U.S. patent number 3,608,677 [Application Number 04/764,892] was granted by the patent office on 1971-09-28 for fragmenting tube energy absorber.
This patent grant is currently assigned to North American Rockwell Corporation. Invention is credited to Donald H. Wykes.
United States Patent |
3,608,677 |
Wykes |
September 28, 1971 |
FRAGMENTING TUBE ENERGY ABSORBER
Abstract
A frangible tube energy absorber is described including means
for inhibiting propagation of longitudinal fracture of the tube. In
a preferred embodiment the means for inhibiting propagation of
longitudinal fracture comprises means for applying a radially
inwardly directed force against the outside of the tube also
inducing a higher frequency of transverse cracking to increase the
amount of energy absorbed as well as improve the reliability and
reproducibility.
Inventors: |
Wykes; Donald H. (Downey,
CA) |
Assignee: |
North American Rockwell
Corporation (El Segundo, CA)
|
Family
ID: |
25072084 |
Appl.
No.: |
04/764,892 |
Filed: |
October 3, 1968 |
Current U.S.
Class: |
188/376;
244/100R |
Current CPC
Class: |
F16F
7/127 (20130101) |
Current International
Class: |
F16F
7/12 (20060101); F16f 007/12 () |
Field of
Search: |
;188/1C ;244/100 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Reger; Duane A.
Claims
What is claimed is:
1. A frangible tube energy absorber comprising:
a frangible tube;
a die at one end of said tube and including a die radius for
fracturing said tube into a plurality of discontinuous fragments
upon longitudinal stroking of said tube toward said die;
means for applying an inwardly directed force against the outside
of said tube adjacent said die radius, said means including a
surface portion temporarily movable longitudinally with said tube
opposite the die during fracturing of said tube, and wherein said
surface portion is temporarily movable longitudinally in a
direction opposite to the direction of said tube stroking, whereby
propagation of longitudinal cracks above the die portion is
inhibited for preventing premature failure of said tube; and
comprising an elastically stretchable torus around said tube and
having an internal surface portion engaging said tube.
2. An absorber as defined in claim 1 wherein said torus comprises
an elastomeric ring having sufficient elastic elongation to roll
along said tube.
3. A frangible tube energy absorber comprising:
a frangible tube;
a die at one end of said tube and including a die radius for
fracturing said tube into a plurality of discontinuous fragments
upon longitudinal stroking of said tube toward said die;
means for applying an inwardly directed force against the outside
of said tube adjacent said die radius, said means including a
surface portion temporarily movable longitudinally with said tube
opposite the die during fracturing of said tube, and wherein said
surface portion is temporarily movable longitudinally in a
direction opposite to the direction of said tube stroking, whereby
propagation of longitudinal cracks above the die portion is
inhibited for preventing premature failure of said tube; and
comprising a plurality of rollers around said tube each of said
rollers having a concave tube engaging face with a constricted
center portion and enlarged ends.
4. An absorber as defined in claim 3 wherein said rollers each
comprise a plurality of raised portions on said concave face for
controlling fracture patterns of said tube.
5. An absorber as defined in claim 4 wherein said raised portions
include ridges arranged transverse to the axis of said tube for
inhibiting propagation of longitudinal fractures of said tube above
said die.
6. An absorber as defined in claim 5 wherein said ridges are
continuous substantially entirely along the length of the roller
face.
7. An absorber as defined in claim 5 wherein said ridges are
intermittent along the length of the roller face and are arranged
in rows with raised portions in successive rows being staggered
from raised portions in preceding rows.
8. A frangible tube energy absorber comprising:
a frangible tube;
a die at one end of said tube and including die radius for
fracturing said tube into a plurality of discontinuous fragments
upon longitudinal stroking of said tube toward said die;
means for applying an inwardly directed force against the outside
of said tube adjacent said die radius, said means including a
surface portion temporarily movable longitudinally with said tube
opposite the die during fracturing of said tube, and wherein said
surface portion is temporarily movable longitudinally in a
direction opposite to the direction of said tube stroking, whereby
propagation of longitudinal cracks above the die portion is
inhibited for preventing premature failure of said tube; and said
means for applying a force comprising a plurality of balls bearing
on said tube.
9. In a frangible tube energy absorber having a frangible tube and
a die at one end of said tube for fracturing said tube into a
plurality of discontinuous fragments upon longitudinal stroking of
said tube toward such die, the improvement comprising:
means for both promoting formation of cracks transverse to the tube
axis and for inhibiting propagation of cracks parallel to the tube
axis;
said means for promoting transverse cracks and inhibiting
longitudinal cracks comprising means for applying an inwardly
directed force against the outside of said tube adjacent said die
radius;
said means for applying a force including a surface portion
temporarily movable longitudinally with said tube during
fragmenting thereof; and a plurality of balls around said tube and
bearing on the outside thereof.
10. In a frangible tube energy absorber having a frangible tube and
a die at one end of said tube for fracturing said tube into a
plurality of discontinuous fragments upon longitudinal stroking of
said tube toward such die, the improvement comprising:
means for both promoting formation of cracks transverse to the tube
axis and for inhibiting propagation of cracks parallel to the tube
axis;
said means for promoting transverse cracks and inhibiting
longitudinal cracks comprising means for applying an inwardly
directed force against the outside of said tube adjacent said die
radius;
said means for applying a force includes a surface portion
temporarily movable longitudinally with said tube during
fragmenting thereof; and an elastically stretchable torus in tight
engagement around said tube.
11. A frangible tube energy absorber having a frangible tube and a
die at one end of said tube for fracturing said tube into a
plurality of discontinuous fragments upon longitudinal stroking of
said tube toward such die, the improvement comprising:
means for both promoting formation of cracks transverse to the tube
axis and for inhibiting propagation of cracks parallel to the tube
axis;
said means for promoting transverse cracks and inhibiting
longitudinal cracks comprising means for applying an inwardly
directed force against the outside of said tube adjacent said die
radius;
said means for applying a force including a surface portion
temporarily movable longitudinally with said tube during
fragmenting thereof; and a plurality of rollers around said tube
and bearing on the outside thereof.
12. In a frangible tube energy absorber having a frangible tube and
a die at one end of said tube including a die portion for
fracturing said tube into a plurality of discontinuous fragments
upon longitudinal stroking of said tube toward such die, the
improvement comprising:
indentations formed on the surface of said tube and disposed at an
angle to the tube axis for both promoting formation of cracks
transverse to the tube axis and for inhibiting propagation of
cracks parallel to the tube axis; and
said indentations are stress-raising indentations in a tube
surface, said indentations each extending no more than a short
length.
13. A frangible tube energy absorber as defined in claim 12 wherein
said indentations are formed as said tube is stroked into said
die.
14. A frangible tube energy absorber as defined in claim 12 wherein
said indentations are formed before said tube is assembled onto
said die.
15. A frangible tube energy absorber as defined in claim 14 wherein
said indentations are on the outside surface of said tube.
Description
BACKGROUND
Frangible tube energy dissipation devices have been found
advantageous in many applications because they are highly efficient
in terms of the quantity of energy dissipated per unit weight of
the device. Thus they are often preferable to friction brakes,
fluid-containing shock absorbers, crushable balsa wood, crushable
honeycomb, and the like. A typical frangible tube energy absorber
is described and illustrated in U.S. Pat. No. 3,143,321 entitled,
"Frangible Tube Energy Dissipation" by J. R. McGehee, et al. In
this device a strong frangible tube such as aluminum alloy, steel
or plastic reinforced with glass fiber, boron fibers, graphite or
the like, is forced onto a die or anvil having an outwardly flaring
radius acting against the interior of the tube upon stroking
thereof to enlarge the tube beyond its ultimate strength thereby
causing fracturing of the tube. The die radius rapidly and
nonlinearly flares outwardly so that a biaxial stress is applied to
the tube thereby causing both longitudinal and transverse cracking
and hence fracture into a plurality of fragments. In this process
energy is absorbed both in the plastic deformation and in
fracturing of the tube. The quantity of energy absorbed is
determined by the strength of the tube material, the crack
propagation properties of the material, and the geometry of the
die.
Another type of energy absorber employs a tube that is plasticly
deformable and a die that splits the tube longitudinally with the
ribbons so formed being curled. Longitudinal slits may be provided
in such tubes to promote longitudinal splitting. The energy
absorbed is a function of the yield strength of the tube material,
whereas, in the frangible tube absorber, it is also a function of
the ultimate strength of the tube material. Since the frangible
tube is not only plasticly deformed but also fractured
transversely, greater energy absorption per unit weight may be
achieved. In addition to employing different tube materials for
fragmenting tube absorbers as compared with deformable tube
absorbers, the dies are of somewhat different design to effect the
two diverse types of energy absorption.
In the past the fragmenting tube type of energy absorber has not
reached its full efficiency in terms of energy absorbed per unit of
weight because control over the mode of fragmentation has been
poor. This lack of control results in erratic load fluctuations
during tube deformation and rather poor tube-to-tube
reproducibility. Occasionally a crack will propagate substantially
the full length of the tube or plastic deformation will take place
during stroking resulting in a minimal amount of fracturing and a
relatively low value of energy absorption. These difficulties are
particularly pronounced in high-strength tubes (for example,
250,000 p.s.i. ultimate) and in the tube-wall thickness to die
radius ratios which should give the maximum energy absorption.
SUMMARY OF THE INVENTION
Thus in the practice of this invention according to a preferred
embodiment there is provided a frangible tube energy absorber
comprising a frangible tube, an enlarging die at one end of the
tube for fracturing the tube into a plurality of fragments, and
means for controlling the mode of fracturing the tube.
DRAWINGS
Attendant advantages of this invention will be readily appreciated
as the same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings wherein:
FIG. 1 illustrates, in perspective, a frangible tube energy
absorber incorporating principles of this invention with a
stretchable torus surrounding the tube for inhibiting propagation
of longitudinal fracture;
FIG. 2 shows a longitudinal section of the tube of FIG. 1;
FIG. 3 comprises a cross-sectional view of an energy absorber
having rollers for inhibiting propagation of longitudinal
fracture;
FIG. 4 comprises a top view of the energy absorber of FIG. 3;
FIG. 5 illustrates a roller with ridges for use in the embodiment
of FIG. 3;
FIG. 6 illustrates an alternative ridged roller for use in the
embodiment of FIG. 3;
FIG. 7 shows a frangible tube energy absorber having a pattern of
shallow grooves for controlling fracture;
FIGS. 8A to 8D illustrates alternative patterns of grooves for the
frangible tube of FIG. 7;
FIG. 9 shows a cross section of a frangible tube energy absorber
having a fixed ring for restraining an end of the tube;
FIG. 10 comprises a top view of the energy absorber of FIG. 9;
and
FIG. 11 comprises a cross section view of a fixed ring-restraining
member with a plurality of balls in the ring for engagement with
the tube upon stroking.
Throughout the drawings like reference numerals refer to like
parts.
DESCRIPTION
FIG. 1 illustrates a frangible tube energy absorber incorporating
the principles of this invention. As illustrated in this embodiment
there is provided a die or anvil 10 onto which a frangible tube 11
is pressed during operation of the absorber in substantially the
same manner described and illustrated in the aforementioned U.S.
Pat. No. 3,143,321. Upon stroking of the tube onto the die 10 the
tube is fractured into a plurality of fragments 12 thereby
absorbing energy in the bending and rending of the tube. This
operation can also be seen in FIG. 2 wherein the tube is moved
downwardly onto a die radius 13 on the die 10 (or the die moved
upwardly onto the tube). The die radius 13 nonlinearly flares the
end of the tube 11 as the tube is stroked downwardly thereby
imparting large magnitude biaxial stresses on the tube yielding
large plastic flow and many fractures 14 extending both tranversely
and longitudinally of the tube to produce a plurality of separate
discontinuous fragments 12.
The embodiment of FIGS. 1 and 2 also includes a torus 16 of
elastically stretchable material, such as high-strength rubber, or
other elastomer, surrounding the tube at a point directly opposite
the die radius 13 during operation of the energy absorber or
dissipater. If desired the rubber can be reinforced with glass
fibers or the like to increase the elastic modulus. The original
position of the frangible tube and torus relative to the die before
stroking of the tube is shown in phantom in FIG. 2. Thus it can be
seen that the torus is originally arranged near the end of the tube
near a tapered end 15 (such a tapered end is conventionally
employed to permit gradual increase of force rather than an
instantaneous increase of force as would be obtained with a
blunt-ended tube).
Upon stroking of the tube towards the die the torus is carried
along with the tube down to a point opposite the die radius due to
frictional engagement with the tube. When the torus reaches the
bottom of its possible travel a substantial portion of the surface
is frictionally engaged with the tube as the tube flares outwardly.
This causes a "rolling" of the torus along the length of the tube
as the tube progresses through the stretchable torus. This action
is substantially identical to that commonly obtained in rolling a
rubber O-ring onto an unlubricated shaft. The torus has an internal
surface portion moving with the tube opposite the die radius and
this portion then moves outwardly opposite the outward flare of the
die, then moves upwardly in the direction opposite to the direction
of tube-stroking and finally moves back inwardly to reengage the
tube.
In rolling on the tube opposite the die radius an inwardly and
downwardly directed force is applied to the tube by the torus. This
force constraining outward expansion of the tube at a critical
region at the commencement of the die radius acts to reinforce the
tube in a region subjected to high stresses tending to induce
longitudinal cracking. The presence of the restraining force
directed radially inwardly on the tube by the torus inhibits the
propagation of longitudinal fractures above the die radius thereby
substantially reducing premature tube failure for enhancing the
reproducibility and efficiency of energy absorption from one tube
to another. Inhibiting longitudinal cracking in the tube also
encourages transverse cracking with enhanced energy absorption as
compared with an unrestrained tube. Pressure of the elastically
deforming torus inwardly and downwardly into the die radius forces
the material of the tube to fragment more frequently into smaller
fragments of more consistent size and geometry in both the
longitudinal and transverse directions and restricts longitudinal
fragmentation of the tube to a region well into the die radius,
thereby effectively restraining the propagation of cracks that
yield erratic fragmentation and thereby preventing large
fluctuations in load during tube stroking. Rolling of the torus
along the tube provides a continuing inward force on the tube
without a large increase in friction on the tube so that a
substantial tube length can be used without increasing the
possibility of tube buckling.
FIG. 3 illustrates in cross section an alternative embodiment of
restraining means for the tube in a frangible tube energy absorber
wherein the restraining means, as with the torus, includes a
portion that is temporarily movable longitudinally with the tube
opposite the die radius upon stroking of the tube. Thus in the
embodiment of FIGS. 1 and 2 the portion of the torus opposite the
die radius is temporarily moving with the tube and subsequently
moves out of contact with the tube and around the outside of the
toroidal ring to again engage the tube and move therewith near its
entry into the die radius. In the embodiment of FIGS. 3 and 4 a
similar effect is obtained by providing a plurality of rollers 17
surrounding the tube upon stroking and acting against its outside
surface at a location opposite the die radius to effectively
restrain the tube from expanding radially until well into the die
radius thereby inhibiting propagation of longitudinal cracking of
the tube beyond the die and increasing the frequency of failures to
provide a more uniform stroking force and energy absorption during
the length of the stroke. By increasing the frequency of failures
smaller fragments are obtained and variations between one fracture
and the next are smoothed out so that a more uniform
load-versus-displacement or force-versus-stroke relation is
obtained.
As is more clearly seen in the top view of FIG. 4, each of the
rollers 17 is journaled at each end in a roller support 18 so as to
be maintained in contact with the outside of the tube. The roller
supports 18 are radially extending ribs in which a bearing
extension 19 on the roller is journaled. If desired, the roller
supports need not extend outwardly to the maximum diameter of the
anvil. The roller supports 18 are bolted to the die or anvil along
with a debris cover 20 which serves to prevent fragments of the
tube from jamming between the tube and rollers.
Each of the rollers 17 has a "concave" face bearing on the tube;
that is, the diameter of the roller is larger at the ends than in
the middle so that the face of the roller mates with the outside
diameter of the tube. By selecting a die radius appropriate to the
rollers, the rollers are maintained in contact with the tube during
a substantial portion of the flaring. It will be apparent that in
operation the rotation of the roller due to motion of the tube
cannot be with the same peripheral speed at all points and hence
some portions of the roller face must be slipping relative to the
tube. This slipping and the consequent nonuniform frictional
engagement between the rollers and tube may cause local
deformations which enhance the energy absorption and further
enhance the favorable fracture patterns in the tube.
In order to take full advantage of the nonuniform frictional
engagement between the roller and the tube and promote more
controlled fracturing it is desirable in some circumstances to
employ a roller having a plurality of raised portions to
selectively engage the tube with higher forces in some regions than
in others. A roller so formed is illustrated in FIG. 5. As
illustrated therein, the roller 21 includes a plurality of raised
triangular ridges 22 extending along the face of the roller in a
longitudinal direction. It will be apparent that the longitudinal
ridges 22 on the roller are transverse to the longitudinal axis of
the tube when employed in an embodiment as illustrated in FIG. 3.
In operation, a roller as illustrated in FIG. 5 operates against
the outside surface of the tube with the raised ridges serving as
stress concentration points about which bending may preferentially
occur thereby inducing fracture at corresponding locations in the
tube. The transverse cracks so initiated induce very uniform
fracturing of the tube upon stroking thereof for optimum uniformity
of energy absorption. The transverse cracks also serve to interrupt
longitudinal cracks in the tube thereby effectively preventing
propagation of cracks along the full length of the tube to cause
failure at low stress.
The rollers are journaled so as to fit tightly against the tube
upon stroking, and if desired spring loading can be employed. The
rollers may be hardened steel and sufficiently tight against the
tube to indent the surface. This is not necessary and good results
are obtained with relatively rigid rollers of hard rubber or
plastic. The action of the roller is to afford a stress-raising
point in the manner of a fulcrum and indenting is not necessary for
this in every instance.
An alternative arrangement of roller with an irregular face in the
form of raised ridges is illustrated in the embodiment of FIG. 6.
As illustrated in this embodiment a plurality of short raised
ridges 24 are provided in rows on the face of a roller 25 for
engaging the outside surface of a tube as hereinabove described in
relation to FIG. 3. The raised ridges 24 which are also triangular
in cross section, are arranged in rows with each of the ridges
being of relatively short length compared with the length of the
roller and each of the raised ridges in one row is staggered from
the ridges in the adjacent rows. It will be readily appreciated by
one skilled in the art that other patterns of raised ridges may be
provided on roller surfaces to act as crack-initiating sources to
assure uniform fragmentation.
In order to control fracture patterns of the tube indentations in
the surface may be provided throughout the length of the tube or in
a selected portion thereof rather than relying on raised ridges on
a roller or a combination of both tube indention and roller ridges
may be employed. A tube so formed is illustrated in FIG. 7 wherein
a tube 27 has shallow grooves 28 in the external surface for
increasing stress and controlling fracture patterns. In the tube 27
the grooves are arranged in circumferential lines around the tube
and in short longitudinal segments along the length of the tube
between the circumferential grooves to yield an overall pattern
similar to that commonly employed in laying brick. A pattern of
this nature promotes both longitudinal and transverse fracturing
without providing a direct line for longitudinal cracking of the
tube since the longitudinal grooves are not extensively continuous
and extend only a short length along the tube. Uniform and
reproducible fragmentation is obtained by such texturing of the
tube surface.
It will be apparent that other patterns can be employed on the tube
surface as illustrated in FIG. 8. Thus, as shown in FIG. 8A, a
pattern of uniform squares may be employed on the tube surface.
Similarly, as shown in FIG. 8B, a series of short grooves
transverse to the tube axis may be formed on the surface in a
pattern similar to that formed by the roller hereinabove described
and illustrated in FIG. 6. Still another pattern of surface
texturing is illustrated in FIG. 8C wherein a plurality of short
grooves is provided transverse to the tube axis. In this embodiment
the grooves are arrayed in rows with alternate rows of grooves
having ends overlapping the ends of grooves in the intermediate
rows to inhibit longitudinal crack propagation.
Patterns such as those illustrated are readily machined, embossed,
or chemically etched into the tube surface on either the external
or internal portion. It is also contemplated to provide a pattern
of grooves on the internal surface of the tube since crack
initiation during tube fracturing is initiated at the inner
surface. A pattern of grooves particularly suitable for forming on
the inside of the tube is illustrated in FIG. 8D wherein multiple
long pitch, right- and left-hand threads are overlapped to produce
a diamondlike pattern on the surface. Such a pattern of fracturing
affords high-energy absorption since the relative crack length for
each fragment is greater than the crack length for substantially
square fragment of the same volume.
It will be apparent that many other patterns can be provided on
either the inside or outside of the tube for specific situations
and that various patterns and fragment-promoting sizes of pattern
could be combined sequentially along the length of a tube to change
the load/deformation curve along the length to suit specific
energy-absorbing requirements.
FIG. 9 illustrates in cross section a frangible tube energy
absorber having a fixed ring restraining an end of the tube
adjacent the die radius for inhibiting propagation of longitudinal
cracks in the tube. As illustrated in this embodiment, an anvil or
die 31 is employed for spreading and fracturing a tube 32. A ring
33 is rigidly secured to the die 31 by a plurality of bolts 34 and
is spaced from the anvil at a fixed distance by a plurality of
spacers 36 which are hidden from view in the top view of FIG.
10.
As the tube 32 is pressed onto the die 31 it is constrained in the
die radius 37 by the ring 33 which acts in substantially the same
manner as the rollers hereinabove described and illustrated in
FIGS. 3 and 4. It will be apparent that the ring does not have a
portion moving with the surface of the tube during stroking thereof
into the die and that a substantial friction force may be involved
between the ring and tube.
In order to minimize the friction force between a fixed ring and
the tube a low-friction coating such as Teflon
(polytetrafluoroethylene or fluorinated ethylene propylene) may be
applied to either the tube or ring to lubricate the interface
therebetween. The friction between the ring and tube in the die
radius can be alleviated by providing a plurality of captured balls
within the ring as illustrated in the cross-sectional view of FIG.
11. As illustrated therein a tube 39 is pressed onto a die 40. A
ring 41 surrounds the tube at the die radius for inhibiting
longitudinal cracking and promoting transverse cracking as
hereinabove described. Within internal cavities of the ring 41 a
plurality of freely rotatable balls 42 are provided which bear on
the tube opposite the die radius, upon stroking of the tube. The
balls are preferably closely spaced to provide support for the tube
substantially completely around its periphery and if desired, the
balls on the tube may be coated with a thin layer of Teflon or
other suitable solid lubricant for minimizing friction.
Obviously many variations may be made in the structure described
within the principles of this invention. Thus, for example, the
frangible tube can readily be combined with other types of energy
absorbers in a most efficient manner. A core of crushable honeycomb
or balsa wood can be provided within the tube and the crushing
thereof by the member stroking the tube enhances energy absorption
without substantial increase in weight. Likewise a second frangible
tube can be arranged concentrically with the first to increase
energy absorption without substantial increase in weight. The
frangible tube arrangement provided in practice of this invention
can include a plurality of rolling rings and an outer constraining
cylinder in the manner of the energy absorber developed by
Mechanics Research, Inc., wherein the rolling rings are plastically
deformed between the containing walls for energy absorption.
* * * * *