Mixed Mode Fracture Behavior of a Multi-Directional Plain Weave Composite - an Interface Delamination Between a 0◦/90◦ and a +45◦/-45◦ Weave

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PhD "ZOOM" SEMINAR
Wednesday, June 17, 2020 at 14:00
Mixed Mode Fracture Behavior of a Multi-Directional Plain Weave Composite - an Interface Delamination Between a 0◦/90◦ and a +45◦/-45◦ Weave
Orly Dolev
Ph.D. candidate under the supervision of Professor Leslie Banks-Sills

Currently, the advantages of composite materials, such as high strength and toughness to weight ratios, corrosion and fatigue resistance, make these materials very desirable to work with, especially in the aerostructure industry. However, composite structures are sensitive to the presence of damage such as delamination, which is one of the most typical failure modes in laminate composites. The main problem is that most of composite structural damage is difficult to detect or follow. The lack of accurate and reliable fracture toughness, fatigue and damage tolerance properties, which enable the evaluation of damage growth within a structure, results in an over-designed structure due to the high safety margin regulations. In order to better understand the mixed mode I/II fracture (initiation and propagation) behavior of a carbon/epoxy multi-directional (MD) woven composite containing a delamination between two plain woven plies, with tows in the 0◦/90◦ and +45◦/-45◦ directions, a comprehensive investigation has been performed, involving analytical, numerical and experimental work.
Mixed-mode fracture toughness tests were carried out on an MD laminate making use of the Brazilian disk (BD) specimen, containing a delamination, at various loading angles in order to obtain a wide range of mode mixities. Employing the experimentally and numerically obtained results at fracture, a two and three-dimensional failure criterion were generated. A statistical analysis with a 10% probability of unexpected failure and a 95% confidence was performed, in order to account for scatter in the results. These failure criteria may be used for safer design purposes for the investigated interface.
Fracture toughness tests for delamination initiation and propagation under quasi-static loading were carried out making use of three beam-type specimens: double cantilever beam (DCB), calibrated end-loaded split (C-ELS) and mixed mode end-loaded split (MMELS), with the following modes of deformation: nearly mode I, nearly mode II and one in-plane mixed mode ratio, respectively. Based upon the experimentally and numerically obtained results, a fracture toughness resistance G_iR-curve was generated, for each kind of beam-type specimen. In addition, the critical values of the interface energy release rate for initiation G_ic and steady-state propagation G_iss were determined.
Quantification of the critical energy release rate G_ic values obtained for delamination initiation in all tested specimens, as a function of the in-plane mode mixity, was presented and discussed.

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