I’ve heard it mentioned a few times. Can anyone explain to me what it is and how it applies to glassing?
In woven fiber, yarns go up and down perpendicular yarns so yarns are deviated compared to a flat yarn. This angle of deviation is call crimp angle. In others words woven yarn make wave shape. When you stretch this wave shape yarn it want to go flat so it press on each perpendicular yarns, that’s created shear force wich destroy fiber.
During fatigue test low or no crimp fiber are much better, depend of direction of forces compared to direction of yarns.
Sorry for my frenglish.
You know when you weave something, you go over-and under-and over and under and over… etc…
That means that the cloth is actually taking an “S” path, it, each strand is not perfectly straight, it “weaves”.
That is called crimp.
It’s not something you need to worry about just for glassing - it matters if you get into the engineering of composites though, because it changes how the fibres carry loads.
True uni-directional cloth is just strands layed side by side - they are not woven, so they do not weave over and under each other - they do not have any crimp.
These cloths obviously would just fall apart, as they are not woven together, so instead there is transverse stiching, which simply holds the strands together. Stitching looks ugly, so its not used for highly cosmetic applications.
Some woven cloths are occasionally called “uni-directionals”, but are not really in the strictest sense of the word