What triggers microbranching of fast cracks in brittle amorphous solids
In plain words
A fast crack moving through glass or plastic becomes unstable and develops small side branches at a speed well below that predicted by classical fracture theory. The cause of this threshold in three-dimensional samples is not explained.
Precise statement
In brittle amorphous materials (silicate glass, PMMA, polyacrylamide gels), single cracks become unstable to microbranching at velocities of roughly $0.4 c_R$ ($c_R$ the Rayleigh wave speed), below the branching velocity predicted by linear elastic fracture mechanics. Determine the 3D mechanism and critical velocity from a theory including near-tip nonlinear elasticity and dissipation, and predict the branch length and spacing.
What would settle it
A theory and 3D simulations predicting the microbranching velocity and branch statistics that agree with measurements in at least two materials.
Status in the literature
Unverified note
In thin gel sheets an oscillatory crack instability near the Rayleigh speed was explained by weakly nonlinear elasticity in the 2010s; the 3D microbranching threshold lacks a first-principles explanation (2026).