Investigation on the load-bearing performance and interface stress of plastic pipe–CIPP liner composite Structures

Flexible plastic pipes are widely deployed in urban drainage networks, yet aging and construction irregularities lead to corrosion, leakage, deformation, and joint failures. Cured-in-Place Pipe (CIPP) rehabilitation installs a resin-cured liner inside the host pipe to form a composite system, whose mechanics for plastic hosts remain insufficiently quantified. This study integrates OFDR-based distributed strain sensing, parallel-plate loading tests, and a validated 3D finite-element model with a cohesive interface to interrogate the effects of the liner-to-pipe thickness ratio $\beta$, pipe diameter $D$, and modulus ratio $\eta =E_a/E_b$ on ring stiffness, bending-moment sharing, and interfacial stresses. Results show that interfacial bonding is the key lever for composite action: ring stiffness was observed to increase monotonically with $\beta$, by ≈92 % and ≈210 % in the DN315 PE series relative to the$\beta$ = 0.15 baseline, whereas non-bonded interfaces yield much lower stiffness. Neutral-axis migration with increasing $\beta$ or $\eta$ explains the measured strain patterns. At a fixed deformation, $D$ mainly sets demand, while moment partition is governed by $\beta$ and bonding. Coaction of interface shear with tensile radial stress at the crown/invert was identified as the primary driver for debonding, consistent with the closed-form and FE stress fields. Two simplified relations are proposed for ring-stiffness enhancement and bonding-induced moment amplification; predictions agree with tests and FE trends within the calibrated ranges $\beta \in$[0.15, 0.70], $\eta \in$[0.14, 0.47], and$D$ = 250–500 mm. The findings provide design guidance for specifying liner thickness and verifying interfacial bonding in CIPP rehabilitation of plastic pipelines.