Introduction

More competitive design is required for the wind turbine blades to be lighter, stiffer and more durable, especially in the large size of blades of 5 MW, 8 MW and even higher. The larger blades are more efficient since the wind power is squarely proportional to the blade size. However, as the blade is bigger the mass is also dramatically increasing and placing severe conditions on overall design. However, the current design procedure is in most cases limited to conventional rules and layup sequences. For instance, only three different layup types, i.e., unidirectional laminates (UD), bi-axial laminates (BX), and tri-axial (TX) laminates, are utilized and combined and only thickness are determined. So the current blade performance is that much limited without question.

In the wind turbine blade design, there are many criteria on deflection, ultimate strength, critical buckling, natural frequencies, and, most importantly, ensuring at least 20 years of life and 200 million revolutions. To industry, the cost effectiveness should be considered as importantly as the design requirements. To meet such various structural requirements, designers should use reliable predictive tools to consider wider range of materials of resin and fibers and laminates with various ply angles. The ply angle of composite materials is a key design factor, and its effects on the mechanical performance should be fully understood and utilized. For instance, the bend-twist coupling as a result of the mirror laminate ply angles in the top and bottom spar caps can help reduce aerodynamic loads, increasing fatigue life of the blades.

To achieve a more competitive design, we consider the detailed micro stress distribution of laminates with arbitrary ply angles. It allows us reduce weight with confidence. In this respect, the Micro-Mechanics of Failure (MMF) and Accelerated Test Methods (ATM) have been proposed for predicting the static strength and fatigue life of composites, eliminating vast tests required to characterize material and laminate properties of the non-crimping fabrics (NCF). In-house programs of Hyblade and HyFatigue are currently being developed for the design of wind turbine blades.

The MMF starts with analyzing the constituents of fiber, matrix and interface as shown in Figure 1. A unit cell is modeled to consider the fiber architecture inside the matrix at the micro-mechanical level. This way, various types of fibers and resins can be studied prior to extensive tests. The multi-angled non-crimp fabrics (NCF) is then homogenized as the next step. Using these properties, global structural analysis is performed to obtain the stress and strain distribution of the blade. These macro stresses will change the material behavior of the constituents at the micro level. Finally, based on the microscopic damage assessment, we predict the ultimate strength and fatigue life of the constituents, the plies. laminates and eventually the whole wind turbine blades.

Figure. Design Procedure of Wind Turbine Blades using Hy-series tools