Supplementary information
1.1. Preparation of strain gauge samples and their characterization:
Preliminary approach:
As a first step to fabricate the strain gauge sample, strain sensing composite is synthesized by mechanically mixing the commercially available graphite powder and a silver filled conductive epoxy (Siltech Corporation Inc, 122-c two-component room temperature curing silver filled epoxy conductive adhesive/cement product data sheet, 2013.). The mixture is pasted on glass substrates in different patterns and they are air dried. One such sample is shown in figure 1.
Double enameled copper wires are attached to the two ends of the pattern to carryout electrical measurements.
Particle size in graphite powder is 99.5% passing through 150µm mesh and it has high electrical conductivity. It is stable, resistant to oxidation and thermal shocks. The choice of epoxy is based on its various properties like room temperature curing, Air dry and their highly adhesive to the substrate [1]. Borosilicate glass slides of 75mm x 25mm x 1.4mm dimensions are used as substrate normally it intends for bending experiment so that its elasticity is suitable for performing strain sensing experiments.
During strain characterization of thus fabricated samples it is found that they result in inconsistent GF and large hysteresis. This may be attributed to the unevenness of the pasted strain sensing composite as can be seen from figure 1 and, its rigidity occurring due to the lumped poor assortment of graphite powder in epoxy. On repeated cycles of stress testing, the sensing composite develops cracks and becomes no more usable. These issues are related to the homogeneity of materials in the composite as well as its adhering quality to the substrate.
To address these issues and to understand the piezoresistive behavior of the sensing composite, one of the solutions attempted is to vary its composition. Graphite powder and epoxy are mixed in different proportions (by weight) and they are pasted on glass substrates to fabricate strain gauges. During stress tests, the effect of proportions of the materials in the strain sensing composite is experimentally observed to be as below:
1. If the amount of epoxy is increased, the composite becomes metallic. It results in poor sensitivity because of low piezoresistivity. Also, the resistance of the sample is reduced to very low values (about 2-3Ω).
2. If the amount of graphite powder is increased, adhesion of the composite to the glass substrate gets limited.
Many composites were produced using different mix proportions in order to arrive at a suitable optimized mix ratio. It was observed that, varying the composition of thus prepared sensing
composites does not resolve the earlier issues. Specifically, the change in resistance with applied load is inconsistent. After repeated trails, the composite undergoes permanent deformation.
These issues, related to homogeneity and adhesion of the composite, led to the improved approach discussed in section 3.1.
Figure 1: Graphite based strain sensing mixture pasted on glass substrate.
References:
[1].Jing Z, Guang-Yu Z, Dong-Xia S, “Review of graphene-based strain sensors,” Chinese Physics B, 22(2013) 05770.
Glass Substrate
Graphite Based Strain Sensor
