T.O. 33B-1-1
4-73
gap, paint and primer thickness. Standards for pitting may also be used for exfoliation and intergranular attack.
Standards should also have approximately the same curvature as the part.
4.6.5.11
Inspection Procedure-Corrosion Dectection.
Detection of corrosion with eddy current techniques is applied to aircraft skins when corrosion may occur on
inaccessible interior surfaces. Corrosion usually results in areas where moisture is entrapped. If relatively uniform
thinning is expected, corrosion detection may be simply a matter of thickness measurement. In most instances,
corrosion is confined to smaller localized areas of relatively small diameter. As skin thickness increases, sensitivity to
small areas and shallow depths of corrosion is reduced.
4.6.5.12
Part Preparation.
Prior to inspection, all foreign material should be removed from the area to be inspected. Any roughness, sharp edges,
or protrusions that could damage the probe or cause errors in readings should be removed by light sanding within the
limits of the applicable T.O.'s. The locations of all fasteners, edges and changes in structure on the far side of the
inspection surface should be established and marked with an approved removable marker to aid in the interpretation of
inspection results. Paint removal is not required if it is relatively uniform and not loose or flaking.
4.6.5.13
Inspection Procedures.
Because of the wide variety of corrosion attack, inspection shall be performed in accordance with the applicable T.O.
SECTION VII
CONDUCTIVITY MEASUREMENT
4.7
SPECIFIC APPLICATION — CONDUCTIVITY MEASUREMENT.
4.7.1
Relationship Of Mechanical Properties And Conductivity.
4.7.1.1
Structure Of Metals.
The atoms of a chemical element have a nucleus or center with a positive charge. Around each nucleus are orbiting
electrons. Each element has a different size nucleus surrounded by a characteristic number and arrangement of
orbiting electrons. The distribution and number of the outermost electrons determine the properties of the element,
including its metallic or nonmetallic nature. In a crystalline solid the atoms are stacked in orderly arrangement called
lattices.
4.7.1.2
Mehcanical Properties.
The physical properties of a metal are related to the binding energy between the atoms. Yield strength, tensile strength
and fatigue strength are determined by resistance to plastic deformation. Plastic deformation is permanent distortion of
the metal and results from shearing along layers of atom. Plastic deformation is made easier by the presence of
localized imperfections in the lattice. These lattice imperfections are called dislocations and are present in great
numbers in all commercial metals and alloys. If the resistance to movement of the dislocations can be increased, the
strength of the metal can be increased.
4.7.1.3
Conductivity Of Metals.
Metals are good conductors of electricity. Conductivity is dependent on the arrangement of atoms in each metal lattice
and the distribution and energy of the electrons surrounding each atom. Any variation in the structure of metals that
affects the electronic structure and energy of the atoms changes the conductivity of the metal. For simplicity, a
decrease in conductivity may be associated with obstacles in the path of electron flow through a metal. The obstacles to
electron flow may be caused by lattice distortions resulting from dislocations, missing atoms (lattice vacancies), foreign
atoms, or grain boundaries. The presence of particles of different composition also restricts the flow of electrons, a
greater number of smaller particles offering more resistance than fewer larger particles.
