Use of phase spectra for ultrasonic NDE of thin rubber coatings
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This paper describes a technique for ultrasonic nondestructive evaluation (NDE) of a thin rubber coating on a thin substrate; by thin we mean the thicknesses of both the coating and the substrate are less than the wavelength of the ultrasonic wave used to interrogate them. Since the acoustic impedance of rubber is about the same as that of water, the conventional water-immersion methods could not be used. Instead, the coating side was backed by air; while the substrate side was in contact with water. A plane longitudinal wave normally incident upon the substrate side was considered. The transfer function in reflection, H*(), was derived. Only the phase of H*() was used in this work; the magnitude is unity at all frequencies. A systematic analysis of the sensitivity of the phase to the thickness and wavespeed of the coating and the substrate has been carried out. This technique was used to measure the thickness and wavespeed of thin rubber coatings on a thin stainless steel substrate. The Simplex method was employed to determine simultaneously the coating properties through a comparison of the theoretical and measured phase spectra. Reasonably good comparison between the theory and experiment was observed.
