## 1 Introduction  
Fiber-reinforced plastics (FRPs) are hybrid materials composed of reinforcing fibers and polymer matrices [1; 2]. High specific stiffness and strength characterize FRPs, and their excellent resistance to fatigue loads of glass fiber-reinforced plastics (GFRPs) make these composites particularly suitable for the fabrication of wind-turbine blades with higher wind power efficiency [3; 4; 5; 6]. During service, wind blade systems are exposed to a variety of environmental conditions, such as high humidity [7; 8], elevated temperature [9; 10], freeze-thaw cycles [11; 12], and ultraviolet (UV) radiation [13; 14], which can affect their performance. Ultraviolet radiation is detrimental to polymers, initiating photodegradation, which usually degrades their mechanical properties [13]. [PERSON] et al. [15]. exposed epoxy to UV light for up to 200 h and then observed the surface by scanning electron microscopy. Microcracks occurred on the surface exposed to UV radiation, and the damage became more severe as the surface cracks enlarged, with increasing amounts of accumulated energy. Various combinations of composites were tested for up to 100 days of photodegradation [16]. The ultimate tensile strength and modulus of FRP decreased with increasing UV exposure. Therefore, for the maintenance and residual life evaluation of wind turbine blades, it is essential to predict changes in the fracture behavior of FRPs exposed to UV radiation during their life cycle. It is important to establish efficient monitoring technology due to the difference between natural exposure and laboratory conditions. Acoustic emission (AE) of transient elastic waves in solids result from crack formation (plastic deformation) due to aging, temperature gradients, and mechanical forces [17]. Acoustic emission testing is a noninvasive, passive technique that can monitor the evolution of conditions in a structure [18; 19; 20; 21]. It can be used to evaluate the crack state based on simple parameters, such as hit frequency (hits per second) and amplitude. Amplitude is related to crack size, while hits are related to crack growth. This technology has been used for structural health monitoring (SHM), especially in the context of seismology [22; 23], and construction (i.e., concrete structures [24; 25; 26]). [PERSON] et al. [27], introduced the \(b\)-value, derived from the amplitude-cumulative hits distribution, to evaluate the state of a crack (earthquake). The \(b\)-value is defined as the slope of the cumulative hit with respect to the amplitude. When cracks grow, a signal with a large amplitude is generated and the \(b\)-value is designed to decrease. [PERSON] et al. [28], improved the \(b\)-value by using both the mean and variance, and applied it to the analysis of cracks in rocks. Finally, [PERSON] et al. [29], proposed a standard for evaluating cracks in a concrete structure based on the \(lb\)-values. Our previous study [30; 31; 32] used \(b\)-values to analyze a UV-irradiated FRP structure. The previous study [31] investigated the signal propagation behavior (i.e., attenuation) under UV irradiation conditions and studied the damage initiation and accumulation behavior using \(b\)-value analysis. In another piece of research [30; 32], we studied the failure behavior and frequency-dependent attenuation behavior in CFRP, finding that \(lb\)-value is continuously decreasing. The conventional \(b\)-value is helpful in damage analysis. However, it is not suitable for structural health monitoring (not monotonously changing in FRP structure). The feasibility of \(lb\)-value in FRP structural health monitoring is required, especially under environmental degradation. We investigated the fracture behavior of a UV-irradiated GFRP under tensile loading. Technical issues, and the accompanying methodology, are schematically shown in Figure 1. The GFRP specimens were prepared with a circular notch to establish the crack location. In the tensile test, the \(\Delta\)E hits and amplitude distribution were used to analyze the crack condition.