T.O. 33B-1-1magnitude of crack response as scanning speed is increased. The reduction in needle def lection withincreased speed of scanning is shown in Figure 4-46. Manufactures may produce equipment manuals thatsuggest a fast scanning speed, however, scanning speeds using manual scanning techniques SHALL notexceed 1/4 to 1/2 inch per second.4.5.6.3AutomaticorSemiAutomaticEquipment.Automatic eddy current equipment in conjunction with high speed recorders is capable of operation atextremely high speeds. The upper limits of scanning speed are based on the operating frequency and thesampling rates of the recorder or readout. The principal use for automated eddy current equipment by themilitary is for the inspection of bolt holes. In this application, rotational speeds of 100 rpm can be obtained bythe inspection system.4.5.6.4UseofRecordersorOscilloscopes.The use of recorders or oscilloscope (CRT) type eddy current instruments permits increasing the speed ofmanual scanning to the limits imposed by the reaction time of these instruments. Generally, otherrestrictions related to guiding the probe in the prescribed scanning pattern become the controlling factorwhen recorders or oscilloscopes are employed.4.5.6.5ScanningPattern.The scanning pattern required for eddy current inspection is based upon the possible initiation site of thecrack, the orientation of the cracks, and the size of the cracks which must be detected. If cracks initiate froman edge in thin material (0.050 inch or so), eddy current inspection is usually limited to a single scan of theedge. For thicker materials, scans might be required on both surfaces adjacent to the edge and one or morescans of the material between the edges. When cracks initiate beneath the heads of nonremovable fasteners,the pattern usually consists of a single scan around the protruding head of the fastener to detect cracksgrowing outward from the hole. If cracks can occur at a variety of positions and orientations, as is possible onf lat surfaces, in radii, and on cylindrical surfaces, scanning must be performed in a manner which will assuredetection of the smallest cracks required to be found. For these types of inspection areas, the direction ofscanning, the number of scans, and the distance between scans should be specified.4-58Change 3
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