Session: 12-26-01: Mechanics and Materials of Soft Electronics

Paper Number: 150775

150775 - Failure of Intrinsically Stretchable Conductors: Computational Modeling and Experimental Characterization 

Novel stretchable conductor materials (e.g. silver and carbon filled pastes) allow for a greater variety of applications due to superior integration with biological materials and textiles compared to traditional (e.g. metallic) conductors. Understanding their coupled electro-mechanical cyclic and failure behavior will be critical for future applications. Multiple fabrication methods exist for such materials, but screen printing is a low cost and adaptable method, in which conductive inks and protective encapsulants can be deposited on stretchable substrates, such as thermoplastic polyurethanes (TPU). In this work, the deformation response of bare substrates and encapsulated conductive traces fabricated using the screen printing process were studied. Different material combinations were tested, as TPU films from different manufacturers showed qualitatively different stress responses. For some combinations of TPU film, ink, and encapsulant, the ink and encapsulant had minimal effect on stiffness and mechanical behavior of the substrate, while for other material sets, they increased the stiffness of both straight and serpentine traces compared to bare substrates. Constitutive material models for TPU substrates were calibrated to match experimental results from quasi-static testing of bare substrate and of double encapsulated conductive traces.  A robust model allows for simulation of ink and substrate compatibility and stretchability along with common modes of failure. Additionally, changes in electrical response of the traces were measured under cyclic loading. The results indicate sensitivity of long-term performance of such traces are distinct from those using standard, non-stretchable, conductors and encapsulants. This is explained by the fact that the deformation in stretchable conductors is dominated by the substrate, while in traditional conductors their higher stiffness means that the influence of substrate is minimal. Therefore, the local deformation maps in a traditional conductor trace is not unlike those in a bare conductor wire structure. Consequently, the history of changes in electrical resistance of traces through cyclic deformations demonstrate observable differences with those for traditional conductor traces.  This hypothesis was validate using extensive computational analysis that demonstrated that the evolution of different constitutive damage parameters are quite sensitive to the stiffness contrast between ink and substrate, as well as the geometry of the conductor. For stretchable pastes with low stiffness, the volumetric strain would be a suitable damage parameter and it was shown that it grows more substantially in serpentines, making straight traces more robust. In contrast if the material is stiff and plastic, the proper damage parameter may depend more on J2 stress invariant. It was shown that for copper and other high stiffness ink representatives, the serpentine traces will bend and limit the increase of J2. Based on the experimental observations and computational arguments, the designs of traces that are meant for intrinsically stretchable conductors may need to be greatly modified and simplified and care must be taken to avoid transferring expectations meant for traditional conductors without computational and experimental study of the new material choices.

Presenting Author: Alireza Amirkhizi University of Massachusetts, Lowell

Presenting Author Biography: Alireza Amirkhizi is an associate professor of mechanical engineering at University of Massachusetts, Lowell. He received his PhD in Applied Mechanics from UC San Diego in 2006. His research interests include modeling and characterization of mechanics of materials, structures, composites, and metamaterials in dynamic and unsteady conditions.

Authors:

Kailey Miller University of Massachusetts, Lowell
Vahidreza Alizadeh University of Massachusetts, Lowell
Alexander Krueger University of Massachusetts, Lowell
Joey Mead University of Massachusetts, Lowell
Alireza Amirkhizi University of Massachusetts, Lowell