Non-collinear wave mixing to determine physical ageing in PVC pipelines

The ultrasonic signal amplitudes obtained from non-collinear wave mixing has previously shown significant changes for physically aged Poly-Vinyl Chloride (PVC) pipes, demonstrating the method's effectiveness in assessing the ageing condition of PVC. However, the exact relationship between the measured amplitudes and the extent of physical ageing in PVC has yet to be established. Additionally, it has not yet been validated against established methods or applied to determine the service lifetime of PVC pipes. This study outlines a three-step process to validate the effectiveness of the non-collinear wave mixing technique for assessing the physical ageing of PVC pipelines. Firstly, the non-collinear wave mixing results are compared with the established laboratory method of differential scanning calorimetry, to validate the scattered wave amplitude measurements. Secondly, employing the Arrhenius relation, service lifetime estimates derived from non-collinear wave mixing results align within the expected magnitudes, emphasizing the technique's potential for in-situ pipeline inspections. Finally, a material model is utilized to estimate the Third Order Elastic Coefficient (TOEC) $m$ based on the measured amplitudes, demonstrating a linear correlation with annealing time. A 50% change in TOEC $m$ signifies a brittle state, indicating a high risk of failure. A combination of experimental and analytical analyses highlighted TOEC $m$ variations as the primary factor influencing non-collinear wave mixing amplitude changes during physical ageing. This comprehensive approach also highlights the importance of incorporating diverse factors to precisely forecast and oversee the performance of PVC pipes in real-world environments.