Structural Analysis of Composite Wind Turbine Blades

The main objective of this work is the development of finite element models capable of predicting the structural damping and the damped structural response of laminated composite blades and beams. The theoretical framework presented in the current work consists of two main parts. Firstly, the material coupling effect on the static and modal characteristics of composite structures is investigated. New damping terms encompassing strong material coupling effects are formulated and incorporated into a new 3-D beam finite element capable of predicting the modal characteristics of composite structures. The second part deals with the inclusion of nonlinear effects due to large rotations and initial stresses, the prediction of nonlinear damped structural dynamics and the characterization of the damping of laminated composite strips subject to large in-plane tensile and compressive loads. The nonlinear section mechanics were incorporated into a new nonlinear tubular beam finite element and a research finite element analysis code which enable the computational prediction of nonlinear characteristics of composite blades. The finite element is first applied and experimentally validated for the case of composite strips subject to initial tensile and compressive loads. Based on the successful validation of the nonlinear strip element, an extended nonlinear tubular beam finite element for the damping prediction in more complicated composite structures, such as wind-turbine blade models, is also formulated and presented.

The present chapter describes the work reported in the area of structural analysis and structural dynamics of composite blade structures. Emphasis is given in the literature survey on analytical beam finite element models which assume linear kinematic assumptions in the constitutive equations. The effect of material coupling terms both in stiffness and damping structural matrices is also presented in the following paragraphs. However, the maximization of wind-turbine blades performance through longer rotor blades in combination with the development of new more flexible helicopter blade designs made the development of new nonlinear models essential regarding their structural analysis.

The current chapter presents the theoretical background for the damped dynamic analysis of composite beams and blades encompassing material coupling effects. The formulation includes composite material coupling effects, first in the crosssection stiffness and damping matrices and finally into the structural stiffness and damping matrices of the blade. In the following sections, new coupling damping cross-section terms associated with non-negligible ply stiffness and damping terms are formulated. In detail, this chapter consists of seven subsections. Firstly, a brief description of the developed damped beam finite element is presented. The second subsection reports the constitutive equations as well as the straindisplacement relations of the composite ply. Accordingly, the third subsection deals with the blade cross-section mechanics and the formulation of the respective linear stiffness, damping and mass terms of the cross-section. In the fourth subsection, building upon the damping mechanics, an extended beam finite element is developed capable of providing the stiffness and damping matrices of the structure, which contain new material coupling terms, essential for describing the structural dynamics response of composite beams and blades.

The current chapter presents the theoretical framework for the study of the nonlinear response of composite strips. The analysis of composites strips is considered to be an intermediate step before moving to tubular sections and beam elements, which provides valuable insight and understanding of the nonlinear composite damping behavior of these simple structural configurations, moreover offers an opportunity for experimental verification. Nevertheless, there is a void in current literature and technology which is also covered by this chapter. The damping mechanics and nonlinear structural dynamics formulations enable the inclusion of nonlinear effects due to in-plane loads and large deformations on both structural stiffness and damping of laminated composite strips.

Composite material systems are known to provide damping which is beneficial for the passive control of vibratory, aeroelastic and acoustic loads, in a variety of structural applications. Many of such structures may be exposed to large deformations and high tensile and compressive loads.

The necessity for maximization of wind-turbine rotor performance has led to the design of longer and more flexible composite blade configurations. In that direction, new wind-turbine blade designs are introduced which exceed 60m in length and exhibit complex nonlinear structural behavior. These long blades are substantially more flexible and exhibit complex nonlinear geometric behavior which affects their static, dynamic and aeroelastic response, therefore, improvement of current blade modeling tools is required.

This is the closing chapter of the book which presents a brief summary of the conclusions obtained along the current work. It also addresses some open issues and interesting topics for future research, regarding the evolution of the developed nonlinear beam finite element to better describe the structural analysis and structural dynamics of large-scale composite wind-turbine blades.