Plasticity, yielding and fracture of amorphous solids

5 problems

• Universal exponents of plastic avalanches in sheared amorphous solids

  Partially resolved

  A slowly sheared glass flows in bursts, or avalanches, of all sizes. The numbers describing how often avalanches of each size occur differ from simple mean-field theory, and an exact theory in two or three dimensions is missing.

• Is the brittle-to-ductile change in glass yielding a critical point

  Contested

  Glasses cooled quickly yield gradually when sheared, while slowly cooled glasses fail suddenly along a crack-like band. Whether a sharp critical point separates these two behaviours in large samples is disputed.

• What triggers microbranching of fast cracks in brittle amorphous solids

  Partially resolved

  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.

• How shear bands nucleate in metallic glasses

  Open

  Metallic glasses fail by concentrating all deformation into a thin band that then slides like a crack. How scattered local rearrangements organize into such a band, and at what strain, is unknown.

• Is fracture of oxide glasses ductile at the nanometer scale

  Contested

  Window glass breaks in a brittle way, but microscope images in 2003 suggested that tiny cavities open ahead of a slowly moving crack, as in a metal. Later work found no such cavities, and whether glass is ductile at the nanometer scale is disputed.