Composites can be very impact-resistant compared to other materials. Based upon their makeup, different composites materials will offer different degrees of resistance to impact. Once the threshold to impact resistance is passed, cracking will occur. Localized impact can form a crack in the weakened material, and vibration and additional loading can keep the cracks spreading. There is a relatively simple solution to stopping this.
I am currently working on repairing an SMC composite truck hood, which has various forms of damage including cracks. I want to repair the cracks, but also must keep them from spreading. So the solution is …
Drill a Hole!
A hole that is drilled at the end of a crack in the material will stop it from spreading. There is no place for it to restart. It cannot travel any farther because there is not any high-stress area that is weaker than the surrounding surface.
In my example, I am working on a complete repair, not just stopping the crack from spreading. I drilled the hole, reinforced the back side, and sanded the surface to accept filler. The filler will fill this low spot over the crack as well as the hole that was drilled. It can be filled right back in but will still retain its crack resistance!
Another way for cracks to start is from high-stress areas such as square corners. When making a hole in composites, it is important to always avoid sharp corners. Round holes are the best, but if the opening must be rectangular, the corners should have some radii incorporated into the corners.
A cracked laminate is a sign of failure, and it must be addressed before it gets worse.