FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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Any recognizable fibre damage may be interpreted as cause for rejection, or at
least further investigation.
THE VALUE OF THE TEST
Many utitlities in both Canada and USA either perform the test for themselves,
contract out to testing agencies or are on the point of implementating a testing
program. Frequency of testing varies from semi-annual, consistent with dielectric
testing, to only at rebuild time, every ~8 years. There is no doubt
that defects are discovered, but primarily in metal components, and there is
some debate as to whether improvements to maintenance and inspection procedures
would be of greater benefit than an acoustic emission test. However, acoustic
emission seems to be the only reliable monitor of the structural integrity of
the boom.
Industry experience is that structural problems with the boom are rara, and
personal injury related to such problems is rarer still. However, it may be
argued that safety is not an issue that can be weighted against the cost of
downtime, but the question must be asked as to whether the acoustic emission
test enhances safety. Is the frequency of testing such that the test will
identify incipient failure before it can progress to actual failure? The answer
is probably yes if the vehicles are used within the design limits specified by
the manufacturer and enforced through the codes [7], but no if the vehicle is
liable to be subjected to abuse«
One USA utility has attempted to address this problem by installing on-board
monitors. These are single channel, battery powered acoustic emission systems
which are attached to the critical area of the upper boom. In an overload
situation which results in acoustic emission the monitor produces an audible and
visual alarm. Unfortunately, this is not a panacea as false alarms do occur
which may require the vehicle being taken out of service for verification, only
a very small portion of the critical areas are covered and it is possible that
the device can generate a false sense of security in the work crew. On the
other hand, the most common failure site is at the lower insert, Figure 11,
which is well covered by the onboard monitor, and there is no question that
enormous amounts of acoustic emission will be generated even prior to a sudden
avalanche failure in tension.
A recent exhaustive inspection of forty-three manlifts and forty-nine digger
derricks [4] discovered no mechanical defects classed as critical or needing
immediate attention associated with frp boom components. However, very many
other defects were discovered but, again, whether acoustic emission testing
would have uncovered all of these (eg, missing bolts holding pedestal to truck)
or is even the appropriate procedure is debatable.
CONCLUSIONS
There is no question that the frp material used for manlift booms generates
acoustic emission at the early stages of structural degradation. Both compressive
and tensile modes of failure are acoustically "noisy".