Mechanical Weakening
A reduction in the breaking force of a yarn occurs when a knot is tied in the specimen. This loss is known as knot strength decay, and it reflects the fiber’s vulnerability to multi-axial stresses. It is a critical factor for yarns used in knitting and netting.
Stress Concentration
The presence of a knot introduces sharp bends and high shear forces that concentrate stress in a small area. This localized stress forces the outer filaments to carry the entire load while the inner filaments are compressed. The yarn ruptures at a lower total force than a straight, unknotted strand, because the shear forces exceed the transverse strength of the polymer chains.
Testing Protocol
Testing is performed by tying a standard overhand knot in the center of the yarn specimen before clamping it in the tensile tester. Comparing the knotted breaking force to the straight-strand breaking force reveals the percentage loss. High-performance fibers often show a larger percentage drop due to their high rigidity.
Industrial Impact
Netting manufacturers must select fibers that minimize this reduction. A high straight-strand strength is useless if the net fails at the knotted joints during service. Consequently, yarn specifications must balance high tenacity with sufficient flexibility to maintain knotted integrity.
