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Abstract

Accurate measurement of rock deformation is essential in geophysical and geotechnical studies. However, conventional extensometers combined with roller-chain tend to overestimate circumferential strain under large-deformation conditions due to chain inclination caused by local axial displacement. To overcome this limitation, we developed a novel ball-chain extensometer that incorporates spherical rolling elements, accommodating both axial and radial deformation to always maintain a circular chain configuration. Its performance was evaluated through a series of uniaxial compression tests on granite samples, with comparisons made against conventional roller-chain extensometers and strain gauges under different servo-control modes. Results show that the ball-chain extensometer provides accurate circumferential strain measurements in the elastic stage and exhibits improved stability and fidelity during inelastic deformation. Its reduced sensitivity to chain inclination makes it a promising tool for laboratory studies involving fracture reactivation and highly localized deformation, where traditional measurement approaches face significant limitations.