Biaxial Inflation Stretch Test for Flexible Electronics

Printed flexible ink manufacturers typically report electrical performance under uniaxial strains. Under observed real‐world conditions, flexible ink systems 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. A novel test method is proposed in which a device is clamped around its periphery and subjected to an inflation pressure to induce a biaxial strain‐state. The electrical performance of the ink is monitored via in situ resistance measurement. 3D digital image correlation and finite‐element analysis are used to validate the calculated stress–strain state. Comprehensive analytical and numerical models are developed and compared with experimental results. The following findings are reported: 1) a commercial silver‐based stretchable ink screen‐printed onto a thermoplastic polyurethane substrate can withstand upward of 70% uniaxial strain without electrical failure, yet experiences a loss of conductivity at just 12% biaxial strain, 2) the proposed inflation test setup successfully creates a biaxial strain‐state in the substrate–ink system while measuring the system's electrical performance, and 3) models, based on 3D percolation theory and the mechanics of pressurized membranes, accurately describe the electrical and mechanical behavior of the substrate–ink system.