PSI - Issue 52

Yingwu Li et al. / Procedia Structural Integrity 52 (2024) 709–718 Author name / Structural Integrity Procedia 00 (2023) 000–000

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To address this challenge, Structural Health Monitoring (SHM) has emerged as a solution to assess the structural integrity of CFRP components. By implementing SHM, it becomes possible to continuously monitor the health of CFRP structures during their operational life, thereby reducing maintenance expenses and enhancing overall safety Giannakeas et al. (2022). SHM provides real-time data on crucial parameters, such as strain, temperature, and load, to evaluate the safety of composite structures. In recent years, the Optical Frequency Domain Reflectometry (OFDR) has emerged as a prominent method in the realm of SHM and condition monitoring due to its capacity for real-time monitoring and early detection of po tential defects or damage in various structures. The fundamental principle of this technique involves exploiting the interference phenomenon between Rayleigh backscattering and incident light to characterize the loading state along a single-mode fiber (SMF) Froggatt and Moore (1998). Rayleigh backscattering originates from random fluctuations in the refractive index of the SMF during its manufacturing process, resulting in a detected interference signal that exhibits remarkable stability and uniqueness. External mechanical or thermal loads induce a frequency shift in this interference signal. To detect this shift, cross-correlation analysis is performed between the reference and loading states in the frequency domain D´ıaz-Maroto et al. (2018). Consequently, the measurement principle of OFDR can be mathematically expressed as Equation 1. In this equation, the frequency shift caused by external loads is represented by ∆ v , while ∆ T and ∆ ε signify the external thermal and mechanical loads, respectively. The temperature-frequency shift coe ffi cient and strain-frequency shift coe ffi cient are denoted as K T and K ε , respectively. For detailed expressions of these coe ffi cients, readers can refer to Kwon et al. (2019). The unique ability of OFDR to provide distributed measurements with sub-millimeter spatial resolution makes it a valuable tool for investigating a wide range of samples including composites. Therefore, the significance of consistency analysis pertaining to OFDR measurements bears paramount impor tance, as it directly influences the credibility and validity of data Li et al. (2019). Through the meticulous evaluation of OFDR measurement consistency, researchers and engineers gain confidence in the accuracy and reproducibility of their findings, thereby enhancing the robustness of further investigations, including load monitoring Iele et al. (2018), shape sensing Xu and Sharif Khodaei (2020), and damage detecting Kuang (2015). Such scrutiny facilitates the identi fication and mitigation of factors that may introduce variability or errors into the measurements, encompassing noise, environmental conditions, and calibration inaccuracies. However, limited research has been conducted on the consis tency analysis of OFDR measurements in various scenarios, including di ff erent structures, measurement times, and loading conditions. This may lie on di ff erent reasons including specialized equipment and expertise, complexity and cost, limited awareness and prioritization, and data availability and sharing. This paper presents a thorough examination of the measurement consistency of OFDR, encompassing three pri mary sections. Chapter 2 focuses on investigating the strain-frequency shift coe ffi cient consistency under di ff erent test scenarios, including tensile experiments and three-point bending experiments. Chapter 3 delves into the consis tency of temperature-frequency shift coe ffi cient, considering scenarios including surface mounted and in a free state. Lastly, Chapter 4 conducts a type A evaluation of standard uncertainty to assess the uncertainty of strain and tem perature measurements. The findings presented in this paper provide valuable reference and insights for a wide range of applications that rely on the distributed measurement capabilities of OFDR. The conclusions of this resaeerch are summarized in Chapter 5. ∆ v = K T ∆ T + K ε ∆ ε (1)

2. The measurement consistency of strain-frequency shift coe ffi cient

2.1. The strain-frequency shift coe ffi cient in various experiments

Since the measurement of OFDR is presented with frequency shift, the strain is obtained by a strain-frequency shift coe ffi cient according to Equation 1. In this research, the reading of strain gauge is adopted as reference strain, which is adopted to calculate the strain-frequency shift coe ffi cient. Therefore, the strain-frequency shift coe ffi cient under di ff erent test scenarios is obtained. An illustrative example of obtaining strain-frequency shift coe ffi cients from