Stress Glass Breakage Pattern — Thermal

The crack lines are smooth and continuous . There is no rough, hackly, or granular surface at the origin point (unlike a blunt impact, which leaves a crushed zone). Why the Pattern Looks Like a Fern The feathering pattern is a result of fracture mechanics. As the primary crack accelerates across the pane, it reaches critical velocity (approximately 1,500 m/s in glass). At this speed, the crack becomes unstable and branches. Each branch further subdivides, creating a fractal, tree-like pattern.

The critical condition for thermal stress breakage is . If the glass were free-floating, it could expand without restraint. However, glass in a window is typically held by a frame, gaskets, or setting blocks. If the expanding center pushes against a restrained, cold edge, the tensile forces concentrate dangerously. The "Arc and Feather" Fingerprint When a piece of annealed (non-tempered) glass fails due to thermal stress, it almost never produces a starburst pattern (which indicates impact). Instead, it produces a signature that glaziers call the "arc and feather" pattern. 1. The Origin Point (The Arc) The break nearly always initiates at the edge of the glass, not the center. Look for a smooth, curved crack that resembles a semicircle or a shallow "C" shape starting at the edge. This is the arc . The arc forms because the crack seeks to propagate perpendicular to the maximum tensile stress, which runs parallel to the edge. 2. The Propagation (The Feather) As the initial crack (the arc) relieves local stress, the remaining energy sends cracks racing across the pane. From the ends of the arc, two distinct cracks shoot inward, often in a sweeping curve toward the opposite edge. These are the feathers . Between these two main cracks, secondary branching cracks will form, creating a pattern that looks uncannily like a bird's feather or a fern leaf. The crack lines are smooth and continuous