Current
research
Current
Research (at University College London)
For the continuous nondestructive monitoring
of remote and difficult-to-access structures it would be advantageous to
permanently attach monitoring devices that run autonomously, i.e., independent
of external energy supply, and transmit data about the condition of the
structure wirelessly. An advantage of the permanent attachment of the device to
the structure is the possibility of comparative measurements. Taking
measurements at different stages in the lifecycle of the structure, an emerging
defect can be detected more clearly by comparison to an initial, defect free,
measurement, so increasing the sensitivity of the device. Application areas
include offshore oil platforms, which are subject to adverse weather conditions,
and thus should be inspected regularly for corrosion or the development of
cracks. Such structures often consist of large plate-like parts which can be
efficiently monitored using guided waves. Guided waves can propagate
over large distances of up to hundreds of meters in one-dimensional structures
like pipelines, allowing for an efficient nondestructive testing. In plates
the guided waves can propagate in two dimensions. Therefore not only is it
necessary to achieve a distinction between the different Lamb wave modes, but the angular resolution of the array also has to be sufficient to
distinguish between features in different directions on the structure.
The aim of the project described here is the
development of such a permanently attached, autonomous device for monitoring the
condition of a large area of plate-like structures from a single position of the
device, resulting in a large ratio of the monitored surface to the area occupied
by the device. In an array of single transducer elements, ideally each element
selectively excites or receives the desired Lamb wave mode in the plate. For an
omni-directional inspection of the surrounding plate, the guided wave propagates
radially outwards from the excitation source, thus decreasing in amplitude and
effectively limiting the inspection range to several meters in a plate.
Preliminary measurements were made on a 5
mm thick aluminum plate (2.45m by 1.25m) in the laboratory. The array layout
used consists of two concentric circles, an outer circle with 32 receiving elements equally spaced on a diameter of 70 mm, and an inner
circle with 16 excitation elements on a diameter of 50 mm. The circular array design was introduced to achieve the same performance in all
directions.
The setup shown above was
used, employing standard excitation and measurement devices. The excitation
signal was a 5 cycle toneburst with a center frequency of 160 kHz modulated by a
Hanning window. Multiplexing units were used to switch between the different
excitation and receiving transducers. A time trace containing 10000 points was
stored for each combination of excitation and receiving transducer.
The data processing is done in the wavenumber domain,
providing effective dispersion compensation. Taking the Fourier transform of
each time trace and employing the known dispersion relation
for the plate, the wavenumber spectrum is calculated. A phased addition
algorithm is used to synthesize a guided wave beam that can be steered in any
direction from the array. The data
is then Fourier transformed to the angular order domain and a deconvolution
algorithm applied, which improves the angular selectivity of the array
significantly. The results are converted back by means of an inverse
two-dimensional Fourier transform to obtain an omni-directional B-scan in the
radial-angular domain.
The resulting omni-directional B-scan for the undamaged
plate is shown below (a), with the position of the array and the plate edges
marked. The amplitude is normalized to the maximum reflection (occurring at the
closest plate edge) and shown on a color scale down to –15 dB. The measurement shows the reflections of the guided wave at the four
sides and the four corners of the plate. The plate edges are only seen in the
direction where they are normal to the waves propagating radially from the
array. The data processing algorithm is designed to pass signals transmitted and
received along the same radial line and to reject signals from other directions. An artificial model defect was introduced into
the plate by drilling a through hole with a diameter of 30 mm at a distance of
0.36 m from the sensor, marked below (b). The scattering of the guided wave at
such a model defect can be calculated and measured. An additional reflected
signal from that defect is visible with an amplitude about 12 dB lower than the
maximum reflection at the plate edge. This allows the detection of such a model
defect.
Current Address:
Dr. Paul Fromme
Department of Mechanical Engineering
University College London
Torrington Place
London WC1E 7JE
United Kingdom
p.fromme@ucl.ac.uk
