FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 4. Metal Structure, Welding, and Brazing | 2. Testing of Metals | 4-21. Indentations
9/8/98
AC 43.13-1B
a. Fields of Application. Microhardness
testing is capable of providing information re
garding the hardness characteristics of materi
als which cannot be obtained by hardness tests
such as the Brinell or Rockwell, and are as
follows.
(1) Measuring the hardness of precision
work pieces that are too small to be measured
by the more common hardness-testing meth
ods.
(2) Measuring the hardness of product
forms such as foil or wire that are too thin or
too small in diameter to be measured by the
more conventional methods.
(3) Monitoring of carburizing or ni
triding operations, which is sometimes accom
plished by hardness surveys taken on cross
sections of test pieces that accompanied the
work pieces through production operations.
(4) Measuring the hardness of individ
ual microconstituents.
(5) Measuring the hardness close to
edges, thus detecting undesirable surface con
ditions such as grinding burn and decarburiza
tion.
(6) Measuring the hardness of surface
layers such as plating or bonded layers.
to the shape of the indenter, indentations of ac
curately measurable length are obtained with
light loads.
(2) The Knoop hardness number (HK)
is the ratio of the load applied to the indenter
to the unrecovered projected area of indenta
tion. The formula for this follows.
HK = P / A = P / Cl2
Where P is the applied load, in kg; A is the
unrecovered projected area of indentation, in
square mm; l is the measured length of the
long diagonal, in mm; and C is 0.07028, a con
stant of the indenter relating projected area of
the indentation to the square of the length of
the long diagonal.
4-21. INDENTATIONS. The Vickers in
denter penetrates about twice as far into the
work piece as does the Knoop indenter. The
diagonal of the Vickers indentation is about
one-third of the total length of the Knoop in
dentation. The Vickers indenter is less sensi
tive to minute differences in surface conditions
than is the Knoop indenter. However, the
Vickers indentation, because of the shorter di
agonal, is more sensitive to errors in measuring
than is the Knoop indentation. (See fig
ure 4-1.)
b. Indenters. Microhardness testing can
be performed with either the Knoop or the
Vickers indenter. The Knoop indenter is used
mostly in the United States; the Vickers in
denter is the more widely used in Europe.
(1) Knoop indentation testing is per
formed with a diamond, ground to pyramidal
form, that produces a diamond-shaped inden
tation with an approximate ratio between long
and short diagonals of 7 to 1. The indentation
depth is about one-thirtieth of its length. Due
FIGURE 4-1. Comparison of indentation made by Knoop
and Vickers indenters in the same work metal and at the
same loads.
Par 4-20
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