Mechanical performance and microstructural behavior of ultrasonically welded PVC-coated polyester fabric seams at subzero temperature

Ultrasonic welding is increasingly used to join PVC-coated polyester fabrics in membrane structures where seam reliability is critical, particularly under cold-service conditions. This study systematically investigates the effects of welding pressure (0.5,1.5 and 2.5 bar), coating thickness (0.6 and 0.8 mm), seam margin (6 and 12 mm), and temperature (20 °C and - 20 °C) on the mechanical performance and failure behavior of continuously ultrasonically welded seams. A full-factorial experimental design was employed, and seam strength (N/5 cm), extension at failure, work of rupture, and seam efficiency were evaluated using load-extension analysis. Increasing welding pressure from 0.5 to 2.5 bar produced a 2.5-5 fold increase in seam strength and substantially enhanced energy absorption. Increasing thickness from 0.6 to 0.8 mm improved seam strength. Furthermore, at a pressure of 2.5 bar, increasing the seam margin from 6 to 12 mm raised the seam strength from 546.0 to 1190.4. At - 20 °C, seams exhibited higher strength but reduced ductility, reflecting a stiffer and more brittle response. Fractographic analysis revealed a transition from predominantly adhesive failure at low pressure to cohesive failure at 2.5 bar, consistent with improved interfacial fusion. FTIR and EDS analyses confirmed that ultrasonic welding primarily induced morphological rather than major chemical changes. Regression-based predictive models were developed and validated, demonstrating reliable strength prediction under independent processing conditions. The results provide quantitative guidance for optimizing ultrasonic welding parameters to ensure durable seam performance in PVC-coated textile structures operating under both ambient and subzero environments.