Operating Manual

Chapter 6: Fastener Geometry
Guide to Ultrasonic Inspection of Fasteners Page 21
Chapter 6: Fastener Geometry
As explained throughout Chapter 1 of this guide, many
of the calculations made by the BoltMike rely directly on
user-input fastener dimensions. A fastener’s material
type, nominal length, average diameter, and effective
length (also known as working or grip length) must be
input in order for the BoltMike to perform all calculations.
While material types and the constants that define their
properties are described in Chapter 7, this chapter deals
with the geometric properties that define a fastener’s
shape. Some of a fastener’s geometric properties have
little effect on certain BoltMike calculations, while others
have a significant effect. It is important to understand
how each geometric property affects the BoltMike’s out-
put.
6.1 Approximate Length
In the BoltMike, the approximate length is the total length
of the fastener. In terms of ultrasonics, this is the dis-
tance from the ultrasonic transducer to the opposite (re-
flecting) end of the fastener. The approximate length is
used to determine the distance at which the BoltMike’s
receiver is enabled.
While the accuracy of the quantity entered for total fas-
tener length does not directly affect the accuracy of the
BoltMike readings, entering a significantly incorrect value
for total length may result in unstable or no readings at
all. If the value entered for approximate length is too
large, the first echo that returns from the bolt will be ig-
nored. If the value entered for approximate length is too
short, the BoltMike will not detect the correct returning
echo. These two cases are shown in Figure 6-1.
6.2 Determining Effective Length
When a fastening system is tensioned, the length of the
fastener to which the tensile load is applied is known as
its effective length. When considering a constant applied
load, the amount of fastener elongation is directly pro-
portional to a fastener’s effective length. In other words,
if two fastening systems are identical in all ways, includ-
ing the tensile load on the fastener, except that the ef-
fective length of the first fastener is twice the effective
length of the second, then the elongation of the first fas-
tener will be twice the elongation of the second.
The effective length must be entered into the BoltMike
in order to make any measurement other than the refer-
ence length. However, the accuracy of the value en-
tered as the effective length has almost no influence on
the accuracy of the elongation measurement. And then,
the affect on elongation measurement is only noticeable
at very high tensile loads, approaching the material’s
yield strength. Because the measurement of elongation
is virtually independent of the effective length, tension
loading is specified in terms of elongation in applications
where the ability to accurately determine effective length
is questionable.
However, the accuracy of the value entered for effective
length has a direct influence on the accuracy of mea-
sured stress and load. If the value entered for effective
length is ten percent less than the actual value, the er-
ror in load and stress measurements will be ten percent.
FIGURE 6-1—The value of approximate total length is used only to set the position of the gate(s) on the A-scan
display screen.