Mechanical response of the carbon fiber reinforced polymer composite sandwich structures with pyramidal lattice core

Abstract

Composite materials are widely used for many years in various industries. Depending upon technological developments, fiber reinforced composite materials such as carbon, glass and aramid fiber, have been introduced. Also, they can be classified based on the matrix type or the reinforcement type. Matrix-based composite materials can be divided by ceramic, organic, metal matrix; while, reinforcement based classification covers fiber-reinforced, particulate and structural composite materials. It is also explicitly known that sandwich structures are established by a core, upper and lower skin (a.k.a facing or face sheet). Core of a sandwich structures conventionally is chosen as honeycomb or foam. The sandwich structures which includes foam or honeycomb, are remarkable common as structural composite. Surely, this yields having plenty of research in the literature. In the context of thesis herein, a special group of structural composites rather than foam or honeycomb, have been investigated. It is the carbon fiber reinforced polymer composite sandwich structures with pyramidal lattice (CPL) core. Along with demanding to high stiffness or strength to weight ratio, it is needed to arise to replace conventional cores by their counterparts. Promising ones might be given as Kagom'e core, X-Type core, Y-Type core, V Type core, tetrahedral, diamond textile, diamond collinear, square collinear, pyramidal. As to CPL core, it has been examined by a number of researcher from different perspectives such as compressive behaviour under quasi static loading, shear behaviour, bending behaviour, enhancing its analytical models, improving its manufacturing method, hierarchical CPL core, its node designing, searching appropriate failure criteria or etc. It is also indicated that works related to the CPL core in literature are not extensive. As the other core types, the CPL core could be characterized by its relative density. This parameter is basically defined as the ratio of volume occupied by the material within a cell to volume of the cell. Also, it determines failure mode of the CPL core. For instance, the CPL core with lower relative density, tends to fail by means of Euler buckling in the case of compression loading. However, it is possible to have delamination failure if the CPL core has higher relative density. Within the scope of the current thesis a relative density formulation has been derived. This is exact definition despite of that approximate formulations have been introduced. Moreover, it is note that the CPL cores which have been studied in present thesis, have square cross section. This fact has been holds by the exact definition of relative density. Also, it can be indicated that relative density is a parameter which can be utilized by comparing any type of core between each other. Subsequently, two different mechanical behaviour of sandwich structures with the CPL core, have been studied in this work such as out of plane compression and flexural based shear. For each behavior, the specimens have been designed so as to have two different relative densities like 2.863% and 0.725%. While the former relative density stands for Design 1, the latter relative density represents Design 2.