A tensile test pulls a fabric in one direction. Knitted fabrics, nonwovens and laminates are rarely stressed that way in use — a knee or an elbow pushes outwards in every direction at once. That is what a bursting test reproduces: the specimen is clamped over a rubber diaphragm, the diaphragm is inflated, and the pressure at rupture is recorded.
The diaphragm can be inflated by liquid or by air. Which one you use, and how you correct for the diaphragm itself, decides whether your figure can be compared with anyone else's.
Three documents, two principles
| Standard | Pressure medium | Notes |
|---|---|---|
| ISO 13938-1 | Hydraulic (liquid, constant-rate pumping) | Strength and distension |
| ISO 13938-2 | Pneumatic (compressed air) | Strength and distension |
| ASTM D3786 | Hydraulic or pneumatic diaphragm tester | Bursting strength |
Both ISO parts were revised in 2019 and cover knitted, woven, nonwoven and laminated fabrics, tested conditioned or wet. ASTM D3786 accepts either type of diaphragm tester within a single method.
When hydraulic and pneumatic agree
The ISO scope gives a useful rule: there appears to be no significant difference between hydraulic and pneumatic results for pressures up to 800 kPa. That range covers most general apparel. For speciality textiles that need higher bursting pressures, the standard considers the hydraulic apparatus more suitable.
In practice this means:
- For T-shirt jerseys, underwear and most fashion knits, either principle will give comparable numbers — provided the other conditions match.
- For technical fabrics, coated or laminated materials and heavy knits that burst well above 800 kPa, choose hydraulic. A pneumatic system storing a large volume of compressed air also releases that energy suddenly at rupture, which is a handling consideration in its own right.
Correction 1: the diaphragm is not free
Part of the pressure you read is spent stretching the rubber diaphragm, not the fabric. The methods require this to be corrected for: the pressure needed to inflate the diaphragm alone, without a specimen, to the same distension is subtracted from the total bursting pressure.
The common mistakes:
- No correction at all. On a light knit the diaphragm share is a meaningful fraction of the result. Uncorrected figures look stronger than they are.
- One correction value used forever. Rubber ages, hardens and thins. A correction determined when the diaphragm was new is wrong six months later. Re-determine it on the schedule the standard sets and whenever the diaphragm is replaced.
- Correction at the wrong distension. The diaphragm pressure must be taken at the distension the specimen actually reached at burst, not a fixed value.
Most current instruments run the correction automatically, but someone still has to trigger the blank run. Make it part of the start-of-day routine.
Correction 2: the test area
Bursting pressure depends on the area of fabric under load. ISO 13938-1 allows several test areas, with 50 cm² used as the common reference; ASTM D3786 is typically run on a much smaller test area.
A kPa value without its test area is incomplete. Two labs using different clamping rings can report different pressures for the same fabric, and neither is wrong. Put the test area on every report and in every specification limit. If a customer specification gives a limit without an area, ask before testing.
Correction 3: the rate
A bursting test is fast, and rate matters. ISO 13938 controls the rate at which the volume increases, or — on testers without volume regulation — the time to burst. ASTM D3786 has its own rate requirement. Running faster than specified usually gives higher pressures.
If your instrument allows rate adjustment, set it from the standard, write it on the set-up sheet, and lock it if the software allows. Rate drift between operators is a classic source of week-to-week scatter.
Other places results go wrong
Clamp slippage. If the specimen draws in from under the clamping ring, the distension increases and the burst does not occur where it should. Bursts that start at the clamp edge are not valid; record and exclude them as the standard requires.
Wet testing. Both ISO parts allow testing in the wet state. A wet result is a different property from a conditioned one and must be labelled as such. Specimens for conditioned tests should be brought to the standard atmosphere of ISO 139.
Distension ignored. ISO 13938 reports bursting distension as well as strength. Two fabrics with the same bursting pressure can differ greatly in how far they bulge before failure — a relevant quality signal for knits that bag at the knee. If your instrument measures it, report it.
Diaphragm or ball?
Ball bursting (ASTM D3787, ISO 9073-5 for nonwovens) pushes a polished steel ball through the fabric on a mechanical tester. It is a separate family of methods with its own results, and there is no conversion from ball burst force to diaphragm pressure. If a specification says "bursting strength" without naming a method, ask which principle it means.
Before you report
- ISO 13938-1, ISO 13938-2 or ASTM D3786 — and does the expected pressure exceed 800 kPa?
- Was the diaphragm correction determined recently, at the correct distension?
- Is the test area stated?
- Was the rate set from the standard, not from habit?
- Were clamp-edge bursts excluded?
- Conditioned or wet — and is that on the report?
Our hydraulic bursting tester works up to 7000 kPa with a standard 50 cm² test head, which comfortably covers the high-pressure range where the hydraulic method is preferred. It is manufactured in our own workshop in Istanbul.