Durability of Tapered Composite Laminates under Static and Fatigue Loading

Abstract

A micromechanics based computational approach is put to use to assess the durability of composite laminates with ply drop features commonly used in wind turbine applications. Ply drops are used in composite joints and closures of wind turbine blades to reduce skin thicknesses along the blade span. They increase localized stress concentration, which may cause premature delamination failure in composite and reduced fatigue service life. The use of computational simulation in the design of tapered composites structures will reduce risk of failure under service. Durability and damage tolerance (D&DT) is evaluated utilizing a Multi-scale MicroMacro Progressive failure analysis (PFA) technique. The PFA approach is finite element based and is capable of detecting all stages of material damage including initiation and propagation of delamination. Two different approaches have been used within the PFA to investigate these issues. The first approach is Virtual Crack Closure Technique (VCCT) PFA while the second one is strength based approach. The PFA assesses multiple failure criteria and includes the effects of manufacturing anomalies (i.e., void, fiber waviness). In the work presented here, constituent stiffness and strength properties for glass and carbon based material systems are reverse engineered for use in D&DT evaluation of coupons with ply drops under static loading. Lamina and laminate properties calculated using manufacturing and composite architecture details matched closely published test data. Similarly, resin properties are also determined for fatigue life calculation. The simulation not only reproduced static strength and fatigue life as observed in the test, it also showed composite damage and fracture modes that resemble those reported in the test. The results presented in the paper show that computational simulation can be relied on to enhance the design of tapered composite structures such as the ones used in turbine wind blades.