The principle objective of the kinds of demonstration tests that are discussed is to try to verify whether or not carbon fibers that are released by burning composite parts in an aircraft-fuel fires can produce failures in electrical equipment. A secondary objective discussed is to experimentally validate the analytical models for some of the key elements in the risk analysis. The approach to this demonstration testing is twofold: limited end-to-end test are to be conducted in a shock tube; and planning for some large outdoor burn tests is being done

Pride, R. A.

English

NASA Technical Reports Server

END-TO-END TESTING 9 
Richard A. Pride 
NASA Langley Research Center 
With all the testing that has been done to determine vulner- 
ability and failure of equipment, we still have not identified an 
occasion in which electrical or electronic equipment has failed 
because of carbon fibers released by the burning of composite 
parts in an aircraft-fuel fire. Therefore, the principal objec- 
tive, figure 1, of the kinds of demonstration testing that I will 
discuss is to try to verify whether or not carbon fibers that are 
released by burning composite parts in an aircraft-fuel fire can 
produce failures in electrical equipment. A secondary objective 
is to experimentally validate the analytical models for some of 
the key elements in the risk analysis: source, dissemination, 
transfer function, and vulnerability of equipment. 
The approach to this demonstration testing (figure 2) is two- 
fold; we are going to be conducting limited end-to-end tests at 
Dahlgren, Virginia, in the Naval Surface Weapons Center shock tube, 
and we are planning for some large outdoor burn tests at the 
Army's Dugway Proving Ground in Utah. There are certain qualifi- 
cations for the large outdoor tests which will be discussed later. 
For now, I want to indicate that we do have these two types of 
tests in various stages of planning and development. 
RISK ANALYSIS DEMONSTRATION TESTING 
OBJECTIVES 
@ VERIFY WHETHER CARBON FIBERS RELEASED BYBURNING COMPOSITE MATERIAL PARTS IN 
A REAL AIRCRAFT-FUEL FIRE WILL PRODUCE FAILURES IN ELECTRICAL EQUIPMENT 
0 EXPERIMENTALLY VALIDATE ANALYTICAL MODELS FOR 
SOURCE 
DISSEMINATION 
TRANSFER FUNCTION 
VULNERABILITY OF EQUIPMENT 
Figure 1 
125 
https://ntrs.nasa.gov/search.jsp?R=19790014033 2020-03-20T19:01:36+00:00Z
RISK ANALYSIS DEMONSTRATION TESTING 
APPROACH 
DAHLGREN SWC - SHOCK-TUBE TESTS 
ARMY DUGWAY PROVING GROUND - LARGE OUTDOOR BURN TESTS 
Figure 2 
Our objectives at Dahlgren (figure 3) are to verify the vul- 
nerability of equipment to fire-released fibers and to identify 
some of the problems that would be associated with end-to-end 
tests that will be even more significant if we go outdoors. Our 
approach is to develop a burn and exposure facility in the shock 
tube for doing these end-to-end tests and then to subject typi- 
cal vulnerable equipment to critical exposures of fire-released 
fibers along with the associated soot and smoke. When jet fuel 
is burned in open fires, a lot of smoke and soot is generated 
due to the fuel-rich type of combustion. The photograph from 
Vernon Bell's source paper of the outdoor pool fire at China 
Lake showed that it was certainly generating a lot of smoke. 
It was not clean burning by any means. Our smaller JP-1 fuel 
fires in the Dahlgren shock tube were also very smoky and sooty. 
The Dahlgren shock-tube tests provide an opportunity to ex- 
perimentally validate only a few of the elements of the risk 
analysis flow chart, figure 4, but these are probably the most 
important elements. For the source fire, we are using a controlled 
burning rate JP-1 commercial jet fuel. There is not much differ- 
ence in combustion with JP-1, JP-4, or JP-5. We did try to make 
it realistic for the civil aviation situation by specifying JP-1. 
We are going to burn graphite composite parts from structural 
test programs at Langley. Combustion temperatures in the vicin- 
ity of those parts will be controlled from about 930 to 980°C 
126 
DAHLGREN SWC - SHOCK TUBE TEST 
OBJECTIVES 
0 VERIFY VULNERABILITY OF EQUIPMENT O FIRE-RELEASED FIBERS 
0 IDENTIFY PROBLEMS ASSOCIATED WITH END-TO-END TESTS 
APPROACH 
0 DEVELOP BURN-EXPOSURE FACILITY FOR END-TO-END TESTS 
0 SUBJECT VULNERABLE QUIPMENT O CRITICAL EXPOSURES OF 
FIRE-RELEASED FIBERS WITH ASSOCIATED SMOKE AND SOOT 
Figure 3 
RISK ANALYSIS FLOW 
OPTIONAL IMPACT OF RESIDUE AMPLIFIER TRANSFER 
DAHLGREN TUBE TESTS 
SIX AMPLIFIERS 
MEASUREMENTS 
27 DEPOSITION 
34 EXPOSURE 
AIR VELOCITY 
TEMPERATURE 
12 FAILURE MONITORS 
Figure 4 
127 
(1700-1800°F), and we will have a facility that has the option of 
agitating the residue after it has been burned, in order to study 
the difference in the amount of fibers that are released from 
just a burn compared to those released from an agitation towards 
the end of the burn. Because we are working inside a tube, we 
will not be getting much information about dissemination. We 
will not be able to measure plume parameters or downwind trans- 
port of the fibers in the sense that we can verify models that 
are developed for outdoor atmospheric dissemination. We will be 
disseminating fibers, but they will be constrained to stay within 
the walls of the shock tube at Dahlgren. 
We will be getting some limited transfer function data, 
primarily the transfer function that is associated with the fibers 
being pulled inside the case of the fan-ventilated stereo ampli- 
fiers that we expect to be exposing for vulnerability at the 
target table in the shock tube. Our present plans are to expose 
a sample of six amplifiers. Failure characteristics have been 
established for these amplifiers based on chamber testing. They 
are reasonably low cost pieces of electronic equipment, and we 
think we understand pretty well the kinds of failure modes that 
are most apt to occur when they are exposed to carbon fibers. 
These amplifiers are representative of a class of generic circuit 
board equipment. They will allow us to determine failure rates 
when exposed to fire-released fibers for comparison with those 
rates that have been previously established for chamber-released 
fibers. In the shock-tube tests we will be measuring both depo- 
sition and exposure levels for fibers in the vicinity of the 
amplifier locations. We will be measuring air velocity, air 
temperature, and fire temperature, and we will have up to 12 
failure monitors associated with the 6 amplifiers. 
Figure 5 is an aerial photograph of the conical shock tube 
at Dahlgren which was built back in the mid-60's to study the 
effect on equipment of the pressure wave from a simulated atomic 
blast. The facility is approximately 0.8 km (a half-mile) long. 
It starts with four 16-inch naval gun barrels butted end-to-end, 
and expands to a 7.3-m (24-foot) diameter tube at the exit end. 
We are using approximately half the length, starting opposite 
the white building shown in figure 5, where we opened up the 
tube by taking out a couple of sections to provide an inlet. 
The fire pan for the simulated aircraft-fuel fire is located 
about 60 m (200 feet) from the inlet. The airborne effluent 
from the fire is pulled out through a filter system by a group 
of exhaust fans at the exit end of the tube. The filter system 
has been shown to filter out all the fibers and the heavier 
particles of soot, and essentially allow the discharge of only 
very-fine-particle soot and smoke into the atmosphere. We have 
monitored the particle content of the filtered smoke, and we have 
determined that the size of soot particles is less than 4 microns 
in diameter. We think our filtering system is very effective in 
taking out any particle sizes greater than 4 microns. 
128 
DAHLGREN NSWC - SHOCK TUBE 
Figure 5 
Figure 6 is a photograph of the inlet end of the tube. At 
this point, the cross section of the tube opened to the atmosphere 
is 3 m (10 feet) in diameter, which permits sufficient fresh air 
to be drawn in to feed the fire. The fire pan is shown in 
figure 7. We have two sizes of pan built up together, a 1.2-m 
(4 feet) square inner pan and a 2.4-m (8 feet) square outer pan, 
giving us the capability of building two different size fires, 
depending on the particular experiment requirements. Fuel is 
pumped at a controlled rate in through the bottom of the pan, 
which is kept flooded with water. Behind the fire pan is an array 
of tubing supporting a series of thermocouples that were used to 
survey and monitor flame temperature at locations downwind of 
the fire. Figure 8 is a photograph taken, from inside the tube, 
of one of the development fires. Modifications made since that 
time include a chimney around the fire pan to get the fire to 
stand more nearly vertical. With the addition of a chimney we 
have been able to provide a larger area in the flames in the 
930-980°C (1700-1800OF) temperature range in which to mount the 
composite specimen for burning. 
The combustion products and fibers from the burned composite 
are pulled through the tube approximately 210 m (700 feet) to the 
location of the target table, figure 9. This is a table that is 
about 5 m (16 feet) wide and 9 m (30 feet) long. The vulnerable 
electronic equipment will be exposed on this table and connected 
129 
SHOCK-TUBE INLET 
SHOCK-TUBE FIRE PAN 
130 
- .- -....- ..-.. . . ._... ,, .- 
DEVELOPMENT FIRE l.N SHOCK TUBE 
Figure 8 
SHOCK-TUBE TARGET TABLE 
Figure 9 
to the electrical switch boxes shown. There are some wires 
standing up from the surface of the table. Those wires.hold the 
sticky cylinders that we use to measure the fiber exposure. 
A propeller anemometer is also on the table to measure local air 
velocity. This particular view is looking up the tube towards 
the location of the fire pan. The opening that can be seen at 
the end is where the section of the tube was removed for the in- 
let. Note that there is an indication of deposition of soot on 
the upper portions of the tube wall. There appears to be a fair 
amount of stratification in the airflow coming down the tube. 
The bulk of the soot tends to be near the top., but there is an 
ample deposition of soot all over the table and on the floor of 
the tunnel in that location. 
Figure 10 is a picture of the filtering system. In the back- 
ground is a framework which supported an initial attempt at 
building a filter wall which consisted of fiberglass furnace 
filters. They were very effective in taking out particles of 
soot and fibers; however, they became clogged in about three 
minutes of operating time, stopping the airflow and choking the 
fire upstream. We are looking for operational times on the order 
of 20 minutes on a particular burn. In the foreground is the 
current filter system which is basically a fire-fighting type 
water-spray fogging nozzle. A fire hose is connected to the in- 
clined supply pipe supported on the scaffold. The nozzle is at 
the upper end. This nozzle emits an effective fog of fine par- 
SHOCK-TUBE FILTERING SYSTEM 
Figure 10 
132 
title water which washes fibers and soot particles greater than 
4 microns in diameter out of the airstream. We think it also 
washes out a considerable number of particles under 4 microns, 
but it does not capture all of them. The washed out fibers, soot, 
and water collect on the floor of the tube and then are filtered 
between a series of baffles shown in figure 11. We have a skim- 
ming system which allows the water to run under these baffles in 
a controlled manner and retains the soot and fibers that are 
floating on the surface. The water drains through a hole in,the 
floor of the shock tube. We have been monitoring the output of 
particles and fiber by deposition on sticky cylinders in the 
exhaust end of the tunnel. The filtering system has been working 
very effectively. 
Figure 12 is a photograph of the exhaust end of the shock 
tube. Six exhaust fans were installed in the bulkhead in the end 
of the tube. Access to instrumentation is through a steel door 
which is closed and locked prior to testing. The six exhaust 
fans have variable speed control, providing a wide range of air- 
flow ranging from essentially zero up to a maximum of about 3400 
cubic meters per minute (120 000 cubic feet per minute). Typically 
we are operating at about 1130 cubic meters per minute (40 000 cubic 
feet a minute) which gives us an average velocity over the target 
table of about 0.5 m/set (1.2 miles per hour). 
DRAIN BAFFLES IN SHOCK TUBE 
Figure 11 
133 
- 
SHOCK-TUBE EXHAUST 
Figure 12 
One of the first graphite composite samples that was burned 
in the shock-tube fire was monitored for fiber release with 
sticky cylinders at the target table. Sticky cylinders are made 
by rolling a 50-mm (2 inch) square piece of sticky paper into a 
cylinder with the sticky surface on the outside and mounting it 
on a wire with the cylinder axis perpendicular to the airflow. 
Figure 13 is an enlarged photograph of a portion of one of the 
sticky cylinders after exposure and after cutting it open to 
flatten it. This part of the sticky cylinder shows a cluster 
of about six fibers ranging in length from 3 to 15 mm. Those six 
fibers on that particular sticky cylinder represent an exposure 
of about lo4 fiber-seconds/cubic meter for a 20 minute fuel 
burn. That is not enough for the levels that are needed to gen- 
erally produce electrical failures, but considering that it was 
our first attempt at burning composite in that facility, we were 
quite encouraged. 
Figure 14 is a photograph of a circuit board out of an 
amplifier that failed in chamber testing. It represented an 
electrical short caused by a fiber getting across a critical 
element in the amplifier, which then caused both the transistor 
and a resistor to burn out. There is a lot of smoke, oil, and 
soot associated with the burn out, which created a failure in 
that particular case and is an indication of the kinds of fail- 
ures that can occur on this type of equipment in chamber test- 
ing. I should say that, in connection with our preparation 
134 
GRAPHITE FIBERS DEPOSITED FROM FIRST COMPOSITE SPECIMEN 
BURNED IN JET FUEL FIRE AT DAHLGREN 
Figure 13 
AMPLIFIER CIRCUIT BOARD FAILURE 
Figure 14 
for testing in the shock tube, we have exposed these amplifiers 
without fibers in the tube for periods up to an hour to determine 
if smoke and soot would cause a failure. There was no indication 
of failure. 
The typical failure curve that has been obtained on these 
stereo amplifiers in the chamber testing is shown in figure 15. 
For this equipment, 27 tests produced 27 failures with a mean ex- 
posure for the failures of 8.48 x lo5 fiber-seconds/cubic meter. 
These were tests at various fiber lengths, ranging from 3 to 14 
millimeters. The indications from those early sticky cylinders 
* exposed in the shock tube are that the fibers we have seen so 
far range in length from less than 1 mm to about 15 mm. But again 
this is preliminary data. Future effort will be directed towards 
increasing the number of released fibers to achieve exposure 
levels comparable with chamber tests. 
The second part of the two-fold approach to demonstration 
testing would be to go to Dugway Proving Ground for outdoor, end- 
to-end tests, figure 16. Dugway is not the only place in the 
United States where this type of test might be conducted; however, 
it does provide a location which appears to be quite satisfactory 
for doing large outdoor burns of graphite composites for the pur- 
pose of verifying the kinds of risk analysis elements that are 
PROBABILITY 
OF 
FAILURE 
STEREO AMPLIFIER FAILURES IN CHAMBER TESTING 
I 
8,118 X lo5 IS MEAN EXPOSURE FOR 27 FAILURES 
AT VARIOUS FIBER LENGTHS FROM 3 TO 14 MM 
I 
0 1 2 3 x 106 
EXPOSURE, FIBER-SECONDS/METER3 
Figure 15 
136 
DUGWAY PROVING GROUND END-TO-END TESTS 
OBJECTIVE 
LARGE OUTDOOR BURN FOR VERIFICATION OF RISK ANALYSIS ELEMENTS 
APPROACH 
USE ACCIDENT EXPERIENCE TO DEVELOP CREDIBLE FIRE SCENARIOS 
USE TEST EXPERIENCE AND RISK ANALYSIS TO SELECT EXPOSED ELECTRICAL EQUIPMENT 
PREPARE PRE-TEST PREDICTIONS OF: FIBER RELEASE 
FIBER DISPERSION AND PENETRATION 
PROBABLE EQUIPMENT FAILURES 
MEASURE ALL VARIABLES KNOWN TO INFLUENCE FINAL RESULTS 
COMPARE PRE-TEST PREDICTIONS WITH TEST RESULTS 
MANDATORY PREREQUISITES 
ESTABLISHED CONFIDENCE IN: FIRE PLUME PREDICTIONS 
FIBER RELEASE PREDICTIONS 
MODELS FOR FIBER DISSEMINATION 
CONCERNS: IF WIDESPREAD ISSEMINATION FOOTPRINTS - TEST MAY BE IMPRACTICAL 
Figure 16 
shown in figure 17. In this case if we go outdoors, we not only 
have the capability of verifying source but we can also verify 
outdoor dissemination. We can verify in a more effective manner 
a number of transfer functions and the vulnerability of suitable 
electrical equipment. Our approach is to use the accident 
experience that is being generated by the three commercial air- 
plane manufacturers, using their data to select a creditable 
scenario for a fire for the source of fiber release. We will use 
our test experience in the risk analysis program to select the 
appropriate kinds of electrical equipment to be exposed. We want 
to be able to prepare pretest predictions for the amount of fibers 
to be released, their dispersion and penetration, and the probable 
equipment failures that will result from a test of that sort. We 
need to have the capability of measuring all the variables that 
are known to influence the final results. That is one of the 
reasons Dugway is considered to be a favorable location. In 
their past experience with airborne release of a number of chemi- 
cal agents for the army, they have built up a very comprehensive 
network of meteorological stations, so they have a knowledge of 
the weather conditions over the entire range at the time of re- 
lease as well as before and after release. We will be able then 
to modify our pretest predictions with the weather conditions 
actually present. 
137 
RISK ANALYSIS FLOW 
REDISSEMINATION 
DUGWAY PROVING GROUND 
OUTDOOR TESTS 
Figure 17 
We do have a couple of mandatory prerequisites to conducting 
the end-to-end test. We need to have already established confi- 
dence in our fire plume predictions, in the fiber release pre- 
dictions, and in the models for fiber dissemination. Until we 
have a good level of confidence established, it is probably 
futile to go outdoors and start burning composites. Our chances 
of finding fibers, knowing where they are going, and catching 
them again would be quite uncertain. One concern is that if we 
get a widespread dissemination footprint, the available fibers 
are scattered over such a large area that the whole test may be- 
come impractical to perform from the experimental point of view. 
That is not to say that the problem is not there; it just becomes 
very difficult to try to track down the results. 
138 
Question: 
What is the rationale for burning fiber first and then exploding 
it? Isn't this kind of the reverse of what it should be? 
Answer: 
The impact, or explode, and then burn seems to be the general 
scenario for commercial aircraft accidents. We do not have the 
final answers in from the aircraft manufacturers, but the pre- 
liminary indications are that for most of the commercial aircraft 
accidents the plane crashes and burns. Then, if there is an 
impact, it will be a low-order type impact. It will not be an 
explosion. A major impact like a crash occurs before the fire 
and really does not have anything to do with disseminating carbon 
fibers. The crash would scatter parts of the airplane around, 
but the fibers can not be scattered until after they have been 
released from the composite by burning out the matrix. You have 
to have a fire to burn the matrix out before the impact or ex- 
plosion is going to have a significant effect on scattering fiber. 
Most of the civil accidents that have been investigated do not 
have the kind of explosion that occurs after the fire has burned 
out the epoxy matrix. I am sure there are going to be a number of 
exceptions to the above crash and burn situation, but I think, 
for civil aircraft, you generally do not have the burn and then 
explode situation. 
139 


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