What Does Delamination Mean?
Delamination refers to the loss of coating adhesion to a surface or between coating layers.
Delamination can occur in:
- Composite materials
- Reinforced concrete structures
- Laminated materials
Delamination tends to destroy a coating's strength and durability, as well as the material's aesthetic appearance. This can make the substrate vulnerable to corrosion.
When corrosive media have direct access to a metal substrate, electrochemical reactions will take place at the metal-coating interface in the presence of water, causing delamination to occur. Therefore, a good protective coating requires tight bonding between the coating and metal substrate to resist the penetration of water to the interface. The mechanical bond is really important in terms of ensuring good coating adhesion. It is considered a good engineering practice to increase the surface roughness of metals in order to increase the contact area and improve adhesion between the coating and substrate metal.
The mechanism of a coating failure is influenced by a number of parameters. These include not only the properties of the coating, interface, and substrate, but also the distribution of stresses. If the coating is under a residual tensile stress, there are at least three possible mechanisms by which failure can occur. For example, a brittle coating may fracture by the development of cracks through the thickness of the film. Tougher coatings may fail by delamination along the interface or even by the propagation of a crack within the substrate. The failure mechanism associated with a compressive stress in the coating happens due to simultaneous buckling and delamination.
Because the use of composite materials in aviation is increasing, delamination is increasingly an air safety concern, especially in the tail sections of airplanes.
Concrete delamination is found most frequently in bridge decks and is caused by the corrosion of reinforcing steel or freezing and thawing.
Corrosionpedia Explains Delamination
Delamination failure can be of several types, such as:
- Fracture within the adhesive or resin
- Fracture within the reinforcement
- Debonding of the resin from the reinforcement
In this last instance, it is the debonding that leads to delamination. To be clear about the distinction, debonding refers to when two materials stop adhering to each other, whereas delamination refers to when a laminated material becomes separated.
However, the cause of delamination is weak bonding. Thus, delamination is an insidious kind of failure as it develops inside of the material, without being visible on the surface, much like metal fatigue.
Delamination failure may be detected in the material by its sound:
- Solid composite has a bright sound, while a delaminated part sounds dull
- Reinforced concrete sounds solid, whereas delaminated concrete will have a light, drum-like sound when exposed to a dragged chain pulled across its surface
Other nondestructive testing methods used include:
When dealing with concrete delamination, it occurs when the fresh concrete surface is sealed or densified by troweling while the underlying concrete is still plastic and continues to bleed and/or to release air. The resulting delaminations are generally thicker than those caused by improper finishing. Delaminations are difficult to detect during finishing but become evident after the concrete surface has set and dried.
Delamination in a laminated composite structure without flaws can be simulated. The delamination process is divided into two parts; delamination initiation and subsequent growth. To predict a starting delamination, the first-ply failure criterion is used. Growth of the starting delamination is simulated using the VCCTi.e virtual crack closure technique
Practical experience and laboratory observations indicate that corrosion-affected reinforced concrete structures are prone more to cracking and delamination of the concrete than to the loss of structural strength. The proposed method is based on fracture mechanics and is based on cracks opening in concrete as the criterion for its delamination. An analytical model is derived to determine the crack width in concrete. The corrosion rate is the most significant single factor that affects the time to concrete delamination. The proposed method can be used for the prediction of time to concrete delamination for reinforced concrete structures located in chloride-containing environments. In principle, this provides a means for judging timely intervention and, as such, has the potential to prolong the service life of corrosion affected concrete structures.