Elastics fail quietly. The colour is still right. The width is still 30 mm. The waist no longer comes back. That is set (permanent deformation), not a sewing error. A buyer’s job is to name elongation, recovery and ageing before the mill ships a pretty braid that was never meant to survive 40 °C laundry.
TrimGem puts those numbers on elastic webbing quotes the same way peel sits on a fusible: method, specimen, and a pass/fail that a lab can repeat.
The properties that matter
Tensile and elongation. Maximum force and elongation at break say whether the tape survives a yank. They do not say whether a waist still fits after lunch. Use them as a floor, not as the product.
Elastic recovery. The percentage of original length regained after a stated stretch and a stated wait. High recovery is the point of the article.
Permanent set. What is left when recovery is incomplete. A waistband with 8% set after a wear cycle is a different garment.
Fatigue. Repeated stretch-relax cycles. Underwear and sports elastics live here. A single pull test will not catch a core that dies at cycle 800.
Heat. Modulus changes in a dryer, a mould, or a fusing press nearby. Some cores take a set in the heat that they would not take at 20 °C.
Chemicals. Sweat, chlorine, alkaline wash, skin creams. Latex and some polyurethanes care a lot.
Colour. Dry/wet crock and light. A recovered waist that crocks onto a white shirt is still a return.
Dimensional stability after wet heat: shrinkage of the cover versus the core is how you get a wavy tape.
Cores you will be asked to age: natural latex, synthetic elastomers, polyester/polyurethane systems, silicone composites. Structures: covered yarns, braids, laminates. The test does not care about the marketing name. It cares about the core.
Methods that actually get cited
Labs stitch together apparel and industrial methods. Typical building blocks:
- ASTM D4964 — constant-rate-of-extension tensile on elastic fabrics. A common setup: gauge ~200 mm, speed ~500 mm/min. After pre-cycles, stretch to a marked length, hold (~60 s), release, wait (~180 s), measure set.
- ISO 23733 — staged recovery: preload, hold at a stated elongation, free recover, measure residual.
- Fatigue — cyclic stretch on a machine (examples in the trade: thousands of cycles at a stated amplitude and frequency). ISO 7854 method B is one fatigue language used on coated and elastic articles (e.g. 50–150% elongation, thousands of cycles). Knitted apparel elastics are often recovered in a 50–300% elongation window; record set after a stated cycle count.
- Light — xenon (AATCC 16.3 / ISO 105-B02 families) and a ΔE, not “didn’t fade much.”
- Chemical soak — ISO 1817-type immersion when the tape will see disinfectant or industrial chemistry.
Wash remaining: a 40 °C × 20 wash dimensional check is a reasonable apparel screen. UV 72 h with a strength-loss cap is a reasonable outdoor screen. None of these replace a wear trial on the actual waist.
Instruments are ordinary tensile frames with proper grips (elastics slip), plus xenon and, if you are serious, a cyclic rig. Resolution on displacement matters more than a large force capacity.
How to write the spec
Name construction and core (see the elastic webbing note), then:
- Elongation at a stated force (or force at a stated elongation) — the wear modulus.
- Recovery and set after a stated hold and wait, after N pre-cycles.
- Fatigue: cycles, elongation limits, frequency, remaining recovery.
- Wash: temperature, cycles, remaining width and recovery.
- Heat if the garment is tumble-dried or pressed.
- Crock and light if the cover is dyed.
“Good bounce” is not a number. Neither is “like last season” unless last season’s tape is retained as a retain sample with the same method.
If the mill only offers a hand-stretch in the office, they are selling hope. TrimGem’s sampling notes record the method on the quote line so a waistband is tested as a waistband, not as a ribbon.