Bachelor's thesis defense session (Mr. Akbar Taheri Afarani)

Title: Design, Fabrication, and Analysis of Novel Mechanical Metamaterials Based on Truss Structures

Presenter: Akbar Taheri Afarani

Supervisors: Assoc. Prof. Reza Jafari Nedoushan

Advisors: 

Reviewers: Prof. Saleh Akbarzadeh, Assoc. Prof. Mahdi Javanbakht

Abstract:

Metamaterials are engineered materials that have unique properties that are not abundant in nature. Today, the design of new structures with unique features such as negative Poisson's ratio, zero and negative thermal expansion coefficients, and negative stiffness has attracted the attention of many researchers and engineers due to their ability to improve mechanical properties. These features allow structures to perform more optimally against various loads or to demonstrate performance that is contrary to the imagination of most materials found in nature, and for this reason, the use of these types of structures in civil and industrial projects is increasing day by day. The development of these types of structures can lead to improved quality of life, reduced maintenance and operation costs, and increased energy efficiency, and help create smart and resilient infrastructures that can be used for future generations.

In the present study, a three-dimensional structure is designed based on simple trusses, the main goal of which is to achieve negative Poisson's ratio and negative thermal expansion coefficient with minimal stress. This type of design requires a detailed and comprehensive study of various geometric parameters and structural materials. For a detailed and comprehensive study of various geometric parameters and structural materials, the finite element modeling method is used, which is one of the powerful tools in analyzing the mechanical behavior of structures. For this purpose, the structure is first designed parametrically in Abaqus using Python scripting. To do this accurately, mathematical equations and software commands are used, and then the set of these items is provided as Python code for execution by the software with maximum flexibility and the ability to make changes to the original design. Therefore, with each execution of the code and change in the parameters, the effects of that change on the coefficient of thermal expansion and Poisson's ratio are observed.

In addition, a device was designed for step-by-step temperature changes to increase the temperature of the structure isothermally. The temperature increase by this device was verified by a standard alcohol thermometer. The measurement method by image analysis was also examined by an aluminum sample. In this way, this sample with a known thermal expansion coefficient was measured by image analysis to check how the thermal expansion coefficient number extracted through this measurement method differs from this number in the case of standard measurements. At this stage, the measurement method by imaging was successful and the error was within 5%. After examining and analyzing the results, the following conclusions were drawn. The results of the analyses showed that this three-dimensional structure exhibits the property of negative Poisson's ratio well and when the distance between the two-unit cell planes is 5 and 10 mm in the y direction, which is the direction perpendicular to the two-unit cell planes, a negative thermal expansion coefficient is achieved for the structure.