Improving the efficiency of aerospace structures involves reducing their weight and maintaining their safety throughout their life cycle. To avoid oversizing, a deep knowledge of the damage mechanism in composite materials is crucial, and interlaminar fracture, or delamination, is one of the most critical mechanisms. Understanding delamination behavior and characterizing the Interlaminar Fracture Toughness (IFT) of composites is key to designing safe and efficient structures. Standard tests to quantify IFT are restricted to unidirectional (UD) specimens with delamination propagation parallel to the fiber. However, structures are built using Multi-Directional (MD) laminates, where delamination may appear at any interface and propagate in any direction, and IFT may differ from that obtained by standard tests, leading to oversized, inefficient structures. The main problems that prevent IFT test for MD laminates are: elastic couplings, thermal residual stress, finite width effects, and delamination migration. Currently, there is no understanding of whether migration can be reliably avoided in mode I tests, while it can be prevented in mode II and mixed mode I/II tests. Since migration depends on the state of shear stress at the delamination crack tip, our objective is to find a way to design MD specimens for the mode I test that could avoid migration.
Abstract Improving the efficiency of aerospace structures involves reducing their weight and maintaining their safety throughout their life cycle. To avoid oversizing, a deep knowledge [...]
Long fibre composite laminates are excellent materials for structures. Their mechanical behaviour can be tailored through constituents selection and stacking sequence design, but this flexibility is rarely exploited. Simple approaches (e.g., quasi-isotropic laminates as a one-fits-all solution) and design rules (e.g., stack symmetry to achieve uncoupling) are often used to reduce design complexity, eating up on design space and potential performance. Researchers actively look for strategies to better tap into this flexibility while keeping complexity reasonable. As an example, the Double-Double laminate design approach [1] is one strategy currently debated in the community.
Another promising strategy is the use of Quasi-Trivial (QT) solutions [2,3]. These are sequences that ensure uncoupling, bending-extension homogeneity, or both together. QT solutions yielding uncoupling are a much larger set than symmetric sequences (a subset of the former). Also, bending-extension homogeneity is not otherwise obtainable with simple strategies. Despite their potential, adoption of QT solutions remains limited due to their little understanding and by their limited availability. In this work we present recent developments regarding QT solutions, namely: I) a novel analytical framework to describe and manipulate them, II) newly found properties, III) novel algorithmic strategies to obtain them. Hopefully, this work will increase the knowledge and understanding of QT solutions, providing tools for their exploitation.
Abstract Long fibre composite laminates are excellent materials for structures. Their mechanical behaviour can be tailored through constituents selection and stacking sequence design, [...]
Investigating how to avoid delamination migration in mode I delamination tests with multidirectional specimens 
