Crash resistant container

A container for hazardous materials capable of protecting the enclosed materials from high speed impact. Energy absorption is provided by a multiplicity of crushable layers of either wire mesh or perforated metal sheets which thin and flow together under impact loading. Layers of a higher tensile strength material are interspersed within the crushable layers to confine them and increase performance.

BACKGROUND OF THE INVENTION 
This invention relates to an impact-resistant energy absorbing container 
for transporting a load, particularly a load of hazardous materials. 
The storage and transportation of hazardous materials presents a number of 
unique challenges. Some of the prior art reflects solutions to the problem 
of shielding nearby objects from the effects of explosion of the hazardous 
material by designs that damp the shock waves and confine the debris 
produced by the explosion. Other references show construction details of 
various containers that shield a load from impacts, fire, puncture, 
immersion, and the rest of the variety of adverse conditions that can 
adversely affect the load during storage and transportation. The need for 
a container that can withstand all of the more normal types of impacts 
contemplated by the prior art structures as well as those contemplated by 
new government regulations covering the air transportation of nuclear 
materials has now created a requirement for a container with capabilities 
to withstand very high speed impacts with subsequent fires and immersion 
without the escape of the material within the load into the environment. 
The present invention is the only container design known that is capable 
of meeting this increased level of protection. 
SUMMARY OF THE INVENTION 
The load may be either a solid (granulated, powder or a continuous solid) 
or a fluid that is contained within a relatively simple vessel that is not 
necessarily designed for impact resistance. This load is surrounded by the 
inner wall of the container of this invention which restrains the load 
from any but insignificant movement of the load relative to the container. 
An intermediate region of the container then surrounds the inner wall. 
This intermediate region has a large number (at least 200) of layers of a 
plastically deformable metallic mesh or perforated sheets in which are 
interspersed a smaller number of layers of a high tensile strength mesh 
that act to confine the plastically deformable layers so that ruptures in 
the lower strength deformable layers do not propagate past the 
interspersed high tensile layers. For applications that require shielding 
from external fires, additional layers of non-combustible material are 
provided in this intermediate region or at the outer perimeter. An outside 
wall is provided for the container to shield the intermediate region from 
normal wear and tear and also puncture and fluid infiltration in the event 
of a high speed impact. Such a container is able to withstand impacts at 
over 400 feet per second without unacceptable damage to the load.

DETAILED DESCRIPTION OF THE INVENTION 
The container described herein is designed to meet new requirements for 
impact, fire and puncture resistance and to be able to be scaled up or 
down to meet a wide range of requirements for various contents and 
regulations. It utilizes a very robust primary container for protection 
and confinement of an inner containment vessel which carries the actual 
contents/load. This primary container has an outer wall or shell, an 
intermediate region with multiple layers of plastically-deformable 
metallic wire mesh or perforated sheets and high-tensile strength 
materials, and an inner wall surrounding the inner containment vessel 
carrying the load. This provides energy absorption for the load as well as 
thermal protection. The use of intermittent layers with high tensile 
strength results in a limiter which remains in place during accidental 
impact events and can be relied upon to provide subsequent puncture and 
fire protection. In addition, the outer wall around the energy absorbing 
region is provided for handling and weather protection. 
The design has been validated by scoping tests with various material 
samples, wall samples and partially modeled prototypes. Finite element 
analyses have been performed to evaluate various design features. 
Multiaxial compression and tension experiments were performed on various 
candidate materials to obtain appropriate material properties for the 
model. Scale model containers were subjected to side and end impacts for 
analysis. Prototype scale model packages were fabricated and subjected to 
129 meters/second side impact and 200 m/s end impact tests. Test results 
indicated that the container would remain intact throughout a worst case 
accident and that structural loads on the inner containment can be limited 
to the extent necessary to maintain its integrity. 
The container was designed to satisfy and exceed the requirements for a 
large plutonium air transport package as prescribed in NUREG-0360 and the 
more recent Public Law 100-203. The sequential test environments in 
NUREG-0360 require that a load weighing more than 227 kg must be subjected 
to and not release an A2 quantity of material in one week are: (1) a 129 
m/s perpendicular impact onto a flat unyielding target in the most severe 
location, (2) a 3-m drop onto a conical steel puncture probe in the most 
severe location, (3) two slash tests by a 45-kg section of structural 
steel dropped 46 m onto the package, (4) a fully-engulfing JP-4 fire test 
for a period of no less than one hour, and (5) a 1-m submersion test in 
water for a period of 8 hours. Recent U.S. legislation (U.S. Public Law 
100-203) also requires that foreign shipments of plutonium through U.S. 
airspace be able to withstand a worst-case aircraft crash; therefore, the 
requirements for containers used for these applications are expected to be 
even more severe. Although designed for this application, this invention 
will find a variety of other uses for other types of loads, be they 
hazardous materials or whatever. In these other applications, 
modifications to the specific embodiment discussed below may be made to 
provide for a better match to these other applications. For example, there 
may be situations for which fire protection is not necessary. 
The container has three elements. The first is the inner wall or shell. If 
the load contents are confined by an inner containment vessel of 
sufficient strength, this inner containment vessel wall can serve as the 
inner wall of the container. It would be more usual however to provide for 
a separate inner wall for the container which surrounds the wall of the 
inner containment vessel and closely fits around it to prevent significant 
relative movement between the two. The second element is the intermediate 
energy absorbing region. Multiple layers of wire mesh or perforated sheets 
are provided as the primary energy absorbing element. This material may 
have various wire sizes, various mesh spacings in the case of the mesh, 
various thicknesses, hole diameters and spacings for the perforated 
sheets, and may be aluminum, corrosion resistant steel, titanium, or other 
suitable material depending on the requirements. Within the wire screen or 
perforated sheet layers are interleaved layers of high tensile strength 
fabric which act to confine the wire layers in an impact and for puncture 
protection. This material may be polyaramid fiber cloth, S-2 glass, 
graphite, or other suitable cloth-like material depending on the 
application. Layers of insulation material may also be included if 
necessary for thermal protection. This may be interleaved with the wire 
mesh layers and also employed as multiple layers at the outside surface of 
the intermediate region. The radial thickness of the energy absorbing 
region will normally range from about 6 inches to 24 inches or more 
depending on the anticipated impact. Finally, the third element is the 
outer wall or shell which covers and protects the materials in the 
intermediate region. This outer wall may be made of corrosion resistant 
steel, aluminum, resin impregnated cloth or other suitable material. For 
air transport containers resistant to very high speed crash impacts, the 
weight ratio of load to container will normally be less than about 15%. 
For ground transport containers, the ratio can range much higher to about 
75% depending on impact speeds. Clearly, the higher speed impacts require 
greater thicknesses for the energy absorbing regions. 
The actual energy absorption happens through the crushing and plastic flow 
of the metallic mesh or the perforated sheet. In the mesh, the wires flow 
together and end up as a very much flattened and reduced thickness, 
quasi-continuous solid layer in their end state. The perforated sheets 
thins as the material in the sheets flows into the holes and again forms a 
continuous solid sheet in its end state. 
A specific embodiment capable of carrying 7.8 kg of plutonium was developed 
and is shown in the drawing figure. The container 1 utilizes a robust 
inner wall 12 fabricated from a titanium alloy with a 2.5 cm sidewall that 
can carry various configurations of inner containment vessels 20. It is 
desirable to provide for the insertion of packing material between the 
inner containment vessels 20 carrying the load and the inner wall 12 of 
the container to prevent significant relative movement between these two 
elements. This packing is not shown in the drawing. The intermediate 
energy absorbing region is here realized as three subregions, the lateral 
subregions 7 in which the layers are oriented parallel to the longitudinal 
axis 16 of the container and the two stepped end plugs 5, 5' in which the 
layers are oriented perpendicular to the longitudinal axis. The layers in 
the lateral subregion are 60 cm thick at the widest portion, and the 
layers in the end plugs are a maximum of 120 cm thick. The end plugs are 
held in place by keyed pins 13 and bolts 15. The outer wall has three 
separate subelements, upper end 17, lower end 11, and lateral area 10, 
corresponding to the three subregions of the intermediate energy absorbing 
region. The outer wall was made of 1.5 mm thick 304 stainless steel. 
Many static tests were performed on small samples and wall sections of 
various wire mesh and high-tensile strength cloth materials. Dynamic tests 
were then performed on scale model prototypes. The preferred material for 
air transport applications was found to be aluminum wire mesh. The 
high-tensile strength cloth materials had less utility as energy absorbers 
but were very necessary to provide confinement of the wire mesh, to spread 
the load from a puncture environment over a much larger area, and to 
provide a degree of thermal protection for the contents of the 
intermediate energy absorbing zone. 
Several radial sample wall sections with polyaramid cloth included a 
multiple locations in the wire mesh were tested to failure by crushing in 
the same configuration as a dynamic side impact test. A significant 
improvement in confinement was observed with the addition of the 
polyaramid cloth, approximately a factor of four over a configuration 
without the cloth. 
The data from these tests were used to design and fabricate a simple 
quarter-scale wire mesh model capable of carrying 8 kg of PuO2 but 
actually filled with lead shot. The construction of this model was 
somewhat simpler than that shown in the drawing in that this model did not 
have an outer shell. During manufacture of the quarter scale model, the 
layers of the energy absorbing region were wound around the inner 
containment vessel rather than stacked as they were for the multiaxial 
compression tests. The winding process generates a compressive stress 
between the layers, and the layers are therefore precompressed prior to 
the impact event. The amount of precompression will depend on the winding 
tension and radial location in the winding. Layers nearer the inner 
confinement vessel will be precompressed significantly more than those 
near the outer surface of the structure. The cylindrical portion of the 
model, corresponding to the lateral subregion 7 of the drawing, contained 
30 layers of Kevlar.RTM. aramid fabric and 208 layers of aluminum screen 
wire interleaved together. The aramid fabric layers alone will give the 
energy absorbing region an initial tensile strength of 8615 psi in a 
circumferential direction. As the layered material is crushed, the spacing 
between the aramid fabric layers will decrease as the aluminum screen 
layers are crushed. This will act to increase the total tensile strength 
of the crushable region if the aramid layers are not damaged. Simulations 
indicate that tensile loading on the aramid layers for 425 feet per second 
impacts will not exceed the breaking strength of these layers. The overall 
diameter of the model was about 12 inches, with the inner containment 
vessel being about 2 inches in diameter. 
This model was subjected to a side impact reverse ballistic test at the 3 
km rocket sled track at Sandia National Laboratory-Albuquerque. A 273 kg 
steel target mounted to a rocket sled with a catcher box was impacted onto 
the test model at 129 m/s (approximately 425 feet per second). The 
intermediate energy absorbing region or overpack remained completely 
intact, and the inner containment vessel carrying the load, which was 
fabricated of low carbon steel, sustained minimal (approximately 3% 
maximum at the center of the cylinder) deformation. The energy absorbing 
region deformed radially about 2.75 inches from its original radial 
dimension of about 5 inches. The low carbon steel shell around the 
simulated load ovalized and deformed 0.082 inches near its midplane. 
A modified quarter scale model of the air transport container shown in the 
drawing was subjected to an end impact with an impact velocity of 650 feet 
per second (fps) at the rocket sled test facility. The model omitted the 
outer shell and included aluminum spreader plates as part of the end caps. 
These spreader plates were located at the level of the locking pins 13 and 
extended radially to about the position of these pins. Also, a single shot 
container was used, and the remaining vertical space at the interior of 
the container was filled with further layers of aluminum screen/aramid 
fabric as well as two spacers made of multiple layers of 0.060" perforated 
aluminum sheets having 0.125" diameter holes spaced 0.200" apart located 
above and below the simulated load container. The perforated spacers 
performed in crush tests much like a rigid foam but have the advantage of 
nonflammability. The overall diameter was 14.5 inches and the overall 
height was 32.0 inches. The end caps were made by simply stacking up the 
layered material inside a stainless steel shell and manually compressing 
the layers an undefined amount. These end caps had approximately 60 layers 
of the wire mesh and 4 layers of aramid fabric per inch of thickness. An 
end-on impact test at 650 fps was conducted. The titanium inner 
confinement vessel containing the lead shot had its maximum outside 
diameter increased 0.10 inches due to liquification of the lead shot which 
caused a great increase in hydrostatic pressure resulting in the bulge. 
Subsequent analysis indicated that the deformation would not have occurred 
absent this melting of the lead. 
A similar quarter scale air transport container model as constructed for a 
side impact test at 428 fps. This model omitted the aluminum spreader 
plates and the perforated aluminum spacers but included a 304 stainless 
steel shell. The cylindrical portion corresponding to the lateral portion 
7 in the drawing contained 24 layers of aramid fabric and 374 layers of 
aluminum screen wire. The aluminum mesh had a wire diameter of 0.011 
inches and a tensile strength of about 75 pounds per inch of width. The 
Kevlar.RTM. fabric had a thickness of 0.017 inches and a tensile strength 
of about 1400 pounds per inch of width. Simulation indicated that at least 
some of the aramid layers would fail at the higher 650 fps impact 
loadings, and it is recommended that more aramid fabric layers be included 
for these conditions. The inner containment vessel was made from titanium 
for this model and did not deform at the 428 fps impact level. 
Thermal tests were also conducted. A partial one dimensional test article 
with the same composite makeup as an actual prototype was fabricated for 
each test. The test articles were subjected to a thermal environment for 
30 minutes and for 60 minutes to establish the preliminary thermal 
properties for the container. Initial results indicate that, depending on 
the heat load generated by the load itself (fissile materials generate 
heat in the load), the inner containment vessels will remain below the 
maximum allowable temperatures. Analyses performed on a package with 
approximate dimensions of only one fourth those for this package indicate 
temperatures would be below the allowable temperatures for elastomeric 
seals on the inner confinement vessel in a 30 minute fire. The results 
were conservative and did not account for heat flow through the outer wall 
of the container, indicating that the temperature rise in a fire is not a 
major design problem so long as the inner containment vessel remains 
surrounded by the overpack.