STITCH TEAR STRENGTH
This strength is determined to know the strength of small seams of leather. During the manufacture of football leather, garment, shoes, bags etc., two or more pieces of leather are stitched together. If the strengths of seams are high, it is believed that stitched leathers will not separate out due to failure of leather from stitched regions. There are two methods of determination of this test
The double hole stitch tear strength can be defined as the load (Kg) required to tear the sample of leather between two holes of 2mm. Diameter each and whose centers are 6mm apart expressed on its unit thickness (cm).
Stitch tear strength = tearing load (Kg)/Leather Thickness (cm)
The sample test is carried out in both parallel and perpendicular directions to the backbone and the test specimen is a rectangular piece of size 50*25 mm.
Two holes of 2mm diameter are punched at a distance of 6mm from one 25mm long end of the specimen. The distance between the centers of the two holes should also be 6mm and the two holes should be equdistance from the central line, parallel to the 50mm long edges of specimen. The mean thickness of the specimen is determined at the area of the leather of 6mm length throughout its 25mm width. A soft 1mm dia. Steel wire about 10cm length and bent into a U-shape, is introduced into the holes from the grain side in such a way that the two ends of the wire project from the two holes respectively at the flesh side. The projected ends of the wire are wrapped and clamped to a jaw of the tensile strength testing machine. The other end of the specimen is clamped to the other jaw of the machine. The machine is run and the tearing load is noted from the dial of the machine.
Basically the tensile strength and the stitch tear strength determination tests are identical except that in the former the braking load is applied in a direction parallel to the surface of the leather, whereas in stitch tear test the force acts at right angles to the surface of the leather.