This is a 2D cross-section through a crystal: a square lattice of atoms joined by directional bonds, viewed edge-on so a cleavage plane appears as a straight crack line instead of a 3D surface. An applied tensile stress resolves onto every bond exactly like in the 3D model — only bonds aligned with the pull axis feel any load:
F_nominal(bond) = σ · |cos θ| (θ = angle to pull axis; 0 or 90° on a square grid)
The 2D view adds real crack-tip physics the flat 3D threshold rule leaves out: once a bond in a line breaks, the stress it was carrying doesn't vanish — it concentrates on the surviving bonds right next to the gap, exactly as predicted by Inglis's 1913 stress-concentration formula for a crack tip, σtip ≈ σ·(1 + 2√(a/ρ)):
a = bonds already broken along this same line (crack length)
ρ = one lattice spacing (tip radius)
K(a) = 1 + 2·√a
bond breaks when σ · K(a) > bond strength
That single extra term is what makes a crack propagate instead of failing one independent bond at a time: the first break in a line lowers the stress needed to break its neighbour, which lowers it further for the next, producing the sudden, accelerating snap of real fracture rather than a slow uniform crumble.
- Halite (NaCl) — equal bond strength on both axes, so a crack that starts anywhere races straight across in one avalanche: a single clean split, the textbook "cubic cleavage."
- Mica — bonds along Y (between silicate sheets) are ~6× weaker than along X (within a sheet). Pull along Y and the whole lattice peels apart at very low stress; pull along X and it barely responds — "perfect basal cleavage in one direction only."
- Quartz — strong, disordered bonds with no weak direction. It survives far higher stress, and because no single line is weakest, several cracks nucleate at once and zig-zag — the ragged, curved break real quartz shows (conchoidal fracture, no true cleavage).
Stress slider / Strike ratchets the applied load up; at every level the model re-checks every bond, including the concentration effect, until no more break (a quasi-static equilibrium) before waiting for more stress. Heatmap colors intact bonds from weak (red) to strong (cyan). Reset rebuilds an intact lattice.