original is dangerous.
The danger is that one material can have
greater tensile strength than another but less compressive strength.
For example, the buckling and torsional strengths of some sheet metal
and tubular parts are dependent primarily upon the thickness rather
than the allowable compressive and shear strengths of the material.
Therefore, a substitute thinner than the original will reduce the
buckling and torsional strength of a part, even though the thinner
substitute material has higher compressive and shear strengths.
NOTE:
Never substitute a material thinner than the
original material or with cross-sectional area less than
the original. The buckling and torsional strengths of many
sheet metal and tubular parts are dependent primarily upon
the thickness, rather than the allowable compressive and
shear strength of the material.
Therefore, a substitute
thinner than the original will reduce the bucking and
torsional strengths of a part considerably, even though the
thinner
substitute
material
has
higher
allowable
compressive and shear strengths.
Forming.
Be particularly careful when forming (shaping) alloys.
Heat-treated and cold-worked alloys cannot take much bending without
On the other hand, soft alloys can be easily formed
without cracking; however, they are not strong enough for primary
structures.
Strong alloys can be formed in their annealed state
first and then heat treated, in the desired shape, to develop their
strength.
In some cases, if annealed metal is not available, the metal can be
heated and quenched according to regular heat-treating practices and
formed before hardening sets in. However, forming must be completed
in approximately half an hour after quenching because the material
may become too hard to work. When a brake is used to form the metal,
be sure to use a thin piece of soft metal on the brake jaws to
prevent the metal's surface from being marred.
Rivet Size and Number. The rivets in the next parallel row inboard
on the wing or forward on the fuselage can be used to determine the
size of rivets needed for any repair. Another method is to multiply
skin thickness by three and use the nearest larger size rivet
corresponding to that result.
For example, if skin thickness is
0.040 inch, multiply that by three.
The result is 0.120; use the
next larger size rivet, 4/32 inch (0.125 inch).
Each repair takes a specific number of rivets to restore
the original strength. This number will vary with the thickness
of the material being repaired and the size of the damage.
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AL0992
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