Bursting strength testing is a fundamental quality control procedure for apparel fabrics, evaluating how well a textile withstands multi-directional forces applied perpendicular to its surface until rupture occurs. Particularly essential for knitted fabrics used in underwear, T-shirts, sportswear, and other next-to-skin garments, this test is governed by internationally recognized standards such as ISO 13938, ASTM D3786, and GB/T 7742. This article explains the technical principles, sample requirements, testing methods, result interpretation, and practical applications of fabric bursting strength testing.
Technical Overview
Bursting strength is defined as the maximum force or pressure a fabric can resist when a load is applied perpendicular to the fabric plane and distributed over a defined circular area until the specimen ruptures. Unlike uniaxial tensile testing, which evaluates strength along a single direction such as warp or weft, bursting strength simulates the complex multi-directional stresses that garments experience during actual wear, for example at the elbows, knees, seat, and shoulders.
For knitted fabrics, whose loop structure allows deformation in all directions, bursting strength is widely regarded as the most representative mechanical performance index. It is specified as a mandatory requirement in numerous national and industry product standards for knitted apparel, and it is routinely verified by accredited third-party laboratories during product certification and trade acceptance.
Why Bursting Strength Matters
- It evaluates overall fabric durability under realistic, multi-directional loading conditions rather than a single direction.
- It supports compliance with product standards, buyer specifications, and regulatory requirements for knitted garments.
- It reveals weaknesses caused by inferior yarn quality, insufficient fabric density, poor loop formation, or damaging finishing processes.
- It is applicable to fabrics, such as fine knits and nonwovens, for which strip tensile methods are difficult to perform or less meaningful.
Governing Standards
- ISO 13938-1: Textiles — Bursting properties of fabrics — Part 1: Hydraulic method for determination of bursting strength and bursting distension.
- ISO 13938-2: Textiles — Bursting properties of fabrics — Part 2: Pneumatic method.
- ASTM D3786: Standard Test Method for Hydraulic Bursting Strength of Textile Fabrics Using Diaphragm Bursting Strength Tester.
- ASTM D3787: Standard Test Method for Bursting Strength of Fabrics — Constant-Rate-of-Traverse (CRT) Ball Burst Test.
- GB/T 7742.1 and GB/T 7742.2: Chinese national standards corresponding to ISO 13938-1 and ISO 13938-2.
- GB/T 19976: Determination of bursting strength of knitted fabrics by the steel ball method.
Testing Samples
Knitted Fabrics
Weft-knitted and warp-knitted fabrics are the primary sample type for bursting strength testing. Typical examples include single jersey, rib, interlock, and fleece fabrics made from cotton, polyester, nylon, modal, or blended yarns, which are used for underwear, T-shirts, hoodies, leggings, and sportswear. Because knitted fabrics stretch readily in every direction, a bursting test reflects their in-use behavior far better than a directional tensile test.
Woven and Nonwoven Fabrics
Certain woven fabrics with balanced constructions, as well as nonwoven materials such as interlinings, medical dressings, filtration media, and hygiene textiles, are also evaluated by bursting strength when their end use involves distributed perpendicular loading. For most conventional woven apparel fabrics, however, strip tensile and tear strength tests remain the dominant methods.
Sample Preparation and Conditioning
Specimens should be taken from representative positions across the full width of the fabric, avoiding selvedges, fabric edges, creases, wrinkles, and visible defects. At least five valid specimens are required for each laboratory sample under most standards. Specimens are cut large enough to cover the clamping device completely, commonly 100 mm × 100 mm or equivalent circular pieces. Before testing, samples must be conditioned in a standard atmosphere of 20 ± 2 °C and 65 ± 4 % relative humidity in accordance with ISO 139 (or the equivalent ASTM atmosphere) until moisture equilibrium is reached, typically for a minimum of 24 hours.
Testing Items
Depending on the method selected, the following parameters are measured and reported:
- Bursting strength expressed in newtons (N), obtained from the steel ball or ball burst method.
- Bursting pressure expressed in kilopascals (kPa), obtained from the hydraulic or pneumatic diaphragm method.
- Bursting distension (expansion height) expressed in millimeters (mm), describing how far the fabric deforms before rupture.
- Diaphragm correction value, determined by pre-testing the diaphragm without a specimen so its own resistance can be subtracted from the total pressure.
- Statistical results, including the arithmetic mean, range, and coefficient of variation of all valid specimens.
- Optional evaluations after domestic laundering cycles, for example following ISO 6330 or AATCC procedures, to assess strength retention after repeated washing.
Testing Methods
Hydraulic Diaphragm Method (ISO 13938-1, GB/T 7742.1, ASTM D3786)
A circular specimen is securely clamped over an elastic rubber diaphragm. Hydraulic fluid pressure is then increased at a controlled, steady rate, causing the diaphragm and specimen to distend together until the fabric ruptures. The maximum pressure at rupture is recorded, and the true bursting pressure is calculated by subtracting the diaphragm correction pressure. ISO 13938-1 and GB/T 7742.1 commonly employ a test area of 50 cm², while ASTM D3786 uses a smaller aperture of approximately 31 mm in diameter, which is one reason results between these standards are not directly interchangeable.
Pneumatic Method (ISO 13938-2, GB/T 7742.2)
The pneumatic method follows the same clamping and distension principle, but compressed air is used instead of liquid to inflate the diaphragm. It offers clean operation without fluid contamination of the specimen, which is advantageous for fabrics that must remain dry, such as those intended for subsequent chemical analysis or certain technical applications.
Steel Ball Method (GB/T 19976, ASTM D3787)
In the steel ball method, a polished steel ball of 25 mm diameter is driven through a specimen clamped in a ring with a 45 mm internal diameter at a constant rate of traverse of 100 mm/min per GB/T 19976 until the fabric ruptures. The maximum force recorded, expressed in newtons, is the bursting strength. This method is particularly common in the verification of Chinese national product standards for knitted apparel and yields results familiar to most domestic and many international buyers.
General Test Procedure
- Verify instrument calibration and select the specified test area, clamping pressure, and loading rate.
- For diaphragm methods, determine the diaphragm resistance without a specimen and record it as the correction value.
- Mount the specimen flat and free of tension or distortion, then securely clamp it to prevent slippage during testing.
- Apply the load until rupture occurs and record the maximum force or pressure, along with the distension value where required.
- Repeat the test for all specimens, discarding results affected by specimen slippage or rupture at the clamp edge in accordance with the standard.
- Calculate the arithmetic mean and, where applicable, apply the diaphragm correction before reporting.
Result Interpretation
Results must always be reported together with the test method, standard designation, test area, and instrument type, because bursting values obtained by different methods and apertures are not directly comparable. A fabric deemed acceptable under the steel ball method in newtons cannot be judged against a hydraulic result in kilopascals without a documented correlation, which is why specifications should always name the exact test standard.
Applications
- Quality control and incoming fabric inspection at garment manufacturers, ensuring rolls meet internal strength limits before cutting.
- Compliance verification for knitted apparel product standards, such as those covering knitted underwear, knitted T-shirts, and knitted sportswear, which commonly specify minimum bursting strength values in the approximate range of 135 N to 240 N depending on product category and fiber content.
- New product development and benchmarking, comparing yarn counts, loop structures, finishes, and fabric weights for strength optimization.
- Trade settlement and buyer acceptance, providing an objective, standardized basis for resolving quality disputes between suppliers and purchasers.
- Failure analysis of returned or burst garments, identifying whether the root cause lies in raw material selection, knitting parameters, or wet processing damage.
- Certification support, where manufacturers submit samples to accredited third-party laboratories for independent testing reports required by retailers, marketplaces, or regulatory bodies.
- Wash durability studies, measuring strength retention after repeated laundering cycles to validate care label claims.
FAQ
Why is bursting strength preferred over tensile strength for knitted fabrics?
Knitted fabrics deform in all directions because of their loop construction, so a unidirectional tensile test can underestimate or misrepresent their real performance. The bursting test applies load simultaneously along every in-plane direction, closely mimicking how stress is distributed when a garment is stretched over the body, snagged, or pressed at a point.
Which method should be chosen: hydraulic diaphragm or steel ball?
The choice depends on the governing product specification and the target market. Chinese knitted apparel standards frequently reference the steel ball method of GB/T 19976, whereas many international buyers and ISO-based specifications call for the hydraulic method of ISO 13938-1 or ASTM D3786. When a specification names a specific standard, that standard must be followed exactly.
Are results from different bursting methods interchangeable?
No. The steel ball method reports force in newtons while diaphragm methods report pressure in kilopascals, and test areas, loading geometries, and rates differ between standards. Results should only be compared within the same method and test conditions unless a validated correlation has been established.
How many specimens are needed, and what causes a result to be discarded?
At least five valid specimens are typically required. A specimen is discarded and replaced when it slips in the clamps, ruptures within the clamped edge zone, or fails due to an obvious pre-existing flaw, provided the standard permits such exclusion and the event is documented.
What factors influence bursting strength results?
Key factors include yarn tenacity and twist, fabric grammage and density, loop structure, fiber blend ratio, finishing and dyeing damage, conditioning atmosphere, clamping pressure, test area size, and loading rate. Consistent instrument settings and proper conditioning are essential for reproducible results.
Can bursting strength be tested after washing?
Yes. Fabrics can be laundered for a defined number of cycles under ISO 6330, AATCC, or GB/T standard washing procedures and then conditioned and tested again. Comparing bursting strength before and after laundering is a common way to demonstrate wash durability for quality claims and product development purposes.
