Novel hydrophilic polymer couplant for application in ultrasonic non destructive testing

Abstract

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.