Proceedings Article | 30 March 2009
KEYWORDS: Transducers, Ultrasonics, Reflection, Receivers, Scattering, Target detection, Acoustic coupling, Wave plates, Prototyping, Data acquisition
A steel box beam in a monorail application is constructed with an epoxy grout wearing surface, precluding visual
inspection of its top flange. This paper describes a sequence of experimental research tasks to develop an ultrasonic
system to detect flaws (such as fatigue cracks) in that flange, and the results of a field test to demonstrate system
performance. The problem is constrained by the fact that the flange is exposed only along its longitudinal edges, and by
the fact that permanent installation of transducers at close spacing was deemed to be impractical. The system chosen for
development, after experimental comparison of alternate technologies, features angle-beam ultrasonic transducers with
fluid coupling to the flange edge; the emitting transducers create transverse waves that travel diagonally across the width
of the flange, where an array of receiving transducers detect flaw reflections and flaw shadows. The system rolls along
the box beam, surveying (screening) the top flange for the presence of flaws.
In a first research task, conducted on a full-size beam specimen, we compared waves generated from different transducer
locations, either the flange edge or the web face, and at different frequency ranges. At relatively low frequencies, such
as 100 kHz, we observed Lamb wave modes, and at higher frequency, in the MHz range, we observed nearlylongitudinal
waves with trailing pulses. In all cases we observed little attenuation by the wearing surface and little
influence of reflection at the web-flange joints. At the conclusion of this task we made the design decision to use edgemounted
transducers at relatively high frequency, with correspondingly short wavelength, for best scattering from flaws.
In a second research task we conducted experiments at 55% scale on a steel plate, with machined flaws of different size,
and detected flaws of target size for the intended application. We then compared the performance of bonded transducers,
fluid-coupled transducers, and angle-beam (wedge) transducers; from that comparison we made the design decision to
use wedges, which beam the wave to increase the scattering from flaws. We also compared the performance of wired
transducers using fluid coupling to that of wireless (inductively coupled) transducers mounted permanently. Although
the wireless transducers achieved flaw detection, the necessary spacing (determined experimentally) would have
required an impractical number of transducers. Therefore, we made the design decision to use wedge transducers with
fluid coupling.
In a third research task we developed and tested a rolling system with a water channel for acoustic coupling, including a
study of its sensitivity to misalignment, and in a fourth task we devised a data display to create a pattern of reflections or
shadows that could be easily interpreted as evidence of a flaw. Finally, we conducted a field test on the full-size system
in a region containing bolt holes, which act as a physical simulation of a flaw, and show successful detection of
reflections and shadows from those holes.
