PSI - Issue 80

Marilyne Philibert et al. / Procedia Structural Integrity 80 (2026) 65–76 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 1. Printing parameters optimisation using conductive ACI FS0142 silver ink.

The traces consistency was limited by some broken traces for low dispense pressure (below 300) and high print speed (above 800 mm/min). Therefore, the optimised print speed was set to 750 mm/min and the optimised dispense pressure was set to 400, for the ACI FS0142 silver ink. The traces of length 100 mm were printed the same day and achieved an average resistance of 3.08 Ω and standard deviation of 0.18 Ω (6%), with an average measured line width of 0.23 mm. Despite all efforts in improving printing consistency, the sheet resistance of printed traces was found to differ from one print day to another, suggesting non-consistent traces thicknesses. 2.2. Experimental setup For assessment of monitoring performances, the strain gauges are printed onto Kapton films, cured at recommended curing profile (Table 1), attached onto the structure to monitor (metal or composite plate), and then wires are installed. The metal plate is a 30 cm by 10 cm aluminium 5251 plate with a 1 cm hole in the middle. The composite plate is a 30 cm by 5 cm plate with stacking sequence [0/+45/-45/90] 2s . The thickness of both plate is 2 mm. The printed strain gauges are attached onto the structure using thermoplastic film and a heated vacuum table (Nabuurs Global G-Sub 1310) at 140ºC for 1 hour. Commercially available strain gauges (TML FLAB-1-350-11) with unstrained resistance of about 350 Ω are attached using the same method on the opposite side of the structure for comparison. For wiring the printed strain gauges, adapted solder wire (SnBiAg) provided by Voltera is used on the printed and cured silver ink pads. Manual bending of the substrate with printed strain gauges allows quick strain testing to highlight resistance change. Consistent displacement is attempted for comparable measurements. Moreover, fatigue testing is performed using Instron 250 kN hydraulic universal test machine for tensile loading under multiple cycles at different frequencies. The strain, , can be obtained from the length of the specimen, , and the absolute change in length, ∆ , given by the Instron test machine as the displacement induced during fatigue testing. 2.3. Quality assessment Electrical testing is performed using 4-wire resistance measurements. The resistances of the different printed traces are measured using digital multimeter (Keysight 34465A 6½). The width of the traces is measured through image processing performed using Python from images obtained by the camera integrated in the printer. The sheet resistance is then obtained from the length and the measured resistance and width. Consistency is evaluated by testing the sheet resistance for a batch of printed strain gauges. For data acquisition of multiple strain gauges, the 4-wire resistance of each sensor is obtained using data acquisition system (Keysight DAQ973A with DAQM902A multiplexer). For validation of the performances of printed strain gauges, sensitivity, repeatability and fatigue resistance are evaluated. The resistance is measured continuously during strain testing to observe resistance changes. The sensitivity, corresponding to the gauge factor, is obtained through measurement of the change in resistance for a known strain. The linearity can be estimated by the increase in ratio of relative resistance change with increased applied strain. Repeatability is evaluated from cyclic tensile testing, observing drift in unstrained resistance or change in sensitivity.