sewing thread loop & knot strength

Loop & knot strengths of sewing threads are important in measuring seam efficiency

Loop strength is measured by the load required to break a certain length of thread which is looped through another length of the same (construction) thread.

The life of a durable seam depend upon a loop produced by a thred, which is just enough to be caught by another thread to make a stitch. While the ultimate target is measuring seam performance & durability, the formation of a good loop bears a direct relationship to stitch breaking strength and hence seam breaking strength.

The strength of thread in the loop is expected to be twice of that obtained with the simple tensile test, as the force applied being borne by both arms of the threads. But at the point of intersection in the shape like alphabet U, the bent portions of the thread becomes week due to the excessive stretching of fibers in the outer layer of the bent portions while the fibers in the inner layer are compressed, so the tenacity of thread in loop form is always less than that obtained from the simple tensile test. 

The ratio of tenacity and loop strength of a thread is termed as loop strength ratio. Loop strength ratio is used to find out the efficiency of sewing thread in the stitch.

Loop strength can be calculated as multiplying the breaking strength of Spun polyester thread with a constant 1.3 and for Polyester corepsun the constant will be 1.4

Brittleness of thread equally plays vital role in measuring seam strength. The knot strength of a sewing thread is considered as a measurement of brittleness of the thread. Reduction in knot strength reflects the performance of a thread after stitching.