FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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Sensors used on the major metallic components will have a maximum sensitivity in
the range 100-400 kHz. However, attenuation in frp at these frequencies would
demand an unacceptable number of sensors necessary for coverage of the complete
boom. For this reason, it is more usual to employ sensors for the boom with
peak response in the 20-50 kHz range. This is strictly a compromise as the
lower frequency increases the susceptibility to extraneous mechanical noise.
It is rare to use "time of flight" source location on either the boom or metal
components. Sources of acoustic emission that may occur are simply identified
with the particular component. In this way, two sensors may be fed into a
single amplifier chain. This reduces the capital cost of equipment at the
expense of discrimination.
Sensor spacing may be quite important. An artificial acoustic emission source,
such as a puiser or pencil lead break [5], must be used to ensure that the
required degree of coverage is achieved. However, this rigorous process need
only be performed once for a given model of aerial lift truck. Sensitivity,
which includes sensor performance, couplant and amplifier gain, must be checked
and equalized for each test. This is a simple process of stimulating each channel
with the artificial source and adjusting amplifier gains for equal
response. Figure 7 illustrates commonly used sensor locations.
The boom is then deployed in a manner such that it is representative of working
configurations and such that a controlled load may be applied to the bucket
end. in Figure 8 the load is applied through a cable winch and load cell.
A predetermined load regime is followed consisting of the gradual imposition of
load which is then held for a few minutes and then repeated [6], The applied
load must be consistent with the total vehicle design [7], but this may be as
high as three times, but more typically twice, rated load [8].
TEST RESULTS
For each stage of the load profile acoustic emission activity is recorded for
each channel. The data are plotted in a number of ways, but would normally
include count versus load, Figure 9, amplitude distributions, Figure 10 and
perhaps an indication of individual channel activity through the test.
Experience plays a large part in both conducting the test itself and in interpreting
the results. At the present, there are no solid, universal accept/
reject criteria. However, if acoustic emission continues into the hold period
or recurs on the second load, there is strong indication that a problem exists.
If the general activity, that is the overall count, is unusually high, concern
would depend on which channels were active. For example, this type of indication
could be due to a worn or poorly lubricated pin. The shape of the count
versus load plot is also a significant factor used in interpretation.
Fibre breakage represents high energy acoustic emission generation. By observation
of the amplitude distribution, Figure 10, some assessment of fibre breakage
during the test is available.