Experimental and numerical investigation of the load-deformation behavior of socket-and-spigot FRPM pipe joints under vertical loading

To investigate the load-deformation behavior of socket-and-spigot Fiber Reinforced Polymer Mortar (FRPM) pipe joints under vertical loading, a full-scale four-point bending test was conducted on DN800 (nominal internal diameter 800 mm) socket-and-spigot FRPM pipes. In addition, a refined finite element model incorporating double rubber sealing rings was established in ABAQUS to investigate the vertical displacement response and von Mises stress distribution characteristics of the socket section, spigot section, and rubber sealing rings. The experimental results showed that leakage first occurred at the joint at an applied load of 64 kN, corresponding to a vertical displacement of 16.9 mm. No visible structural damage to the FRPM pipe components was observed experimentally even when the applied load increased to 420 kN; however, the inner rubber sealing ring exhibited more pronounced deformation and damage than the outer sealing ring. The numerical simulation results indicated that the maximum von Mises stress in the socket section was concentrated at the inner side of the pipe crown, reaching 177.9 MPa, whereas the maximum stress in the spigot section was concentrated at the pipe invert with a value of 148.8 MPa. The maximum equivalent stress in the rubber sealing rings was concentrated in the bottom region, and the stress level of the inner sealing ring was significantly higher than that of the outer sealing ring. The numerical results are used as qualitative support for interpreting structural deformation and equivalent-stress concentration and are not intended as direct predictions of leakage initiation, sealing-contact loss, rubber-ring cracking, or composite failure. This study provides engineering insight into the deformation behavior of large-diameter flexible composite pipe joints under vertical loading.