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.comAlthough 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 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 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