A void between fiber layers in a composite. In this case, a viscous resin was unable to penetrate the area between two layers of fiber.
A void is the unintended third phase present in nearly all composite materials. Voids are unoccupied pores in the material – essentially, “empty” space filled with gas instead of solid material. Typically, they are the result of imperfections in the manufacturing process and are seen as undesirable because they can reduce the mechanical properties and therefore the lifespan of the finished composite. As defects, they can act as crack nucleation sites and, following propagation of said cracks, can cause unpredictable behavior – even catastrophic failure – in the composite.
Depending on the properties of the matrix and the type of manufacturing process, a few different types of voids can be formed in a composite material. In particular, a very viscous matrix resin is likely to produce voids in the finished composite because it is difficult for the resin to penetrate the space between the fibers (this is especially true if the fibers are packed tightly together). If the matrix cannot penetrate the space between the fibers, then the air currently there will never be pushed out and replaced with resin. Instead, it stays inside the composite and causes serious void issues. There are also simple manufacturing mistakes which are more likely to produce voids in the final product. For example, a low curing temperature might not allow for a complete degassing to occur, but if the temperature is too high then gelation might happen too quickly and the gas will not have enough time to diffuse from the material. In both situations, the gas remaining in the material collects into voids which could compromise its mechanical properties.
Because of their adverse effects, voids are generally considered to be defects and therefore should be reduced during the manufacturing process. The most common method to achieve that for thermoset composites is by using a vacuum bagging system combined with an autoclave under pressure and heat. This works because the vacuum system (with the help of the autoclave) physically removes the voids from the system. However, if the resin is too viscous, this may not be the best method for void removal because it will be difficult for the voids to travel through and out of the matrix.
We know that voids, essentially defects, can cause unpredictable behavior – even catastrophic failure – in the composite. Through APP’s exacting standards and rigorous testing, we have perfected the science and engineering of using composite materials to produce the industry’s most effective safeguards against pipe corrosion.
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