Investigation on Failure Mode of Short Carbon Fiber-Reinforced Thermoplastic Composites Joints of Nonmetallic Pipeline

Pipe joints are weak points in nonmetallic pipeline systems, and their mechanical strength is essential for ensuring safe pipeline operation. By reinforcing with short carbon fiber-reinforced thermoplastic composites (SCFRTCs), the mechanical strength of pipe joints shows great improvement. Understanding the failure mechanisms of SCFRTCs joints is crucial for further structural enhancement and safe operation. In this study, the failure mode of SCFRTCs joints is first investigated through bursting tests. Microstructural analysis using scanning electron microscopy (SEM) and computed tomography (CT) reveals that stress concentration at the SCFRTCs joint initiates cracks formation. The progressive propagation of these cracks, leading to ultimate failure, is identified as the primary failure mechanism. Subsequently, a finite element model of SCFRTCs joints, incorporating contour integration for crack propagation, is constructed, and the energy release rate for crack propagation of SCFRTCs is experimentally determined. Simulation results confirm the stress concentration-induced crack initiation. Furthermore, the effect of joint wall thickness on structural strength of the SCFRTCs joint is analyzed. Increasing wall thickness improves the mechanical strength of SCFRTCs joint. However, most of the SCFRTCs in the joint remain at low stress levels, far from yielding when the crack initiates. As wall thickness increases, the strength utilization rate of SCFRTCs gradually decreases from 49.0% to as low as 14.9%, indicating that increasing wall thickness is not an effective method for enhancing structural strength, particularly under conditions of large wall thickness.