Determination of Damaged Length in Vertical Burning Tests: Principles, Standards, and Procedures

The determination of damaged length in vertical burning tests is a fundamental flammability measurement used to quantify how far flame-induced damage propagates through a material after a controlled ignition source is removed. This article explains the testing principle, relevant standards such as GB/T 5455, ASTM D6413, ISO 15025, and UL 94, specimen requirements, measurement procedures, and typical industrial applications, serving as a practical reference for manufacturers, quality engineers, and accredited third-party laboratories.

Technical Overview

The damaged length, also referred to as char length in some standards, is the maximum extent of the region of a test specimen that has been charred, melted, or otherwise thermally degraded following exposure to a standardized flame in a vertical orientation. During the test, a rectangular specimen is suspended vertically inside a ventilated combustion chamber, and a calibrated gas burner flame is applied to the lower edge or the surface of the specimen for a precisely defined period. After the ignition source is withdrawn, the specimen is observed to record the afterflame time, the afterglow time, and any flaming molten drips, after which the damaged length is measured along the length of the specimen.

Damaged length is one of the most widely reported flammability indicators because it directly reflects the propensity of a material to propagate flame. A shorter damaged length indicates that combustion or thermal degradation remained confined to the immediate area of flame impingement, which is the desired behavior for flame-retardant and protective materials. Conversely, a long damaged length, or damage extending to the upper edge of the specimen, signals poor flame resistance and potential failure against product-specific acceptance criteria.

• Damaged length quantifies the vertical propagation of thermal damage and is expressed in millimeters.
• It is evaluated together with afterflame time and afterglow time to give a complete picture of burning behavior.
• Principal standards include GB/T 5455 for textiles, ASTM D6413 for textile flame resistance, ISO 15025 for protective clothing, JIS L 1091 for Japanese textile requirements, UL 94 and IEC 60695-11-10 for polymeric materials, and EN ISO 11612 or NFPA 2112 as end-product performance specifications.
• The test is destructive, relatively fast, and well suited to routine quality control as well as certification testing.

Testing Samples

Textile Fabrics

Woven, knitted, and nonwoven fabrics are the most common subjects of damaged length determination. Typical examples include flame-retardant finished cotton fabrics, inherently flame-resistant aramid and modacrylic fabrics, workwear and protective clothing materials, curtains and drapery fabrics, bedding textiles, tent and awning fabrics, and children's sleepwear materials. For GB/T 5455 and ASTM D6413, specimens are customarily cut to approximately 300 mm in length and 80 mm in width, with the long dimension corresponding to the warp or weft direction being evaluated.

Coated, Laminated, and Composite Materials

Coated fabrics, laminates, and multi-layer assemblies used in transportation interiors, military equipment, and industrial applications can also be evaluated. For such materials, the direction of the face and back surfaces, the layer orientation, and any seams or trim should be documented, because these factors can strongly influence the measured damaged length.

Plastic and Polymeric Materials

Rigid and flexible plastics are evaluated using the vertical burning configuration of UL 94 and IEC 60695-11-10, in which smaller bar-shaped specimens are subjected to two successive flame applications. Although these standards emphasize afterflame time, flaming drips, and burn-up distance rather than a numeric damaged length, the same principle of measuring the extent of flame damage along the specimen applies.

Sampling and Conditioning

Specimens must be representative of production material and free from creases and contaminants. Depending on the governing standard, three to ten replicates are usually tested for each fabric direction, each surface, and each test condition. Prior to testing, specimens are conditioned in a standard atmosphere, typically 20 ± 2 °C and 65 ± 4 % relative humidity for GB/T 5455 or the equivalent conditioning atmosphere specified in ASTM D1776 for ASTM D6413, for a minimum of 24 hours. Some specifications additionally require testing after a defined laundering or dry-cleaning cycle to assess the durability of flame-retardant treatments.

Testing Items

A vertical burning test for damaged length determination typically reports the following parameters:

• Damaged length: the maximum distance, in millimeters, from the edge of the specimen exposed to the flame to the furthest point of charred, melted, or embrittled material, measured after removal of loosened char.
• Afterflame time: the time, in seconds, during which the specimen continues to burn with a visible flame after removal of the ignition source.
• Afterglow time: the time, in seconds, during which glowing combustion without a visible flame persists after flaming ceases.
• Flaming debris and molten drips: whether burning fragments or flaming drops detach from the specimen, and whether they ignite an indicator material placed below.
• Surface flash: the occurrence of rapid flame spread across the specimen surface without penetration through the thickness.
• Extent of damage relative to the holder: whether flame or damage reaches the upper edge, clamp, or securing pins of the specimen holder.

Testing Methods

Apparatus

The test is performed in a dedicated vertical burning chamber or ventilated fume enclosure that provides controlled airflow while protecting the specimen from external drafts. Key components include a specimen holder or clamp that suspends the specimen vertically, a laboratory gas burner supplied with a specified fuel gas such as methane, propane, or butane, gas flow and pressure controls to stabilize the flame height, a timing device accurate to at least 0.1 s or 0.2 s, a ruler or scale graduated in millimeters, and auxiliary items such as absorbent cotton indicator pads and marking threads where the standard requires them. Interior chamber lighting and dark observation conditions may be needed to judge the end of afterglow accurately.

Specimen Mounting

Each conditioned specimen is secured in the holder so that the marked length direction is vertical and the lower edge is positioned at the specified distance from the burner barrel, typically around 20 mm. The burner is adjusted so that the flame impinges on the center of the lower edge for edge ignition, or is angled against the specimen face for surface ignition as defined in ISO 15025. Flame height is verified with a ruler before each series; for example, 40 ± 2 mm in GB/T 5455 and 38 mm in ASTM D6413.

Test Procedure

• Verify chamber ventilation, burner alignment, and flame height in accordance with the governing standard.
• Mount the conditioned specimen and close the chamber.
• Apply the ignition flame to the specimen for the specified contact time, commonly 12 s in GB/T 5455 and ASTM D6413, or two successive 10 s applications in the UL 94 vertical procedure.
• Withdraw the burner and immediately start timing the afterflame; record the time at which flaming ceases.
• Continue observing and record the afterglow time, flaming drips, surface flash, and whether damage reaches the holder or upper edge.
• Extinguish any residual smoldering safely, commonly by moistening the damaged area, before handling the specimen.

Measurement of Damaged Length

After the specimen has cooled and any residual combustion has been extinguished, the damaged length is determined. The accepted technique, described in both GB/T 5455 and ASTM D6413, is the tear-removal method: the specimen is laid flat, folded through the midpoint of the damaged region, and the embrittled or charred material is carefully removed by tearing or gentle mechanical action along the line of maximum damage. The damaged length is then measured from the original lower edge of the specimen to the apex of the tear, that is, the furthest point at which the material retains sufficient integrity to resist tearing. Measurements are made to the nearest millimeter. For materials that melt or shrink rather than char, the damaged length is taken as the extent of visible melting, distortion, or hole formation, and the observation basis is stated in the report.

Expression of Results

Individual and averaged damaged length values are reported for each test direction, surface, and conditioning state, together with afterflame times, afterglow times, and all auxiliary observations. Results are compared against the acceptance criteria of the applicable product standard; for instance, protective clothing fabrics evaluated under NFPA 2112 with ASTM D6413 are limited to a maximum char length of approximately 100 mm, while the U.S. children's sleepwear regulations in 16 CFR Parts 1615 and 1616 impose strict average and individual char length limits on ten specimens. Because of the statistical nature of flammability testing, testing multiple specimens and reporting the full dataset is essential for defensible conformity assessment.

Applications

Damaged length determination in the vertical orientation underpins flammability conformity assessment across many industries:

• Protective clothing: verification of flame spread limits for firefighting garments, industrial workwear, and arc-rated clothing against EN ISO 11612, NFPA 2112, and related specifications.
• Children's sleepwear: demonstrating compliance with the char length requirements of 16 CFR Parts 1615 and 1616.
• Home and contract furnishings: evaluation of curtains, draperies, upholstery, and bedding fabrics for public buildings, hotels, and hospitals.
• Transportation interiors: screening of seat coverings, wall linings, and insulation materials for aircraft, rail, and road vehicles under applicable aviation and transport regulations.
• Plastics and electrical equipment: classification of polymeric materials into UL 94 V-0, V-1, or V-2 categories for enclosures and components.
• Flame retardant development: comparison of damaged length before and after treatment, and after repeated laundering, to optimize formulations and demonstrate durability.
• Quality control: routine production monitoring by manufacturers and verification testing by accredited third-party laboratories supporting market access and certification.

FAQ

What is the difference between damaged length and char length?

The two terms describe the same fundamental quantity, namely the maximum extent of flame-induced damage measured along the specimen. Char length is the terminology used in ASTM D6413 and the U.S. regulatory framework, while damaged length is the terminology used in GB/T 5455 and much of the international textile community. Both are measured after removing loosened char from the damaged region.

Why is the specimen tested in a vertical orientation?

Vertical mounting represents the most severe common exposure scenario because the flame is applied at the bottom edge, so that the specimen is preheated above the flame and the buoyant hot gas stream is directed upward along the specimen, promoting flame propagation. This orientation therefore provides a conservative assessment of flame spread behavior for hanging fabrics and vertical installations.

How many specimens should be tested?

The number of replicates depends on the governing standard and specification. GB/T 5455 typically requires testing of several specimens in each fabric direction, ASTM D6413 commonly uses five specimens per condition, and children's sleepwear certification requires ten specimens per unit. Testing both lengthwise and crosswise directions, and both face and back surfaces, is strongly recommended whenever the material is anisotropic.

What does it mean if the afterflame time is short but the damaged length is long?

This pattern suggests that although visible flaming ceased quickly, thermal degradation continued through smoldering, glowing, or heat transfer, leaving an extensive damaged zone. It is often observed in cellulosic fabrics with inadequate flame-retardant treatment. Many product standards therefore impose simultaneous limits on afterflame time, afterglow time, and damaged length rather than relying on a single parameter.

Can damaged length be determined on plastics and coated materials?

Yes, in principle. For plastics, the UL 94 and IEC 60695-11-10 vertical procedures assess damage through afterflame duration, ignition of a cotton indicator by flaming drips, and whether the specimen burns up to the clamp, and a V-0, V-1, or V-2 rating is assigned accordingly. For coated and laminated textiles, the tear-removal measurement is adapted to record the full extent of char, melt, and delamination, and the observation basis is documented in the test report.

How should results be interpreted for conformity assessment?

Results must be compared with the explicit acceptance criteria of the applicable product standard or regulation, considering the required number of specimens, averaging method, and any individual-specimen maximum limits. Retesting is warranted when borderline values, atypical burning behavior, or specimen preparation anomalies are identified, and certification decisions should be based on testing performed by laboratories accredited for the relevant standards.