was fully understood the heat treatment of the part was often blamed for the cracking. The heat treat
operation did, however, deserve some of the blame by leaving the part with high residual stresses.
f. Plating Cracks. Plating can introduce high residual stresses at the plated surface and thus create the
potential for cracking. The hot galvanizing process itself may also produce cracks in surfaces
containing residual stresses by the penetration of hot zinc into the grain boundaries. Copper
penetration during brazing may result in similar cracking if the parts contain residual stress. (See
Figure 3-66. Magnetic Particle Indications of Plating Cracks.
When performing magnetic particle inspection on landing gear parts the paint
SHALL be removed.
The fourth major classification of discontinuities comprises those that are formed or produced after all fabrication has
been completed and the part has gone into service. The objective of magnetic particle testing to locate and eliminate
discontinuities during fabrication is to put the part into service free from defects. However, even when this is
accomplished, failures in service still occur as a result of cracking caused by service conditions.
a. Fatigue Cracks. As a source of discontinuities, the phenomenon of fatigue is a prolific one. Fatigue
strength 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.