8. 50 The above methods are primarily used to detect material flaws: cracks, voids, porosity, inclusions, weld disbonds, etc. Some limited success in measuring residual stresses has been achieved with ultrasonic and Barkhausen noise techniques (8.21), but there has been little progress beyond the laboratory to date. VIII.8.2.2: Details of Selected Inspection Methods: The fail-safe design eliminates the need for continuous operational inspections for surface cracks in the plates, but a general inspection of all components, welds, and fasteners is needed to eliminate nuisance failures and to provide feedback data for the manufacturing and assembly processes. Ultrasonic and eddy current techniques can both detect flaws as small as 1.27 mm (0.05 in) under optimum inspection conditions (8.23), (8.24). Under nonideal practical conditions, sensitivities of about 5 mm (0.2 in) have been achieved at 90% reliability (90% of flaws present actually detected) for both methods (8.25). The rapid development of NDE methods should bring considerable improvement in the practical sensititivies, especially when the equipment can be optimized for inspecting large numbers of nearly identical parts as in the pr~totype. The prototype is designed with the assumption of 0.1 in (2.5 mm) detectable flaws; i.e., with NDE sensitivities somewhere between the current ideal and typical practical results. Weld defects are best detected by ultrasonic delta-scan techniques (8.14), (8.25), where one transducer illuminates the general weld area with an ultrasonic beam, and a second transducer receives any ultrasonic energy reflected from a flaw. Fastener holes are the one inspection requirement best met by eddy current methods (8.23). The small search area of eddy current probes becomes an advantage when inspecting fastener holes for local cracks. Recently developed electromagnetic ultrasonic transducers would be ideal for use on the colony (8.26), (8.27). These transducers generate a static magnetic field and also have a high frequency excited coil which generates eddy currents in the surface of the metal being inspected. The eddy currents react with the static magnetic field to produce acoustic surface waves from local stresses
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