How Is Bursting Strength Tested in Knitted Fabric?

Bursting strength is tested by clamping a fabric over a circular opening and loading the exposed area until it ruptures. In diaphragm methods, hydraulic liquid or pneumatic air expands a flexible diaphragm beneath the specimen, creating pressure across the surface. In a ball-burst method, a steel ball is driven through the clamped fabric by a constant-rate-of-extension tensile tester. The reported result can be bursting pressure, bursting distension, or force, depending on the standard. Values from different methods are not directly interchangeable.

Why a multiaxial test is often used for knitted fabric

A strip tensile test pulls a narrow specimen mainly along one axis. Many knitted fabrics deform through loop rotation, extension, yarn movement, and load transfer in more than one direction when pressed by an elbow, knee, shoulder, hand, or rounded object. A burst test subjects the clamped central area to multidirectional stress and can therefore be useful for knitted, nonwoven, and other fabrics whose service deformation is not adequately described by one strip direction.

The test does not reproduce every garment event. Clamping restrains the specimen edge; the pressure field or steel ball has a defined shape; test speed and opening size are controlled; seams, fashioning, ribs, trims, body movement, repeated wear, abrasion, snagging, and fatigue may be absent. Bursting strength is one performance indicator, not a universal simulation of durability. Its value is in consistent comparison under a named method.

Knitted structures can carry load through different mechanisms. In a stable compact fabric, several loop rows may share the stress before one yarn segment breaks. In an open, highly extensible fabric, loops can enlarge and redistribute load over a large distension before rupture. A tuck, transfer, float, cable, local reduction in active needles, damaged filament, weak splice, or incipient hole can concentrate stress. Two fabrics can therefore reach a similar peak result through different deformation and failure sequences.

Hydraulic, pneumatic, and ball-burst methods create different test systems

Hydraulic diaphragm testing

ISO 13938-1 specifies a hydraulic method. The specimen is clamped over an extensible diaphragm and liquid pressure is increased at a controlled pumping rate until the fabric bursts. The instrument determines bursting pressure and bursting distension according to the method. The diaphragm itself requires correction because some pressure is needed to expand it without the specimen. Test area, clamping, diaphragm characteristics, rate, conditioning, and whether the specimen is dry or wet all form part of the result.

Pneumatic diaphragm testing

ISO 13938-2 uses compressed air beneath the diaphragm. ASTM D3786/D3786M covers hydraulic or pneumatic diaphragm bursting-strength testers for textiles and is widely used for knitted and nonwoven fabrics as well as suitable woven fabrics. Pneumatic and hydraulic results can agree within stated ranges and conditions, but the standards do not license an assumption that every machine, material, pressure range, and laboratory is equivalent. A change of instrument or medium should be verified with comparative material.

Constant-rate-of-extension ball burst

ASTM D6797 specifies a ball-burst procedure using a constant-rate-of-extension, or CRE, testing machine. A polished steel ball advances through a fabric clamped in a ring, and the peak force required to cause rupture is recorded. This makes use of a tensile testing platform but produces a multiaxial puncture-like deformation. Ball diameter, support fixture, clear opening, speed, preload, and specimen preparation are method variables. The result is force; it cannot be converted into diaphragm bursting pressure through a simple universal factor.

A specification should select a method before setting a limit. “Burst strength: 500” is incomplete without standard, edition, units, apparatus, test area, specimen state, and statistical basis. If a customer protocol modifies a standard method, the deviation should be explicit. Historical results can remain useful only when their procedure is traceable.

Specimen control begins before the fabric enters the tester

Sampling should reflect the material being accepted. For yardage, specimens are distributed across relevant width and length while avoiding folds, creases, obvious handling damage, and normally excluded selvedges unless the protocol states otherwise. Repeated specimens should not overlap the same yarn paths more than necessary. For shaped panels or garments, location is critical: plain body, intarsia boundary, fashioning line, elbow, underarm, pocket, rib transition, and seam-adjacent zones are not structurally equivalent.

Conditioning and state must be controlled. ISO 139 provides a standard atmosphere for textile conditioning and testing. Some methods permit testing in a conditioned or wet state; the report must identify which. Fibre moisture, residual finish, recent pressing, washing, tumble drying, or thermal setting can change yarn strength, friction, loop mobility, and specimen dimensions. A wet result cannot be substituted for a conditioned-dry result simply because both use the same machine.

The operator checks the diaphragm, clamps, pressure system or CRE load cell, fixture, calibration status, and appropriate measurement range. The specimen is mounted flat without intentional prestretch, wrinkles, trapped folds, or slippage. Clamping pressure must prevent movement without cutting or weakening the edge. A rupture at or immediately beside the clamp may indicate jaw damage or an invalid stress concentration and is treated according to the chosen standard, not accepted automatically as fabric performance.

Several specimens are tested and individual results retained. The report includes method and edition, apparatus and medium, test area or fixture, rate, conditioning, wet or dry state, specimen locations, number tested, units, mean, variability, any rejected results, and observed failure mode. Photographs can be useful when rupture location, yarn pullout, laddering, loop enlargement, coating separation, or clamp effects help explain the number.

A high number is not the complete performance brief

Bursting performance depends on fibre strength and elongation, yarn twist and ply, linear density, number of ends, yarn uniformity, filament damage, stitch length, loop density, structure, plating, reinforcement, finishing, moisture, and relaxation. Needle damage or recurrent needle-line faults can establish a weak path. A coating may restrict loop movement and raise stiffness while changing the way rupture initiates. Washing may either relax and redistribute loops or weaken a fibre, finish, or junction.

Higher bursting strength is not automatically better design. Increasing yarn mass, shortening loop length, adding ends, bonding layers, or tightening the fabric may raise the result while reducing drape, softness, breathability, extension, or recovery. The target should correspond to garment zone, expected load, structure, material, and care route. A sheer engineered knit and a compact base layer should not share a limit merely because both are knitted.

Distension and failure mode help distinguish fabrics with similar peak values. One fabric may withstand pressure through controlled extension; another may resist movement until an abrupt yarn break. If only peak pressure or force is preserved, that difference is lost. Variability also matters. A strong mean with one very low specimen can indicate yarn damage, a local construction fault, or unrepresentative sampling that deserves investigation.

Comparisons must be like-for-like: same standard and edition, machine configuration, test area, rate, conditioning, fabric state, finishing history, and sampling logic. ASTM D3786 notes that interlaboratory differences can require comparative testing because bias may exist between laboratories. For supplier transfer or factory change, testing matched material in both facilities is more defensible than assuming apparatus equivalence.

Performance requirements belong in a broader technical file with yarn identity, machine and gauge, structure, stitch length, finished dimensions, mass per unit area, thickness where relevant, stretch and recovery, shrinkage, pilling or abrasion requirements, care route, and visual standard. The Process page connects such testing to sampling and production handoff. Construction examples appear in Work; Anne's role is outlined in About, and technical project information can be requested through the contact form.

Common questions about bursting strength

The method is part of the result

A burst number becomes transferable only when pressure or force, apparatus, test area, rate, specimen state, sampling, and failure evidence travel with it. That discipline prevents a single peak value from obscuring the structural choices and local faults that determine how a knitted surface actually carries load.