt or if other paths of investigation are not fruitful. Failures of Shafts Revised by Donald J. Wulpi, Metallurgical Consultant Stress Systems Acting on Shafts The file is downloaded from www.bzfxw.com