Abstract
Strain gauges are widely applied to measure mechanical deformation of structures and specimens. While metallic foil gauges usually have a gauge factor slightly over 2, single crystalline silicon demonstrates intrinsic gauge factors as high as 200. Although silicon is an intrinsically stiff and brittle material, flexible and even stretchable strain gauges have been achieved by integrating thin silicon strips on soft and deformable polymer substrates. To achieve a fundamental understanding of the large variance in gauge factor and stretchability of reported flexible/stretchable silicon-on-polymer strain gauges, finite element and analytically models are established to reveal the effects of the length of the silicon strip, and the thickness and modulus of the polymer substrate. Analytical results for two limiting cases, i.e., infinitely thick substrate and infinitely long strip, have found good agreement with FEM results. We have discovered that strains in silicon resistor can vary by orders of magnitude with different substrate materials whereas strip length or substrate thickness only affects the strain level mildly. While the average strain in silicon reflects the gauge factor, the maximum strain in silicon governs the stretchability of the system. The tradeoff between gauge factor and stretchability of silicon-on-polymer strain gauges has been proposed and discussed.
Highlights
Strain gauges are widely used across all engineering fields to measure mechanical deformation of a solid object
We describe finite element and analytical modeling of thin silicon strips bonded to polymer substrates of wide ranges of Young’s modulus and thickness
There is no uniform function of interface shear stress distribution that is applicable to wide ranges of ĒSi/Ēs, L/h, and H/h, which is the case for stretchable silicon-on-polymer strain gauges
Summary
Strain gauges are widely used across all engineering fields to measure mechanical deformation of a solid object. Candidates of strain sensing materials for flexible/stretchable strain gauges include metals, silicon, piezoresistive elastomers, and even carbon nanotubes and graphene. We will focus on the mechanical responses of flexible and stretchable gauges based on polymer-supported thin silicon strips in this work. We describe finite element and analytical modeling of thin silicon strips bonded to polymer substrates of wide ranges of Young’s modulus and thickness. Both gauge factor and stretchability can be predicted and effects of material and geometric variables are revealed.
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