BALLISTIC IMPACT FACILITY - CONSIDERATIONS AND EXPERIENCES
Peter H. Bull and Jørgen A. Kepler
Aalborg University
Pontoppidanstraede 101
DK 9220 Aalborg East
Denmark
[email protected]
Abstract
A facility for testing of ballistic impact has been designed and developed. The goal was to develop a facility which was relatively easy to
use and modify as the requirements for the testing equipment changed with different research projects and students projects. The setup
consisted of a compressed air gun with an exchangeable barrel, a speed trap for measuring the incident velocity and a kinematic pendulum
which both served as a means to measure the residual energy of the projectile and to catch the projectile from further travel. The aim of this
paper was to cover some of the experiences encountered during the design and development high velocity impact test equipment.
Introduction
Facilities for testing of ballistic impact are regularly used, but usually, and of obvious reasons, only briefly described as common testing
equipment in papers covering ballistic impact tests of different types. The most common facility utilizes some kind of gun shooting
projectiles horizontally, but Nilsen et al. [1] used a facility which was vertically oriented shooting downwards. It was a compressed air gun
using a 10 bar pressure. The projectile was a 25 mm rod with hemispherical head that was instrumented with strain gauges. A high speed
camera and a reference grid were utilized to measure the impact velocity. Wen et al. [2] and Reddy et al. [3] used a compressed air gun for
their tests, and measured the velocity of the projectile on both sides of the test specimen using an electrical device. Roach et al. [4-5] used a
gas gun with nitrogen as propellant, and Wu and Chang [6] used a similar setup with an undisclosed compressed gas as propellant. In a
study of the influence of environment temperature on the impact damage, López-Puente et al. [7] utilized a SABRE gas gun which used
helium gas as propellant. The gun shot 7.62 mm projectiles into a climatic chamber which was encased within a steel box. Ulven et al. [8]
used a gas gun with a 3 m barrel which was connected directly to a capture chamber. The gun was used to propel 12.7 mm, or cal .50,
projectiles up to a velocity of 200 m/s. Incident and residual velocity of the projectile were measured with optical devices placed in front of
and behind the test specimen. A gun powder propelled apparatus was used by Flanagan et al. [9] and Flanagan [10]. It was located inside a
chamber which was evacuated of air during tests in order to reduce the influence of air shock waves on the measurement equipment. The
projectile velocity was measured inside the barrel of the gun, at the muzzle of the gun, and behind the test specimen. A soft catcher system
was designed to catch the bullets without damaging them. Awerbuch and Bodner, [11] and Marom and Bodner [12] used a setup with a
base to fix different types of guns, e.g. 7.62 mm standard military rifle, 0.22 caliber sport rifle and 9 mm semi-automatic gun. Photo diodes
were used to measure incident velocity and a wire grid which the projectile short circuited was used to measure the residual velocity of the
projectile. Bull and Hallström [13] used a 40 mm Bofors L70 anti aircraft gun at Bofors shooting range to achieve up to 1000 m/s impact
velocity with 40 mm inert projectiles on sandwich panels for marine applications. The incident and residual velocities were measured using
a Doppler radar, and the impact events were monitored using a high speed camera capable of a frame rate of 20.000 frames per second.
While the list of investigations mentioned above is not exhaustive it suggests that experiences from the development and use of ballistic
test facilities are usually left out of the description of the test facilities in papers concerning ballistic impact testing. The aim of this work is
to describe a facility for ballistic impact testing and the experiences gathered through its development so that other researchers can find
some help from this if they whish to build or develop their own facility for ballistic impact testing.
The facility
A chamber in the basement of a laboratory below a 1.5 m thick concrete floor contains the test facility. The main parts of the equipment are
shown in figure 1. They are; a compressed air gun, a blast shield to reduce the amount of air and possible debris following the projectile, a
speed trap used to measure the incident velocity of the projectile, a frame which is used to fix test specimens in, and a kinematic pendulum
which is used to catch the projectile and measure the residual velocity of the projectile.
The camber was initially intended to house a facility for testing turbines, thus the walls and roof were clad with 25 cm thick logs of
timber to stop turbine blades that may separate from the test apparatus. For reasons unknown the test facility was unused and found to be
suitable to house a facility for ballistic impact testing instead.
In the gun end of the chamber it was closed off with a pair of sliding doors constructed from a steel frame filled with the same timber
as the roof and walls. In order to reduce the possibility for unauthorized access during tests the doors were equipped with security cut off
switches which cut the power to the compressor feeding the gun.
The ceiling in the chamber was used as a floor for fixing test equipment in the laboratory above on the floor above. Thus the ceiling
had ample fixing points which was where the major equipment was fixed; the gun, the frame for test specimens and the kinematic
pendulum. In addition a pair of rails was bolted to the roof between the blast shield and the frame for fixing test specimens. It was used to
support the fixture for the speed trap, but could also be utilized to hold other test or measuring equipment in place.
FIGURE 1. Schematic of the facility for ballistic testing. From the left; the compressed air gun consisting of pressure chamber,
barrel and blast reducer, blast shield, speed trap, frame for fixing test specimens, and kinematic pendulum.
The gun
The main equipment was the compressed air gun, which is described in closer detail by Kepler [14]. The gun was fixed to the roof in the
gun camber, figure 2. It was built up in a modular way in order to relatively easily allow for modifications as new needs for testing would
arise. An important parameter was relative ease of use so that also students [15] would be able to use the gun with an absolute minimal
amount of risk. Therefore it was decided to use compressed air from a scuba dive compressor rather than e.g. gun powder. The use of
compressed air puts some limitations to the maximum possible projectile velocity, but by using of a lightweight air valve between the
pressure chamber of the gun and the barrel it was still possible to reach velocities up to 680 m/s using a 10 mm caliber barrel. By using an
over sized barrel with a smaller caliber projectile supported by a sabot it might be possible to increase the maximum velocity somewhat.
FIGURE 2. Compressed air gun with pressure chamber in the foreground, a box for a high speed camera and the rig for the
kinematic pendulum can be seen in the background.
One very practical aspect of using compressed air was that it put relatively small demands on ventilation in the gun chamber, and no
demands on safe storage when the gun was not used.
The gun was designed to use exchangeable barrels. At the time of writing, barrels for three different calibers were used; 10 mm, 20 mm
and 50 mm. Thus the range of projectile mass could be varied from about 5 g for the 10 mm caliber barrel to about 1 kg for the 50 mm
caliber barrel. A standard scuba dive compressor capable of a maximum pressure of 25 MPa was used to feed compressed air into the
pressure chamber at the rear end of the gun. A lightweight wedged valve, which would be quickly propelled back by the air pressure, was
used to keep the air within the pressure chamber until the gun was triggered.
Measuring equipment
In the receiving end of the chamber there was a kinematic pendulum similar to the one used by Robbins [16] and also described by Johnson
[17]. It served a double duty as it was used both to measure the residual energy of a penetrating projectile and it was also used to stop the
projectile from further travel. A linear voltage displacement transducer (LVDT) coupled to a HBM Spider 8 multi channel PC
measurement unit was used to measure the travel of the pendulum, figure 3. From the measured displacement the vertical travel, h, could
be calculated and used to calculate the residual energy of a projectile after passing through a test specimen according to the following
expression.
⎛
2l 2 − a 2 ⎞
⎟
h = l ⎜⎜ 1 −
4l 2 ⎟⎠
⎝
Where l is the effective length of the supports for the pendulum and a is the measured secant length of the arc traveled by the
pendulum. By measuring the travel with an LVDT there was a small error compared to the actual length of the secant of the arc traveled.
This was because the point of rotation of the LVDT, which was the point were it was fixed to the measurement rig, could not be coincident
with the point it was fixed to on the pendulum at rest. The error, however, was found to be less than 0.6 % within the movement range of
the kinematic pendulum.
FIGURE 3. Close up of the kinematic pendulum with the displacement transducer mounted to its side shown as a small cylinder
parallel to the pendulum.
The rig that held the pendulum, figure 4, was also used for fixtures for test specimens. The rig was manufactured from 80 mm square
cross section steel tubes and was fixed to the concrete roof using M30 bolts. Currently two different fixtures have been used, one which
could simulate fixed or simply supported boundary conditions, and one four point bending rig which was used to pre stress test specimens
in order to simulate more realistic boundary conditions for the test specimens.
FIGURE 4. Rig for mounting test specimens seen from the rear and the kinematic pendulum, with a steel box for mounting the high
speed digital camera to the right.
In front of the rig used to fix the kinematic pendulum and the test specimens a Shooting Chrony Master Gamma optical speed trap was
mounted, figure 5. This model of speed trap is commonly used on shooting ranges, thus it was capable of measuring the velocities
encountered, and it was also easy to use and relatively compact. It turned out, however, that the blast following the projectile damaged the
housing of the speed trap rather severely so that it had to be repaired repeatedly. Therefore the speed trap was mounted underneath a 22
mm thick plywood plate with holes cut out for the speed trap to “see” through. Because the camber is relatively dark some difficulties with
getting the speed trap to measure the projectile velocity was encountered. This was solved by placing a 150 W work light on each side of
the speed trap and putting a white plate in the roof above it.
FIGURE 5. Speed trap in the foreground in front of mounting frame for test specimens with the safety box for the high speed
camera to the left. The mirror used for the camera is shown in a fold back position in front of the camera box.
In addition to the fixed measuring equipment mentioned above the chamber was equipped with rails in the ceiling which could hold
other equipment. Further the HBM Spider 8 measurement unit which was used to sample the LVDT used to measure the travel of the
kinematic pendulum could also be used to sample e.g. strain via strain gauges fixed on test specimens or the fixture.
Test specimens and fixtures
The maximum size of the test specimens was limited to 60 cm x 60 cm. This was mainly governed by the height to the ceiling; if it was not
necessary to hit the test specimen in the centre, a larger test specimen could be used. A wider test specimen could also be used, but that
would limit the possibility to move around inside the chamber since it is relatively narrow. High speed camera footage of tests on
composite and sandwich plates showed that for test specimens 30 cm x 30 cm in size the boundary conditions had no or very limited effect
on the behavior of the test specimen. Therefore the test specimens were usually limited to that size when mounted in the fixed frame.
Most real life structures are subjected to some kind of loading during service. It was assumed that a pre loading of a test specimen
could affect the amount of damage sustained by impact damage; therefore a four point bending test rig was designed, figure 6. It was
designed so that it could load the test specimens both symmetrically and asymmetrically. The size of the test specimens was limited to 30
cm in height and 40 cm in width.
FIGURE 6. The rig designed to preload test specimens in bending.
High speed camera
An Olympus i-speed 2 high speed digital camera was purchased to monitor the impact events. The camera was capable of frame rates up to
33.000 frames per second. Ideally one would want as high a frame rate as possible when filming ballistic impact events. With the impact
velocities encountered in this facility the projectile would usually travel about 50 cm per millisecond. A frame rate of 33.000 frames per
second would therefore capture the projectile every 15 mm of its travel. The camera had a basic image resolution of 800 x 600 pixels.
However, the resolution would drop with increasing frame rate so that the resolution would be only 96 x 72 pixels at a frame rate of 33.000
frames per second. Another important detail discovered was that very much light was needed in order to film at high frame rates. Some
testing and evaluation of the results showed that using a frame rate of 8.000 frames per second would still capture relatively good pictures
of the traveling projectile. At that frame rate the image resolution was 256 x 192 pixels, which was found to yield quite good images.
Figure 7 shows an image of a cylinder of POM hitting a bar of lead at 425 m/s. The image is captured using a frame rate of 8.000 frames
per second. Two photo lamps of 650 W each were used, and aperture and shutter speed were adjusted for as good an image quality as
possible.
FIGURE 7. A cylinder of POM hitting a bar of lead at 425 m/s; captured using an Olympus i-speed 2 digital high speed camera.
Conclusion and discussion
The main aim for developing a facility for ballistic impact testing is obviously to suit ones needs for such test equipment. Therefore the
facilities vary according to whatever physical and financial limits exist, as well as the developers taste and needs. A limiting factor in the
design of the facility described here was the amount of available space. It put physical limits to the size of the gun and the distance between
the gun and the test specimen. To some extent it also limited the maximum velocity of the projectile, or rather the maximum kinetic energy
of it. Since the test facility is inside a building the projectile has to be stopped before it hits the buildings walls.
The use of propellant would put certain requirements on the facility. If gun powder was used, some kind of safe storage of the gun
powder would be required, as well as procedures for safe handling of it. In addition, again since this facility was inside, good ventilation
would be required. Good ventilation and safe storage would also be required if a compressed gas such as nitrogen or helium were to be
used since the human body’s warning systems do not react to high levels of these gases. By using compressed air there were limited
requirements to ventilation, and since a scuba dive compressor was hooked directly to the pressure chamber of the gun there were no
demands of safe storage of the compressed gas. The compressor would need 5 to 10 minutes to pump the pressure chamber up to the
required pressure, but the pressure would be released within seconds after the wanted pressure level was reached.
It was mentioned earlier that the blast of air from the gun following the projectile seriously damaged the unit used to measure the
velocity of the projectile. This happened even though the barrel of the gun was fitted with a blast reducing device. In order to reduce the
influence of the blast on the test specimen and the kinematic pendulum, a blast shield was placed between the gun and speed trap as shown
in figure 1. Tests using chalk did however show that the blast following the projectile was focused as a thin jet with a diameter of about the
same size as the diameter of the projectile, figure 8.
FIGURE 8. Blast of air following the projectile captured using chalk and a high speed camera.
The kinematic pendulum used was obviously affected by the blast of air following the projectile. It did, however, turn out to be very
difficult to separate the jet of air from the projectile. Some tests using a baffle with a pre drilled hole with the same diameter as the
projectile were conducted, but high speed photography showed that the projectile would not pass cleanly through the hole in the baffle. The
jet of air did not seem to affect the damage in test specimens; therefore it was assumed that the effect of the air blast on the kinematic
pendulum would be the same as long as the geometry of the test specimens was the same.
A high speed digital camera turned out to be a very valuable tool to use to study the impact event, but it was found that filming at high
frame rates put very high demands on the amount of light. In addition to a high frame rate one would want to use as small an aperture as
possible in order to get as good a depth of field as possible. A short shutter time would “freeze” the motion of the moving objects in each
frame thus making it easier to study the event. Together with as high a frame rate as possible, this was found to put very high demands on
the amount of light on the object. Obviously one would want to fill the chamber with as much light as possible, but even disregarding the
financial impact it would not necessarily be a good idea. During tests of pre stressed sandwich plates with face sheets of carbon fiber
laminate the two 650 W halogen photo lamps used heated up the carbon fiber laminates so much that they failed. Since composites can be
relatively sensitive to heat, some different approaches were investigated to solve the problem with the excessive heat. The most obvious,
and simplest, was to limit the time the test specimen was exposed to the light by keeping the light off until it was actually needed. In
addition the carbon fiber laminates facing the photo lamps were painted white.
Alternative light sources were also investigated, but all had some shortcomings which were not easily countered. A high power
electronic flash could emit sufficient amounts of light, but the duration of the emitted light was very short. An electronic flash emits light
for up to 0.5 milliseconds [18]. Ideally it should emit light for about 10 – 20 milliseconds, which would allow for the projectile to pass
through the image. A classic style flashbulb would emit light for about the specified time, but it took a certain amount of time to fully
ignite. A trigger system was mounted on the gun which could ignite Meggaflash PF330 flashbulbs [19]. It was, however, found that the rise
time for the flash bulb was longer that the time it took for the projectile to travel through the barrel of the gun and through the test
specimen. Thus, a trigger mechanism which first triggered the flash and then the gun was needed. Because of the short time span involved,
~10 milliseconds, this was not possible without serious redesign of the trigger mechanism of the gun. One possibility not yet tested would
be to use high intensity discharge (HID) lamps rather than halogen lamps. HID lamps emits up to five times the amount of light of a
halogen lamp with the same power consumption [20]. This would reduce the need for using many lamps and also the emitted heat.
Acknowledgements
The authors whish to acknowledge the contribution from the Danish research and development consortium Komposand. Classic style
flash bulbs were provided by Meggaflash Technologies Ltd.
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