Investigation of the Mechanical Behavior of Non-Conventional Laminates: Anisotropy of Tensile Strength in Coupons with Quasi-Isotropic Stiffness Characteristics

Advanced composites have emerged as viable structural solutions over the years and have therefore become implemented in several applications, notably aerospace structures. In modern aircraft structures, they have been gradually introduced to both secondary and primary components. Laminated composites used in such applications have generally been restricted to those with straight fibers, that are aligned to only a handful of pre-selected fiber orientations; 0˚, 45˚, 90˚ and -45˚. Recent advances in the technology of laminated composites have however indicated the possibility to harness significantly higher structural gains by allowing the use of other fiber orientations, and even implementing curvilinear fibers in composite structures. These gains have been demonstrated by improvements in performance metrics such as stiffness and static strength. However, the introduction of non-conventional fiber orientations significantly increases the complexity of manufacturing of such laminates, their analysis, and the uncertainties involved in understanding their mechanical behavior. While the former has been alleviated by the development of highly precise fabrication systems such as Automated Fiber Placement (AFP), research into the latter still needs more illumination to comply with the stringent verification requirements of the aerospace industry. Related research has demonstrated the significant contribution of interlaminar stresses in determining the tensile strength of quasi-isotropic laminates, whose fiber orientations are not restricted to the conventional (0˚, 45˚, 90˚ or -45˚) types. This therefore provides a basis for a research into the mechanical response of quasi-isotropic laminates with non- conventional angles, with a view to gain fundamental insights that would be extended into the real-world implementation of laminates with various fiber orientations. This thesis presents investigations into the response of a number of selected quasi- isotropic laminates subjected to in-plane uniaxial tensile loading. Coupons made of the selected configurations are tested using standardized methodologies for experimentally characterizing the static response of laminated composites. Each coupon is characterized by studying its stiffness and ultimate tensile strength. In addition, simple theoretical models are implemented to gain preliminary insight into the response of these laminates, relative to one another. In order to further elucidate their behavior, interfacial stress distributions from three-dimensional Finite Element Analyses (FEA) are studied for each selected laminate, and this is followed by a rudimentary look into the possible predominance of initial delamination on laminate strength. Results are presented for analyses and testing, and finally, their discrepancies are discussed