3D Failure Envelopes for Thermoplastic Composite Pipes under Thermomechanical Loading

Thermoplastic Composite Pipes (TCPs) are lightweight and flexible tubular structures made from fiber-reinforced composite materials, such as glass or carbon fibers, in a thermoplastic matrix, which exhibit high strength and durability in extreme environments. These characteristics make them ideal for applications in offshore natural gas pipelines and oil extraction, where they are subjected to high pressures and temperatures in corrosive conditions. Due to their complex mechanical behavior, predicting failure in these materials is challenging, especially in the absence of comprehensive models. In this context, failure envelopes stand out as widely used graphical representations to verify material safety under different loading conditions by delimiting their safe operational regions. This work aims to develop a methodology that combines numerical analysis and automated implementation for generating 3D failure envelopes for TCP models commonly found in the literature, considering internal pressure, external pressure, and axial load as the axes of the 3D space. The proposed methodology is based on two automated scripts: one focused on obtaining the distribution of local stresses through the thickness of the TCP, and the other on data processing and generation of the 3D failure envelope based on specific failure criteria for composite and isotropic materials. The stresses are obtained through numerical simulations using ABAQUS software, employing C3D8T elements — 3D solid elements with 8 nodes, linear displacement, and thermal coupling — suitable for the thermomechanical analysis of the problem. Using the simulated data, the second script constructs the 3D envelope and enables the calculation of safety factors related to the loading conditions. The methodology is validated through comparison with literature studies on the safety of TCPs under various operational conditions. The results demonstrated the effectiveness of the proposed methodology in generating 3D failure envelopes, validated through comparisons with literature data and numerical analyses. Different shapes and behaviors were observed in the envelopes for each adopted failure criterion, including shrinkage and translation of the surfaces with increasing temperature. The sensitivity study revealed a tendency for rotation and enlargement of the failure surfaces with an increasing number of layers and variations in the laminate stacking angle. The use of 3D failure envelopes proved fully capable of representing the operational limits of TCPs.