Abstract <p>This study investigates the application of nonlinear ultrasonic technology for characterizing ductile failure mechanisms and damage evolution in polyethylene (PE) pipes under sustained hoop stress. An integrated experimental framework combining hydrostatic pressure testing and ultrasonic detection was developed to replicate in-service damage evolution. Online monitoring captured continuous nonlinear ultrasonic responses during accelerated damage, while offline measurements provided discrete validation points across progressive damage stages. The methodology employs pulse-echo methods with third-harmonic analysis to quantify acoustic nonlinearity parameters at critical deformation sites. Results demonstrate that nonlinear ultrasonic parameters exhibit sensitivity to microstructural degradation during plastic deformation. A characteristic peak phenomenon consistently emerges in the nonlinearity evolution curve at a critical damage threshold, serving as a reliable precursor to failure. Offline measurements using pulse-echo and V-scan methods corroborate online findings, confirming measurement reproducibility. Furthermore, impact strength testing establishes a correlation between acoustic nonlinearity parameters and material degradation. This work establishes nonlinear ultrasonics as a viable methodology for real-time structural health monitoring of PE pipes. The nonlinear ultrasonic technology bridges microscopic damage progression with macroscopic mechanical behavior, enabling early detection of ductile damage before macroscopic failure occurs. The findings provide a foundation for developing condition-based assessment protocols in pipe integrity management.</p>