# Reliability Analysis of CFRP-Packaged FBG Sensors Using FMEA and FTA Techniques
## 1 Introduction  
As one of the most common renewables, wind-generated electricity has increasingly received great interest abroad. The wind turbine blade (WTB) is the key part of the wind turbine, and its quality and reliability are the basis of the system's safe and stable operation [1, 2]. WTB full-scale structural testing (see Figure 1) is a commonly used method to check the design and manufacture of a new blade before delivery [3, 4]. In WTB full-scale structural testing, comparison of the measured strain of the blade surface and its theoretical value is the basis on which to judge whether the blade fails or not; thus, an accurate strain measurement is required. Currently, gauges are often used to measure strain in WTB structural testing (see Figure 2), where their extra wiring, heavy accumulated weight, and high failure rates have brought about inconvenience for long-term monitoring. Considering this situation, FBG sensors are applied in WTB full-scale structural testing due to their light weight, anti-electromagnetic interference, anti-corrosion properties, and their ease of setup. FBG sensors are usually packaged with carbon fiber-reinforced plastics (CFRP) before use (see Figure 3) because the FBG sensors are thin and easy to break and CFRP has a good compatibility with the blade surface's material [5, 6]. Due to working conditions, complicated alternating loads, and other uncertain factors, the packaged FBG sensors will be subject to performance degradation and function failures as time goes on. [PERSON] et al. [7] found that the metallized FBG sensors have obvious attenuation of temperature sensitivity, spectral characteristics, and experience power failure in short-time high temperature testing, which indicates that a degradation of spectral and reflection peak power in the FBG sensors exists. [PERSON] [8] studied the fatigue characteristics of adhesive-packaged FBG sensors by a series of tests, and found that the sensitivity, repeatability, and consistency of the adhesive-packaged FBG sensors degenerated after a period of vibration. [PERSON] [9] studied the strain transfer characteristics of the FBG sensor by establishing a strain transfer mechanical model, analyzed the influences of thebonding layer on the average strain transfer ratio of the FBG sensor, and verified that the deterioration of the bonding layer would cause the degradation of the strain transfer ratio. According to the literature, the quality and performance of packaged FBG sensors are not stable in practice; consequently, the reliability analysis of FBG-packaged sensors is very important. Our previous study [10] showed that the bonding layer greatly affects the strain transfer ratio of the CFRP-packaged FBG sensors applied in WTB full-scale structural testing, and the reliability of the CFRP-packaged FBG sensors are influenced by adhesive types, adhesive thickness, etc. However, the study only considered the influence of the bonding layer, and ignored the influences of the packaged layer, welding quality, inherent reliability of FBG sensors, etc. Consequently, the measurement accuracy and long-term availability of the CFRP-packaged FBG sensors still cannot be ensured, and a much more in-depth reliability analysis is needed. Among all reliability analysis methods, the failure mode and effects analysis (FMEA) and fault tree analysis (FTA) techniques are commonly used. FMEA is a systemic process to ascertain critical failure items, such as the failure modes, failure causes, failure effects, etc., and it adopts the risk priority number (RPN) to assess the risk of one failure item [11; 12; 13; 14]. FTA is a deductive process to determine the root causes of a failure item, and the results can show how different component failures or certain working conditions can combine together to cause failure [15; 16; 17].