thumb|Delamination of [[carbon fiber–reinforced polymer under compression load]]
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. 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.
Delamination resistance testing methods
Coating delamination tests
ASTM provides standards for paint adhesion testing which provides qualitative measures for paints and coatings resistance to delamination from substrates. Tests include cross-cut test, scrape adhesion, and pull-off test.
Interlaminar fracture toughness testing
Fracture toughness is a material property that describes resistance to fracture and delamination. It is denoted by critical stress intensity factor <math>K_c</math> or critical strain energy release rate <math>G_c</math>. For unidirectional fiber reinforced polymer laminate composites, ASTM provides standards for determining mode I fracture toughness <math>G_{IC}</math> and mode II fracture toughness <math>G_{IIC}</math> of the interlaminar matrix. During the tests load <math>P</math> and displacement <math>\delta</math> is recorded for analysis to determine the strain energy release rate from the compliance method. <math>G</math> in terms of compliance is given by
where <math>dC</math> is the change in compliance <math>C</math> (ratio of <math>\delta /P</math>), <math>B</math> is the thickness of the specimen, and <math>da</math> is the change in crack length.
Mode I interlaminar fracture toughness
thumb|Schematic of deformed double cantilever beam specimen.
ASTM D5528 specifies the use of the double cantilever beam (DCB) specimen geometry for determining mode I interlaminar fracture toughness. Multiple test architectures have been proposed for use in measuring interlaminar shear strength, including the short beam shear test, Iosipescu test, rail shear test, and asymmetrical four-point bending test. The goal of each of these tests is to maximize the ratio of shear stress to tensile stress exhibited in the sample, promoting failure via delamination of the fiber-matrix interface instead of through fiber tension or buckling. The orthotropic symmetry of fiber composite materials makes a state of pure shear stress difficult to obtain in sample testing; thin cylindrical specimens can be used but are costly to manufacture. Sample geometries are thus chosen for ease of machining and optimization of the stress state when loaded.
In addition to manufactured composites such as glass fiber-reinforced polymers, interlaminar shear strength is an important property in natural materials such as wood. The long, thin shape of floorboards, for example, may promote deformation that leads to vibrations.
Asymmetric four-point bending
thumb|(top-bottom) The point forces, shear stresses, and moments acting on an asymmetric four point bend test sample
Asymmetric four-point bending (AFPB) may be chosen to measure interlaminar shear strength over other procedures for a variety of reasons, including specimen machinability, test reproducibility, and equipment availability. For example, short-beam shear samples are constrained to a specific length-thickness ratio to prevent bending failure, and the shear stress distribution across the specimen is non-uniform, both of which contribute to a lack of reproducibility. Rectangular samples can be used with or without notches machined at the center; the addition of notches helps to control the position of the failure along the length of the sample, but improper or nonsymmetrical machining can result in the addition of undesired normal stresses which reduce the measured strength.