Effect of Double-Pass Stitching Along the Same Seam Line on Seam Opening of Upholstery Fabrics Under Cyclic Loading

Seams in upholstery fabrics are subjected to repeated mechanical loading during use, which may lead to progressive seam opening. This study evaluated a double-pass stitching variant in which the second pass follows the same nominal seam line as the first, with the aim of improving local seam stability without changing the basic seam geometry or adding reinforcing material. Four commercial upholstery fabrics were investigated: a plain-woven cotton–linen fabric (T1), a jacquard fabric containing chenille yarns (T2), a predominantly recycled-polyester twill fabric (T3), and a jacquard bouclé fabric (T4). The fabrics were tested in the warp and weft directions using single-pass and double-pass stitching. Three specimens per fabric × direction × stitching combination (48 in total) were subjected to a modified ISO 13936-2 procedure consisting of 10–180–10 N loading cycles up to 1000, with seam opening and specimen displacement recorded at 180 N every 100 cycles. Repeated-measures ANOVA evaluated the effects of fabric, loading direction, stitching variant, loading cycles, and their interactions. T1 was analysed descriptively because all weft specimens failed before the first scheduled measurement. For fabrics T2–T4, stitching variant had a strong effect on seam opening (p < 0.001; ηp2 = 0.984), with the double-pass variant showing approximately 38–81% lower mean seam opening. At 1000 cycles, the largest difference was observed for T4 in the weft direction (13.97 ± 0.64 mm vs. 2.56 ± 0.38 mm). A significant Cycles × Stitching interaction (p < 0.001; ηp2 = 0.504) showed different seam-opening development between the two stitching variants. The effectiveness of double-pass stitching depended on fabric type and loading direction, and inferential conclusions are limited to T2–T4 and the investigated loading conditions.