Further measurements of the individual signals may

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martensitic transformations. Further measurements of the individual signals may provide a much improved understanding of this aspect of the martensite transformation. Measurement of the rate at which emission is detected also is a measurement of the rate at which laths form and may help explain the kinetics of the transformations and particularly the autocatalytic phenomena where they occur. Although almost all solid state phase transformations result in the development of internal stresses, only those associated with rapid martensitic transformations appear to generate detectable elastic waves directly. However, plastic deformation and even fracture are sometimes induced by internal stresses, providing indirect detection of their development. For example, rapidly cooled high carbon steels have a very high internal stress that is normally relieved by a tempering treatment that allows dislocation and impurity atom migration. Using acoustic emission techniques, it has been observed that isothermal tempering at too low a temperature will result in microcracking, which may be caused by locking of dislocations by impurity atoms. 107 Similarly, although the development of stress in and around a second phase precipitate has so far not proven directly detectable by acoustic emission techniques, signals have been observed 108 from hydride precipitates of niobium, tantalum and vanadium as they relax this localized stress by fracture and adjacent lattice plastic deformation. These results suggest that there may be merit in using acoustic emission techniques to detect the point during aging when the coherency stresses of small precipitates are relaxed by the formation of interfacial dislocation structures around semicoherent precipitates (as when θ ´´ becomes θ ´ in the aluminum copper system). The prismatic punching of dislocation loops during precipitation and cooling in other alloy systems (such as molybdenum carbon steels) might also be worthy of investigation. Magnetic Effects Magnetic domain walls in ferromagnetic materials can be induced to move by the application of magnetic fields. This 77 Fundamentals of Acoustic Emission Testing F IGURE 29. Total number of acoustic emission signals per unit volume generated by martensite. Total is dependent on carbon content in iron base alloys. M s temperature decreases with increasing carbon concentration. Total acoustic emission (10 4 counts per cubic millimeter) 2 1 0 100 150 200 250 300 350 (212) (302) (392) (482) (572) (662) Temperature, °C (°F) Legend A. 0.8 percent carbon by weight. B. 0.6 percent carbon by weight. C. 0.4 percent carbon by weight. D. 0.3 percent carbon by weight. A B C D
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movement is accompanied by electromagnetic emission (the barkhausen effect). The motion of these walls under the action of monotonic or alternating magnetic fields has also been found to generate elastic radiation (acoustic emission), the details of which have been found to depend on microstructure variables such as dislocation density and carbide distribution.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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