Composites fail by coming apart in layers
Delamination, and why it is invisible from outside.
Composites fail by coming apart in layers because a laminate is really a stack of separate fibre sheets held together only by a thin film of resin between each one, and that bond can fail internally, at the interface between two layers, while every individual fibre and every outer surface of the part remains completely intact and undamaged.
A stack of strong sheets glued by a weak film
A metal part fails by a crack that starts somewhere and grows through solid material, and that crack is usually visible or at least detectable at the surface long before it becomes dangerous. A laminated composite has a different weak point built into its own construction. It is many thin layers of fibre, each one individually strong, bonded face to face by a resin film that is doing a completely different job from the fibres themselves. That resin bond can separate, allowing two adjacent layers to slide apart internally, without a single fibre breaking and without any crack reaching either outer surface. The layers on either side of the separation each remain intact, so the part can look, from outside, exactly as sound as it did before the bond between two of its internal layers gave way.
Two loose halves bend four times as easily
The damage matters far more than the missing area suggests. A bonded stack bends as one thick section, and the stiffness of a plate in bending rises with the cube of its thickness, so doubling the thickness makes it eight times stiffer. If a laminate splits cleanly along its middle, the two halves can slide over each other and bend as two separate thin plates, each an eighth as stiff as the whole, which together give only a quarter of the original stiffness. Split it into four loose layers and the figure drops to a sixteenth.
A pack of paper shows this in the hands. A thick notepad glued along every sheet, or a single board of the same thickness, resists bending firmly, while the same number of loose sheets flops over the edge of a table because each sheet slides past its neighbours instead of being stretched or squashed. A delaminated patch of a composite panel has quietly reverted from one strong stack into several weak, independently sliding layers, which also makes it far easier to buckle under compression, and it does this right where the surface still looks undisturbed.
An old identity card peeling at one corner
An old laminated card, a library card or an identity card that has begun to delaminate at one corner, shows the failure in an object almost everyone has handled. The two plastic layers pull gently apart at the corner while the flat face of the card still looks and feels normal everywhere else, giving no hint that the bond has failed until the corner is flexed or picked at. The failure happened at an internal interface that was never meant to be examined, only trusted to stay bonded. Flexing the card at the corner is often the only way to notice anything is wrong at all, a small give and a faint loss of stiffness at that spot, which is a poor substitute for a proper inspection and a worse one on a structural part, where missing the same clue means a component failing under a load it was assumed to be carrying safely.
Finding a gap nobody can see
Because delamination gives so little visible warning, a composite part cannot be reliably inspected by looking at the surface for cracks, dents or discolouration, and a part with a serious internal separation can pass a purely visual check without difficulty. Composite structures that matter are therefore checked by methods that sense a hidden internal gap. The simplest is tapping the laminate with a coin or small hammer and listening for the duller, hollower sound a separated area produces compared with a solidly bonded one, and more demanding applications use ultrasound to find the internal boundary directly. Tapping in a grid across a suspect panel and marking each spot where the note changes lets an inspector draw the outline of the damaged patch on the surface, which is the first step towards deciding whether it can be repaired.
It is also why an impact that leaves no visible mark on a composite panel, a dropped tool or a stray knock during handling, is treated with far more suspicion than the same knock would deserve on a metal part. On metal, a light blow either leaves a dent or leaves nothing. On a laminate, the energy of that blow can separate layers internally without breaking the surface enough to leave a mark anyone would notice by eye.
None of this means every composite part is one light knock away from failure. A well-designed laminate, properly cured under even pressure with a resin chosen to bond well to its fibres, resists delamination under the loads it was designed for. The concern is damage from outside that design envelope (an impact, a flaw at the layup stage, or a repair that reintroduced a weak interface), while the ordinary behaviour of a correctly made part within its intended loads is predictable and safe.