ASTM D6797

What the test does

The moving member does the work here. A 25.4 mm polished steel ball rides on the crosshead and advances, at a rate held constant throughout, into a disc of fabric under 45 mm across that the ring clamp holds still and grips right round its rim. Forced to follow the crown of a sphere, the cloth thins and tightens until it opens. The height of the force trace at that instant is the whole result; nothing else is reported.

What it measures, and why it matters

The reported quantity is bursting force: the peak the frame reaches before the ball goes through.

Read plainly, that is a fabric's tolerance for a concentrated out-of-plane load — the elbow inside a knitted sleeve, the blunt corner met by a nonwoven liner, the minimum burst written into a protective-textile specification. The figure is biaxial by construction, and that is both its use and its limit: because no yarn direction can be loaded on its own, a burst result cannot be decomposed. It says the structure as a whole gave way at that force in that geometry, not which system went first. It is an acceptance and ranking number, not a design allowable, and does not convert to a pressure rating.

Specimen

Specimens are plain cut squares of at least 125 × 125 mm, five per laboratory sample, taken across width and length so that no two share the same yarns and none carries a crease or selvedge. Conditioning is to moisture equilibrium in the Practice D1776 standard atmosphere, 21 ± 1 °C and 65 ± 2 % relative humidity. Knits should be laid into the clamp relaxed; pre-tensioning while closing the ring biases every number that follows.

What the machine must be capable of

A constant-rate-of-extension frame is required, with the ball on the moving member and the ring clamp fixed. The distinction is not pedantry: results are stated not to be interchangeable with those from the constant-rate-of-traverse machine of the neighbouring method. Crosshead speed is 305 ± 13 mm/min (12 ± 0.5 in./min); textiles respond to rate, so that is a controlled variable, not an operator preference.

Nowhere does the method name a capacity, so sizing rests on the precision work: the averages published with the superseded edition sit near 766 N and 584 N on two woven fabrics. A low-single-digit-kN frame has ample headroom above that. Choosing the cell is the harder decision, since the reading that matters arrives at the very top of the trace, where resolution is set by the range picked before the run.

An accuracy class cannot be stated with confidence. The 2015 edition invoked no machine specification and no accuracy class at all, unlike its constant-rate-of-traverse counterpart, which cites one directly; whether the 2024 revision added one could not be confirmed, so read the current text rather than assume the counterpart's requirement carries across.

The fixture is fully specified where the frame is not: ring bore 44.450 ± 0.025 mm, ball 25.400 ± 0.005 mm, polished. Those tolerances are what make one laboratory's burst comparable with another's; alter either and the cloth follows a different curvature through a different opening. The method uses no extensometer, and the conditioned atmosphere is itself a requirement.

What goes wrong in practice

Mixing the two ball-burst methods is the commonest error, and it happens in the reporting rather than in the laboratory. A figure quoted as ball burst with no machine class named is unusable, because the two classes return different numbers on the same fabric and both documents say so.

Clamping is next. Too little, and slick or coated fabrics creep inward as the ball advances, giving a low peak on a long, soft curve; too much, and heavy fabrics are crushed at the ring, so the tear starts at the clamp line and the force belongs to the fixture. A specimen burst around the clamp rather than over the ball's crown should be repeated, not averaged in.

Concentricity is easily lost when the fixture is swapped between jobs. An off-centre ball loads one arc of the ring first, and scatter widens before anyone suspects alignment.

Related and equivalent standards

ASTM D3787 is the neighbour, and the drive separates them, not the fixture. Its constant-rate-of-traverse machine advances the clamp at a set rate while the load-measuring element deflects, so a fabric that stiffens as the ball enters is no longer strained at the rate set; the constant-rate-of-extension frame required here holds that rate regardless. Each document names the other, and both forbid exchanging the figures.

The diaphragm methods — ASTM D3786/D3786M and ISO 13938-1 hydraulically, ISO 13938-2 pneumatically — are often treated as alternatives to ball burst and are not: they load a far wider clamped area with fluid pressure and report a pressure. Choose by what the fabric meets in service, a blunt object arguing for ball burst and a pressure differential for a diaphragm method.