Delamination is a mode of failure where a material fractures into layers. A variety of materials, including laminate composites and concrete, can fail by delamination. Processing can create layers in materials, such as steel formed by rolling and plastics and metals from 3D printing which can fail from layer separation. Also, surface coatings, such as paints and films, can delaminate from the coated substrate.

In laminated composites, the adhesion between layers often fails first, causing the layers to separate. For example, in fiber-reinforced plastics, sheets of high strength reinforcement (e.g., carbon fiber, fiberglass) are bound together by a much weaker polymer matrix (e.g., epoxy). In particular, loads applied perpendicular to the high strength layers, and shear loads can cause the polymer matrix to fracture or the fiber reinforcement to debond from the polymer.

Delamination also occurs in reinforced concrete when metal reinforcements near the surface corrode. The oxidized metal has a larger volume causing stresses when confined by the concrete. When the stresses exceed the strength of the concrete, cracks can form and spread to join with neighboring cracks caused by corroded rebar creating a fracture plane that runs parallel to the surface. Once the fracture plane has developed, the concrete at the surface can separate from the substrate.

Processing can create layers in materials which can fail by delamination. In concrete, surfaces can flake off from improper finishing. If the surface is finished and densified by troweling while the underlying concrete is bleeding water and air, the dense top layer may separate from the water and air pushing upwards. In steels, rolling can create a microstructure when the microscopic grains are oriented in flat sheets which can fracture into layers. Also, certain 3D printing methods (e.g., fused deposition) builds parts in layers that can delaminate during printing or use. When printing thermoplastics with fused deposition, cooling a hot layer of plastic applied to a cold substrate layer can cause bending due to differential thermal contraction and layer separation.

There are multiple nondestructive testing methods to detect delamination in structures including visual inspection, tap testing (i.e. sounding), ultrasound, radiography, and infrared imaging.

Visual inspection is useful for detecting delaminations at the surface and edges of materials. However, a visual inspection may not detect delamination within a material without cutting the material open.

Tap testing or sounding involves gently striking the material with a hammer or hard object to find delamination based on the resulting sound. In laminated composites, a clear ringing sound indicates a well bonded material whereas a duller sound indicates the presence of delamination due to the defect dampening the impact. Tap testing is well suited for finding large defects in flat panel composites with a honeycomb core whereas thin laminates may have small defects that are not discernible by sound. Using sound is also subjective and dependent on the inspector's quality of hearing as well as judgement. Any intentional variations in the part may also change the pitch of the produced sound, influencing the inspection. Some of these variations include ply overlaps, ply count change gores, core density change (if used), and geometry.

In reinforced concretes intact regions will sound solid whereas delaminated areas will sound hollow. Tap testing large concrete structures is carried about either with a hammer or with a chain dragging device for horizontal surfaces like bridge decks. Bridge decks in cold climate countries which use de-icing salts and chemicals are commonly subject to delamination and as such are typically scheduled for annual inspection by chain-dragging as well as subsequent patch repairs of the surface.