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.comPhotoelastic 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