Probe Selection

T.O. 33B-1-1 4-71 4.6.3.4 Probe Selection. As with many other flaw detection applications, the use of small diameter, radiused probes is recommended. These probes permit better visibility of probe coil location and permit more flexibility in establishing spacing between the probe and the fastener. Radiused probes are also less susceptible to liftoff variations with changes in probe to surface angle than flat surface probes. 4.6.3.5 Standards For Nonremovable Fastener Holes. Whenever possible, the standards for inspecting around the heads of nonremovable fasteners should duplicate as closely as possible the conditions of the inspection area. If cracked specimens representing the minimum crack size to be detected are not available, sawed notches or EDM slots cut at the edges of holes in the reference standard can be employed. Material, geometry, hole size, and fastener type, and installation should be the same for the reference part as for the inspection area, unless prior correlation with other available references has been established. Duplication of part geometry in the reference minimizes differences in response between references and cracks in the part. 4.6.4 Fillets And Rounded Corner. 4.6.4.1 Crack Occurrence. Repeated bending loads applied to fillets and radii (rounded corners) of a part can lead to fatigue cracks. Fatigue cracks usually lie parallel to the radius. In formed radii, cracking usually occurs near the center of the radius where maximum thinning is obtained. In machined fillets or radii of extruded shapes where part thickness is greater at the center of the radius, fatigue cracks are more likely to occur at the tangent point of the radius. Stress corrosion cracking can sometimes occur in the radii and fillets of machined parts where tensile stresses are applied or areas of residual tensile stresses are exposed. Stress corrosion cracking is often promoted by the collection of moisture in these fillets and radii. 4.6.4.2 Equipment Requirements For Fillets And Radll. In general, no special equipment is required for the inspection of fillets and radii. Adequate inspection can be performed using eddy Current instruments with a radiused tip probe or an equivalent test system. The radius of the probe tip must be less than the radius of the fillet to be inspected to ensure relatively constant contact between probe and part and thereby avoid excessive changes in lift-off. For inspection of the edges of radii or fillets, a thin plastic straight edge is desirable to maintain Probe-to-edge spacing in the fillet. Occasionally, a fixture similar to those employed for the bead seat radii in wheels can be employed for fillets and radii. Fixtures decrease inspection time and assure complete coverage. 4.6.4.3 Reference Standards For Fillets. The best reference standard for any part is one that represents the configuration of the part to be tested. Therefore, it is preferable to have a standard of the same material, finish, and radius as the fillet to be tested. An EDM notch representing the smallest crack required to be detected should be placed in the radius. Flat standards can be used if a standard of the required configuration is not available. Response from flat standards differs very little from response from cracks or slots in fillets or curved surfaces if a radiused probe is used. Slots at edges are not interchangeable with slots located away from the edge. 4.6.5 Corrosion. 4.6.5.1 Types Of Corrosion. Corrosion is the deterioration of metals by chemical action. Corrosion occurs where two different metals are in contact via a conducting material. For corrosion to occur, electrons must be moved from one metal to another. This movement can occur through any conductive material, including air, water, or conductive adhesives. As a solution's conductivity increases, its potential to cause corrosion increases. Thus, corrosion is more prominent in humid or saline environments. Although corrosion may be classified in many ways, for purposes of detection, five principal forms are considered: (1) uniform etch, (2) pitting, (3) intergranular attack, (4) exfoliation, and (5) stress corrosion cracking.