Operating Manual

Chapter 6: Fastener Geometry
Page 24 Guide to Ultrasonic Inspection of Fasteners
6.3 Fastener Cross-Sectional Area
The cross-sectional area is the average area of that
portion of a fastener that is subjected to tensile loading.
In other words, it’s an average cross-sectional area taken
over only the fastener’s effective length. The cross-
sectional area in threaded portions of the fastener should
be calculated based on the thread’s minor diameter. The
accuracy with which cross-sectional area is entered only
affects the BoltMike load calculation. It has no effect on
the stress or elongation measurement.
The accuracy of the value entered for cross-sectional
area has a direct influence on the accuracy of measured
load. If the value entered for cross-sectional area is ten
percent less than the actual value, then the measured
value of load will be ten percent lower than the actual
value.
If a fastener’s geometry is more complex, with varying
values of cross-sectional area along its effective length,
the various areas over the effective length may be aver-
aged to arrive at an overall average cross-sectional area.
In the case of a hollow fastener, the area of the hole
must be subtracted from the overall average cross-sec-
tional area to determine the actual cross sectional area.
To calculate the average cross-sectional area of a fas-
tener, multiply the length of each segment along the ef-
fective length of the fastener by the cross-sectional area
of each specific segment. (Figure 6-6) Add all of the re-
sulting values, and then divide the total by the sum of
the lengths.
In the appendix of this manual, you will find tables of
average cross-sectional areas for various types and sizes
of common fastener.
For the best accuracy of load readings, calibrate the
BoltMike for the specific application. This will cancel er-
rors due to cross sectional area uncertainty. In this ap-
proach a calibration group is formed (using fasteners
that are the same or similar to the ones being tested).
The fasteners are inserted in a fixture that loads them at
the same effective length with a known quantity of load.
FIGURE 6-6—Follow this procedure to determine the average cross-sectional area over the effective length of an
irregular fastener.