Sewn Seam Tensile Strength Testing
topic
Sewn seam tensile strength testing (ISO 13935-1 grab and ISO 13935-2 strip) produces specimens by sewing standard seam construction (lockstitch stitch type 301, stitch density 4–5 stitches/cm, 12 mm seam allowance, thread: matched count corespun polyester) parallel to specimen short axis, then testing in identical manner to parent fabric tensile test — seam positioned centrally in gauge length. Failure mode classification: fabric failure adjacent to seam (structure fails before seam, indicates good seam construction); seam thread failure (thread breaks before fabric — insufficient thread strength or stitch density); seam slippage (yarns pull out from seam, not thread break — indicates slippage problem, test should be reported separately per ISO 13936). Seam efficiency calculation: SE% = (seam breaking force / parent fabric breaking force) × 100 — measured from ISO 13935-1 versus ISO 13934-2 (same method, different specimen). Specification examples: woven cotton shirting (fabric strength 280 N grab): minimum seam strength 196 N (SE ≥ 70%); workwear cotton twill (fabric 600 N): minimum seam 480 N (SE ≥ 80%); EN 364 personal fall arrest equipment webbing seam: minimum 15 kN (SE > 90%). Stitch density effect on seam strength: 3 stitches/cm provides SE = 60–70% for average apparel fabric; 5 stitches/cm SE = 75–85%; 7 stitches/cm SE = 85–90% but increases thread consumption 40% and risk of fabric needle damage (needle holes weakening fabric 5–10% adjacent to seam). Thread-to-fabric strength matching: thread breaking strength should be 60–80% of fabric grab strength — under-strength thread fails before fabric in seam (thread break failure mode); over-strong thread causes fabric needle-hole tearing adjacent to seam (fabric fail mode near seam).
Role
Seam tensile strength testing is the structural integrity verification for garment and technical textile seam construction — with seam efficiency falling below 70% indicating thread specification or stitch density inadequacy, and below 50% indicating construction fault that would cause premature seam failure in normal consumer use, triggering garment rework or rejection in quality audit.