Operating Manual

Chapter 1: Ultrasonic Measurement of Fasteners
Guide to Ultrasonic Inspection of Fasteners Page 9
the stressed length is so small, little or no measurable
elongation of the fastener occurs.
In the same way, it is difficult to measure the effects of
very low loads. Negligible elongation occurs when ten-
sile stress levels are less than about 10% of the material’s
ultimate tensile stress. The small errors in measurement
introduced by removing and replacing the transducer
(as described in section 2.2) become very significant when
trying to measure such a small amount of elongation.
1.3.3 Fastener End-Surface Configuration
The ends of bolt heads and threaded sections (bolts or
studs) must be prepared before the fastener is fit for
ultrasonic inspection. The fastener end that will be mated
with a transducer must be machined to a very flat, smooth
surface to allow for proper coupling of the transducer.
The ideal finish for the transducer coupling point is be-
tween 32 to 63 micro inch CLA (0.8 to 1.6 micro meter
Ra). Refer to section 2.1 to learn more about the re-
quirements of fastener end-surface preparation.
Similarly, the surface at the opposite end of the fastener
(known as the
reflective surface ) must be parallel to the
surface that supports the transducer. This parallelism
allows the reflective surface to reflect the ultrasound back
to the transducer. While the finish of the reflective sur-
face is not as critical, very rough or uneven finish can
produce errors. Problems with surfaces are indicated by
poor signal quality on the waveform display.
1.3.4 The Limitations of I.P. and M.E.
Measurement Modes
Because M.E. measurement mode determines the
elapsed time between two consecutively returning ech-
oes, it eliminates some inconsistencies introduced in I.P.
mode such as variation of couplant thickness and probe/
instrument zeroing.
However, because M.E. mode relies on the second re-
turning echo, and the quality of ultrasonic signals dimin-
ishes substantially with each returning echo, there are
certain conditions under which the subsequent return-
ing echoes will be distorted beyond acceptable limits and
M.E. mode will not be effective. For instance, ultrasonic
interference resulting from echoes off of the fastener’s
sidewalls increases the level of distortion present when
the second returning echo is received. To some extent
the sidewall distortion effect can be compensated for with
the use of a larger diameter transducer. Similarly, the
effects of frequency dispersion, attenuation, and sidewall
distortion can also be compensated for by using a lower
frequency transducer. In general lower transducer fre-
quencies produce greater-amplitude returning echoes.
Ultimately, however, some small-diameter, longer-length
fastener measurements must be conducted in I.P. mode.