PSI - Issue 52

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

714

6

Table 2. The consistency assessment results of the strain-frequency shift coe ffi cients. Test scenarios Cronbach’s alpha ( α )

McDonald’s omega ( ω )

Split-half reliability coe ffi cient

Tension Fatigue

0.991 0.997 0.992 0.997

0.998 0.999 0.997 0.999

0.993 0.991 0.992 0.997

Three-point bending

All three types

Table 3. The experiment details in temperature measurement. Test scenarios

Temperature load range ◦ C Temperature load step ◦ C Coating material

Sensor state

Test quantities

First Group Second Grou Third Group

-50 ∼ 50 -50 ∼ 50 -30 ∼ 80

5 5 5

Acrylate Acrylate Polymer

Surface mounted Surface mounted

8 7 7

Free

the strain-frequency shift coe ffi cients measured in di ff erent test scenarios Spiliotopoulou (2009); Cortina (1993). The results of the consistency assessment of the strain-frequency shift coe ffi cients presented in Section 2.1 are summarized in Table 2. Based on the results presented in this table, the strain-frequency shift coe ffi cients demonstrate high consis tency across di ff erent test scenarios, including temperature variations , fatigue, and three-point bending experiments.

3. The measurement consistency of temperature-frequency shift coe ffi cient

3.1. The temperature-frequency shift coe ffi cient indi ff erent experiments

In this section, we conducted a comprehensive analysis of the temperature-frequency shift coe ffi cient derived from data collected during twenty-two independent temperature experiments. These experiments were organized into three distinct groups. The first group encompassed temperature measurements of single-mode fiber (SMF) sensors with acrylate coating in their free state, consisting of eight independent experiments. The temperature load in this group is set as -50 ◦ C to 50 ◦ C with a step of 5 ◦ C. The second group involved seven independent experiments with SMF sensors (acrylate coating) installed on the surface of composite panels. The temperature load in this group is set as -50 ◦ C to 50 ◦ C with a step of 5 ◦ C. The last group included seven independent experiments with SMF sensors (polymer coating) in their free state. The temperature load in this group is set as -30 ◦ C to 80 ◦ C with a step of 5 ◦ C. The details of these three groups of experiments are summarized in Table 3. The SMF sensors with acrylate coating were model 1550BHP from THORLABS, while the SMF sensors coated with polymer were model SM1500(9 / 125)P, manufactured by FIBERCORE. All distributed temperature sensing data were obtained using the ODiSI-B system, manufactured by LUNA Ltd. The temperature-frequency shift coe ffi cients obtained from the experiments in the first group are visualized using a Cumming plot in Figure 6. The x-axis represents T1 to T8, corresponding to test 1 to test 8, while the y-axis denotes the values of the temperature-frequency shift coe ffi cient (denoted by ‘T-frequency shift coe ffi cient’). The di ff erence between T1 and the rest of the experiments is presented at the bottom of this figure, indicating that the mean di ff erence is less than 0.07 ◦ C / GHz. The installation of single-mode fiber (SMF) sensors (acrylate coating) on the surface of composite panels sig nificantly a ff ects the temperature-frequency shift coe ffi cient compared to their free state counterparts. Based on the temperature-frequency shift coe ffi cients obtained in the experiments of the second group, the value of this coe ffi cient decreases from -1.55 ◦ C / GHz (free state) to -1.04 ◦ C / GHz (surface mounted). The detailed measurement data is presented in Figure 7. Notably, the maximum mean di ff erence after installation decreased to 0.03 ◦ C / GHz compared to the free state situation (0.07 ◦ C / GHz). This observation indicates that the installation of SMF sensors improves the measurement stability of temperature.