Damage Initiation and Growth in Composite Laminates of Wind Turbine Blades

The physical properties of composite materials, such as high strength-to-density and stiffness-to-density ratios, are key properties for wind turbine blade structure. These materials are however vulnerable to damage during service. The static failure of composites occurs in two stages: 1) onset of damage and 2) damage evolution that leads to final failure. The response of damaged composites depends upon a mixture of mechanisms that take place at the micro level, i.e., in the fiber and the matrix. Consequently a model is proposed for predicting ultimate strength of composite laminates based on the constituent's properties; the fiber, the matrix, and the interface. For onset of damage, the Stassi Equivalent stress model sigma(eq,m) is used for the matrix. This model take cares of different tensile and compressive strengths of the matrix. For the fiber, the failure criterion for onset of fiber breakage is related to fiber strength. Once failure occurs, gradual degradation of material properties is used, i.e., D-m = 1-exp [gamma(1-sigma T-q,m/(i))]. The analysis is carried out on a three dimensional representative unit cell of the composite. The ultimate strength predictions were in reasonably good agreement with the test data for E-glass/epoxy laminates used in wind turbine blades.