SUMMARY

Over the last decade, there has been a thrust by the flexible electronics community to develop materials and devices with appropriate mechanical properties to maintain electrical performance while subjected to high levels of strain. Before these soft and stretchable devices can be used in everyday life, their safety, performance, and durability must be demonstrated. Research in academia and industry has typically focused on reliability of flexible electronics under uniaxial strains. However, under observed real-world conditions, flexible electronic systems often experience strain in multiple dimensions. This indicates a potential for misrepresentation of the failure modes of these materials in application if strictly characterized using uniaxial methods.

In this research, an innovative test method is proposed in which a polymer substrate with printed elements is clamped around its periphery and subjected to an inflation pressure to induce a biaxial strain-state. The electrical performance of the printed element is monitored via in-situ resistance measurement. The stress-strain state of the inflated system is calculated both analytically and numerically, and is validated using digital image correlation and pressure-volume measurements. The printed element designs consist of several screen-printed stretchable composite conductors. The electrical performance of these conductors under strain is compared to predictions by classical percolation theory, forming a basis for improved understanding of strain-induced resistance increase. To facilitate numerical modeling and characterize the material constitutive behavior, the viscoelastic and hyperelastic parameters of the flexible substrate and stretchable ink are characterized using the developed biaxial test system. Additionally, a method is proposed for examination of the conductor microstructure under uniaxial and biaxial strains to explore failure mechanisms and damage evolution. Monotonic, cyclic and relaxation loading regimes are explored. Parameters such as biaxial strain ratio, sample dimensions, strain rate, and temperature are examined to determine their effect on the electrical and mechanical performance of the printed element. Recommendations are also made for standardizing the developed techniques, allowing the flexible electronics community to reliably measure and compare the performance of emerging materials and systems under biaxial strains.