Studies On Delamination Characteristics And Fatigue Life Of Helicopter Composite Blades

The prediction of fatigue life for the rotor blades is a key issue in the design of helicopters because they are the most important dynamic components for helicopters. Because of the complex alternating stress, the defects of the composite blade could be resulted from internal inclusions and air bubbles, and this would cause the layer crack initiation and propagation, eventually, catastrophic damage of helicopters can happen. However, it is hard to explain the delamination damage of composite blades from the material fatigue point of view using the traditional S- N curve. Thus, the objective of this study is to effectively predict the delamination fatigue life of composite blades. The delamination crack tip singular stress fields, three-dimensional fracture criterion, fracture threshold value and the damage accumulation criterion were extensively studied by using the anisotropic interfacial fracture mechanics theory and the interface element method. Several mathematical models for the blades and the composite fatigue life were established and used for the effective numerical simulation purpose. This study provides a theory basis of predicating of life for the rotor blades used in helicopters.A generalized two-dimensional displacement equation to describe the helicopter blade was established by studying stress distribution in the helicopter blade and adopting a nonlinear blade structural model. The displacement equation consists of two parts: the first part is to decipher the homogeneous solution for the generalized plane strain deformation and the second part is to describe the non-homogeneous solution for the deformation generated by the axial strain, flap bending, lag bending and torsion. On the basis of the Stroh theory and Lekhnitskii anisotropic elastic theory, the crack tip singular fields of the skin and the cohesive layer between skin and spar were by simplifying the main fatigue section of composite blade (blade root) as a solid beam. A method was proposed to obtain the homogeneous solution and the non-homogeneous solution of the asymptotic crack tip fields, and their corresponding coefficients values.The first term of the homogeneous solution was assumed to be the crack tip singularity strength. Based on Rice's small-scale contact theory, the stress intensity factors with the same dimension as that of the the traditional fracture mechanics was achieved by introducting a reference length (r|∧) .The boundary collocation method was used to calculate the static and dynamic delamination crack tip singular stress fields and stress intensity factors.Because of the oscillation characteristic of composite delamination crack tip, the normal stress and shear stress must occur at the delamination crack tip even if the composite bear a unity loading. In other words, there was no single form of plane damage in the composite blade. A method to compute three-dimensional fracture toughness and fracture threshold values based on the phase anglesψ₁ andψ₂, was provided accordingly. The validity of the method was verified by comparing the numerical results of this study with the experimental results from literature.In this thesis, besides above novel numerical method proposed for the calculation of three- dimensional delamination fracture toughness and fracture threshold value, another new method used for the calculation of the interface element stresses, which includes interface strength and displacement jump occurred in the three types of fracture modes (modeâ… , modeâ…¡, modelâ…¢) was created. The delamination growth resistance was incorporated into the new model, which greatly improved the traditional fatigue crack growth rate model for layered composite materials. Furthermore, an equation for the calculation of the cohesive zone length in the composite materials was derived on the basis of the interfacial fracture mechanics. Application of this equation together with the new developed delamination growth rate model to estimate the delamination fatigue life of composite materials significantly improved its forecast accuracy.The Miner cumulative damage criterion was applied to predict the delamination fatigue life of composite blade. Meanwhile, the relationships between the damage-life and the strain energy release rate-life were established for the constant-amplitude fatigue loadings system by setting the Miner criterion as a prerequisite. Finally, on this basis, a method to predict delamination fatigue life of the composite materials under more complicated circumstances, e.g. variable-amplitude fatigue loadings was also proposed. The delamiantion fatigue life of composite blade was simulated by using the proposed method. The simulation results are consistent well with experimental data, which confirms the validity of the numerical models developed in this thesis.