Delamination
A variety of materials, including laminate composites[1] and concrete, can fail by delamination.
[6] 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).
[7] 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.
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.
[8] In steels, rolling can create a microstructure when the microscopic grains are oriented in flat sheets which can fracture into layers.
[4] 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.
[10] 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.
[11] 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.
[15] For unidirectional fiber reinforced polymer laminate composites, ASTM provides standards for determining mode I fracture toughness
is recorded for analysis to determine the strain energy release rate from the compliance method.
ASTM D5528 specifies the use of the double cantilever beam (DCB) specimen geometry for determining mode I interlaminar fracture toughness.
[17] A double cantilever beam specimen is created by placing a non-stick film between reinforcement layers in the center of the beam before curing the polymer matrix to create an initial crack of length
Using the compliance method, the critical strain energy release rate is given by
Typically, equation 2 overestimates the fracture toughness because the two cantilever beams of the DCB specimen will have a finite rotation at the crack.
can be calculated experimentally by plotting the least squares fit of the cube root of the compliance
Mode II interlaminar fracture toughness can be determined by an edge notch flexure test specified by ASTM D7905.
If the test is performed with the initial crack (non-precracked method) the candidate fracture toughness
[19] 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.
[20] 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.
[21] 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.
[22] 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.
[23][24] 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.
