A classic thermal stress crack travels perpendicular to the glass edge for a short distance before meandering or branching out.
The most defining characteristic of thermal breakage is the origin. In a thermal stress fracture, the crack almost invariably initiates at the edge of the glass. Furthermore, the crack typically propagates away from the edge at a perpendicular angle (90 degrees). This "square" entry point is the forensic smoking gun for thermal stress, distinguishing it from impact damage (which often enters at an angle) or bending stress (which often creates curved crack paths). thermal stress glass breakage pattern
Thermal cracks almost always start at the edge of the glass, hidden under the frame or gasket. A classic thermal stress crack travels perpendicular to
Because glass is an elastic material (up to a point), the hot zone goes into (pushed together), while the cold zone is pulled into tension (stretched apart). Glass is exceptionally strong in compression—about five times stronger than in tension. Failure occurs when the tensile stress at the cold edge exceeds the glass’s ultimate tensile strength. Furthermore, the crack typically propagates away from the
: The crack typically travels straight for approximately 13mm to 50mm (0.5 to 2 inches) before it begins to branch or meander. Branching Patterns :
The next time you see a cracked window that looks like a fern growing from the frame, you are not looking at vandalism or a stray baseball. You are looking at a classic case of thermal stress. The smooth arc at the edge and the branching feather pattern tell a silent story of expansion denied, of hot centers pushing against cold shadows. Recognizing this pattern is the first step toward a proper remedy—not just replacing the glass, but fixing the constraint or shading issue that caused the break in the first place.