Effect of ply drops on the failure mechanism of double-double tapered laminates under tensile loading: Experimental and numerical studies

Understanding the influence of ply drops (PD) on the failure mechanisms of tapered laminates is essential for accurately predicting structural performance. However, as a novel layup design strategy, research on the mechanical and failure mechanisms of DD tapered laminates is currently limited. In this paper, the failure mechanisms of three types of ply drop configurations in DD tapered laminates under tensile loading are investigated, and compared with test results from QUAD tapered laminates with equivalent in-plane stiffness to better illustrate the influence of PD. Digital Image Correlation (DIC) is employed for in-situ observation of the macroscopic failure process on the specimen surface and for capturing crack initiation through strain distribution. In addition, the progressive damage evolution of the structure is analyzed through interrupted tensile test, and 3D X-ray tomography is used to detect internal damage in the tapered region of the specimen after interrupted loading. In parallel, numerical simulations are performed to further reveal the failure mode and mechanism of tapered laminates. The analysis results indicate that stress concentration at the PD location is the main factor leading to occurrence of different failure behavior, and the design of layups and PD location will affect the failure mode and delamination propagation paths. In addition, compared to traditional external PD design, placing PD discretely inside the structure and improving the homogeneity of the tapered region can significantly enhance the mechanical of the structure.