Mode-Dependent Strain-Rate Sensitivity of Filled Polymer Yield Under Shear and Uniaxial Deformation

Filled polymers yield differently under shear and uniaxial deformation, but filler and rate effects remain sparsely characterized. We study a glassy Kremer–Grest polymer with frozen filler beads at N=100, loading ϕ∈[0,0.30], and strain rate ε˙∈[10−4,10−2]τLJ−1 by non-equilibrium molecular dynamics. All ten strain-rate-sensitivity confidence intervals exclude zero (uniaxial m∈[0.17,0.30], shear m∈[0.14,0.16]). Uniaxial yield is more rate-sensitive than shear, but the mode gap is loading-dependent: Δm≈0.08–0.16 (p≥0.98) for ϕ≤0.10, attenuating to 0.02–0.04 for ϕ≥0.20 as filler loading increases. Frozen filler softens rather than reinforces (shear yield −78%, plateau modulus −43%); density-matched neat controls attribute most of the shear softening to density dilution (89–97%). At ϕ=0.30, shear develops a low-rate crossover and the yield ratio σyuniax/σyshear rises to [1.30,1.40], below the von-Mises reference. Density dilution dominates, but both-mode controls isolate a mode-selective filler contribution at ϕ=0.30 (resolved at the higher rate): 35–42% of the uniaxial versus ∼10% of the shear yield.