Failure mechanisms of fusion-bonded reinforcement joints in reinforced thermoplastic pipes under uniaxial tensile conditions

As deep-sea oil and gas extraction technologies advance, the demand for high-performance joints in reinforced thermoplastic pipes (RTPs) has increased. This study introduces a novel fusion-reinforced joint for RTPs and analyzes its tensile failure mechanism. Two user-defined material (VUMAT) subroutines were developed for unidirectional fiber composites and plain fabric composites to analyze the damage of RTPs and joints. The tensile damage mechanisms were evaluated using the 3D Hashin failure criterion, the maximum strain failure criterion, the residual stiffness model, and the cohesive zone model (CZM). To validate the numerical model, fusion-reinforced joints were designed, machined, and subjected to uniaxial tensile tests. Findings suggest that matrix damage in RTPs is the primary factor contributing to stiffness degradation. The shear stress in the adhesive layer at both ends of the joint reaches the shear strength, resulting in the failure of the adhesive. The tensile process can be divided into four distinct stages: the no-damage stage, the bonding damage stage, the matrix damage stage, and the failure stage. Initially, damage in the adhesive layer leads to a minor decrease in tensile stiffness, followed by significant matrix damage in the RTPs. Failure of the adhesive layer at both ends of the joint gradually propagates to the middle, culminating in the failure of the fusion zone. The time required to reach maximum strain in the central joint region is longer than at the ends.