Sepide Aghajani

Grade:  Master

Thesis Title:

Life Prediction Of wind Turbine Blades Using Multi-scale Damage Model

Year: Sept. 2014- Mar. 2018.

Abstract:

Since the evolution of fatigue damage in composite materials currently used for large structure, including wind turbine blade, was not well understood, large safety factors were employed which lead to highly conservative designs. In order to improve and enhance the design confidence, these structures were tested in different conditions, which would lead to a lot of time and cost. To advancing, researchers used analytically and simulation methods to design structures. A lot of analytical methods have been proposed for designing and estimating the life of composite structure, one of which was estimation of stiffness reduction in material because of damage. This study investigates one simulation method based on multi-scale damage mechanic, Computational micromechanics was coupled within a continuum damage mechanics (CDM) framework, and implemented through a user-defined subroutine within commercial finite element software, for evaluating sub-critical damage evolution and stiffness degradation of the structure. Finally, VUMAT user subroutine of ABAQUS is used to determine the constitution equation in continuum damage mechanics and then is applied to wind turbine blade to investigate fatigue damage mechanism.using modified Puck’s criteria, damage is studied in a layer level of a fatigue loaded wind turbine blade. Damage is investigated in fiber direction and matrix. The model is implemented in a user material finite element subroutine to calculate damage parameter in layer level at any cycle of loading. Then, calculations are extended to laminate using classic layer theory. New damage parameter and elastic modulus is computed at any element and finally wind turbine blade life is computed with a stop criterion.damage parameter of layers and layers constant are obtained to be applied on [0/90/45/-45] and [0/90] laminates. Then applying specified load to blade and using user subroutine code of synergistic damage mechanic, saturated crack density of layers is calculated and compared to similar results of related journals. Crack density of 90˚, 45˚, and -45˚ plies are 1.1, 0.75 and 0.36 respectively. According to results of analysis, reduced stiffness and damage parameters are calculated. Finally, applying fatigue damage multi-axial in the multi-directional composite based on modified Puck’s criteria, life is estimated 950000 cycles causing fracture in -45˚ ply at mode C. In fracture cycle, damage parameter is calculated in 90˚, 45˚, -45˚ plies which results in 0.99,0.82 and 0.59 respectively.

Keywords: synergistic damage mechanic, Computational micro-mechanic, Continuum damage mechanics, fatigue damage, Matrix cracking.