What was the load on the component at the time of failure? Was the correct material used and manufacturing/processing sound? Was the part designed properly? Did the environment influence the failure? Of course, many situations may involve thin sections and/or very ductile materials, where the conditional constraint for LEFM may not apply. In this case, the measure of toughness is plane-stress fracture toughness(K)and requires the use of elastic-plastic fracture mechanics(EPFM), as the process of unstable fracture involves some plasticity. Plane-stress fracture toughness(Ke)is higher than plane-strain fracture toughness(Kl), but when thinner sections and more ductile materials are involved net-section instability becomes a factor. See the appendix in the article Fracture Appearances and Mechanisms of Deformation and fracture for more information on the use of fracture mechanics in failure analysis Metallographic examination Metallographic examination of polished and of polished-and-etched sections by optical microscopy and by electron- optical techniques is a vital part of failure investigation and should be carried out as a routine procedure when possible Metallographic examination provides the investigator with a good indication of the class of material involved and its structure. If abnormalities are present, these may be associated with undesirable characteristics that predispose to early failure. It is sometimes possible to relate them to an unsuitable composition or to the effects of service, such as aging in low-carbon steel that has caused precipitation of iron nitride, or gassing in copper. Microstructural examination may also provide information as to the method of manufacture of the part under investigation. It can reveal the heat treatment and possible deficiencies in heat treatment such as decarburization at the surface. Microstructural inspection can also reveal possible overheating through coarsening of carbides of superalloys and solution and precipitation of manganese sulfide Other service effects, such as corrosion, oxidation, and severe work hardening of surfaces, also are revealed, and their extent can be investigated. The topographical characteristics of any cracks, particularly their mode of propagation, can be determined, for example, transgranular or intergranular. This provides information that can be helpful in distinguishing between different modes of failure. For example, fatigue cracks always propagate perpendicular to the maximum cyclic tensile stress while stress-corrosion cracking may propagate along grain boundaries Only a few general directions can be given as to the best location from which to take specimens for microscopic examination, because almost every failure has individual features to be taken into account. In most examinations however, it must be determined whether the structure of a specimen taken adjacent to a fracture surface or a region at which a service defect has developed is representative of the component as a whole. This can be done only by the examination of specimens taken from the failure region and specimens taken from other locations. For instance, in the case of ruptured or bulged boiler tubes in which failure is usually restricted to one portion only, it is desirable to examine specimens taken from both sides of the fracture, from a location opposite the affected zone, and also from an area as remote from the failure as the size of the sample permits in order to determine whether the failure has been due to a material defect or to overheating-and, if the latter, whether this was of a general or localized nature. In investigations involving general overheating, sometimes the original condition of the material can be ascertained only from a sample cut from a part of the tube many feet away from the affected zone Metallographic specimens should be taken perpendicular to the fracture surface, showing the fracture surface in edge w. In cases where metal cleanliness may be an issue, the specimen orientation must be selected properly to determine inclusion density and morphology. This type of examination must be performed on the unetched metallographic In the investigation of fatigue cracks, it may be desirable to take a specimen from the region where the fracture originated to ascertain if the initial development was associated with an abnormality such as a weld defect, a decarburized surface, a zone rich in inclusions, or, in castings, a zone containing severe porosity. Multiple fatigue-crack initiation is very typical of both fretting and corrosion fatigue and may form in areas where there is constant stress across a section. Similarly, with surface marks, where the origin cannot be identified from outward appearances, a microscopic examination will show whether they occurred in rolling or arose from ingot defects, such as scabs, laps, or seams. In brittle fractures, it is useful to examine a specimen cut from where the failure originated, if this can be located with certainty. Failures by brittle fracture may be associated with locally work-hardened surfaces, arc strikes, local untempered martensite, and so forth For good edge retention when looking at a fracture surface, it is usually best to plate the surface of a specimen with a Ei lishing and can be included in the examination. Alternative means include hard metal or nonmetal particles embedded the mount adjacent to the edge Analysis of Metallographic Sections. As with hardness testing and macroscopic examination, the examination of metallographic sections with a microscope is standard practice in most failure analyses, because of the outstanding capability of the microscope to reveal material imperfections caused during processing and of detecting the results of a variety of in-service operating conditions and environments that may have contributed to failure. Inclusions, Thefileisdownloadedfromwww.bzfxw.com· What was the load on the component at the time of failure? · Was the correct material used and manufacturing/processing sound? · Was the part designed properly? · Did the environment influence the failure? Of course, many situations may involve thin sections and/or very ductile materials, where the conditional constraint for LEFM may not apply. In this case, the measure of toughness is plane-stress fracture toughness (Kc) and requires the use of elastic-plastic fracture mechanics (EPFM), as the process of unstable fracture involves some plasticity. Plane-stress fracture toughness (Kc) is higher than plane-strain fracture toughness (KIc), but when thinner sections and more ductile materials are involved net-section instability becomes a factor. See the appendix in the article “Fracture Appearances and Mechanisms of Deformation and Fracture” for more information on the use of fracture mechanics in failure analysis. Metallographic Examination Metallographic examination of polished and of polished-and-etched sections by optical microscopy and by electron￾optical techniques is a vital part of failure investigation and should be carried out as a routine procedure when possible. Metallographic examination provides the investigator with a good indication of the class of material involved and its structure. If abnormalities are present, these may be associated with undesirable characteristics that predispose to early failure. It is sometimes possible to relate them to an unsuitable composition or to the effects of service, such as aging in low-carbon steel that has caused precipitation of iron nitride, or gassing in copper. Microstructural examination may also provide information as to the method of manufacture of the part under investigation. It can reveal the heat treatment and possible deficiencies in heat treatment such as decarburization at the surface. Microstructural inspection can also reveal possible overheating through coarsening of carbides of superalloys and solution and precipitation of manganese sulfide. Other service effects, such as corrosion, oxidation, and severe work hardening of surfaces, also are revealed, and their extent can be investigated. The topographical characteristics of any cracks, particularly their mode of propagation, can be determined, for example, transgranular or intergranular. This provides information that can be helpful in distinguishing between different modes of failure. For example, fatigue cracks always propagate perpendicular to the maximum cyclic tensile stress while stress-corrosion cracking may propagate along grain boundaries. Only a few general directions can be given as to the best location from which to take specimens for microscopic examination, because almost every failure has individual features to be taken into account. In most examinations, however, it must be determined whether the structure of a specimen taken adjacent to a fracture surface or a region at which a service defect has developed is representative of the component as a whole. This can be done only by the examination of specimens taken from the failure region and specimens taken from other locations. For instance, in the case of ruptured or bulged boiler tubes in which failure is usually restricted to one portion only, it is desirable to examine specimens taken from both sides of the fracture, from a location opposite the affected zone, and also from an area as remote from the failure as the size of the sample permits in order to determine whether the failure has been due to a material defect or to overheating—and, if the latter, whether this was of a general or localized nature. In investigations involving general overheating, sometimes the original condition of the material can be ascertained only from a sample cut from a part of the tube many feet away from the affected zone. Metallographic specimens should be taken perpendicular to the fracture surface, showing the fracture surface in edge view. In cases where metal cleanliness may be an issue, the specimen orientation must be selected properly to determine inclusion density and morphology. This type of examination must be performed on the unetched metallographic specimen. In the investigation of fatigue cracks, it may be desirable to take a specimen from the region where the fracture originated to ascertain if the initial development was associated with an abnormality, such as a weld defect, a decarburized surface, a zone rich in inclusions, or, in castings, a zone containing severe porosity. Multiple fatigue-crack initiation is very typical of both fretting and corrosion fatigue and may form in areas where there is constant stress across a section. Similarly, with surface marks, where the origin cannot be identified from outward appearances, a microscopic examination will show whether they occurred in rolling or arose from ingot defects, such as scabs, laps, or seams. In brittle fractures, it is useful to examine a specimen cut from where the failure originated, if this can be located with certainty. Failures by brittle fracture may be associated with locally work-hardened surfaces, arc strikes, local untempered martensite, and so forth. For good edge retention when looking at a fracture surface, it is usually best to plate the surface of a specimen with a metal, such as nickel, prior to mounting and sectioning, so that the fracture edge is supported during grinding and polishing and can be included in the examination. Alternative means include hard metal or nonmetal particles embedded in the mount adjacent to the edges. Analysis of Metallographic Sections. As with hardness testing and macroscopic examination, the examination of metallographic sections with a microscope is standard practice in most failure analyses, because of the outstanding capability of the microscope to reveal material imperfections caused during processing and of detecting the results of a variety of in-service operating conditions and environments that may have contributed to failure. Inclusions, The file is downloaded from www.bzfxw.com