Burning length testing measures the extent of flame-induced damage on a material specimen after exposure to a standardized ignition source, providing a quantitative index of flammability and flame resistance. It is a core fire-safety evaluation for textiles, plastics, foams, and composite materials used in aviation, rail, automotive, and consumer products. This article explains the test principle, sample requirements, measured parameters, standard test methods, and typical acceptance criteria applied by accredited third-party laboratories.
Technical Overview
Burning length—also referred to as damaged length or char length depending on the material and standard—is defined as the distance from the edge or point of flame application to the farthest evidence of damage on a test specimen. Damage includes partial or complete consumption, charring, embrittlement, or melting caused by flame impingement, measured after combustion has ceased and the specimen has cooled.
The test principle is straightforward: a specimen of specified dimensions is mounted in a defined orientation (typically vertical, horizontal, or at 45 degrees) inside a ventilated combustion chamber and exposed to a calibrated gas burner flame for a fixed duration. After the ignition source is removed, the after-flame time and afterglow time are recorded, and the specimen is subsequently examined to determine the damaged or burning length. Because the result is a physical distance expressed in millimeters (or inches), burning length provides a direct, reproducible measure of how far a material self-propagates flame damage.
Burning length is frequently the decisive pass/fail parameter in regulatory schemes. For example, aircraft interior materials tested under the vertical method of FAR 25.853 (Appendix F, Part I) must not exceed a burn length of 152 mm (6 inches). Textile standards such as ASTM D6413, ISO 15025, and GB/T 5455 use damaged length together with after-flame time to classify flame-retardant performance. Accredited third-party laboratories routinely perform these tests for certification, quality control, and product development purposes.
Testing Samples
Textile Fabrics and Apparel Materials
Woven, knitted, and nonwoven fabrics—including protective clothing textiles, workwear, curtains, and bedding—are the most common samples. Typical specimens measure 300 mm × 76 mm (ASTM D6413) or 300 mm × 80 mm (GB/T 5455) and are cut separately in the lengthwise and widthwise directions because flame propagation can differ significantly between warp and weft.
Plastic Materials and Molded Components
Rigid plastics, engineering resins, and finished molded parts are tested under UL 94, IEC 60695-11-10, and GB/T 2408. Vertical specimens are commonly 125 mm × 13 mm bars, while horizontal burning (HB) tests measure the rate of flame spread between 25 mm and 100 mm gauge marks, from which the effective burning distance is derived.
Flexible Foams and Padding Materials
Seat cushions, upholstery foams, and padding materials are evaluated for burn length and mass loss because foam is often the primary fuel load in seating assemblies. Aviation standards require full-scale burner exposure of seat constructions, while component-level tests provide screening data during material selection.
Aircraft, Rail, and Marine Interior Materials
Wall and ceiling panels, partitions, thermoset composites, elastomers, and decorative laminates used in transport interiors are subjected to 12-second or 60-second vertical flame exposure per FAR 25.853 and analogous European (EN) specifications. Burn length limits are strict because escape time in a cabin fire depends directly on flame spread over interior surfaces.
Coated Fabrics, Films, and Tarpaulin Materials
PVC-coated fabrics, tent materials, awnings, and industrial sheeting are tested under ISO 15025 and JIS L 1091-type methods. The damaged length after surface or edge ignition indicates whether the coating and substrate provide adequate flame retardance for the intended application.
Testing Items
A burning length test report typically includes the following measured parameters, evaluated for each specimen and reported as individual values and arithmetic means:
- Burning length (damaged length): the distance, in millimeters, from the original edge exposed to the flame to the farthest point of consumption, charring, or embrittlement.
- After-flame time: the duration, in seconds, for which the specimen continues to flame after removal of the ignition source.
- Afterglow time: the duration, in seconds, of glowing combustion without flame after flaming ceases.
- Ignition behavior: whether the specimen ignites, whether ignition occurs at the surface or the cut edge, and whether flaming is sustained.
- Flaming drips and debris: whether burning fragments fall and whether they ignite a layer of absorbent surgical cotton or indicator paper placed beneath the specimen.
- Flame spread rate or spread time: for horizontal and 45-degree methods, the time or speed at which the flame front travels a defined gauge distance.
- Char characteristics: the nature and extent of the charred zone, including whether the damaged boundary is clean enough to permit reliable length measurement.
Results are normally reported to the nearest millimeter (or 1 second for time values) and interpreted against the pass/fail thresholds of the applicable standard.
Testing Methods
Specimen Preparation and Conditioning
Specimens are cut to the exact dimensions required by the standard, with clean edges and no fraying or delamination that could influence flame behavior. Textile specimens are conditioned in a standard atmosphere—such as 20 ± 2 °C and 65 ± 4 % relative humidity per ISO 139 or the equivalent atmosphere defined in ASTM D1776—for a minimum of 24 hours before testing. Plastic specimens under UL 94 and IEC 60695-11-10 are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity, with additional dry-oven conditioning for hygroscopic materials. A sufficient number of specimens—commonly five per direction for textiles, or five bars for plastics—is prepared so that statistical evaluation and retesting rules can be applied.
Vertical Burning Method
In the vertical test, the specimen is clamped in a frame with its long axis vertical and its lower edge positioned a defined distance above a gas burner. A methane, propane, or natural gas flame—typically 40 mm (GB/T 5455) or approximately 38 mm (ASTM D6413) high—is applied to the bottom edge for a specified time, usually 12 seconds. After the burner is withdrawn, the laboratory records after-flame and afterglow times and observes any flaming drips. The vertical orientation is the most severe configuration because hot combustion gases rise along the specimen and preheat unburned material, so it is used wherever strict flame resistance must be demonstrated, including aviation interiors and flame-retardant protective textiles.
Horizontal Burning Method
For the horizontal method, the specimen is clamped with its long axis horizontal and the burner flame is applied to one free end. Gauge marks at 25 mm and 100 mm allow the technician to record the time at which the flame front passes each mark, yielding a linear burning rate in millimeters per minute. This method applies to plastics under UL 94 (HB rating) and GB/T 2408 and to automotive interior trim materials, where regulations limit the horizontal burn rate of occupant-compartment components. The measured distance burned between the gauge marks is the key length-based outcome.
45-Degree Ignition Method
Specimens mounted at 45 degrees are exposed to a flame applied at the surface or near the lower edge. This geometry, used in apparel flammability regulations such as 16 CFR Part 1610 and in related ASTM procedures, records the time required for the flame to spread a defined distance along the inclined surface. It evaluates the tendency of clothing fabrics to propagate surface flash, with minimum spread times defining acceptable classification levels.
Measurement of Burning Length
After the specimen has cooled, loose charred residue and carbonized threads are gently removed, usually with brushing or by lightly stripping debris, so the true boundary of damage becomes visible. The length is then measured along the longitudinal axis from the original specimen edge (typically the lower edge in vertical tests) to the apex of the damaged region. Where the damaged boundary is irregular, textile standards employ a tear-propagation technique: the specimen is folded along the damaged line, a hooked weight or controlled tear is applied at a specified offset below the boundary, and the fabric is torn upward along the damage line; the distance to the highest point of the tear is recorded as the damaged length. Measurements are taken to the nearest millimeter, and the mean of the required number of specimens is calculated.
Typical Acceptance Criteria
- Aviation interiors (12-second vertical, FAR 25.853): average burn length not exceeding 152 mm (6 inches), average flame time after burner removal not exceeding 15 seconds, and flaming drips not exceeding 3 seconds.
- Flame-retardant textiles (ASTM D6413, GB/T 5455): damaged or char length within the limit set by the product specification or classification grade, together with defined after-flame and afterglow time limits.
- Plastics (UL 94 / IEC 60695-11-10, HB class): flame front extinguishing before the 100 mm gauge mark, or a burning rate within the specified maximum for the material thickness.
- Automotive interiors: horizontal burning rate not exceeding the regulatory maximum specified for occupant-compartment materials.
Applications
Burning length testing supports material qualification, regulatory compliance, and product safety across many industries:
- Aviation: qualification of cabin panels, seat dress covers, insulation, ducting, and cargo liner materials against interior flammability requirements.
- Rail and mass transit: fire-performance assessment of interior fittings and seating materials under European EN fire-safety frameworks for railway applications.
- Automotive: verification that headliners, door trims, seat fabrics, and instrument panel materials meet burn-rate and damaged-length limits.
- Personal protective equipment: evaluation of flame-retardant workwear and industrial garments where damaged length after flame exposure correlates with predicted skin burn injury.
- Home furnishings and contract textiles: assessment of upholstery, drapery, and bedding fabrics for flame-safety labeling and market-access requirements.
- Electrical and electronic products: classification of enclosure plastics and insulating parts under UL 94 and IEC 60695 fire-hazard testing series.
- Tents, awnings, and architectural membranes: demonstration of limited flame spread for temporary structures and public venues.
FAQ
What is the difference between burning length, char length, and after-flame time?
Burning length (damaged length) is a distance—the farthest extent of visible damage from the flame-application point. Char length is the equivalent term used primarily in textile vertical tests such as ASTM D6413. After-flame time is a duration—the number of seconds the specimen continues to burn after the ignition source is removed. Standards typically require both parameters to be within limits for a material to pass.
Why are specimens tested in both lengthwise and widthwise directions?
Flame propagation in fabrics and anisotropic materials depends on fiber orientation, yarn structure, and coating distribution. Testing both directions ensures that the slower-burning direction is not used alone to hide a fast-burning direction, and regulatory classifications consider the worse-performing orientation.
How many specimens are required for a burning length test?
Most textile standards require five specimens per direction (ten in total), while plastics tests under UL 94 typically require five molded bars per condition. If results are inconsistent or near the limit, standards define additional retest procedures and mean-value calculations.
Can washing or dry cleaning affect burning length results?
Yes. Many flame-retardant finishes are durable only to a limited number of laundering cycles. Product standards often require specimens to be tested after a specified number of wash or dry-cleaning cycles so that the reported damaged length reflects performance over the intended service life.
What flame source is used, and does the gas type matter?
Tests use laboratory burners fueled by methane, propane, or natural gas, with the flame height calibrated to the standard's tolerance (for example, 40 mm in GB/T 5455). Because gas composition affects flame temperature and shape, standards specify acceptable fuels and calibration procedures to keep results comparable between laboratories.
How should a sample be submitted for testing?
Submit sufficient material to cut the required number of specimens with trim allowance, ideally at least 0.5 m × 1 m of fabric or an equivalent panel or set of molded parts, along with the target standard, material description, thickness or grammage, and any required preconditioning such as laundering cycles. This allows the laboratory to complete specimen preparation, conditioning, and testing without delays.