Electrical Reliability of Conductor Printed on Stretchable Substrate Under Biaxial Fatigue Loading

Existing literature has mostly focused on assessing the reliability of flexible and stretchable materials under monotonic uniaxial stretching. Although most wearable patches and smart textiles undergo biaxial stretching, there is limited literature that examines the performance and reliability of these materials under biaxial stretching. The existing literature on biaxial stretching has also confined itself to monotonic stretching and does not discuss fatigue biaxial stretching. Thus, this work aims to illustrate the electrical performance and the reliability of flexible materials under fatigue biaxial stretching. This paper outlines an approach to perform fatigue biaxial stretching experiments to test the performance and reliability of stretchable materials before deployment. In this work, the fatigue biaxial stretching parameters such as the mean strain and the strain amplitude have been selected as a function of the monotonic failure strain. By conducting tests on stretchable thermoset substrate samples that were screen-printed with a silver micro-flake conductor ink, this work has developed a unique predictive model that relates the fatigue life of the printed conductor as a function of the applied mean strain and applied the strain amplitude. In addition, this work has also examined the average rate of resistance increase and how it is related to the applied mean strain and the applied strain amplitude. In this work, it is found that the failure of the samples under biaxial fatigue loading is dependent on both the mean strain and strain amplitude. However, it is also found in this work that the number of cycles to fatigue failure has a much higher sensitivity to the strain amplitude than to the mean strain. It is further seen that when the strain amplitude is kept constant, the rate of increase in the conductor resistance per cycle is higher when the mean strain is increased.