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Delamination is what happens to a composite material when its layers begin to separate. While this term typically refers to layered composites, it is also used to refer to non-layered composites that break down in layers. Delamination is caused by a weakening of the bonds holding the layers together, often meaning that an adhesive begins to break down. Since this happens inside a material, the substance won’t necessarily show any signs of wear, making its breakdown unexpected.
A composite material is any material that is made of multiple substances. The constituent pieces are bonded together either physically, chemically or through use of adhesives. The goal of a composite is making an item that has superior properties to those of any of the constituent materials.
In many cases, a composite substance is formed in tight layers, like plywood. In other cases, they are simply multiple items that make up a greater whole, such as reinforced concrete. Regardless of the actual construction of the composite, they all suffer from some form of delamination.
Composites are common in nearly all parts of the world. Common composite building materials, such as the plywood and concrete mentioned above, make up a huge part of modern construction techniques. Several types of plastic have glass or carbon fiber implanted within them to give them a higher rigidity. Items as disparate as fishing poles and car body panels are made of various composites.
These substances are bonded together in some way. Delamination starts when the bonds that hold them together begin to break down. In some cases, this process starts almost immediately. For instance, many adhesive-based materials begin to lose adhesion almost from the moment they are made.
Several factors increase the speed of delamination. Rapid increases or decreases in pressure and temperature will break the substances down much faster. Adhesives are particularly susceptible to humidity, and physically connected composites are weak against impacts.
These breakdowns happen within the composite. The actual process varies based on how the composite is made, but in general, the delamination process is very advanced by the time the substance shows outward signs. Chemical- and adhesive-based composites could be nearly to the point of collapse before they even show signs of wear.
In many cases, it is possible to tell that delamination is occurring in a substance by the way it sounds when it is struck. Most composites will respond with a loud reverberating sound. A delaminating composite will sound dull on impact, with a low non-reverberating sound. This is a result of gaps occurring inside the material.