FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 5. Nondestructive Inspection (NDI) | 1. General | 5-5. Flaws
AC 43.13-1B
9/8/98
(10) Incomplete weld penetration is a
failure of the weld metal to penetrate com-
pletely through a joint before solidifying.
similar processes. The following are brief de-
scriptions of some secondary processing or
finishing flaws.
(11) Incomplete weld fusion occurs in
welds where the temperature has not been high
enough to melt the parent metal adjacent to the
weld.
(12) Weld undercutting is a decrease in
the thickness of the parent material at the toe
of the weld caused by welding at too high a
temperature.
(13) Cracks in the weld metal can be
caused by the contraction of a thin section of
the metal cooling faster than a heavier section
or by incorrect heat or type of filler rod. They
are one of the more common types of flaws
found in welds.
(14) Weld crater cracks are star shaped
cracks that can occur at the end of a weld run.
(15) Cracks in the weld heat-affected
zone can occur because of stress induced in the
material adjacent to the weld by its expansion
and contraction from thermal changes.
(16) A slag inclusion is a nonmetallic
solid material that becomes trapped in the weld
metal or between the weld metal and the base
metal.
(17) Scale is an oxide formed on metal
by the chemical action of the surface metal
with oxygen from the air.
d. Secondary Processing or Finishing
Flaws. This category includes those flaws as-
sociated with the various finishing operations,
after the part has been rough-formed by roll-
ing, forging, casting or welding. Flaws may be
introduced by heat treating, grinding, and
(1) Machining tears can occur when
working a part with a dull cutting tool or by
cutting to a depth that is too great for the mate-
rial being worked. The metal does not break
away clean, and the tool leaves a rough, torn
surface which contains numerous short dis-
continuities that can be classified as cracks.
(2) Heat treating cracks are caused by
stresses setup by unequal heating or cooling of
portions of a part during heat treating opera-
tions. Generally, they occur where a part has a
sudden change of section that could cause an
uneven cooling rate, or at fillets and notches
that act as stress concentration points.
(3) Grinding cracks are thermal type
cracks similar to heat treating cracks and can
occur when hardened surfaces are ground. The
overheating created by the grinding can be
caused by the wheel becoming glazed so that it
rubs instead of cutting the surface; by using
too little coolant; by making too heavy a cut;
or by feeding the material too rapidly. Gener-
ally, the cracks are at right angles to the direc-
tion of grinding and in severe cases a complete
network of cracks can appear. Grinding cracks
are usually shallow and very sharp at their
roots, which makes them potential sources of
fatigue failure.
(4) Etching cracks can occur when
hardened surfaces containing internal residual
stresses are etched in acid.
(5) Plating cracks can occur when hard-
ened surfaces are electroplated. Generally,
they are found in areas where high residual
stresses remain from some previous operation
involving the part.
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