One of the more consequential facts in composite engineering is that a laminate can lose a substantial fraction of its compression strength to an event that leaves almost no mark. A dropped tool, a hail strike, a ground-handling bump against a stand — impacts well below the threshold of anything anyone would report — can create internal delamination across several ply interfaces while the surface shows a faint indentation or nothing at all. The condition has a name, barely visible impact damage, and the gap between how little it shows and how much it costs is what makes it a recurring subject of inspection-adequacy disputes.
Why the damage goes inward rather than outward
A laminate is strong in the fibre directions and comparatively weak between plies, where load transfers through a resin-rich interlaminar region only tens of micrometres thick. An out-of-plane impact drives a stress wave that the fibres largely resist and the interlaminar regions do not. The result is matrix cracking and delamination distributed through the thickness, while the outer ply — which took the hit — springs back with little permanent deformation to show for it.
The damage grows wider with depth
Impact delamination characteristically forms a roughly conical volume that is smallest near the impacted face and widest at the back. Each successive interface delaminates over a larger area than the one above it, with the individual delaminations tending to align with the fibre direction of the ply beneath. The practical consequence is severe: the largest damage is furthest from the only surface anyone is looking at.
Why compression suffers so much more than tension
In tension, a delaminated laminate still has continuous fibres carrying load, and the loss is modest. In compression, the delamination has divided the laminate into thinner sublaminates, each with far less resistance to buckling than the full-thickness section. One sublaminate buckling locally can trigger a failure at a load well below the undamaged laminate's rated capacity. This is why compression-after-impact testing, rather than tensile testing, is the standard way to quantify what a given impact actually cost.
What visual inspection can and cannot do
Visual inspection reliably finds impacts energetic enough to leave a clear dent, fibre breakage or surface cracking. It does not reliably find the class of damage that produces a barely perceptible indentation, and it finds nothing at all where the surface has fully recovered. A surface that passes a visual walkaround is therefore not evidence that a laminate is undamaged — a distinction that matters enormously when an inspection programme's adequacy is the question.
What does find it
Ultrasonic inspection is the workhorse: a C-scan maps delamination area and depth across a region, and hand-held units can screen a specific location in the field. Computed tomography gives the fullest three-dimensional picture of the damage volume where a part can be brought to a scanner. Thermography — flash or lock-in — covers area quickly and is useful for screening large structures, though it is less definitive about depth. The choice is usually driven by whether the structure can be moved and how much area needs covering.
Establishing when the impact happened
Damage extent alone does not date an event. What often does is the surrounding evidence: paint or coating disruption at the impact site, corrosion or moisture ingress into the damage volume implying time, and matrix crack surfaces that have or have not been exposed long enough to show contamination. Maintenance and handling records, ground-equipment logs and incident reports frequently carry more weight than anything measurable on the part itself.
Damage tolerance is a design property, not just an inspection problem
Structures intended for service where impact is foreseeable are normally designed to retain adequate strength with damage up to a defined size present and undetected. Whether a given structure met that intent is a design question, answered against the design allowables, the assumed detectable damage threshold, and the inspection interval. A structure that failed with damage smaller than its own damage-tolerance assumption raises a different question than one that failed with damage well beyond it.
What sectioning destroys
Cutting into or near suspected damage before it has been imaged permanently forecloses mapping its true extent and shape, and the conical geometry that distinguishes impact from a manufacturing defect is exactly what gets lost. Image first, non-destructively, across a region larger than the suspected damage, and section afterwards through what the imaging has already located.
What to preserve
The structure as found, uncut. The impact history and any handling, maintenance or incident records covering the period. Where available, the manufacturing traveller for the specific part — cure cycle, layup and any process deviations — since a manufacturing defect is the main competing explanation and needs to be addressed rather than assumed away.
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.