Additive Manufacturing of Strain Gauges : A Study of the Feasibility of Printing Strain Gauges Using Inkjet Printing

Sammanfattning: Additive manufacturing (AM) also commonly known as 3D-printing is a manufacturing method which creates parts from adding layer into another. In the field of printed electronics Inkjet printing (IJP) and Aerosol Jet printing (AJP) are the most common AM techniques. IJP and AJP are non-contact-based printing techniques where ink is deposited on a surface with droplets. AJP aerosolizes the ink into a mist which is deposited on a surface according to the predetermined pattern. IJP instead produces singular droplets when printing. These printing methods have been used for manufacturing various printed electronics such as strain gauges which has been the focus of this project.  The purpose of this thesis was to investigate the feasibility of printing strain gauges. Through a literature study the overall function and use for strain gauges and various printing methods were investigated, as well as previous studies related to printed strain gauges using AJP and IJP. To further investigate one of these techniques, strain gauges were printed using Inkjet printing. The sensors were printed using two different inks, one containing silver particles and the other containing constantan particles. The strain gauges were also printed on various substrates such as Polyimide (PI) and Polyetheretherketone (PEEK), to determine the best material combination. The silver strain gauges were then sintered in an oven while the constantan sensors were sintered using photonic sintering. To evaluate each ink-substrate combination several tests was performed throughout the printing and sintering process. A tape test was used to determine adhesion, SEM analysis was performed to study the effect of the sintering process and the resistance was measured to calculate the conductivity and study the printability on different substrates. To characterise the printed strain gauges a bending test was performed where the change in resistance was measured with changing strain. The output was also studied over time to determine the stability of the printed sensors.  The silver ink showed overall better properties compared to the constantan ink, which could be due to that the silver ink has been more developed than the constantan ink. The resistivity of the silver ink was calculated to 7.0E-07 Ωm and the constantan ink to 2.23E-05 Ωm. The average gauge factor for the silver ink printed on PI was calculated to GFavg~1.6 at low strain and GFavg~2.1 at high strain, the silver samples printed on PEEK was GFavg~2.4 at low strain and GFavg~2.3 at higher strain, and the constantan samples was determined to GFavg~2.7 during loading at low strain and GFavg~17 at high strain due to deformation. Some of the samples printed with silver ink showed quite linear behaviour while the samples printed with constantan deformed when applying high stress. The silver samples printed on PEEK showed more hysteresis compared to the silver samples printed on PI, but the PEEK samples showed a better stability over time compared to PI.  The thesis shows that it is possible to manufacture strain gauges, but the result depends a lot on the ink and substrate material chosen. Silver inks has been developed over a long period and thus making it easier to handle and the result is better compared to newer inks such as constantan.