Effect of azimuthal prestress on kinetic penetration into soft matter

This study introduces a novel experimental technique for probing how prestress affects penetration dynamics in soft matter. The superimposed-shear impact (SSI) test introduces torsional preloading to an annular gel sample using a Taylor–Couette cell (TCC) prior to projectile impact. To validate the approach, two triblock copolymer gels of differing stiffness were subjected to four levels of TCC inner cylinder rotation ( Ω i ). Steel spheres of two diameters were used as projectiles. High-speed imaging tracked the projectile’s depth-time trajectory from contact through cavity pinch-off and rebound. Results demonstrate that increasing Ω i consistently reduced the maximum depth of penetration (DoP), independent of gel formulation or projectile size. The stiffer PMMA 19 gel exhibited consistently lower DoP values than the PMMA 9 gel for a given projectile size and Ω i . The smaller projectile produced shallower penetration and shorter interaction times with the material. Pre-shear also influenced cavity symmetry and projectile rebound behavior: higher Ω i caused pinch-off to occur more abruptly and, in some cases, enabled projectile escape via enhanced elastic recoil. Lastly, the classical elastic Froude number ( F e ) was reformulated into a nonlinear elastic Froude number ( F n e ) to account for strain stiffening effects that are ubiquotous in soft matter. Plotting normalized DoP against F n e resulted in the data from all test conditions collapsing onto a single curve, aligning with established DoP- F e scaling trends. The SSI technique thus provides a framework for studying penetration mechanics in preloaded viscoelastic solids that can support understanding, modeling, and control of biological tissues, engineered soft materials, and impact-resistant protective systems.