Knitted Bag Tensile Strength Testing: Which Force Figure to Specify, Strip or Grab, and What a Knit Loop Structure Does to the Result

The Numbers First: What a Tensile Figure Actually Contains

A tensile result on a bag fabric is not one number; it is four. There is the maximum force in newtons, the elongation at that force as a percentage, the direction tested, and the specimen condition. A report that gives only the first is a report that cannot be compared with another one, and most disputes about tensile figures turn out to be disputes about one of the other three.

Maximum force is the figure that gets written into specifications because it governs acceptance. Elongation at maximum force is the figure that should get more attention on a knit, because it distinguishes a stable construction from one that distorts under load, and two panels with identical force and very different elongation behave completely differently on a bag.

Direction matters more on a knit than on anything else in the bag. A knitted panel is strongly anisotropic: the course direction and the wale direction can differ by a factor of two or more in both force and elongation, and the weaker of the two is the one that governs how the bag performs in service.

ISO strip method

ISO 13934-1, 50 mm specimen width frayed to an exact yarn count

ISO strip gauge length

200 mm, reduced to 100 mm where elongation at maximum force exceeds 75 percent

ISO strip extension rate

20 mm/min where elongation is below 8 percent, 100 mm/min from 8 to 75 percent

ISO grab method

ISO 13934-2, partial-width gripping on a wider specimen

ASTM grab method

ASTM D5034, 100 mm by 150 mm specimen, 25 mm jaws, 75 mm gauge, 300 mm/min

ASTM strip method

ASTM D5035, narrower strip on the same constant-rate principle

Machine type

Constant rate of extension, abbreviated CRE, in both the ISO and ASTM systems

Specimen condition

Conditioned in the standard atmosphere under ISO 139, with wet-state testing available

Typical reporting

Maximum force in newtons and elongation at maximum force, reported per direction

Strip Method Versus Grab Method: The Numbers Do Not Match

The strip method clamps the full width of a prepared specimen, so every yarn across that width is loaded. The grab method clamps only the centre portion of a wider specimen, and fabric outside the jaws still contributes to the measured force. The two therefore answer different questions and produce different numbers on the same fabric.

On a woven fabric the gap is meaningful but understood. On a knitted one it is larger, because a loop structure transmits load sideways far more readily than a plain weave does, so more of the ungripped width participates in a grab test. A grab figure on a knit can run materially above the strip figure on the same panel.

That has a direct consequence for specifications. Writing a minimum of 600 N without naming the method invites a supplier to report whichever method produces the higher number, and both reports will be entirely truthful. Name the method, the specimen width, the gauge length and the extension rate, and the ambiguity disappears.

Gauge Length and Extension Rate Move the Result

Two machine settings that look like administrative detail actually change the number. Gauge length sets how much material is under test, and a longer gauge includes more of the weak points that a shorter one might miss, so results fall as gauge length rises. Extension rate sets how quickly the load is applied, and a faster rate generally reads higher.

ISO 13934-1 handles this by tying the rate to the expected elongation: 20 mm/min where elongation at maximum force is below 8 percent, and 100 mm/min between 8 and 75 percent, on a 200 mm gauge. The rate has to be chosen before the run, which means the band has to be estimated in advance rather than read off afterwards.

This is why two competent laboratories can disagree without either being wrong. If one ran 20 mm/min and the other 100 mm/min on the same knit, the faster run will report a higher force. Requiring both settings on the report converts a suspected error into an explainable difference.

What a Knit Loop Does to a Force Curve

A woven fabric under tension shows a fairly straight load-extension region followed by a break. A knitted panel does not, because the loop has geometry to give up before the yarn itself is loaded. The first part of the curve is loop deformation, the second is yarn rotation and jamming, and only the third is the yarn carrying load.

The practical outcome is high elongation. A knitted bag panel routinely reaches elongation at maximum force well above what a woven of comparable mass would, and figures of 30 to 60 percent in the course direction are normal rather than alarming. A buyer who reads that number against a woven frame of reference will conclude the fabric is weak when it is simply knit.

The second outcome is recovery. A knit that has been loaded to near its maximum force may not return fully to its original dimensions, which is why tensile testing on a bag should sit alongside dimensional stability testing to ISO 5077. Strength without recovery produces a bag that survives the load and then looks wrong.

Direction, Conditioning and Wet State

Test both directions and require both on the report. On a knitted panel the course direction generally extends more and the wale direction generally carries more force, though the exact relationship depends on the stitch structure. Specifying only one direction is common and leaves the governing one unstated.

Condition in the standard atmosphere before testing. ISO 139 sets the temperature and humidity for textile conditioning, and moisture content changes both force and elongation measurably. A report that omits conditioning may be competent, but two reports prepared differently cannot be compared.

Wet-state testing is available under ISO 13934-1 and is worth specifying for a bag that will be used outdoors, carried in the rain, or laundered. A panel that loses a significant share of its dry force when wet is a real risk for a beach or market bag, and the dry figure alone will not reveal it.

Setting the Force Target: Where the Number Comes From

There is no correct universal newton figure for a bag panel, and any table that suggests otherwise is hiding the end use. The defensible route is to derive the target from load: estimate the worst realistic load the bag will carry, apply a safety factor, and require the panel to hold that with margin in the weaker direction.

As an order-of-magnitude orientation, a knitted body panel from a 300D polyester typically reports in the several hundred newtons range by the strip method, while a base panel in a 600D yarn sits materially higher. Those bands move with gauge, stitch structure and finishing, which is precisely why they are orientation rather than a specification.

The better calibration is a bag already in the range. Test a bag the buyer sells and is satisfied with, under the same method, then set the new target at or above that result. One afternoon of testing produces a number that is defensible against a retailer's question, which a borrowed figure never is.

Tensile Versus Tear: Why ISO 13937 Often Fails on Knits

Buyers often ask for tear strength alongside tensile strength, and the reflex is ISO 13937, the Elmendorf pendulum method. It is a good standard and it is written primarily for woven fabrics. On a knitted structure the tear path follows the loop rather than running straight, and specimens frequently fail by yarn slippage instead of tearing.

The method itself anticipates this. Where a defined number of specimens show slippage rather than tearing, the result is invalid and the method is considered unsuitable for that material. On a knit this outcome is common enough that commissioning ISO 13937 as a matter of routine is often money spent on an unusable report.

Where tear propagation genuinely matters for a knitted bag, tongue or wing tear methods, or a combination of tensile to ISO 13934-1 and seam strength to ISO 13935-2, produce the more defensible picture. Both measure what actually happens at a loaded join, which is where bags fail.

Reading a Tensile Report: Seven Fields

A usable tensile report carries the method and edition, the specimen width and gauge length, the extension rate, the direction per result, the conditioning statement, the number of specimens, and the individual results rather than only a mean. Seven fields, and each one hides something when it is missing.

Individual results matter most. Five specimens at 620, 640, 650, 660 and 680 N describe a consistent fabric. Five at 380, 470, 650, 810 and 950 N describe the same mean and a very different one, and only the full list tells a buyer which fabric is being quoted.

Discard rules matter too. A specimen that breaks within a short distance of the jaw is a grip failure rather than a fabric failure and should be excluded under the method. Asking whether any specimens were discarded, and why, is a reasonable question and one that competent laboratories answer without hesitation.

Writing the Tensile Clause

The clause needs the method and edition, the specimen geometry, the extension rate, the direction, the conditioning, the minimum force, and the minimum elongation. Eight fields sounds like a lot and compresses into four lines, and every one of them prevents a conversation that would otherwise happen after the goods have shipped.

Add the construction line and the clause is complete. Requiring the tested construction to carry the same gauge and yarn count as the approved sample closes the last route by which a strong result on a different fabric can satisfy a written requirement.

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