Seam Testing

Sewn Seam Failure Mode Predictor (ASTM D1683)

Adding stitches strengthens a seam right up to the moment the fabric becomes the weak link. After that it just makes holes.

Fabric Reference width
N/25mm
mm
Stitching Sewing room
stitches/cm
N
no.

Two for a lockstitch: needle and bobbin.

×

Share of thread strength realised in the seam; sewing damage and obliquity reduce it.

Governing Seam Strength

— N/25mm

The lower of thread and fabric — what the seam actually holds

Failure Mode

Thread-Derived Seam Strength
— N/25mm
Seam Efficiency
— %
Stitches in the Reference Width
— no.
Stitch Density for Fabric Rupture
— stitches/cm
Fabric over Thread Strength
— ×
Thread Shortfall Against Fabric
— N/25mm

A ratio above 1 means the thread gives first and the seam is under-stitched; below 1 the fabric ruptures and more stitching adds nothing but needle damage. Thread utilisation is the weakest assumption here — needle heat, stitch obliquity and thread abrasion in sewing all cut it, so fit it from broken seams rather than assuming half.

Sewn Seam Failure Mode Predictor (ASTM D1683) — free, with the formula and a worked example, at Textile School.