Underwather shearography – consideratons and first results

Shearography is an optical measurement technique based on the interference between two laterally displaced images of the region of interest. Due to its quasi-equal-path configuration, shearography is robust enough for applications outside the laboratory. In fact, shearography has been successfully used for nondestructive testing in composite materials mainly in the aerospace, boating and, more recently, in the oil and gas industries. There are several demands in the oil and gas industry for nondestructive testing on subsea metallic elements coated with composite materials to prevent or mitigate the effects of corrosion. The present work deals with the underwater shearography for nondestructive testing in metal parts coated with composite materials. The paper begins with considerations about the propagation of light in underwater environments and a discussion on the elements that disturb the measurements with shearography such as changes in wavelength, absorption, dispersion, distortions caused by refraction in the air-glass-water interfaces among others. The different loading possibilities to reveal defects in the tested elements are analyzed: by pressure loading, thermal loading by radiation and thermal loading by induction. Exploratory experiments carried out inside water tanks involving the three loading techniques are presented and discussed. The results are compared and the combinations that led to the best results are emphasized. Finally, future works that involve a fully immersed system for underwater shearogaphy are briefly announced.