Finding a delamination answers less than it appears to. A laminate can carry an interlaminar flaw of considerable size indefinitely without consequence, and it can also carry a smaller one that is growing steadily toward the size at which the structure fails. Those two situations look identical in a single inspection and support opposite conclusions about whether an inspection interval was adequate, whether a repair was required, and whether anyone had an opportunity to intervene. Distinguishing them is a fracture-mechanics question rather than a detection question.
Growth is an energy problem
A delamination extends when the energy released by extending it exceeds the energy required to create the new surface. The first quantity depends on the applied load, the laminate geometry and the flaw size; the second is a material property, the interlaminar fracture toughness. Growth therefore is not a property of the flaw alone — the same delamination can be entirely stable under one loading condition and unstable under another, which is why the loading has to be established alongside the damage.
Mode I and Mode II behave differently
Interlaminar loading separates into opening and shearing components, and laminates resist them differently — typically far better in shear than in opening. The double cantilever beam test under ASTM D5528 measures the opening-mode toughness, and end-notched flexure under ASTM D7905 measures the shearing mode. Real loading is usually mixed, so both are needed, and the ratio between them in the actual structure often determines whether growth was plausible at all.
Free edges and ply drops are where growth starts
Interlaminar stresses concentrate wherever the laminate is discontinuous. A cut edge, a hole, a ply termination or a thickness taper all generate through-thickness peel and shear stresses that the interior of the laminate never sees, and these locations initiate delamination even in a laminate with no manufacturing defect. A delamination that begins at such a feature and extends inward is showing its own initiation site, which is a considerable head start on the analysis.
The damage boundary records the history
A delamination that grew leaves evidence of having grown. Progressive extension under cyclic loading commonly produces a boundary region with a different appearance from the original flaw — a change in surface texture, in fibre bridging, or in the degree of contamination on the exposed surfaces. A flaw that formed in one event and never moved has a comparatively uniform boundary. Reading this requires the delaminated surfaces to be examined, which means sectioning after imaging rather than instead of it.
Contamination as a clock
Surfaces exposed for a long period accumulate moisture, oxidation products and environmental contamination; surfaces created moments before the structure came apart do not. Where a delamination shows a heavily contaminated central region and a comparatively clean perimeter, the sequence is legible: the centre was open for a long time and the perimeter opened recently. This is one of the more direct available indications that a flaw was actively extending.
Fatigue and environment accelerate each other
Cyclic loading drives growth below the static critical value, and matrix toughness generally falls as the laminate takes up moisture and as temperature rises. A delamination that was stable under the design's assumed conditions can therefore become unstable in a hot, wet structure late in its life without anything about the loading changing. Establishing the actual hygrothermal condition of the laminate matters as much as establishing the load spectrum.
What testing on the actual material adds
Published toughness values are starting points, not answers. The relevant number is the toughness of this laminate, in its current condition, at the interface where the delamination sits — which can differ substantially from handbook data because of resin batch, cure state, moisture content and the ply orientations either side of the interface. Coupons cut from undamaged regions of the same part are the most defensible source for that measurement.
What the analysis supports and what it does not
Done properly, this work establishes whether growth was mechanically possible under the loads the structure saw, and often whether it occurred. It rarely produces a date. Claims about precisely when a delamination reached a given size should be treated with care, because growth rates depend on a load spectrum that is usually reconstructed rather than recorded, and small errors in that reconstruction propagate substantially.
What to preserve
The structure uncut and unrepaired, with enough surrounding material to establish the load path and any free edges or ply terminations near the damage. The inspection history for the location, including previous scans that may show the flaw at an earlier size — which is the single most valuable evidence of growth and the most commonly discarded.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.