Damage Evolution and Failure Mechanism in Flexible, Printed Conductors under Monotonic and Cyclic Bending
Author(s)
Chen, Rui
Advisor(s)
Sitaraman, Suresh K.
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Abstract
Flexible electronics, due to its conformability, lightweight, increasing functionality, easy tailorability, innovative fabrication methods, new materials, and low cost, have attracted significant interest in academia and industry. They have been widely used in wearable devices, flexible displays, solar cells, and other areas. However, printed flexible electronics, which is one crucial type of flexible electronics, is still in its early stages of development and growth. The performance of printed flexible electronics under various deformation conditions is not fully understood. Such deformations include monotonic and cyclic stretching, bending, twisting, and combined deformations under practical operating conditions. Compared to traditional “rigid” electronics, flexible electronics undergo much larger deformations. Therefore, it is essential to identify the electromechanical behavior of flexible electronic products under these mechanical deformations.
This thesis aims to study the mechanical and electrical behavior of printed silver conductors under a wide range of bending modalities, using conductors printed on polyimide (PI), polyethylene terephthalate (PET), and thermoplastic polyurethane (TPU). These samples have been subjected to the mandrel bend test, the varying-gap adaptive flexure test, and the reciprocating adaptive flexure test. In all the tests, the resistance of the printed conductor was monitored continuously by a 4-wire method. The relationship between the resistance and the bending strain has been obtained from the mandrel bend test and further been used to understand the resistance change of the printed conductor in the varying-gap and reciprocating adaptive curvature flexure tests, where the strain distribution along conductor length is non-uniform. By performing these three bend tests under cyclic conditions, the resistance change with loading cycles has been investigated and related to the microstructure changes through SEM imaging. Besides, analytical formulations of the various bending configurations and finite-element simulations have been performed to obtain a comprehensive understanding of the stress and strain distribution in the conductor and to determine damage accumulation in the printed conductors. With adaptive curvature and mandrel bend tests, predictive models for damage evolution with strain amplitude have also been explored.
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Date
2021-05-01
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Text
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Dissertation