Investigation of mechanical behavior of sandwich panels with lattice structure produced by 3D additive manufacturing method

Additive manufacturing is a production method in which 3D model data is used to produce objects by depositing the material layer by layer. Fused deposition modeling is the most widely used method compared to other various additive manufacturing technologies. Fused deposition modeling as a 3D additive manufacturing method is defined as layer-by-layer deposition of thermoplastic material along the designated path to create an object after melting it at a set temperature. The usage of composite materials is considered important to manufacture structures with sufficient strength and reduced total weight in modern structures. Sandwich composites are one of the materials used by industry. Various industries use sandwich composites to benefit from their high flexural strength, vibration damping, energy absorption and heat insulation properties in addition to their light weight. The use of additive manufacturing for production of sandwich structures provides a wide variety of advantages. Additive manufacturing not only provides an option to manufacture periodic lattice structures, which are difficult or impossible to produce by conventional methods, but also allows programming of functional features on a large scale by adjusting the structure and geometry. Furthermore, periodic lattice structures have many additional advantages such as high stiffness, high flexural strength and high-energy absorption at low density. Because of the geometrical freedom given by additive manufacturing, a wide variety of lattice materials can be used as the core in sandwich panels and the desired properties can be achieved by changing the cell geometry. Due to these advantages, the mechanical properties of lattice structures produced by additive manufacturing were investigated in this thesis. In this study, different unit cell geometries were used as the core structure in the sandwich compression and flexural test samples. The unit cell shapes used in the core section of sandwich samples are horizontal honeycomb, vertical honeycomb, horizontal truss, vertical truss, cubic, gyroid and skeleton. The dimensions of every unit cell was selected as 5mm × 5mm. All lattice structures were designed with the help of a computer-aided design program to have relative densities of 20%, 40% and 60%, by only adjusting the wall thickness of cellular structures. The mechanical properties of the compression and flexural test samples with different relative densities and unit cell shapes were compared and best performing unit cell shapes were determined. Compression test samples were produced from polylactic acid (PLA) and carbon fiber reinforced polylactic acid (CFR-PLA) using the Ultimaker 3 Extended which is a three-dimensional additive manufacturing device. However, flexural test samples were produced only from PLA material using same device. Next, carbon fiber composite plates were manufactured by vacuum infusion method and then sandwich composite flexural test specimens were obtained by glueing the carbon fiber composite plates to the lower and upper surfaces of lattice core structures produced by additive manufacturing. Compression and flexural tests were carried out on Shimadzu universal testing machine with the capacity of 250kN in accordance with ASTM C365 and ASTM C393/C393M, respectively. In this thesis, the changes of compression properties were investigated depending on the shape of the unit cell, relative density and material. The highest compressive strength and modulus of elasticity were observed in samples with vertical truss, vertical honeycomb and cubic unit cells, which exhibit stretching-dominant deformation behavior. In addition, increasing the relative density increased the mechanical properties to a certain extent. The changes of the flexural properties of the samples were investigated depending on the unit cell geometry and carbon fiber plate reinforcement. This study found that carbon fiber reinforced composite plates significantly increased flexural strength, modulus of elasticity, toughness and energy absorption.