Pressure loading on a luggage container due to an internal explosion J.A. Gatto, S. Krznaric Office of Aviation Security Research and Development, FAA Technical Center, Atlantic City International Airport, NJ 08405, USA Abstract The Federal Aviation Administration has experimentally determined the explosive loading on a unit load device (luggage container) resulting from a bomb detonated within the luggage. A steel test fixture was constructed with the internal dimensions of LD-3 luggage containers, as specified in the I ATA Technical Manual. This test fixture was instrumented with nine Endevco piezoresistive and three PCB piezoelectric pressure transducers and eight temperature transducers. Ten tests were conducted using C-4 explosive with locations varying from the top bag to the third bag down into the luggage. Tests were conducted with the fixture 50% full of luggage and repeated with the fixture 75% full of luggage, with the piece of luggage containing the explosive located in the same relative positions. Two tests were conducted to determine the effects of venting in the LD-3 containers. One test was conducted with the rear panel removed to allow the test fixture to immediately vent. The other test was conducted with a rear panel constructed of plywood, allowing the test fixture to vent after a small time delay. The results quantify the dynamic and quasi-static loading on the LD-3 container. The peak pressures show a 35% reduction from bare charge values when the explosive is in the top piece of luggage and a 99% reduction when the explosive is in the third piece of luggage from the top. The quasi-static overpressure shows a 40% and 98% reduction from bare charge values for the respective cases above. 1 Description of the Problem The Federal Aviation Administration (FAA) is supporting the development of hardened luggage containers, containers designed to suppress the explosive ef-
62 Structures Under Shock And Impact fects from the detonation of an undetectable quantity of explosives. These containers are anticipated to bridge the gap between the amounts of explosives that can currently be detected and the amounts of explosives necessary to destroy an aircraft. The environment inside a luggage container makes it difficult to predict the pressure loads generated from an internal explosion. An explosive device would be located in a piece of luggage and loaded into a luggage container, surrounding it by other pieces of luggage. This environment is similar to an anisotropic, porous, absorptive media which attenuates the initial shock wave produced by the blast and reduces the quasi-static pressure. The magnitude of this attenuation depends on the location of the explosive device within the luggage container, the quantity of explosive material and the amount of luggage in the container. The loading on the container must be identified before any design specification can be developed or prototypes can be built. The literature is full of analytical and experimental pressure data for bare explosives detonated in free air and in internal volumes. There is also data available for cased explosives, as found in warheads. But there is very little literature that addresses explosive wave propagation through luggage, or luggage type materials. With little existing data available, the FAA decided to experimentally measure the internal pressure loads of a luggage container subjected to an internal explosion. Ten tests were designed to satisfy the following four objectives: (1) empirically obtain the pressure loading on a LD-3 container due to an internal detonation under several content conditions and charge locations, (2) measure the attenuation of the initial shock wave and the quasi - static pressure provided by the luggage, 2 Test Matrix (3) measure the change in the pressure time history due to venting, and (4) identify parameters for more extensive testing. Some limiting factors were considered in setting up the test matrix. First, the largest charge size to be tested would be the smallest charge size detectable by the current explosive detection systems. Also, since one piece of luggage would cause a reduction in the initial shock pressure, the worst case would be a bare explosive detonated inside an empty container. Finally, the most blast at-
Structures Under Shock And Impact 63 tenuation would occur when the container was full of luggage and the explosive was located in the center of the container. Ten tests were conducted holding the charge size constant (due to the sensitive nature of this research, the charge size is not identified). The location of the explosive device was always in the center of the container, as viewed from the top, but varied in relative height, either in the top bag, the second bag down, the third bag down or the center of the fixture. The luggage capacity in the container varied between empty (0% full), 50% full and 75% full. Venting effects were also investigated by conducting tests with the steel access door (the side opposite the sloping panel on the container) replaced by a plywood door, to give some finite time of containment prior to venting, and with the door removed to provide instantaneous venting. Table 1 gives the full test matrix. TABLE 1. TEST MATRIX Test # Capacity (%) Location Door 1 0 COF Steel 2 50 Top Bag Steel 3 50 2nd Bag Steel 4 50 2nd Bag Steel 5 50 3rd Bag Steel 6 75 Top Bag Steel 7 75 2nd Bag Steel 8 75 COF Steel 9 0 COF Plywood 10 0 COF None COF = Center Of test Fixture 3 TEST SETUP An internal blast container was constructed to experimentally obtain the pressure loading on a LD-3 container due to an internal detonation. The interior
64 Structures Under Shock And Impact dimensions of the test fixture were the same as a LD-3 Unit Load Device (ULD) or luggage container. Figure 1 shows the interior dimensions of the test fixture. Figure 1. Interior Dimensions of the Test Fixture. The test fixture was constructed of 2 inch thick mild steel with two deep stiffeners (W24X229 beams) welded to its exterior in one direction and one deep stiffener welded around the other two directions. The door to the fixture was located opposite the sloping panel. The door was also constructed of 2 inch thick mild steel and bolts onto a 4 inch thick flange at the opening using 58 1.25 inch bolts, equally spaced around the perimeter. The fixture is designed for 800 pounds per inch (psi) static internal pressure or 400 psi quasi-static internal pressure. Figure 2 shows an external view of the test fixture with the door removed. Pressure-Time histories were recorded using twelve pressure transducers (nine piezoelectric and three peizoresistive) mounted in the walls of the test fixture. The locations of the gages are shown in figure 3. Gages 10, 11 and 12
Structures Under Shock And Impact 65 Figure 2. Exterior View of the Test Fixture. ^12 in. Figure 3. Pressure Gage Locations.
66 Structures Under Shock And Impact were the piezoelectric transducers. An attempt was made to record temperature-time histories using eight thermocouples located inside the test fixture. However, due to the severe environment inside the container, the instrumentation either did not survive or the records were deemed unreliable. Future testing should provide for either more robust gages or an alternate means of collecting temperature data. In any event, the temperature data was secondary to the pressure data collected, therefore the tests were considered successful. Figure 4 shows the explosive packed into a suitcase prior to testing. The suitcase was first packed with articles of clothing, toiletries and other travel items. Then, the explosive was then placed inside the suitcase, with the detonator wire running through the side of the suitcase. Finally, the suitcase was loaded into the test fixture at the appropriate location. Figure 5 shows the test fixture loaded for test 6. The suitcase with the explosive is located on top and thefixtureis 75% full of luggage. Figure 6 shows the mounting of the explosive for tests with no luggage. The explosive charge was suspended by wire from the ceiling of the fixture, at the appropriate height. Figure 4. Explosive Loaded Inside Suitcase.
Structures Under Shock And Impact 67 Figure 5. Test Fixture Loaded for Test 6. Figure 6. Explosive mounting for no luggage.
68 Structures Under Shock And Impact 4 TESTS RESULTS Discussion of the tests results will involve gages 5 and 10 only, as data from all 12 gages would require much more room than is available for this paper. Comparing data from the same gages will give the discussion a sense of uniformity, i.e. any error due to the transducer is consistent. Gage 5, a piezoresistive transducer, and gage 10, a piezoelectric transducer, are chosen to compare data from two types of transducers. Both gages are located in the top portion of the test fixture. The data is presented in four groupings, chosen to show the effects of charge location, luggage capacity, and venting. Both the initial shock pressure and the quasi-static pressure are compared. Grouping I Grouping I is selected to show the change in pressure loads inside a luggage container due to the charge location. The initial pressure-time histories and the quasi-static pressures are compared for two tests in which the fixture capacity was at 50% full. In one test the charge was located in the top bag and in the other the charge was located in the third bag. Figures 7 and 8 show that the magnitude of the initial shock pressure is drastically reduced, if not eliminated completely, by burying the charge deeper into the pile of luggage. The pressure records for the test with the charge in the top bag show a distinct shock wave arriving at around 0.75 milliseconds (msec). This shock wave is not present in the pressure records for the test with the charge in the third bag. In fact, it's difficult to identify the arrival of any shock wave, as the records look more like high speed gas flows. The initial pressure is reduced by approximately 97% when the charge is in the third bag, compared to when the charge is in the top bag. Figures 9 and 10 show that the quasi-static pressure inside the fixture is essentially unaffected by the charge location. After approximately. 1 sec, the pressure-time histories for both tests are identical. Grouping II Grouping II is selected to show the change in pressure loads inside a luggage container due to luggage capacity. The initial pressure-time histories and the quasi-static pressures are compared for two tests with the charge located in the top bag. In one test the fixture capacity was at 50% frill and in the other test the fixture capacity was at 75% full.
Structures Under Shock And Impact 69 Time from Detonation (sec x 10" ) Figure 7. Gage 10 initial shock comparison for 50% charge locations. luggage capacity at two 100-20 1 2 Time from Detonation (sec x Figure 8. Gage 5 initial shock comparison for 50% charge locations. luggage capacity at two
70 Structures Under Shock And Impact 0.00 0.05 0.10 0.15 0.20 Time from Detonation (sec) Figure 9. Gage 10 Quasi-Static Pressure for 50% charge locations. 0.25 luggage capacity at two o.o 0.5 1.0 1.5 2.0 Time from Detonation (sec) Figure 10. Gage 5 Quasi-Static Pressure for 50% charge locations. luggage capacity at two
Structures Under Shock And Impact 71 Figures 11 and 12 show differing results for the comparison of the initial shock pressure data. Figure 11 shows a significant difference in the initial pressure-time history while figure 12 does not show any measurable difference. As a result, any observations about luggage capacity will be discussed in grouping III. Figures 13 and 14 show that the quasi-static pressure inside the fixture is consistently lower when the test fixture is 75% full than when the fixture is 50% full. The characteristics of the curves appear identical, except for a 40% reduction in pressure when the fixture is 75% full compared to when the fixture is 50% full. Grouping III Grouping III is also selected to show the change in pressure loads inside a luggage container due to luggage capacity. The initial pressure-time histories and the quasi-static pressures are compared for three tests with the charge located in the center of the fixture. In thefirsttest, the fixture capacity was at 0% full (empty), the second test was at a fixture capacity of 50% full, and the third test was at a fixture capacity of 75% full. Data from gage 5 is unavailable for the test at 0% full luggage capacity due to a transducer failure. Figures 15 and 16 compare the results of the initial shock pressure data. Both figures show a significant reduction in pressure as the quantity of luggage is increased. The pressure records for the tests with the fixture capacity at 0% and 50% full show distinctive shock waves. These shock waves are not present in the pressure records for the test with the fixture capacity at 75% full. Once again, it is difficult to identify any shock waves. Compared to the test at 0% full luggage capacity, the initial pressure is reduced by approximately 35% when the container is at 50% full and 99% when the container is 75% full. Figures 17 and 18 show that the quasi-static pressure inside the fixture is progressively lower as the luggage capacity increases. Again, the general characteristics of the curves appear identical. Compared to the test at 0% full luggage capacity, the quasi-static pressure is reduced by approximately 40% when the container is at 50% full and 98% when the container is 75% full. Comparing the tests at 50% and 75% full luggage capacity, the quasi-static pressure is reduced by approximately 80% when the container is at 75% full.
72 Structures Under Shock And Impact Time from Detonation (sec x 10" ) Figure 11. Gage 10 initial shock comparison with the charge in the top bag for 50% and 75% luggage capacity. 100 Time from Detonation (sec x 10 ) Figure 12. Gage 5 initial shock comparison with the charge in the top bag for 50% and 75% luggage capacity.
100 Structures Under Shock And Impact 73 0.00 0.05 0.10 0.15 0.20 Time from Detonation (sec) 0.25 Figure 13. Gage 10 Quasi-Static Pressure comparison with the charge in the top bag for 50% and 75% luggage capacity. 80 i o.o 0.5 1.0 1.5 2.0 Time from Detonation (sec) Figure 14. Gage 5 Quasi-Static Pressure comparison with the charge in the top bag for 50% and 75% luggage capacity.
74 Structures Under Shock And Impact 600 r -100 Time from Detonation (sec x 10 ) Figure 15. Gage 10 initial shock comparison with the charge in the center of the fixture for an empty, 50% full and 75% full fixture. 100-20 Time from Detonation (sec x 10 ) Figure 16. Gage 5 initial shock comparison with the charge in the center of the fixture for a, 50% full and 75% full fixture.
100 Structures Under Shock And Impact 75 0.00 0.05 0.10 0.15 0.20 Time from Detonation (sec) 0.25 Figure 17. Gage 10 Quasi-Static Pressure comparison with the charge in the center of the fixture for an empty, 50% full and 75% full fixture. o.o 0.5 1.0 1.5 Time from Detonation (sec) Figure 18. Gage 5 Quasi-Static Pressure comparison with the charge in the center of the fixture for a 50% full and 75% full fixture. 2.0
76 Structures Under Shock And Impact Grouping IV Grouping IV is selected to show the change in pressure loads inside a luggage container due to venting. The initial pressure-time histories are compared for three tests with different venting characteristics. The quasi-static comparison is not applicable and therefore not presented. In the first test, a steel door is used to simulate the condition in which the luggage container fully contains the blast. The second test uses a plywood door to simulate the condition in which the luggage container fails, or vents, after containing the blast from the explosion for somefiniteperiod of time. The third test was conducted with no door to simulate the condition in which venting is immediately available. This final condition is more of an optimum venting condition rather than a realistic one. Figure 19 compares the results of the initial shock pressure data. This figure shows there is no significant change in the presure-time history for the first 2 msecs. After this time, the pressure records start to diverge as a result of the venting characteristics. Any structural damage that occurs in a luggage container within the first 2 msecs can not be averted by venting techniques. 400 CO (X, PL, 5 300 -! 200-100 - 1 2 3 4 5 6 7 Time from Detonation (sec x 10"*) Figure 19. Gage 10 initial shock comparison with the charge in the center of an empty fixture with three venting conditions.
Structures Under Shock And Impact 77 SUMMARY The Federal Aviation Administration has experimentally determined the explosive loading on a luggage container, resulting from a bomb detonated within the luggage. These results quantify the dynamic and quasi-static loading on the LD-3 container. The peak pressures show a 35% reduction from bare charge values when the explosive is in the top piece of luggage and a 99% reduction when the explosive is in the third piece of luggage from the top. The quasistatic overpressure shows a 40% and 98% reduction from bare charge values for the respective cases above.