FAILURE PREDICTION AND TEST VALIDATION OF TAPERED COMPOSITE UNDER STATIC AND FATIGUE LOADING

摘要

Ply drop off and transition techniques are commonly used in composite joints and closures in the aerospace industry, as well as in wind turbine blades to reduce skin thicknesses along the blade span. Ply drops increase localized stresses, which may cause premature failure in composite or reduced fatigue service life. Durability and damage tolerance (D&DT) is evaluated utilizing a Multi-scale Micro-Macro Progressive failure analysis (PFA) technique that augments commercial FE stress solvers. PFA determines the critical damage events: 1) damage initiation/propagation, 2) fracture initiation/propagation, and 3) the final residual strength. The prediction is validated under static loading: a) Composite Director's Room (DR) Joint was analyzed and verified against joint test observation. The joint was composed of a steel facing bonded and bolted to a tapered composite/balsa core sandwich panel. In order to obtain the limits on the behavior of the adhesive within the joint two analyses were performed; one assuming a perfect bond between the components and the other assuming there was no cohesion across the interface with material overlap prevented by gap elements. PFA micro-mechanical analyses, assumed adhesive layer thicknesses were added to the models as a more accurate representation of the real structure. Prediction is validated under fatigue loading. A complex test coupon with ply drops, intended to be representative of blade structure, was developed. The complex coupon laminates contained various fabrics, resins, and material transition choices under tension, compression and reversed uniaxial loading conditions. Static and fatigue test results are presented for polyester, vinyl ester and epoxy resins and a toughened vinylester, for both plain and complex coupon geometries.