Biaxial Inflation Stretch Test for Printed Electronics
Flexible, stretchable and wearable electronics have many applications that require them to maintain electrical integrity while being repeatedly stretched, bent, and twisted. To study the reliability of these devices, various mechanical test methods have been adapted to include in-situ electrical measurement, including uniaxial tension, twist, and bend, along with several others. One test method which has yet to be fully adapted for flexible and stretchable electronic materials is biaxial stretching. A biaxial stress-state can be achieved by directly inflating the substrate. The research presented here focuses on the development of a biaxial inflation test for stretchable substrates and printed conductors under monotonic and cyclic loading conditions. During inflation, electrical performance is monitored via in-situ four-wire resistance measurement. Inflation pressure and resultant surface strain are monitored concurrently using a pressure transducer and three-dimensional digital image correlation, respectively. Two highly stretchable substrate and screen-printed conductor material combinations are examined using the proposed inflation test: silver-flake polymer ink on thermoplastic polyurethane and polymer carbon nanotube composite on platinum-catalyzed silicone. Experimental results are compared with analytical and finite-element models. Recommendations are then made for material selection, sample design, test procedures, modeling approaches, and analyses of results.