More compressible materials are harder to deform plastically. This is backwards.
Elgailani, Vandembroucq, and Maloney study amorphous solids — glasses, metallic alloys, foams — using elasto-plastic models where the ratio of bulk modulus to shear modulus (K/μ) is a free parameter. Lowering K/μ makes the material easier to compress. It also makes the material resist plastic flow more strongly.
The mechanism is the Eshelby back stress. When a local region of an amorphous solid yields — undergoes an irreversible rearrangement — it creates a stress field that extends into the surrounding material. This far-field stress depends on the Poisson ratio, which is set by K/μ. At lower K/μ (more compressible), the back stress is larger. The yielded region pushes back harder against the matrix, making the next rearrangement more difficult to trigger.
The result is strain hardening that emerges from mechanics alone, without any ad-hoc hardening parameters. Amorphous alloys are known to exhibit enhanced resistance to plastic deformation, which is traditionally attributed to excess free volume — microscopic voids that accommodate shear. This model produces the same enhanced resistance purely from the Poisson ratio dependence of the stress field. The free-volume story may be a consequence, not a cause.
There's also an abrupt transition at the elastic-plastic threshold between kinematic and isotropic hardening modes, arising spontaneously from the mechanics rather than from assumptions about how the material work-hardens. The transition isn't gradual. It's a discontinuity that the standard smooth-hardening models miss.
The counterintuitive relationship — compressibility enables resistance — follows from the mechanics once you trace it. But it challenges the intuition that softer materials yield more easily. In amorphous matter, soft in one direction can mean stiff in another, because the coupling between modes is set by geometry rather than by material strength.