Architecture-induced cascading deformation bifurcation enabling shear-band suppression in lattice metamaterials

Architected lattice metamaterials are attractive for their lightweight load-bearing and energy-absorption capacity, but these are often limited by catastrophic inclined shear bands, analogous to the shear band behavior in conventional continuum solids. Although the macroscopic shear band in continuum solids is typically understood as an instability originating from constitutive softening, the mechanism of shear banding in discrete lattice structures remains elusive. Here, combining experiments, simulations and theoretical analysis, we find that shear-band formation in finite lattices can emerge from an architecture-induced cascading deformation bifurcation, including a boundary-initiated switch from a horizontal collapse mode to an inclined deformation mode, followed by inward cell-to-cell propagation. In typical lattices with positive Poisson's ratio, boundary confinement converts uniaxial compression into local compression-shear states, triggering corner-initiated inclined collapse modes that propagate inward to form a shear band. This phenomenon can be suppressed in representative two- and three-dimensional lattices by engineering the Poisson's ratio, boundary friction, and boundary topology. These findings identify a boundary-triggered, architecture-mediated route to shear-band formation in discrete lattices and provide a mechanism-guided strategy for mitigating catastrophic collapse in mechanical metamaterials.