AFAP-02 smoke generation test for materials

What it is

AFAP-02 (NATO Allied Fire Assessment Publication 2, Edition 3, printed on its own cover as AFAP-2) is a laboratory test method for measuring how much smoke a material produces under specified fire conditions. Promulgated by NATO in July 2010, it does not create a new test from scratch: it takes the civil test standard ISO 5659-2:2006, "Plastics - Smoke generation - Part 2: Determination of optical density by a single-chamber test," and adds the specimen-preparation, calibration, procedure and reporting rules NATO needs to get consistent, comparable results from it across nations and laboratories, plus an additional optional parameter, VOF4, that measures how quickly a material starts producing smoke.

It is the second of a five-part NATO series that scores different fire characteristics of the same kind of candidate material: AFAP-1 sets the policy for pre-selecting materials, AFAP-2 (this document) measures smoke generation, AFAP-3 measures toxicity of fire effluents, AFAP-4 measures surface spread of flame, and AFAP-5 measures heat release rate. NATO is explicit that the series is a comparative screening tool for choosing between candidate materials, not a prediction of how a material behaves in an actual fire and not a pass/fail verdict on any one of them.

Who actually does the work

Three parties run through the document. A fire test laboratory carries out the physical test: preparing specimens, calibrating the apparatus, and recording results. A "Technical Authority", defined in the document as the relevant authority responsible for providing regulations and guidance on reaction-to-fire properties of materials associated with procurement and in-service support, decides things the document leaves open, such as which face of a laminated material to test or whether the specimen-holder grid is used. And a supplier or manufacturer provides the material under test, named in the finished report alongside the laboratory.

AFAP-02 has no force of its own. Nations record their agreement to use it in STANAG 4602, and it reaches an individual supplier only when a tender, contract or materials specification calls it up. The document itself is explicit on this: any enquiry about it "in relation to an invitation to tender or a contract in which it is incorporated" goes to the Technical Authority, not to NATO.

What the test involves

The core method comes from ISO 5659-2: a horizontal specimen is exposed to a radiant heat flux, and the chamber measures the attenuation of light as smoke accumulates, converted into a specific optical density, Ds. AFAP-02 sets three test modes: 25 kW/m2 without a pilot flame, 25 kW/m2 with a pilot flame, and 50 kW/m2 without a pilot flame, each run on three specimens. The fourth ISO 5659-2 mode, 50 kW/m2 with a pilot flame, is stated as not required.

AFAP-02's own additions replace or tighten several parts of the ISO procedure. The termination rule is rewritten: an initial test in each mode always runs the full 20 minutes regardless of when smoke output peaks, so a second peak (such as a layered material burning through to the next layer) is not missed; where that first test shows the peak within the first 10 minutes, later tests in that mode can stop at 10 minutes. A test can also be cut short once Ds reaches the document's own upper limit of 792, a ceiling the document sets because the underlying formula pushes toward infinity at very low light transmission and because readings that low are not considered accurate. If the pilot flame goes out from the smoke itself during a test, that specimen is discarded and a replacement run. VOF4, calculated only if the Technical Authority asks for it, adds up the specific optical density at one, two and three minutes plus half the value at four minutes, to flag materials that produce smoke early. Materials that behave as "intumescent" when tested 25 mm from the cone heater have that result discarded and are retested at 50 mm, with a note that results at the two separations may not be comparable.

Specimen preparation and calibration

Annex 1 gives material-specific preparation rules that are not in ISO 5659-2 at all: paint and coating systems are applied to 75 mm square mild steel panels, cured, then conditioned for 7 days and tested within a further 7; tube and pipe sections are cut into flat strips if flexible, or into a set number of 75 mm-wide curved segments if rigid; upholstered-furniture and mattress composites are built up to a maximum 25 mm test thickness, with individual component layers weighed and required to fall within 95-105% of the mean mass of a set of nine.

On the apparatus side, the working heat flux meter used to set the radiator cone has to be calibrated at the start of every testing day and be traceable, in no more than four steps, to the primary standard held by LNE in France or SP in Sweden. Annex 3 gives a full worked transfer-calibration procedure, including an example calibration equation derived from measured data. The specimen-holder grid described in ISO 5659-2 is not used by default; it only comes into play if the Technical Authority specifies it, for example in a materials specification.

The test report

Section 11 fixes a long list of what a test report must contain: laboratory and supplier or manufacturer identity, test dates, a full material description (including NATO Stock Numbers where known), the specimen construction and which face was tested, per-specimen results (Ds,max, the time it occurred, Ds10, VOF4 if calculated, and the clear-beam correction factor DC), the mean values across each of the three modes, observations and any invalid tests with reasons, and a fixed statement that the results relate only to the specimens tested under the particular conditions of that test. Annex 4 gives an optional pre-formatted datasheet for recording this, carrying the security-marking layout used by the STANAG 4602 materials database that AFAP-1 describes, which suggests results are meant to be kept and reused rather than generated once and filed away.

What it does not cover

AFAP-02 says nothing about which materials need this test on a given programme, what smoke-generation limit a specification should set, or how the result feeds a wider material decision; those are set by the contract, the materials specification, or the Technical Authority, not by this document. It also does not predict how a material will smoke in a real fire: its own series preface is explicit that these tests are a comparative screening tool for pre-selecting candidate materials, run before a design commits to a specific one.

Standards it references

• AFAP-01Background
• AFAP-03Background
• AFAP-04Background
• AFAP-05Background