Materials selection for spacecraft is based on conventional flammability or ignition sensitivity acceptance tests. Current procedures for determining the inter-laboratory repeatability and reproducibility of aerospace materials flammability tests are not considering the dependence of data variability on test conditions and consequently attempts to characterize the precision of these methods were not successful. The inter-laboratory data variability is determined with tests conducted under arbitrary conditions, which consequently may not provide sufficient information to enable adequate determination of a method's precision. For evaluating the precision of NASA's flammability test methods, the protocol recommended includes selecting critical parameters and determining the 50% failure point by considering the specific failure criteria of each method using the critical parameter as a variable. Upon performing inter-laboratory round robin testing using this approach, the laboratories' performance could be evaluated by comparing the repeatability of the 50% failure point and/or the repeatability of critical conditions where the probabilities of passing and failing are unity, i.e., the transition zone repeatability. When a sufficient amount of data has been acquired with this method, an adequate estimation of precision of aerospace materials flammability test methods will be possible

Beeson, Harold D.

Hirsch, David

NASA Technical Reports Server

FG? =-llEW PURPOSES ONLY 
NQT 1 , a .OVFO FOR PUBUCATm Bwm Source of Acquisition NASA Johnson Space Center 
An Approach to Evaluate Precision and Inter-Laboratory 
Variability of Flammability Test Methods 
for Aerospace Mater~als 
David Hirsch and Harold Beeson 
NASA Johnson Space Center White Sands Test Facility, Las Cruces, New Mexico 
Copyright O 2005 SAE International 
ABSTRACT 
Materials selection for spacecraft is based on 
conventional flammability or ignition sensitivity 
acceptance tests. Current procedures for determining 
the inter-laboratory repeatability and reproducibility of 
aerospace materials flammability tests are not 
considering the dependence of data variability on test 
conditions and consequently attempts to characterize the 
precision of these methods were not successful. The 
inter-laboratory data variability is determined with tests 
conducted under arbitrary conditions, which 
consequently may not provide sufficient information to 
enable adequate determination of a method's precision. 
For evaluating the precision of NASA's flammability test 
methods, the protocol recommended includes selecting 
critical parameters and determining the 50% failure point 
by considering the specific failure criteria of each method 
using the critical parameter as a variable. Upon 
performing inter-laboratory round robin testing using this 
approach, the laboratories' performance could be 
evaluated by comparing the repeatability of the 50% 
failure point andlor the repeatability of critical conditions 
where the probabilities of passing and failing are unity, 
i.e., the transition zone repeatability. When a sufficient 
amount of data has been acquired with this method, an 
adequate estimation of precision of aerospace materials 
flammability test methods will be possible. 
INTRODUCTION 
Spacecraft fire safety emphasizes fire prevention, which 
is achieved primarily through the use of fire-resistant 
materials. Materials selection for spacecraft is based on 
conventional flammability or ignition sensitivity 
acceptance tests along with prescribed quantity 
limitations and configuration control for items that are 
nonpass or questionable. The NASA STD 6001 [ I ]  
Test 1 is the major method used to evaluate flammability 
of materials intended for use in the habitable 
environments of U.S. spacecraft; Test 4 is used to 
evaluate flammability of wire insulations. Although round 
robin tests involving up to four aerospace materials test 
laboratories were conducted during the last 10 years, 
attempts to characterize the precision of these methods 
were not successful. Current procedures for determining 
the inter-laboratory repeatability and reproducibility of 
aerospace materials flammability tests are not 
considering the dependence of data variability on test 
conditions. The inter-laboratory data variability is 
determined with tests conducted under arbitrary 
conditions, which consequently may not provide 
sufficient information to enable adequate determination 
of a method's precision. 
The precision of a given test method is interlinked with 
the repeatability of the phenomena being evaluated. For 
example [2], normally the flammability behavior of 
materials follows an S-shape curve when the response is 
plotted versus the magnitude of a critical parameter, 
such as oxygen concentration (Figure 1). Below and 
above certain stimulus levels, the results would be more 
repeatable, and the flammability test evaluation methods 
would provide results that are insensitive to even a large 
change in variables. Tests conducted under transition 
zone conditions should be expected to provide less 
repeatable results, since in this zone smaller changes in 
test variables would have larger effects on results. It 
appears that the test conditions loci, relative to the 
flammability distribution curve (Figure 2, [3]), will dictate 
the repeatability and consequently should be carefully 
considered to prevent drawing erroneous conclusions on 
the statistical confidence of results. 
A study was initiated on a methodology for characterizing 
~recision of test methods used to evaluate aerosDace 
baterials when the test conditions strongly affeci the 
repeatability of the phenomena evaluated. Normally, 
obtaining a relatively high level of confidence on results 
can be obtained only with a large number of samples if 
the test is conducted with sets of conditions that fall 
within the transition zone for a given material. Testing in 
zones removed from the transition zone would yield 
abnormally high precision for the methods, since the 
results would be insensitive to even large fluctuations of 
variables. This paper evaluates the feasibility of a 
method used to evaluate the repeatability and 
reproducibility of a test method by conducting a limited 
number of tests following a rigorous statistical method, 
such as the staircase (or 50%) method [4]. It is believed 
https://ntrs.nasa.gov/search.jsp?R=20070018761 2019-08-30T00:54:29+00:00Z
that the approach proposed would allow meaningful Experimental 
comparisons of laboratories' performance when the test 
conditions are likely to strongiy affect the repeatability of 
the phenomena being investigated. 
30% Temperature; 
Oxygen Concentration 
Figure 1. Flammability Distribution Curve: Generalized Dependency 
of a Flammability Response (i.e., Burn Length of a Standard-size 
sample) on the Magnitude of the Stimulus (i.e. Temperature or Oxygen 
Concentration) 
Figure 2. Expected Trends for Repeatability and Sensitivity to 
Parameter Shift in Relation with the Magnitude of the Stimulus and its 
Effects on Response. 
Materials 
Two materials were used for flammability tests, 
Kydex 100, which is a PVCIPMMA alloy, and Nomex, an 
aromatic polyamide. The flammability test samples were 
approximately 6.4 cm wide and 30.5 cm long, which are 
standard NASA STD 6001 Test sample dimensions. 
The material thicknesses were approximately 1.6 mm for 
Kydex and 0.32 mm for Nomex. 
Test Sysfems and Procedures 
Flammability tests were conducted in a 1400 L test 
chamber connected to a vacuum pump and oxygen and 
nitrogen supplies. The test system met the NASA STD 
6001 Test 1 requirements [I]. All tests were conducted 
at 101.3-kPa total pressure and following NASA STD 
6001 Test 1 procedures, with the exception that no K-10 
paper was used. The test environment was established 
by evacuating the test chamber and back filling with 
oxygen and nitrogen. The test environment was mixed 
by forced convection inside the test chamber and its 
oxygen content was verified prior to testing. 
Initially, the limiting oxygen indices were determined by 
conducting sequential tests, as recommended by ASTM 
D 2863 [5], and using a step size of one percent oxygen 
by volume. The limiting oxygen indices were calculated 
with the "up-and-down method for small samples" [4]. 
This method has been adopted by both IS0 4589 [6] and 
D 2863 for determining the "minimum oxygen 
concentration required to support combustion of 
plastics." The failure criteria consisted of burn length of 
at least 15.2 cm, the same as for NASA STD 6001 Test 
1. 
Upon determining the limiting oxygen indices, the tests 
focused on determining the flammability transition 
interval. Five tests were conducted under conditions of 
increasing and decreasing oxygen concentration 
stepwise by one percent until a probability of unity for 
both passing and failing the burning criteria was 
obtained. 
Results and Discussion 
The test results are shown in Table 1. The limiting 
oxygen indices following the ASTM 02863 logic are 
shown in Table 2. Data for both Kydex and Nomex 
appears to be repeatable, with relatively small standard 
deviations. The preliminary results indicate that both 
materials appear to be suitable for evaluating intra- and 
inter-laboratory repeatability. Additional repeatability and 
round robin testing involving several laboratories will be 
required to acquire sufficient information to determine 
with statistical confidence if flammability data from 
particular laboratories fall outside expected repeatability 
and reproducibility values. The suggested approach 
could be extended to test methods intended to determine 
ignition sensitivity of aerospace materials. 
Table 1. Probability of Failing NASA STD 6001 Test 1 criteria in various oxygen concentrations 
Test Material Oxygen Concentration Samples Tested Probability of Sample Failing 
Vol % 15.2-cm Burn Criteria 
Kydex 100 29 10 0.0 
34 5 1 .o 
Nomex 22 5 0.0 
23 2 0.4 
24 5 0.2 
25 5 0.6 
27 5 0.8 
28 5 1 .O 
Table 2. Upward Flame Propagation in Quiescent Environments; Limiting Oxygen Index, and Repeatability Statistics 
Test Materials Limiting Oxygen Index ~ a l u e s " ~  ~verage' ~ a n g e ~  Standard 
Deviation 
Kydex 100 30.5 32.5 32.5 31.5 32.3 31.9 30.5-32.5 0.77 
Nomex 25.8 26.0 24.7 25.9 25.0 25.5 24.7-26.0 0.53 
CONCLUSION 
Current procedures for determining the inter-laboratory 
repeatability and reproducibility of aerospace materials 
flammability tests are not considering the dependence of 
data variability on test conditions. The inter-laboratory 
data variability is determined with tests conducted under 
arbitrary conditions, which consequently may not provide 
sufficient information to enable adequate determination 
of a method's precision. For evaluating the precision of 
NASA's flammability and ignition sensitivity test methods, 
the following protocol is recommended. Select one or 
two critical parameters for each method. For example, 
oxygen concentration could be considered for all types of 
flammability tests. Upon selecting the critical 
parameter(s), determine the 50% failure point 
considering the specific failure criteria of each method 
and using the critical parameter as a test variable. One 
can also use ASTM D 2863 test logic to determine the 
limiting oxygen index of NASA STD 6001 Tests 1 or 4 
(which in fact provides the 50% failure point). For 
flammability test methods, an alternative or additional 
test method would be to perform testing under critical 
conditions removed stepwise from the 50% point to 
determine the critical conditions where both probabilities 
of passing and failing become unity upon conducting a 
statistically adequate number of tests under each 
condition. Perform inter-laboratory round robin testing 
using this approach. The laboratories' performance 
could be evaluated by comparing the repeatability of the 
50% failure point (or the limiting oxygen index values for 
flammability) andlor the repeatability of critical conditions 
where the probabilities of passing and failing are unity, 
i.e., the transition zone repeatability. When a sufficient 
amount of data has been acquired with this method, an 
adequate estimation of precision of aerospace materials 
flammability will be possible. Additional repeatability and 
round robin testing involving several laboratories will be 
required to acquire sufficient information to determine 
with statistical confidence if flammability data from 
particular laboratories fall outside expected repeatability 
and reproducibility values. The approach suggested will 
be extended to test. methods intended to determine 
ignition sensitivity of aerospace materials. 
REFERENCES 
1. Flammability, Odor, Offgassing, and Compatibility 
Requirements and Test Procedures for Materials in 
Environments That Support Combustion, NASA- 
STD-6001, National Aeronautics and Space 
Administration, February 9, 1998 
2. Hirsch, D. and Beeson, H., Test Method to 
Determine Flammability of Aerospace Materials, 
Journal of Testing and Evaluation, Vol. 30, No. 2, 
March 2002, pp. 156-1 59 
3. Hirsch, D., Test Methods: Phenomenology and 
Statistics, Presented at the 14 '~  International 
Standardization Organization Plenary Meeting of the 
Technical Committee on Space Systems and 
Operations, Moscow, Russia, May 2004 
4. Dixon, W.J., "Up-and-Down Method for Small 
Samples," American Statistical Association Journal, 
1965, pp 967-970 
5. Standard Test Method for Measuring the Minimum 6. IS0 4589 Plastics - Determination of Burnina 
Oxygen Concentration to support Candle-Like 
" 
Behavior by Oxygen Index, International 
Combustion of Plastics (Oxygen index), ASTM D Organization for Standardization, Geneva, 
2863-97, American Society for Testing and Materials Switzerland, 1996 
International, West Coshohocken, PA, 1997 


An Approach to Evaluate Precision and Inter-Laboratory Variability of Flammability Test Methods for Aerospace Materials

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