Glasses fail under tension through plastic deformation — shear transformation zones nucleate, coalesce into shear bands, and the material yields irreversibly. This is the standard picture. Plasticity is the mechanism; irreversibility is the signature. Once a glass begins to fail, the damage cannot be undone.
Kaur, Ottolenghi, Lerner, Richard, and Bouchbinder (arXiv:2603.12964, March 2026) apply high stress triaxiality — hydrostatic tension dominating the shear component — and find a different failure mode. Under triaxial stress, glasses undergo massive nonlinear elastic deformation. The deformation is hyperelastic: the stress-strain curve deviates strongly from linearity, but the material returns to its original configuration when unloaded. No plasticity. No permanent damage.
The failure mechanism is cavitation — microscopic voids nucleate within the strained glass. But the cavitation is itself reversible. The voids open under load and close upon unloading. The glass breaks and heals within the same loading cycle. Only beyond a critical strain does the cavitation become irreversible and the material fail catastrophically.
The key variable is stress state, not material. The same glass that fails through irreversible plastic shear bands under uniaxial tension fails through reversible elastic cavitation under triaxial tension. The material hasn't changed; the geometry of the loading has. Triaxiality suppresses shear — the primary driver of plasticity — and promotes volumetric expansion, which the disordered glass structure accommodates elastically to a degree that crystalline materials cannot.
The structural lesson: failure mode is a property of the loading, not the material. Calling a glass “brittle” or “ductile” is shorthand for “brittle under the loading conditions we usually apply.” Change the conditions, and the same material fails by a mechanism that looks nothing like the standard one — elastic, reversible, and dominated by cavitation rather than flow.