A variable stiffness omnidirectional chain based on positive-pressure fiber jamming

This paper presents the design, analytical modeling, and prototype-level experimental assessment of a variable stiffness omnidirectional chain (VSOC) based on positive-pressure fiber jamming. To address the intrinsic pressure limitation (∼ 1 atm) of conventional vacuum-based jamming, an internal inflatable bladder is used to compact a fiber bundle in a rigid chain link, thereby providing a broader tunable pressure range for stiffness modulation. A mechanics-based model is developed for a fiber jamming rod under bending, defining the jamming, transition, and slipping states; deriving the critical shear forces associated with state transitions; and introducing a pressure-dependent equivalent area moment of inertia to describe the variation in stiffness. This framework is then adapted to the two primary bending modes of VSOC. Three-point bending experiments over 0–300 kPa are used to evaluate whether the model can capture the observed pressure-dependent behavior of the present prototype. Effective parameters, including an inter-fiber friction coefficient (μ = 0.3665) and an effective fiber modulus (E = 4.85 GPa), identified from the jamming pressure p = 0 kPa bending response, are used in the present structure-level model. Within the tested pressure range, the results indicate that critical loads and slipping state stiffness increase approximately linearly with jamming pressure, whereas jamming state stiffness is comparatively insensitive to pressure and is primarily governed by geometry. This work bridges design, theory, and experiment to develop a high-performance variable stiffness structure with significant potential for soft robotics and wearable devices.