Textile Stretch Simulator

The swatch is 20 mm square at rest and is drawn at the yarn level: every loop or every crossing you see is the same unit cell, repeated. Your drag is a force, not a position. The cursor pulls the fabric through a compliant grip, the fabric extends along its own load–extension curve, and the gap between the cursor and the swatch edge is what stiffness feels like. A jersey follows your hand; a poplin pulled on grain hardly moves while the load climbs.

Yarn colour is the second channel. Teal means the structure is carrying the load and the yarn itself is slack. Rust means the geometry has jammed and the yarn is now being strained. The bar under “yarn strain” runs to that yarn’s break elongation; when it fills, the model stops extending and reports that the grip is slipping, not the fabric.

Let go to see recovery. A knit with no spandex snaps back only part way and then creeps, holding some permanent set; a few percent of spandex returns it to rest.

A weft-knit loop is an inextensible curve that changes shape. Pull along the courses and each loop flattens: the head widens, the legs rotate toward horizontal, and yarn migrates from the legs into the width of the loop. Pull along the wales and the opposite happens: the loop grows tall and narrow, and the fabric narrows with it. In both cases the yarn length per loop is constant — the “one loop” panel reports it — which is why a cotton jersey extends 50–100% at a load that would barely register on a woven.

The soft regime ends when the loops jam: legs of neighbouring wales touch, or the head of one loop hooks the feet of the loop above. From there the only way to extend is to stretch the yarn, and the curve turns stiff. Stitch density moves that limit. A tighter knit has less room inside each loop, so it jams sooner and resists more from the start.

In a plain weave every yarn undulates over and under the yarns crossing it. That undulation is crimp, and it is the only structural stretch a woven has on grain. Pull along the weft and the weft yarns straighten while the warp yarns, which have to go around them, crimp more and pull the width in. That is crimp interchange, and it is worth a few percent. Once the pulled yarns are straight the yarn itself takes the load and the fabric is, for practical purposes, rigid. The cross-section panel shows exactly this.

Pull at 45° and the picture changes. The load now has a shear component, the yarns rotate about their crossings like a garden trellis, and the swatch extends along the pull while contracting sharply across it. This is free until the locking angle, where adjacent yarns jam against one another. A tighter weave locks sooner. Bias-cut garments drape and cling for this reason, and it is the reason a woven stretch fabric needs spandex in the yarn to stretch on grain at all.

Spandex is not primarily about how far a fabric stretches. A plain cotton jersey already extends a long way; what it lacks is recovery. Elastane fibres return the structure to rest, so the same jersey with a few percent of spandex snaps back fully and holds no set. In the model, spandex raises the immediate recovery and shrinks the permanent set, and it also lets the yarn regime extend much further before the break elongation — the core-spun yarn in a stretch denim is what gives a woven its 20–30% weft stretch.

Yarn elasticity is the fibre’s own extensibility with no elastane present. Cotton and linen give up after 5–8%; wool and textured synthetics are springy and extend several times that. It only matters once the structure has jammed, which is why it changes the woven curve immediately and the knit curve only past the knee.