Elastomeric Stress
Tension applied to a textile specimen during repeated stretching cycles under standard designation ASTM D2731 measures the mechanical fatigue of elastomeric yarns. Bulk production batches of spandex core spun yarn undergo this procedure to verify that continuous extension does not induce permanent deformation or premature tension loss before commercial knitting proceeds. Mills rely on the resulting hysteresis loops to predict how inner elastic components behave inside finished swimsuits or compression garments during active daily wear.
Testing laboratories mount a conditioned filament between pneumatic grips, subject the sample to repeated elongation cycles at a constant rate of extension, and record the resulting force displacement curves.
Hysteresis Loss
Energy dissipated during each mechanical cycle appears as the area enclosed between the loading and unloading curves on the force displacement graph. Thermal dissipation accounts for a substantial portion of this lost mechanical work because polymer chains inside polyurethane filaments experience internal friction during repeated deformation. Technicians calculate the ratio of recovery energy to input energy from the plotted hysteresis loops to quantify internal structural damping.
High energy loss during initial extension cycles indicates poor molecular recovery, which translates directly to garment bagging over time.
Cycle Fatigue
Progressive degradation of elastic recovery force accumulates across successive stretching repetitions until a steady state condition is reached or physical rupture occurs. Elastomeric yarn specifications mandate a maximum allowable tension decay after five or fifty continuous extension cycles to guarantee garment longevity. Low quality filaments display rapid tension fatigue during the first ten repetitions, whereas premium spandex maintains sufficient retractive force throughout extended wear.
Quality control managers reject yarn lots that exceed specified permanent set thresholds after standardized cycling because such material fails consumer durability expectations.
Permanent Deformation
Residual strain remaining in a tested filament after the release of applied stress represents the non recoverable elongation of the polymer network. Molecular chains slip past each other permanently when external tension exceeds the elastic limit of the polyurethane structure during testing. Fabric manufacturers adjust knitting stitch densities to compensate for measured elongation set and thereby prevent finished garments from stretching out of shape.
Accurate measurement of this residual strain prevents substandard elastomeric yarns from entering the supply chain for high performance sportswear.
