ISO 13938-1

What the test does

A piece of fabric is laid over an expansive rubber diaphragm and clamped round its edge by a rigid ring, leaving a known circle of it unsupported. Fluid is pumped beneath the diaphragm at a controlled rate, so the membrane swells and drives that circle into a growing dome, which rises until the fabric ruptures, abruptly, behind a safety cover. Pressure and dome height are followed throughout, both captured at the instant of burst.

What it measures, and why it matters

Two quantities are reported: bursting strength, the pressure at which the fabric gives way, and bursting distension, how far it rose before it did. Some testers report the volume pumped in instead of height.

Bursting strength is the figure of merit for fabrics not loaded along one axis in service. Knitted, nonwoven and coated fabrics are pushed out of plane by whatever sits behind them, and a multiaxial pressure test represents that better than a strip pulled in one direction. Distension is the more diagnostic result and the more often ignored: two fabrics can burst at the same pressure with one having stretched twice as far getting there, and the stretchier one bags at the knee or blows through a grid the stiffer one bridges.

Specimen

The specimen is simply a piece of fabric large enough to be clamped by the ring, taken so that no two pieces share the same threads and none carries a crease or selvedge. At least five bursts are made per laboratory sample, and any burst within 2 mm of the clamping line is discarded and replaced. Dry testing follows preconditioning and conditioning to moisture equilibrium in the ISO 139 standard atmosphere, 20 ± 2 °C and 65 ± 4 % relative humidity. Wet testing requires neither, the specimen instead being soaked for an hour in grade 3 water, and ISO 139's tropical atmosphere is permitted for it.

What the machine must be capable of

The instrument is not a load frame and measures no force: it is a hydraulic bursting tester whose capacity is quoted as pressure against a test area, so sizing starts from the fabric class. General apparel bursts well below 800 kPa, roughly the range over which hydraulic and pneumatic testers agree; the hydraulic method is chosen where speciality technical textiles need more pressure than that.

The clamping ring is the real specification. The preferred test area is 50 cm², and smaller and larger areas are also provided for; bore diameters are held to a tight tolerance, since a worn bore changes the loaded area and the answer with it. Which area was used must be reported, since the same fabric bursts at a higher pressure over a small circle than over a large one. Beneath the ring sits a rubber diaphragm up to 2 mm thick.

The method pins its own rate: volume must increase steadily at between 100 cm³/min and 500 cm³/min, within ±10 % of the indicated value, or, where volume rate cannot be set, at a rate found by trial to burst the fabric in 20 ± 5 s.

Accuracy is specified per output: pressure to ±2 % of full-scale range above the first fifth of the range, height at burst up to 70 mm to ±1 mm, volume to ±2 %. Metrological confirmation follows ISO 10012; no ISO 7500-1 force class applies, since nothing measures force, and no extensometer is used. The conditioned atmosphere and a guard over the test head are both requirements.

What goes wrong in practice

The commonest invalid result is a burst that runs into the clamp. Ruptures at the clamping line report the ring's condition rather than the fabric's, and they cluster when clamp pressure is too high, when the bore has been nicked, or when a heavy coated fabric is crushed at the edge before it ever domes.

Slippage is the opposite defect and harder to spot. Slick or heavily finished fabrics creep inward under the ring as the dome grows, so distension reads high and pressure low; drag marks outside the clamp line are the giveaway.

Diaphragms get treated as permanent. A membrane cycled for months stiffens and takes a set, and the tester drifts quietly long before the rubber splits.

Related and equivalent standards

ASTM D3786/D3786M is the cross-body equivalent for hydraulic diaphragm bursting, differing in conditioning atmosphere, test areas and reporting convention rather than in physics; figures are similar in character but should always be reported against the document that produced them.

ISO 13938-2:2019 is the sibling part, not a successor, and the more frequent confusion: it measures the same property pneumatically, the two agreeing reasonably on ordinary apparel fabrics.

ASTM D3787 and ASTM D6797 also report a bursting strength, but by forcing a clamped circle of fabric over a polished steel ball on a tensile frame; the two differ in which member moves. That result is a force, not a pressure, and the two families do not convert.