Behavior of CFRP-strengthened short spiral welded tubes under axial load

Abstract

Spiral welded tubes (SWTs) are formed by rolling steel plates at a certain angle and welding the resulting abutting edges. They are extensively utilized because of their large diameter ranges, high production efficiency, and low production cost. Thin-walled spiral welded tubes with large diameter-to-thickness ratios are susceptible to local buckling under compressive load, and their performance can be affected by corrosive environments. Therefore, this paper proposed the use of carbon fiber-reinforced polymer (CFRP) to strengthen tubes by external bonding. There are limited studies on the axial compression behavior of CFRP strengthened spiral welded tubes, particularly studies that consider the initial imperfections and residual stresses in spiral welded tubes. To bridge this gap, this paper first investigated the distributions of initial imperfections and residual stresses in spiral welded tubes, finding that both are significantly influenced by distinct forming and welding manufacturing processes. Subsequently, axial compression tests were carried out on four unstrengthened specimens and twelve CFRP-reinforced specimens to investigate the compressive behavior by considering the diameter-to-thickness ratio, the length of the steel tubes, and CFRP layer orientation. The experimental results indicate that CFRP strengthening delays local buckling, and significantly enhances the yield load, ultimate load, and initial stiffness of spiral welded tubes, achieving a maximum increase of the yield load up to 69.11 % and the ultimate load up to 61.99 %, while its impact on the ductility index remains uncertain and is influenced by variables such as the diameter-to-thickness ratio of the tubes and the CFRP layer orientation. Finally, a comparison of the ultimate loads of the specimens by tests and design codes was conducted.