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Safety
Detonation Tests Evaluate
High Pressure Cells Full scale prototype demonstrates performance of a new cell design by J. P. Weber,l Jacob Savitt,2 John Krc, Jr.,3 Armour Research Foundation; and H. C. Browne, Monsanto Chemical Co.
Safety column in I/EC (October 1961, p. 52 A) featured background and design of a new type concrete high pressure cell developed at Monsanto Chemical Co. To determine how well the design objectives had been achieved, Monsanto asked the Armour Research Foundation of Illinois Institute of Technology to evaluate a prototype cell. Monsanto built the cell at the ARF explosive testing facility at Coal City, Ill., and explosive charges were detonated within the cell to simulate various conditions of sudden energy release. LAST MONTH'S
The program was intended to determine the effects of 8 pounds of TNT detonated within the cell. For convenience in charge preparation, handling, and firing, Du Pont 50% ditching dynamite was used in place of TNT. The heats of detonation of TNT (p 1.59 grams per cc.) and the ditching dynamite are 2010 and 1880 B.t.u. per pound, respectively. The heats of combustion are 6520 for TNT and 2500 B.t.u. per pound for ditching dynamite. The large heat of com1
2 3
Present address, Sandia Corp. Present address, Explosiform, Inc. Present address, Parke, Davis & Co.
128 A
bustion of TNT results from an oxygen deficiency. The afterburning of the detonation products does not contribute to the air-blast pressure, but is of importance in creating rise of static pressure in a completely closed volume. Since the test cell has a large vent area per unit of volume, pressures within the cell are not sustained, and the increa ses due to afterburning will probably not be of sufficient duration to cause much increase in structural damage. F or this reason the heat of detonation more truly represents the sudden energy release and is the better criterion for predicting air-blast damage to the cell and its surroundings. Thus, a given weight of ditching dynamite is only slightly less effective in causing damage than an equal weight of TNT. Electronic and photographic instrumentation was employed to obtain air-blast peak pressure measurements in the immediate neighborhood of the cell, shock velocity measurements in the immediate neighborhood of the cell (for pressure and shock position calculations), and air-blast peak pressure measurements at greater distances. In order to determine directly whether windows in nearby buildings might be broken by air blast, three glass windows were mounted in the test area. These windows
INDUSTRIAL AND ENGINEERING CHEMISTRY
were located at distances of 280, 500, and 800 feet. Two of the windows were scaled down to about one half the actual dimensions. For some of the shots, a 16-mm. Fastax camera was used to observe shock propagation from the cell, and the response of the cell and the safe wall to the blast. Framing rates were between 725 and 4660 frames per second, depending on exposure tirnes required for the existing light conditions. Instru mentatio n
The gages used for obtaining pressure measurements were of the piezoelectric type employing barium titanate. They were 1/ 4 inch in diameter and were flush mounted in the centers of rectangular steel baffle plates having dimensions of 3/ 8 X 4 X 7 inches. The voltage output of the gages is directly proportional to the pressure, and is given by J..- P
a E=-C
where K a is the gage constant, coulombs per pound per square inch; and C, total capacitance of the gage and the circuit in which it is used. Gage constants for the close-in gages ranged from 13.9 to 26.5 micro-microcoulombs per pound per square inch, and for the distant gages they ranged from 76.5 to 196. Calibration of the gages was ac-
complished by subjecting them to several known pressures between 0.8 and 9.2 p.s.i. in the Armour Research Foundation's 4-inch shock tube. The low pressure gages were also checked against a calibrated Altec-Lansing microphone system at pressures as low as 0.0012 p.s.i. generated by a speaker to check the gage liJ?earity in the very low pressure range. The relation between the velocity of propagation and the pressure in a shock wave in air is given by P s = 1.16 Po
C2 ( U~
1)
where P s is excess pressure of the shock front; Pm atmospheric pressure ahead of the shock; U, shock front propagation velocity; and C, velocity of sound in still air ahead of the shock front. The measurement of the shock front propagation velocity, therefore, constitutes a convenient method of determining shock pressure, if the shock pressures are not too low. A foil-break probe system was used to detect the shock front. Holes 2 inches in diameter were drilled about 3/ 4inch deep at 2-foot intervals along a weoden two-byfour. One-half-inch wide aluminum foils, notched down to 1/ 16 inch in the center, were taped across the
openings. Pieces of Saran film were then taped over the foils. Breaking the foils unshorted resistors, resulting in voltage changes on the output cable. The circuit was balanced so that an up-and-down step function resulted as various foils were broken by the shock front. Because only one Tektronix 551 dual-beam scope was available for the first few experiments, the D, E, and F pressure gage signals were amplified and recorded on an additional 4-channel oscillographic unit. A sequence timer was used to synchronize the 'recording camera on the 4-channel unit, the Fastax camera, and the firing of the charge. Shock arrival probes were used to trigger the sweep of the two Tektronix scopes.
Test Procedure
Six I-pound charges were detonated in the cell, followed by one 2-pound charge, one 4-pound charge, three 8-pound charges, one 16pound TNT charge, and a 50-pound charge. The charges were all 10cated near the center of the cell floor and were supported about 2 feet above the floor by a wooden fourby-four. Number 6 electric blasting caps were used to initiate the dynamite charges, and firing voltage
was supplied by two 6-volt "hotshot" batteries in series. To pe~mit synchronization with the instrumentation, the firing circuit was closed automatically at the instrumentation trailer. When high speed photography was employed, the firing circuit was closed at the camera remote-control unit. Positions of the D, E, and F gages were varied for some of the shots, and the A gage was moved in to 100 feet for a few shots. The number and position of shock arrival time probes were also varied during the experiments. The Band C gages were located in the same positions for each of the experiments. The wooden wall in front of the cell, simulating the concrete safe wall, did not withstand the effects of charges of 2 pounds or more. The wall was rebuilt twice, and after the first 8pound shot it was no longer used. Test Results
Except for damage to the side door mounts, no damage to the cell was detected until after the first 8-pound test. In this shot, some concrete fell from an area across the top of the vent opening or mouth of the cell. The discolorations in the concrete in this area indicated that the concrete had been improperly mixed or settled.
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NOVEMBER 1961
129 A
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Schematic diagram of instrumentation system
The second and third 8-pound charges caused only slight additional damage to the cell. The overhead curtain support rail was torn off, a small patch of concrete fell from the inside of the rear wall opposite the charge position, and some additional concrete fell from above the mouth opening. The 16-pound TNT charge caused several cracks in the side walls and the interior wall but no significant deformation' of the basic cell structure. The 50pound charge, which was considerably in excess of the energy design limit of the cell, did cause extensive structural damage; however, frag-
D GAGE
Map shows layout of test area
ments from steel members placed on the charge were not projected from the cell. The motion picture records of the TNT test show afterburning of the product gases outside the mouth of the cell. It is also apparent that much unreacted carbon was present. This information supports the opinion that afterburning within the cell does not result in significant additional damage to the cell structure and that the energy release criterion should be the heat of detonation rather than the heat of combustion. In the experiments involving charges of 4 pounds and larger, the
pressures recorded from the gages located near the cell seemed too low, and the shock propagation velocities indicated by the foil-break probes were only slightly above sonic. Fastax films of the experiment revealed that the recorded pressures were lower than expected because two shocks were produced. A weak shock preceded the strong shock from the mouth of the cell. This weak shock apparently is caused by leakage around the edges of the baffle plate in the cell and through
Continued on page 132 A
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