Modified ball burst-style testing for highly elastic textiles
Fig. 1. The custom burst apparatus.
Senior Staff Technologist SRTX Labs
Amanda Fleury, PhD
Senior Data Scientist SRTX Labs
Samira Khajehi, BSc MBA
VP Corporate Strategy SRTX Labs
After obtaining inconclusive results when using standard burst strength tests (due to high elasticity of the fabric), SRTX Labs developed an alternative burst strength test that more appropriately measures the performance of hosiery. The SRTX burst strength test was designed to allow for up to 300mm of fabric deformation before failure, where standard test setups range from 100-200 mm. Performance of Sheertex Classic Tights was compared against 4 competitors, both before and after the knit structure was damaged via perforations. Sheertex outperformed on all measures, with four times the burst strength before perforation, and nearly three times the burst strength after damage.
Burst strength of textiles is an important indicator of fabric durability, along with tensile and abrasion resistance. A pair of tights that can withstand high forces of this type is less likely to be damaged when putting them on, taking them off, or during normal wear—increasing the longevity of the product.
To show the strength of the Sheertex knit, we pushed the test further by determining if even a degraded garment made with Sheertex fiber is still stronger than that of an undamaged pair of traditional tights.
To adequately compare Sheertex tights to other brands on the market, test samples were selected for closeness in visual appearance to Sheertex Classic tights from a group of ten top brands. These appear as Brand 2, 3, and 4 in the Results section. Additionally, a drugstore brand was also chosen due to its association with disposable tights, and appears as Brand 5 in the Results section. It is the authors’ belief that this grouping of competitors provides a reasonable basis for comparison to Sheertex tights.
Previous attempts to verify these claims of strength have not been without issue. Standardized ball burst tests for textiles (e.g. ASTM D6797 ) are limited by the range of extension of the tensile tester, which is often in the range of 100-200mm. Highly stretchable fabrics may undergo a significant amount of elastic deformation before failure, leading to inconclusive results on standardized burst strength tests.
Standardized testing was completed by an independent contractor in 2022 according to ASTM D6797 to test the strength of Sheertex pantyhose (knit using a proprietary fiber) against 20 other luxury and drugstore pantyhose brands. In this testing, results were inconclusive (i.e. the stretch of the fabric exceeded the dimensional limitations of the tester) for 11 of 21 samples.
We were interested in determining the true burst strength of Sheertex pantyhose, and therefore developed an in-house version of the apparatus for ASTM D6797 to allow for up to 300 mm of fabric deformation before failure. Additionally, to increase confidence in the overall results, an independent testing laboratory constructed an extended-height fabric clamp for the ASTM D6797  method in order to test samples to failure.
We believe that the failure strength of a ball-burst style test for Sheertex, when the fabric is stressed to failure, is at least twice that of competing brands, and the Sheertex fabric remains stronger even after structural fabric degradation than competing brands.
III- TESTING APPARATUS
MODIFIED BALL BURST APPARATUS
A custom testing rig was designed and built to perform burst strength measurements on highly elastic fabrics, including pantyhose and tights.
The testing setup consists of a large 3D-printed thumb (replacing the standard 1” steel ball) mounted to a load cell rated to 4.4kN, which in turn is mounted on the end of a hydraulic actuator (see Fig. 1). This setup is positioned above a fabric clamp with a throat diameter of 101.6mm, and an overall working depth of 300 mm. The entire test apparatus is installed within a 30 ton hydraulic press frame for stability.
A test device was designed and built using standard Amer- ican household finish nails mounted in a custom-cut HDPE plate. The plate is attached to a linearly-movable upper stage which can be automatically translated downwards via two pneumatic actuators.
There are 105 nails over a 225 mm by 225 mm grid which mate into 8 mm holes on a receiver plate. This mating action ensures that the fabric will be perforated by the nails when the upper stage is brought downwards by the actuators.
To ensure even and repeatable sample preparation, a custom frame was constructed to hold the pantyhose samples. An internal support frame is inserted inside the leg of the fabric under test, and subsequently clamped with an external frame to grip the fabric preventing slippage.
Additional notes and discussion on the design choices of these devices can be found in the companion article describing the considerations and construction details of the apparatus built for these tests.
Each test consisted of a square sample, at least 20x20 cm, cut from the leg (size large or equivalent) of the pantyhose under test. The sample was loaded into the frame and pre- tensioned to just below the elastic maximum of the fabric using small clamps. The four rim bolts on the clamp were tightened to the final test level and then the pre-tensioning clamps were removed.
The hydraulic cylinder was extended into the sample at max- imum rate of extension (133 mm/min) and the peak perforation force was recorded. Every brand was tested in this manner. To simulate spontaneous damage to the knit structure, a bed of nails punctured the knit samples in a controlled manner. Three samples from each brand were taken and subjected to five identical puncture cycles. After each trial, the samples were then tested for burst strength using the method described above.
The external laboratory tests, performed on the same se- lection of brands, were completed as per ASTM D6797 . The brands are consistently numbered between tests and also between laboratories.
Fig. 2 shows that Brand 1 (Sheertex) is the best-performing fabric when measured by burst strength. With an average of46.7kgf required for breakage, it is 4.21times stronger than the strongest competitor (Brand 3: 11.1kgf) and 5.84times stronger than the disposable drug-store brand studied (Brand 5: 8.0 kgf).
Post-perforation (Fig. 3), Sheertex is again the best- performing brand, with an average of 22.1kgf required for breakage., which amounts to 2.8 times the strength of the next- nearest competitor (Brand 3: 7.9 kgf).
When compared to the disposable drug-store brand (Brand 5: 3.6kgf), a 6.14times increase in overall strength was observed. In Fig. 4, external testing shows that Sheertex (50.94 kgf) is 3.99 times stronger than the average of all com- petitors (12.76kgf), and 3.23times stronger than the nearest competitor (Brand 4: 15.73kgf). The perforation force for Sheertex after damage was 1.99 times greater than that of the next-nearest competitor when comparing damaged Sheertex fabric to undamaged competitor fabrics.
Fig. 2. Bar graph showing average compression force required to puncture samples.
Fig. 3. Bar graph showing average compression force required to puncture samples damaged by a bed of nails.
Fig. 4. Bar graph showing average break force required during external laboratory testing.
Burst-strength tests comparing the Sheertex knit to com- petitor brands demonstrated the notable resistance to bursting, and thus its strength. This finding is directly applicable to real- life use cases; pulling on or pulling off tights involves similar forces as the garment is worn on the body.
The dramatic evidence of strength—especially following extensive degradation—shows that Sheertex is a fundamentally more durable and resilient textile, which is substantially less susceptible to the types of wear and damage that may occur in more traditional garments.
STANDARD TEST METHOD FOR BURSTING STRENGTH OF FABRICS CONSTAND-RATE-OF-EXTENSION (CRE) BALL BURST TEST, ASTM, D6797-15