Nonlinear Ultrasonics for In-line Quality Monitoring of Polymer Processing Methods

Abstract

Ultrasonic testing is a nondestructive structural characterization technique with limited examples of application for polymeric products due to the high signal attenuation in this class of materials. Recent developments in this thesis on ultrasonics have focused on a guided waves test method and used nonlinear analysis of harmonic frequencies to characterize polyethylene, a semi-crystalline polymer. This sensor technology was demonstrated in the detection of initial plastic deformation and to monitor solvent swelling. Frequency regions of low signal attenuation and a nonlinear ultrasonic parameter using amplitude ratio of harmonic peaks were used to classify different crystalline morphologies, controlled by thermal treatment. With an established connection between the ultrasonic spectrum signal and the internal structure of polyethylene, a quality monitoring tool was developed and applied to a batch rotational molding process. Multiple traditional quality measurements were correlated with the ultrasonic signal using multivariate statistical analysis. Finally, an in-line statistical approach for quality classification and an on-line process monitoring using dynamic process modeling were validated. The results presented in this study demonstrate the relevancy of incorporation of the ultrasonic sensor technology to promote advanced manufacturing practices for the polymer manufacturing industry.