Investigation of failure mechanisms in single-material and interply hybrid composite laminates

Date

2025-7-09

Author

Karataş Demircan, Cansu

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Interply hybrid composite laminates consist of plies reinforced with different fiber types to achieve enhanced properties compared to single-material laminates. Such laminates are used in high-performance applications, including helicopter tail rotor blade flexbeams and wind turbine blades, where high interlaminar stresses under operational loads make them susceptible to delamination. This study investigates the interlaminar properties and failure behaviors of hybrid laminates in comparison with carbon and glass laminates. An experimental program was conducted, including double cantilever beam (DCB) and end-notched flexure (ENF) experiments for Mode-I and Mode-II fracture toughness, short-beam shear (SBS) and curved beam strength (CBS) experiments for interlaminar shear strength (ILSS) and interlaminar tensile strength (ILTS). Shear-loaded curved beam experiments were also performed to further elucidate the failure mechanisms in the CBS experiments. DCB and ENF results indicated that the Mode-I and Mode-II fracture toughness of the hybrid laminates fell between those of carbon and glass laminates. Post-mortem fracture surface examinations revealed cracks propagating predominantly along ply interfaces, with occasional deflections into adjacent plies. SBS and CBS experiments revealed that hybrid laminates exhibited ILSS comparable to carbon laminates, whereas ILTS was comparable to glass laminates. The sequence and mechanisms of failure were examined, revealing differences from the literature. Our observations challenge the conventional definition of delamination as interface separation between layers in unidirectional (UD) laminates. Particularly, in curved laminates under four-point bending, substantial intralaminar failure accompanied interlaminar failure at various layers. Consequently, ILTS calculated based on maximum radial stress location may not accurately represent ply interface strength. Furthermore, finite element analysis of curved beam laminates with only cohesive zone modeling is not sufficient to capture the experimentally observed failure process. A key contribution of this work is demonstrating that the failure mechanisms in advanced aerospace-grade UD composites studied in this program do not conform to the traditional concept of delamination. The absence of a consistent crack-propagation preference along ply interfaces underscores the need to reassess the use of interface strength values obtained from standard tests in UD laminates.

Subject Keywords

Interply hybrid composite laminates, Interlaminar material properties, Fracture toughness, Interlaminar strength, Failure mechanisms

URI

https://hdl.handle.net/11511/115526

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Graduate School of Natural and Applied Sciences, Thesis