# Numerical Analysis of Fatigue Life of Wind Turbine Blades Reinforced with Graphene Platelets
## 1 Introduction
There are cases in which GPLRC-reinforced blades were mainly analyzed as simplified beam models, as in the studies introduced above; however, only a few studies have actually applied them to detailed blade models. With this background, the authors of this paper have conducted a detailed analysis of the application of graphene platelets (GPLs) to wind turbine blade models, examining their bending, vibration, deflection, and stress concentration characteristics due to fatigue cracks, weight reduction, and economic feasibility [14; 15; 16; 17]. Wind turbine blades are constantly exposed to repeated wind loads and irregular environmental conditions, and such environments act as major factors that threaten the structural safety of blades. Specifically, fatigue damage due to cyclic loads initiates with the occurrence of microcracks in the early stage and leads to the fracture of the structure through crack propagation, as shown in Figure 1. Therefore, fatigue life assessment is a very important factor in structural design, along with yielding and buckling. Giventhis importance, various studies have been conducted on fatigue life assessment for wind turbine blades. For example, [PERSON] et al. [18] analyzed the fatigue load spectrum using the S-N curve and [PERSON]'s empirical formula to assess the fatigue life of wind turbine blades and examined whether a design life of 20 years was met. [PERSON] et al. [19] assessed the fatigue life of wind turbine blades under stall and pitch regulation conditions and analyzed the difference in fatigue damage between the two methods. [PERSON] et al. [20] presented a method to more precisely assess fatigue life considering varying wind loads and multi-axial complex stress. [PERSON] et al. [21] introduced a method to assess the fatigue life of wind turbine blades by applying the modified blade element momentum theory (BEMT). Many other studies contributed to improving the fatigue performance of wind turbine blades, but further research is required when cutting-edge new materials are applied to meet the requirements of large wind turbines. For example, [PERSON] et al. [22] assessed fatigue performance by applying basalt fiber-reinforced composites to offshore wind turbine blades, and they confirmed that fatigue life was significantly improved when compared to conventional fiberglass blades. [PERSON] and [PERSON] [23] analyzed the fatigue performance of CNT-reinforced composites using a multiscale finite element model, and they confirmed that the fatigue performance of the composites was significantly improved by CNT reinforcement. This indicated that CNT reinforcement can be effective when large blades, such as offshore wind turbine blades, are used. When new materials are used, it is required to closely examine the fatigue performance of wind turbine blades. It is difficult, however, to find cases in which the fatigue performance of GPL-reinforced wind turbine blades was evaluated using detailed finite element models. Hence, this study aims to assess the fatigue life of GPL-reinforced wind turbine blades by introducing an effective and reliable numerical evaluation method and comparing it with that of conventional fiberglass blades as a follow-up study to previous studies [14; 15; 16; 17] conducted by the authors. In Section 2, the wind turbine blade model and material modeling method used for fatigue load analysis are described in detail, and the validity of the finite element model constructed based on them is verified. In Section 3, the stepwise procedure for fatigue life assessment and applied method are addressed in-depth. The results derived through the same procedure are closely analyzed in Section 4. Finally, based upon the numerical results, future research directions are presented by discussing potential alternatives to overcome the limitations of GPL-reinforced blades in terms of fatigue strength and to improve performance. Figure 1: Fatigue cracks observed on wind turbine blades: (**a**) a crack in the trailing edge, (**b**) cracks in the trailing edge panel and spar cap [24; 25].