Design, Fabrication and Testing of Contact-aided Compliant Cellular Mechanisms with Curved Walls

Open Access
Author:
Cirone, Samantha Ann
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Mary I Frecker, Thesis Advisor
Keywords:
  • stress relief
  • contact-aided compliant mechanism
  • cellular structure
  • micro fabrication
  • ceramic
Abstract:
Contact-Aided Compliant Cellular Mechanisms (C3M) are compliant cellular structures with integrated contact mechanisms. The focus of the thesis is on the design, fabrication, and testing of C3M structures with curved walls for high strain applications. Global strain capability is increased in these compliant mechanisms by replacing straight walls with curved walls in the traditional honeycomb cellular structure. The addition of contact mechanisms also increases cell performance via stress relief. Furthermore, curved walls are beneficial for fabrication at the meso-scale. The curved honeycomb unit cell, defined by a set of variables, is analyzed using finite element analysis. Each unit cell is subjected to an input displacement that is incremented up until the maximum local strain is equal to the allowable strain. The effective maximum global strain of the cell is then calculated from the input displacement. For each cell the non-contact global strain (maximum global strain in a cell without a contact mechanism) and the contact-aided global strain (maximum global strain in a cell with a contact mechanism) are calculated. For the contact-aided cells the ideal contact gap must also be calculated. It was found that curved C3M structures are always capable of larger global strains than straight-walled structures. Analysis of various cell geometries, reveals that the best cells have curved walls, are tall and slender, and do not benefit from a contact mechanism. Two optimization problems were formulated using MATLAB and finite element analysis to find the best non-contact and contact-aided cells. The first problem optimizes global strain in contact-aided cells, where the value of the objective function is found by taking the product of the contact-aided global strain and the increase in global strain that can be obtained due to stress relief. The second problem optimizes global strain in the non-contact cells, where this objective function is equal to the non-contact global strain. The optimized solutions for the curved cells can achieve global strains of up to 32.4% in non-contact cells and 19.7% in contact-aided cells. The lost mold rapid infiltration forming (LM-RIF) microfabrication process was used to fabricate C3M structures from both metallic (mesoscale 316L Stainless Steel) and ceramic (3mol% yttria stabilized zirconia) materials. The LM-RIF process is utilized to directly fabricate structures from CAD models in multiple arrays of C3M parts. After the parts are fabricated, they are tested experimentally using a custom test rig. During testing, a micrometer drives the displacement of the C3M mechanism, and the corresponding force is recorded via a built in force gauge. The displacement and force are both recorded through a computer interfaced to the system. This data gathered from the test rig is verified by calculating the displacements using digital images. Three different types of C3M specimen are tested and the force versus displacement data corresponds to predicted data with Moduli of Elasticity values ranging from 120 to 280 GPa. Additionally, each of the specimens was capable of a global strain equal to about half of the predicted global strain. There are several factors which could explain these errors in the experimental results. Slight variations in the fabrication process can lead to geometric variations, such as bending, warping and inconsistent wall thicknesses, as well as variations in the material properties, including elastic modulus and material strength, for each batch of parts.