Damage identification of pipelines by fusing normalized strain mode shapes with natural frequency change rates

Pipelines are widely used as critical load-bearing members in spatial structures, bridges, and offshore platforms, where they may suffer from local cracks or fatigue damage under random excitations. Structural health monitoring (SHM) systems are essential for identifying such damage. To overcome the common limitations of existing damage identification methods—namely, their reliance on prior information and low sensitivity to minor damage—this paper proposes a novel damage identification method tailored for pipe structures. The method is based on a structural damage indicator (SDI) that fuses normalized strain mode shapes with natural frequency change rates, along with a corresponding damage threshold determination approach. Finite element simulation results demonstrate that the proposed method achieves 100% accuracy in damage localization under both single-damage and multi-damage conditions with 10–60% stiffness reduction, and maintains stable identification capability for minor damage. Through impact excitation tests on a clamped-clamped pipe beam, the proposed method is experimentally verified using quasi-distributed fiber Bragg grating (FBG) sensors. The results confirm that the circular packaged FBG sensor delivers optimal damage identification performance for pipe beam structures. The proposed method requires neither known excitation information nor a complete prior finite element model, offering a reliable and practical damage identification strategy for SHM systems of pipe beam-type structural components.