| Summary: | Ultrasonic Non Destructive Testing (NDT) is used to inspect materials and
structures for defects. Water is commonly used in NDT as a couplant to
improve ultrasonic transfer between an interrogating probe and test piece.
Unfortunately, the presence of water can cause corrosion and/or degradation
of the test piece material. The aim of this investigation was to evaluate
hydrated cross-linked hydrophilic polymers as candidate solid contact
ultrasonic couplant for use in the field of ultrasonic NDT.
The fact that hydrophilic polymers can absorb and retain large quantities of
water suggested that they might demonstrate the desirable ultrasonic
properties of water without the risks associated with conventional water
coupling. To test this, the ultrasonic properties of a range of hydrophilic
polymers were assessed. Excellent results were achieved, attenuation as low
as 0.36 and 0.71dB mm-1 at 5 and 1OMHz respectively being measured.
Great potential for efficient coupling was established due to acoustic
impedance in the region of 1.81VIN S M-3.
A polymer dependant coupling pressure of less that 1kg CM-2 was required to
achieve optimum coupling to a smooth steel block. Mechanical longevity,
evaluated by life testing, showed that polymers of up to 70% equilibrium water
content were best suited for dynamic testing applications. Temperature was
shown to effect ultrasonic properties; a drop from 5 to
-120C caused an
increase in attenuation of 3dB mm-1 and velocity of 350m s-1. Pressure
demonstrated no influence on attenuation but affected an increase in velocity
of 44m s-1 per kg CM-2
.
Further investigation into the unique ultrasonic
properties of hydrophilic polymers showed that the water sorption process
caused an increase in attenuation prior to saturation being reached. This was
attributed to the absorption of sound during the polymer transformation from
the glassy to rubbery condition observed during hydration. Dehydration from
100 to 37% saturation in a 60% equilibrium water content polymer caused an
increase in attenuation of 1.8dB mm-1 at 5MHz.
The research concluded with the design and development of a prototype
wheel probe employing hydrophilic polymer as the tyre. Operation at 5MHz in
pulse echo mode demonstrated results competitive to conventional immersion
testing. An MMA-VP cross-linked hydrophilic polymer of approximately 60%
equilibrium water content was found most suitable to this application.
This thesis suggests that there is a clear role for hydrophilic polymers in
ultrasonic NDT. The success of the wheel probe design developed as a result
of this research has resulted in patent application in both the UK and USA.
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