PSI - Issue 80

Yuhang Pan et al. / Procedia Structural Integrity 80 (2026) 43–52

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YuhangPan / Structural Integrity Procedia 00 (2023) 000–000

1. Introduction

Fatigue crack initiation and propagation remain critical challenges in structural engineering due to their potential to cause unexpected failures with severe safety and economic consequences (Zhou et al. (2025); Xu et al. (2024)). Their progressive nature, often invisible in the early stages, makes conventional inspection based approaches insu ffi cient for ensuring structural integrity (Zhou et al. (2022, 2025)). Traditional non destructive testing (NDT) techniques, such as ultrasonic testing or radiography (Golodov and Maltseva (2022)), are typically conducted periodically, which leaves structures vulnerable to undetected crack growth between inspections. These limitations have motivated the advancement of structural health monitoring (SHM) systems that enable continuous and real-time assessment, thereby reducing the risk of catastrophic failure while improving maintenance e ffi ciency and lowering operational costs (Marques et al. (2021); Chen et al. (2019)). SHM methods can generally be categorized by sensor type and underlying physical principle (Pan et al. (2024)). Acoustic emission (AE)-based techniques capture transient elastic waves produced during crack initiation and propagation, with features such as event counts, energy, and frequency content serving as in dicators of fatigue crack growth (FCG) (Karimian et al. (2020); Chai et al. (2022)). Despite their passive nature and high sensitivity, AE methods are often limited by signal stochasticity and susceptibility to envi ronmental noise. Another widely used approach is based on Lamb wave, where surface-bonded piezoelectric transducers (PZTs) are used to both excite and receive ultrasonic waves. Due to their capability for long range propagation and high sensitivity to small-scale defects, Lamb wave techniques have been extensively applied in aerospace structures. Linear Lamb wave methods typically extract features such as time-of-arrival, attenuation, and reflection characteristics (Wu et al. (2009); Ostachowicz et al. (2009)). In contrast, nonlinear methods exploit harmonic generation or subharmonic responses induced by crack-tip nonlinearity, thereby o ff ering enhanced sensitivity to early-stage damage and, in many cases, enabling baseline-free detection (Sampath and Sohn (2022)). Despite these advantages, several critical challenges remain unresolved. A sig nificant limitation of many SHM methods still requires interruptions to normal operation to acquire data, which limits their applicability for true monitoring in service (Zhao et al. (2023)). AE-based methods are attractive for passive monitoring but are limited by variability and noise contamination. Linear Lamb wave approaches are heavily baseline-dependent, making them vulnerable to environmental and operational vari ability, particularly temperature fluctuations (Pan et al. (2024)). Nonlinear methods alleviate the reliance on baseline data but typically demand stringent excitation conditions, which restrict their practicality in field applications (Pan et al. (2025)). To address these limitations, this study proposes an in-service, baseline-free method for fatigue crack detection by analyzing structural responses under operational loading, aiming to achieve reliable monitoring without the constraints of conventional approaches. The remainder of this paper is structured as follows: Section 2 presents the experimental setup, including the fatigue testing procedure and data acquisition system. Section 3 details the proposed baseline-free monitoring methodology. Section 4 presents the results together with an analysis of the associated uncertainties. Section 5 summarises the main conclusions and outlines future research directions.