Four-Way Stretch Testing: Recovery Standards Buyers Should Request
Four-way stretch on a fabric submission sheet is not a specification, it is an opinion. It says nothing about how far the fabric extends, how much force it takes to get there, or whether it comes back. Buyers who accept it in place of numbers are the buyers who discover half a size of growth after six weeks of wear, when the returns are already booked. Stretch and recovery are measurable, cheap to test, and the easiest thing in a sports bra program to write correctly.
Separate stretch from recovery
Stretch is how far the fabric extends under a defined load. Recovery is how much of that extension is reversible, and recovery is what decides whether the garment still fits next season. A shell can show 120 percent stretch and still be useless if it only recovers 70 percent. Ask for both, in both wale and course direction, because a knit is rarely symmetrical and a cut-out back panel is cut on a different axis than the cup.
The standards to name
For elastane-containing knits the common references are ASTM D4964 for tension and elongation of elastic fabrics, ASTM D6614 for knitted fabrics with low power, and ISO 20932-1 for the elastic properties of fabrics by strip test. Whichever you choose, name it explicitly in the tech pack so the mill's lab and your third-party lab run the same method. State the conditioning as well: ISO 139 or ASTM D1776 at 20 plus or minus 2 degrees Celsius and 65 plus or minus 4 percent relative humidity, with specimens conditioned for at least four hours. Unconditioned results drift enough to start an argument you do not want to have.
Numbers that work for medium impact
A practical window for a medium impact shell is four-way stretch of 80 to 120 percent under the specified load, immediate recovery of at least 90 percent within one minute of release, and permanent growth of no more than 5 percent after a hold under load. For the underband elastic, which carries most of the load in a bra, tighten growth to 3 percent or less and require the same figures after laundering. If the brief includes the larger end of B/C cup grading, ask for the higher end of the modulus range rather than simply more stretch, because support is about resistance to movement, not about how far the fabric travels.
Test after washing, not only before
Elastane degrades with heat, chlorine and time, so a pass on greige or finished fabric is not the whole picture. Add a post-laundering requirement to AATCC 135 or ISO 6330 at the cycle count you want to claim, typically 5 for development and 30 for a durability claim, and require the recovery figure to stay above 85 percent. This is also where you catch a mismatch between components: if the lining shrinks 4 percent and the shell shrinks 1 percent, the cut-out edges will pucker no matter how good the sewing is.
How to use the results
Put the method, the load, the direction, the acceptance values and the wash cycle count directly on the fabric spec, and require a lab report per dye lot for bulk. Where results sit close to the limit, treat them as a fail rather than a pass, because mill-to-mill variation will find that margin. Then protect the variables a lab cannot see with incoming fabric inspection to ANSI/ASQ Z1.4 at AQL 2.5.
Numbers on a spec sheet are the cheapest insurance in this category. A shell that recovers is a bra that still fits, and a bra that still fits does not come back.
