DSS-Th.03.01 — Differential Heating Stress Crack (edge origin)

A differential heating stress crack is a through-thickness (or actively propagating) fissure that initiates when nonuniform heating/cooling creates thermal stress exceeding glass strength, commonly at edges/corners or hardware penetrations. Any confirmed crack is an integrity condition and is out of in-place restoration scope (CAT-5 by condition/location). This DSS defines identification and documentation; it does not prescribe repair. Inspect under VIS lighting/geometry after a Non-Invasive Reveal; record surface numbering and worked-surface status on Forms:

• Filament-thin, continuous line emanating from an edge, corner, hole, or notch root, extending inward; may show kinks/branching where stress redistributes.
• Often associated with local heat sources (space heaters, IR lamps, roofing torches, sun/reflector hot-spots, heated film edges) or shadow lines (curtains/blinds, frit/opaque zones).
• Can appear suddenly after a thermal event; may be accompanied by a soft “tick” sound and immediate growth.

Surface-catch (MSRT).

• Do not pass a stylus across any suspected crack. If needed, a single light, non-marring pass adjacent to the origin chip/edge may document local relief context only. MSRT is auxiliary and never determinative for integrity.

Visual signature

• Phase-stable, continuous filament that persists through small viewpoint shifts; unlike scratches, it can traverse faint films/texture without loss of continuity.
• Origin evidence: edge chip/flare, corner stress focus, hole/notch root, or hot-spot locus (e.g., behind a heater).
• No abrasive chip-flank sparkle typical of scoring.

Viewing / lighting

• Evaluate clean & dry at 12 / 36 / 72 in, using normal incidence and oblique sweep (~60°-120°) with raking light. Where safe, capture a transmitted-light view to emphasize continuity.

Location / access

• Record pane ID and surface numbering (#1-#4; laminate-internal). If between panes or laminate-internal by fixed parallax, it is still CAT-5 by location and must be referred.

• CAT-5 (default): Any confirmed thermal stress crack (edge, corner, hole/notch, or hot-spot origin) → Non-Repairable / Out of In-Place Scope. Document extent, apparent growth direction, origin, and proximity to supports/hardware; refer per project protocol (e.g., replacement/manufacturer/engineer).
• Reassign only if no crack is present:
• If examination finds only surface abrasion (no fissure), classify under M.01 (linear scoring) or M.02 (abrasion fields) as appropriate.
• If it is solely edge micro-chips/spalls at a hole/notch without a continuous fissure, use M.06.01 / M.03.04.

CAT is governed by integrity/location for cracks; acceptance is evaluated separately under VIS §6.

• vs. Linear surface scratch: Scratches exhibit abrasive edges with MSRT catch and chip-flank sparkle; cracks remain phase-stable and should not be stylus-tested.
• vs. Impact star/bullseye (M.04.02): Impact systems have a central nucleus with radial/annular arms; thermal cracks often trace from edges/corners or hot-spot loci without a classic impact nucleus.
• vs. Spontaneous tempered break (Th.03.02): Spontaneous breaks present full fragmentation (“dice”) in fully tempered lites; differential-heating cracks may occur without full fragmentation (until/unless they run).
• vs. Laminate-internal feature / IGU artifact: Use parallax and reflection tracking to confirm ply/cavity location; internal = CAT-5 by location.

• Safety. Ensure area is safe/stable before close documentation; avoid loading a cracked lite.
• Photo set (VIS):
• Overall: 72 in and 36 in, normal incidence, capturing the full fissure path and edges/corners/hardware.
• Oblique/raking: one or more frames showing continuity and origin; add transmitted-light if safe to confirm depth.
• Context: frames showing suspected heat source (heater, IR lamp, reflective façade alignment, curtain/blind edge).
• Record distance/angle in-frame.
• MSRT note. If recorded, note adjacent SC-class only; do not cross the fissure.
• Forms: Pane ID; surface numbering; worked surface Y/N; functional coating on worked surface? (Y/N/Unknown); reveal status; lighting/time; geometry checklist; photo set attached.
• Disposition. Mark CAT-5; note estimated length, origin, stability (blunted vs. sharp tip), and nearest supports. Refer per protocol.

• Th.03.01 Differential Heating Stress Crack (edge origin) - Continuous fissure observed originating at [edge/corner/hole/notch/hot-spot] on surface #___ under VIS geometry (normal + oblique + raking; transmitted view if safe). MSRT not performed across fissure; adjacent reading noted for context. Classification: CAT-5 (integrity/location). Surface numbering recorded; parallax checked. Photo set + forms attached; acceptance per VIS §6 (separate).