Unraveling slow crack growth mode transition in high-density polyethylene pipe

The slow crack growth (SCG) behavior of high-density polyethylene (HDPE) is influenced by the testing temperature and the stress intensity factor K_I. At low K_I values, cracks grow continuously, whereas discontinuous SCG is observed at moderate K_I. Due to the small crack initiation sites in actual HDPE pipes, a mode transition from continuous to discontinuous SCG often occurs within a single SCG process. However, this phenomenon is difficult to explain using existing crack growth models, posing challenges for accurate long-term lifetime estimation in HDPE pipe design. Here, we introduce the first crack layer-based framework that captures the full mode transition by coupling classical crack layer theory with a thermal-activation scheme. This model integrates an auxiliary component into the original crack layer theory, enabling it to reproduce both the continuous-to-discontinuous SCG transition under low loads and the discontinuous SCG behavior at moderate loads. The proposed model successfully reproduces experimental results, providing a more comprehensive understanding of SCG behavior in HDPE. By addressing the limitations of existing models, this study establishes a fundamental framework for improving the accuracy of lifetime predictions in HDPE pipe design and advancing the understanding of crack growth mechanisms.