Session: 12-20-01: Functional Origami and Kirigami-inspired Structures and Metamaterials

Paper Number: 111491

111491 - Improving the Load Carrying Capacity of Highly Tapered Laminates 

In laminated composite structures used aerospace applications, ply termination technique is widely adopted for cost-reduction and weight-saving purposes. Successive ply terminations in a composite part result in 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. If a large number of plies are terminated in a short distance, highly tapered laminates are formed, further increasing the stress concentrations and reducing the performance of the structure. These types of 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, various 2D 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, two different tapered design alternatives with varying stagger distances are investigated to postpone the delamination initiation in the thin section. In the first design, a continuous ply is placed between the dropped plies while all dropped plies are stacked up and no continuous ply is placed among them in the second one. Results have shown that the delamination initiation load in the thin section with the utilization of varying stagger distances can be postponed when there are continuous plies between dropped plies. However, improvement in the load-carrying capacity with the utilization of varying stagger distances is not observed when the dropped plies are stacked up together. It is concluded that varying stagger distance design can improve the load-carrying capacity of the highly tapered structure for some configurations and understanding their potential and limitations requires more detailed investigation. Therefore, varying stagger distance design with continuous plies between the dropped plies will be further investigated to improve the load-carrying capacity of the highly tapered laminates in this study.

Presenting Author: FIRAT ERGIN Middle East Technical University

Presenting Author Biography: Fırat Ergin is an Aerospace Engineer who graduated from Middle East Technical University in 2019 with a ranking third. After receiving his undergraduate degree, he started working as a Structural Design Engineer in Turkish Aerospace Industry and also started his master of science degree in mid-2020. After leaving his position in the Turkish Aerospace Industry, he started working as a research assistant in the aerospace engineering department of Middle East Technical University, and he still works in that position.

Authors:

FIRAT ERGIN Middle East Technical University
Altan Kayran Middle East Technical University