Comparing the tensile strength of highly elastic circular knit textiles before and after damage to the knit structure
Dave Cox
Senior Staff Technologist
Amanda Fleury, PhD
Senior Data Scientist
A modified apparatus was designed to determine the tensile strength of highly elastic circular knit textiles. Five brands of pantyhose were tested to determine the tensile strength of a single leg when stressed to failure, both before and after perforation damage to the knit structure. Average tensile strength of Sheertex pantyhose was four times the strength of competing brands.
Tensile strength of sheer tights is an important measure of their durability. A pair of tights that can withstand more pulling force has a better chance of surviving the rigors of daily life. Tights that are still strong even after a small hole has formed stand a better chance of lasting the day and also ensuing wears afterwards with minimal ’laddering’, which is a common complaint with tights [2].
Standardized tests of tensile strength for textiles (e.g. ASTM D5034 [1]) are generally designed for woven or non-woven fabrics that are non-elastic or slightly elastic, and therefore fail to account for the extensive deformation sustained by elastic knit textiles before failure. When the elastic deformation of a textile exceeds the extension limits of the standard testing apparatus, testing results are rendered inconclusive and it is not possible to accurately determine the breaking strength of the elastic textile using that specific method and appara- tus.Accordingly there is a need for a method and apparatus that can adequately assess tensile strength in highly elastic textiles such as hosiery.
Because of this, we have designed a custom apparatus which goes beyond the standardized testing to get a true picture of the tensile breaking strength of the tights.
To the end of simulating fabric damage, a ’bed of nails’ apparatus was also designed and built with the intent to damage (via puncture) the knit samples in a controlled manner, allowing for tensile strength testing after damage to the knit structure.
We believe the tensile strength of a leg of Sheertex panty- hose is at least four times that of a competing brand, and is greater after structural degredation of the fabric than undam- aged competitors.
A test device was designed and built which uses a calibrated load cell in combination with a towing winch to stretch samples under test. Fabric clamps were also designed and built which use a capstan mechanism to ensure proper clamping and fabric handling, versus more traditional C-grips which promote early breakage at the clamping point of the fabric.
The tension apparatus can withstand loads up to 10kN, has a constant extension rate of 2058mm/min, and an overall working distance of 57.5cm.
A test apparatus was designed and built which uses standard 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 225mm by 225mm grid, which mate into 8mm holes on a receiver plate. This mating action ensures that when the upper stage is brought downwards by the actuators, the fabric will be perforated by the nails. In order to promote even and repeatable sample preparation, a custom frame was constructed to hold pantyhose. With this device, an internal support frame is inserted inside the leg of the hosiery under test, and then an external clamping frame is clamped around the internal frame, gripping the fabric so that it does not slip when under test.
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 apparatuses built for these tests.
Sheertex tights (referred to herein as Brand 1) were tested against competitor tights which are similar to Sheertex tights in sheerness and appearance (referred to herein as Brands 2, 3, and 4), as well as against disposable drugstore tights (referred to herein as Brand 5).
Firstly, the load cell is zeroed. Then, samples consisting of a single leg (size large or equivalent) of a pair of 30-40 denier standard sheer pantyhose were obtained. The untensioned leg was loaded onto the tester by placing the leg between the two halves of the clamp, and rotating the clamp to ensure the pantyhose completed a minimum of two full rotations - though due to the sample length, five or more wraps could be used - to reduce the possibility of stress concentrations at or near the clamp (see the companion article describing apparatus design for more details). This process was completed with the other end of the leg, and then both clamps were rotated to pre-tension the fabric to the limits of its stretch capabilities to ensure testing to failure.
An electric winch (with an operational speed of 2058 mm/min) was used to apply tension to the pantyhose until failure, while a calibrated load cell captured the tensile force on the pantyhose. The test was completed for three samples each of five different brands of pantyhose. Every effort was made to ensure consistent loading across tests.
To simulate spontaneous damage to the knit structure, the perforation apparatus as described above was used to puncture the knit samples in a controlled manner. Three samples from each brand were taken, and subjected to an identical five puncture cycle for each sample puncturing the tights over 500 times. After each puncture trial, the pantyhoses were then tested for tensile strength using the method described above. Tensile strength testing was run until failure of the fabric; here, this is the maximum load seen through the load cell during the test.
Because the fibers of a knit are interconnected, once the first fiber gives way, the entire knit sample generally tears completely in two. Thus, the highest force seen on the load graph indicates the force to first fiber failure within the knit.
A calibrated and zeroed load cell was used to measure the force on the sample throughout each test. The force at failure was averaged across the three trials for that brand.
Fig. 1. Bar graph showing average tensile force required before breakage.
As can be see from Fig. 1, when the knit is still intact, Sheer- tex tights (Brand 1) outperforms competitor tights (Brands 2 to 5) in terms of average tensile force required to break an undamaged pantyhose leg. Sheertex tights, with an average of 180.4kgf to breakage, are about 2.3 times stronger than the nearest competitor (Brand 3: 79.0kgf), and 5.9 times stronger than the disposable drugstore tights (Brand 5, 30.6kgf).
When assessing tensile strength after sustaining damage to the tights (as shown in Fig. 2), the findings are similar. With an average of 116.3kgf to breakage, Sheertex tights are 2.95 times stronger than the nearest competitor (Brand 3: 39.4kgf) and 7.55 times stronger than disposable tights (Brand 5: 15.4kgf) when damaged. Interestingly, damaged Sheertex tights outperform undamaged tights, even after being punctured over 500 times. Damaged Sheertex tights were 1.47 times stronger than the undamaged Brand 3 tights, or 2.28 times stronger than the averaged strength of all undamaged Competitors (Brands 2 to 5).
In addition, in the perforation testing, it was observed that Sheertex tights better resisted damage, with fewer holes sustained and less severe damage, resulting in a 35% loss in tensile strength, where other competitors saw their tensile strength reduced between 30-50%.
Fig. 2. Bar graph showing average tensile force required to break perforated tights.
From the above results, it was found that the Sheertex tights are stronger and more resilient than disposable tights, as well as similar tights with a similar sheerness and appearance, making them more durable and less susceptible to the types of wear and damage than traditional and disposable knits.
Many thanks to Amy Dam, Amanda Fleury, and Charlotte Fauqueux for their assistance with editing and adding clarity to this report.