Operating Manual
Chapter 2: Fastener Preparation
Page 12 Guide to Ultrasonic Inspection of Fasteners
While the surface finish of the reflective surface is not
as critical, very rough or uneven finish can produce
errors. Use care when machining fastener ends. A
common problem occurs when a small peak is left in the
center of a fastener end after facing on a lathe. This
small bump prevents the transducer from achieving
proper contact and greatly reduces the signal amplitude.
NOTE: The use of Multi-Echo measurement mode re-
duces some types of variation and measurement in-
accuracies, especially those that are due to couplant
thickness and instrument/probe zeroing. However,
errors introduced by inconsistent transducer place-
ment or surface preparation techniques are not elimi-
nated with the use of M.E. mode.
2.2 Methods Of Transducer Placement
Unless fastener ends and transducer surfaces are per-
fectly parallel, as discussed in section 2.1 of this manual,
the reflected ultrasonic signal will vary with changes in
the transducer’s orientation, with respect to the fastener.
This condition is illustrated in Figure 2-2. Optimal re-
peatability and accuracy are achieved by leaving the
transducer attached to the fastener, in exactly the same
location and angular orientation, throughout the
tensioning process. As this ideal approach is often not
possible or practical, the next best practice is to consis-
tently return the transducer to the same location and
angular orientation, with respect to the fastener. This
practice improves the chances that the path followed by
the shock wave when the reference length was mea-
sured is identical (or close to identical) to the path fol-
lowed after the fastening system is tightened.
2.2.1 Practical Methods
Several practical methods are used to ensure consis-
tent transducer placement. The most common method
utilizes a magnetic transducer, which is placed in the
center of the bolt’s head. When inspecting bolts with di-
ameters above one inch, refer to Figure 2-3 and follow
these steps:
Step 1:
First measure the reference (non-tensioned)
length by coupling the transducer to the fastener end
and adjusting its orientation, while observing the A-scan
display. Position the transducer in the center of the fas-
tener end and identify the angular transducer position
that returns the A-scan waveform of greatest amplitude.
At this point consider the accuracy of the selected mea-
surement mode. M.E. mode can increase repeatability
and improve accuracy if the subsequent returning ech-
oes are free enough of distortion to be measured prop-
erly.
Step 2:
Mark the transducer location and angular orien-
tation on the fastener end.
Step 3:
Continue with the fastener tightening procedure.
If possible, the transducer should remain connected to
the fastener end in exactly the same position and orien-
tation. If this is not possible, proceed to step 4.
Step 4:
Before proceeding, reconfirm that the position
marked on the fastener end remains the location that
returns the greatest-amplitude waveform and the short-
est length and/or lowest load or stress reading. This step
is important because in some cases, as the fastener is
tensioned, a small amount of bending occurs. When
bending occurs, the angular orientation that returns the
FIGURE 2-2—Changing the transducer’s position with respect to the fastener’s end can change the shape and/or
amplitude of the returned waveform. This effect is especially significant when inspecting long or large-diameter
fasteners.