T.O. 33B-1-13-106was fully understood the heat treatment of the part was often blamed for the cracking. The heat treatoperation did, however, deserve some of the blame by leaving the part with high residual stresses.f. PlatingCracks. Plating can introduce high residual stresses at the plated surface and thus create thepotential for cracking. The hot galvanizing process itself may also produce cracks in surfacescontaining residual stresses by the penetration of hot zinc into the grain boundaries. Copperpenetration during brazing may result in similar cracking if the parts contain residual stress. (SeeFigure 3-66.)Figure 3-66. Magnetic Particle Indications of Plating Cracks.3.7.4.2 ServiceCracks.NOTEWhen performing magnetic particle inspection on landing gear parts the paintSHALL be removed.The fourth major classification of discontinuities comprises those that are formed or produced after all fabrication hasbeen completed and the part has gone into service. The objective of magnetic particle testing to locate and eliminatediscontinuities during fabrication is to put the part into service free from defects. However, even when this isaccomplished, failures in service still occuras a result of cracking caused by service conditions.a. FatigueCracks. As a source of discontinuities, the phenomenon of fatigue is a prolific one. Fatiguestrength will eventually develop cracks, and finally fracture. Fatigue cracks, even very shallow ones,can readily be found with magnetic particles. (See Figure 3-67 and Figure 3-68.)Figure 3-67. Magnetic Particle Indication of a Typical Fatigue Crack.
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