Numerical Investigation of Delamination in Highly Tapered Laminates

  • Ergin, Fırat (Middle East Technical University)
  • Kayran, Altan (Middle East Technical University)

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A large number of successive ply terminations in a laminated composite part result in highly tapered structures, and ply termination locations, also called as ply drop-offs, decrease the load-carrying capacity of the laminate because of the early initiation of delamination originating from high interlaminar stress regions. These types of highly tapered laminates are observed especially in helicopter flexbeams due to high strength requirement on one side and low stiffness requirement on the other. Therefore, it is important to postpone the delamination initiation in highly tapered laminates, and this can be accomplished by improving the interlaminar properties of the material or reducing the stress singularities. Suppression of delamination by improving the interface properties requires new materials or manufacturing techniques; however, stress singularities can be reduced by changing the tapered geometry with tools and materials on hand. Therefore, improving the load-carrying capacity of highly tapered laminates by using variable stagger distances, the distance between consecutive ply-drop offs, is investigated in this study. Gradually changing the stagger distances smoothens the transition geometry and increases the load-carrying capacity of the laminate owing to the reduction in the stress concentrations. However, due to modeling difficulties and complexity in the design, tapered laminates with varying stagger distances are not investigated in detail in the literature. Therefore, to investigate the possible benefits and limitations of varying stagger distance design, parametric finite element models are generated in ABAQUS with a python script. Zero-thickness cohesive elements are utilized to predict the delamination initiation, and intraply failures are not considered since delamination is the main failure mode in tapered laminates with high taper angles. The preliminary parametric study in which the taper angle is gradually increased showed that the delamination initiation load in the thin section of the tapered structure heavily depends on the taper geometry and decreases as the taper angle increases. Therefore, detailed finite element models comparing the effect of resin pocket shape in the highly tapered laminates are investigated. Results showed that as the taper angle increases, the difference in the delamination initiation loads between the approximate triangular resin pocket model and the realistic resin pocket geometry model becomes significant