Permanently installed ultrasonic sensors have the
capability of measuring much smaller changes in the signal
than conventional sensors that are used for ultrasonic inspections.
This is because uncertainties associated with coupling
fluids and positional offsets are eliminated. Therefore it is
potentially possible to monitor the onset of material degradation.
A particular degradation mechanism that we are keen
to monitor is high temperature hydrogen attack; where the
amount of damage is linked to a drop in ultrasonic velocity
which we hope can be monitored for with an ultrasonic array.
The changes introduced in the ultrasonic propagation velocity
are expected to be of the order of 1 % and in practice they
are observable only from a very limited field of view (i.e. from
the outside of a pipe) and therefore the reconstruction is challenging
to accomplish. In order to explore the feasibility of
this, we are investigating the reconstruction of a non-uniform
temperature distribution which allows us to quickly evaluate
the sensitivity of our method to small spatial variations in
ultrasonic velocity of the material. Two reconstruction algorithms
were implemented and their performance compared
in simulated and real measurements. The results of the tests
were encouraging: local temperature differences as low as
10 ◦C could be detected, which corresponds to a local propagation
velocity change of 5 m/s (0.15 % relative velocity
change).