A crack whose extent penetrates the full nominal thickness of a structural element or containment boundary such that it produces a continuous breach (direct communication) between the two faces of the component; unlike surface or partial‑depth cracks, a through‑thickness crack removes the material ligament across the thickness.

Principle

A through‑thickness crack reduces the effective load‑bearing cross section and, for pressure boundaries or impermeable barriers, creates a direct path for leakage; fracture mechanics characterizations (e.g., stress intensity, residual ligament absence) and leak‑integrity assessments must treat through‑thickness cracks as higher‑severity defects than comparable surface cracks.

Demonstration

Illustrative scenario: a flat plate under tensile loading develops a crack that grows across the full thickness. Situation: crack reaches from top to bottom face. Recognition: edges on both faces are open and transport (fluid or stress) communicates through the crack. Action: the component loses capacity and, if load continues, may experience unstable fracture. Consequence: immediate breach between faces and accelerated risk of catastrophic failure under sustained or increasing load.

Misapplication

Describing shallow or surface‑breaking cracks that do not fully penetrate the thickness as through‑thickness; this mistake overlooks remaining ligament capacity and may overstate immediate leakage or structural loss leading to unnecessary replacement or shutdown.

Consequence

A verified through‑thickness crack typically requires prompt mitigation because it compromises containment or load continuity: consequences include leakage, loss of pressure retention, reduced residual strength and the need for repair, reinforcement or component replacement depending on defect size and loading.

Reversal

In some composite or layered systems a nominal through‑thickness feature may not produce direct communication or full load loss if interlaminar bonding, fillers, or separate impermeable layers remain intact; similarly, in porous materials a continuous geometric crack does not always equal fluid communication without sufficient connectivity.

Boundary

Clearly within: a crack that continuously connects the two faces across the nominal thickness. Boundary case: a deep crack reaching near the opposite face but with a thin ligament remaining — evaluation depends on ligament integrity and communication. Clearly outside: surface or subsurface cracks that do not open through the full thickness.

Semantic Tension

Detectability versus criticality — small through‑thickness cracks can be difficult to detect yet carry high criticality for leakage and fracture, forcing tradeoffs in inspection method sensitivity, frequency and cost.

Synthesis

The key distinction for engineering response is not merely crack length but whether the defect breaches the thickness: through‑thickness cracks change the failure mode from reduced capacity to direct loss of continuity (leakage or immediate fracture pathway) and thus demand different assessment and repair criteria than surface defects.