Applicability of nondestructive testing to detect cavities in PVDF polymer

Abstract

The cavitation phenomenon in polymers, commonly known as whitening, has been quite studied in the last two decades due to its relevance to the plastic deformation process and the overall behavior of the material. Its occurrence has been proposed to be either on equatorial or at polar region of the spherulite during deformation. The detection techniques usually employed are small angle X-ray scattering (SAXS) or light scattering, and neither one is suitable for field inspection. The present study focused on nondestructive methodologies to detect the cavity phenomenon in two grades of poly(vinylidene fluoride), comparing simple visual inspection (VI), optical densitometry (OD) and ultrasound velocity (US). The results showed that the last two techniques were able to identify the existence of voids very earlier along the deformation path, about 2%–5% strain, where visual whitening was not even detected. The OD and US showed to be highly effective in distinguishing the effect of the processing conditions on the occurrence of cavitation, via simple and fast methods. The use of ultrasound technique for cavity detection becomes a very promising approach for the production inspection of the pressure sheath of raisers, polymer pipes and other products, as well as for evaluating the integrity of the component that may be affected by occurrence of cavities.

Keywords

• PVDF
• Processing
• Cavitation
• Whitening
• NDT