Seam Strength

Definition and Evaluation Target

Seam strength is a quality characteristic that indicates the strength of a sewn joint, not the fabric itself. Under fixed conditions, the fabric on both sides of the seam is pulled to confirm the strength until the thread breaks, the seam comes apart, or the fabric tears.

There is not just one starting point for damage. In addition to sewing thread breakage, the fabric may tear from the needle holes, the woven yarns may shift and open the seam, or the seam allowance may pull out. For this reason, fabric and sewing conditions are evaluated together with the thread.

Relationship to Thread Material, Twist, and Structure

The material, thickness, twist, and structure of the sewing thread, such as whether it is single yarn or double yarn, affect seam strength. In general, for areas that require strength, choose a thread with the necessary tensile strength that can better handle friction and heat during sewing, according to the fabric and application.

If the twist is insufficient, it may lead to fuzzing and thread breakage, while excessive twist may affect thread handling and the appearance of the seam. In addition, depending on the structure, such as core-spun thread, the balance of tensile strength, stretch, abrasion resistance, and sewability differs.

How to Read Size and Nominal Size

The thickness of sewing thread is expressed by size and nominal size, but depending on the notation method and material, it may not be possible to compare them simply. In general, within the same notation system, thin and thick thread differ in the strength they give to the seam, how visible they are, and the load on the fabric.

Switching to thicker thread does not always increase seam strength. If thread and needle are too thick for a thin or high-density fabric, needle-hole damage, puckering, and a stiff seam are more likely to occur. Thread size is determined together with needle size, stitch density, and seam allowance.

How to Choose by Application and Sewing Precautions

In areas that are repeatedly stressed, such as side seams, crotch seams, armholes, pocket openings, and handle attachments, ensure the seam strength needed for the product application. In workwear, denim, bags, and similar items, in addition to thread selection, seam allowance width, reinforcing stitches, and bartack specifications are also important.

For elastic knit or stretch fabric, the seam must follow the fabric stretch as well as provide strength. With sewing thread or sewing methods that have little stretch, thread breakage may occur during wear. Before sewing, perform test stitching and stretch confirmation on the actual fabric.

• Confirm the combination of construction thread, needle, and fabric with test stitching.
• Set the upper and lower thread tension properly and avoid over-tightening.
• Select a needle suitable for the fabric thickness and structure to reduce damage from the needle holes.
• For high-stress areas, design including stitch density, seam allowance, and reinforcement specifications.

Differences from Similar Terms

Tensile strength is the strength of the fabric or material itself when it is pulled. In contrast, seam strength refers to the strength of the joint composed of the fabric, sewing thread, sewing method, and sewing conditions. Even if the tensile strength of the fabric is high, the seam is not necessarily equally strong.

Slippage is a phenomenon in which the warp or weft yarns shift near the seam of woven fabric, causing the seam to open. It can be related when evaluating seam strength, but slippage is mainly a phenomenon related to the fabric structure and density, and its focus differs from seam strength, which mainly addresses thread breakage.

Frequently Asked Questions