  Metal wire networks rely on percolation paths for electrical conduction, and
by suitably introducing break-make junctions on a flexible platform, a network
can be made to serve as a resistive strain sensor. Several experimental designs
have been proposed using networks made of silver nanowires, carbon nanotubes
and metal meshes with high sensitivities. However, there is limited theoretical
understanding; the reported studies have taken the numerical approach and only
consider rearrangement of nanowires with strain, while the critical break-make
property of the sensor observed experimentally has largely been ignored.
Herein, we propose a generic geometrical based model and study distortion,
including the break-make aspect, and change in electrical percolation of the
network on applying strain. The result shows that when a given strain is
applied, wire segments below a critical angle with respect to the applied
strain direction end up breaking, leading to increased resistance of the
network. The percolation shows interesting attributes; the calculated
resistance increases linearly in the beginning and at a higher rate for higher
strains, consistent with the experimental findings. In a real scenario, the
strain direction need not necessarily be in the direction of measurement, and
therefore, strain value and its direction both are incorporated into the
treatment. The study reveals interesting anisotropic conduction features;
strain sensitivity is higher parallel to the strain, while strain range is
wider for perpendicular measurement. The percolation is also investigated on
direct microscopic images of metal networks to obtain resistance-strain
characteristics and identification of current percolation pathways. The
findings will be important for electrical percolation in general, particularly
in predicting characteristics and improvising metal network-based strain
sensors.
