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limitations have led to assigning this work to future. Temperature compensation is
also required to be done before the instrument
can be
used in non-laboratory
conditions. A method of compensation has been identified based on the linear
relationship between resonances and temperature, but the lack of an environmental
chamber has led to postponing of temperature compensation tests.
Using the RUS instrument, stress can be predicted in a single bolt to± 10% of
the actual stress. For a group of 10 bolts, the average stress prediction accuracy is ±
20%. The current ultrasound pulse-echo instruments which are used to calculate bolt
stress based on the bolt elongation after tightening, have a claimed accuracy of± 5%.
But the pulse-echo instruments require measurement on the bolts before and after
tightening to predict stress. Also the claimed accuracy of± 5% has been found to be
inaccurate by the technicians of Hedley Purvis Ltd at several occasions of testing. The
realistic accuracy of pulse-echo instruments are in the range of ± 10 - 15%. RUS
instrument does not require measurements to be done on the bolt under test before and
after tightening. This instrument relies on the dataset of 'similar' bolt data that has
been built, tested and verified. The value of the RUS instrument is due to its ability to
predict stress in an 'unknown' bolt. A B7 bolt which has been in place for several
years and has no recorded history of pulse-echo data, is a target of the R US
instrument. RUS can also be used in condition monitoring of flanges, where the stress
of individual bolts can be monitored over the course of time to provide accurate
information of change in bolt stress across the flange. British Petroleum has expressed
particular interest in using RUS for condition monitoring.
In this project, a new method has been developed to predict stress in bolts
using shifts in resonances. Apart from stress, other factors that affect resonances have
been investigated and means of compensation identified. A suitable instrument to
excite the bolt and measure resonances to predict stress has been built. An accuracy of
± 20% has been achieved with a group of 10 B7 bolts.
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