Structural element with ribbing mechanically blocked against separation

A structural element suitable for walls, tanks and bulkhead structures comprising opposed, axially extended surface structures and ribbing interposed therebetween in freely movable relation responsive to a temporary first relative dimensional condition of the surface structures and ribbing, and in blocking relation against separation of the surface structures responsive to a persistent second relative dimensional condition of the surface structures and ribbing.

FIELD OF THE INVENTION 
This invention relates to structural elements, ranging from aircraft 
components to enclosures, which are lightweight, extremely strong, widely 
variable in size, configuration and material of construction, easy to 
manufacture with widely available equipment, and free of the problems 
associated with other forms of structural elements serving some of the 
same purposes, such as honeycomb panels. 
More particularly, this invention relates to structural elements having 
first and second axially extended surface structures spaced by ribbing 
mounted to said structures free of bonding agents, the ribbing being 
mechanically blocked against separation from the surface structure or 
unblocked for separation as a function of dimensional changes relatively 
between the ribbing and the surface structure ribbing mounting, e.g. 
temperature responsive expansion of the mounting and shrinkage of the 
ribbing. 
BACKGROUND 
Aircraft components including engine intake ducts, flooring and wing 
sections are frequently made of honeycomb material which is lightweight, 
rigid and capable of being fabricated in many shapes. Honeycomb is, 
however, expensive, difficult to work with, problematic when contour 
changes are required, awkward to attach other parts to, and not readily 
repairable. These drawbacks to honeycomb structural elements have been met 
by reinforcing the element where other parts are to be mounted, by adding 
brackets at junctions of internal members and, in general, beefing up the 
structural element, all at the cost of increasing its weight, thus 
lessening the most significant putative advantage of the honeycomb. 
Honeycomb further suffers the disadvantage of using bonding agents such as 
resin adhesives to bond the components together. Bonded together 
structural elements have increased material costs, possible bonding 
failures, and more complex and, thus, more expensive construction 
procedures. 
SUMMARY OF THE INVENTION 
It is an object, therefore, of the present invention to provide a 
structural element which affords the advantages of honeycomb but avoids 
the disadvantages. It is another object to provide a structural element 
which is widely variable in size and contour with no great change in 
manufacturing complexity or cost, which is lightweight while offering 
great strength, which is readily repaired, and which affords easy 
attachment of other components with no special post manufacturing steps. 
It is another object to provide increased ease of machining or otherwise 
fabricating the component structures, by using a ribbing integrally formed 
with or separately formed from the first and second structures. It is 
further an object to provide structural elements in which no bonding 
adhesives are required, and in which the connection of parts is through 
mechanical interlocking. The mechanical interlocking is typically achieved 
through the divergent dimensional response of the locking parts under 
temporarily selected conditions. Thus interfittable parts are brought to 
different temperatures, e.g. on opposite sides of ambient temperature so 
that the physical dimensions of each of parts to be locked together are 
slightly increased and decreased respectively, for fitting together. Then 
the parts are returned to a predetermined e.g. ambient temperature 
condition, and they return to their normal dimensions at such temperature 
condition, but now interlocked against separation. 
These and other objects of the invention, to become apparent hereinafter, 
are realized in a structural element comprising opposed first and second 
axially extended surface structures, a pattern of ribbing extending 
between and spacing the surface structures, ribbing mounting means on the 
surface structures in registered relation with the ribbing, the ribbing 
being mechanically blocked against separation from the ribbing mounting 
means when the mounting means and ribbing are each at a predetermined 
temperature condition and mechanically unblocked when the mounting means 
and ribbing deviate differentially, i.e. vary in different directions or 
in the same direction but at different rates, from the predetermined 
temperature condition. 
In this and like embodiments, typically, the first and second surface 
structures generally lie in parallel planes; the first and second surface 
structure planes are congruent; the first and second surface structures 
generally lie in cylindrical planes; the first and second surface 
structures in the cylindrical planes are congruent; the first and second 
surface structures in the cylindrical planes are coaxial; the ribbing 
mounting means on at least one of the first or second surface structures 
is integral with the surface structure; at least one of the first or 
second surface structures is locally relieved to define the ribbing 
mounting means; both of the first and second surface structures are 
locally relieved to define the ribbing mounting means; the ribbing is 
formed by local relief of the first or second surface structure; and the 
ribbing comprises individual ribbing segments formed independently of both 
the surface structures. 
In a preferred embodiment, the first and second surface structures lie in 
nested cylindrical planes, and the ribbing comprises plural ribbing 
segments helically disposed between the surface structures in engaged 
relation with the ribbing mounting means on each surface structure; the 
first and second surface structures lie in nested cylindrical planes, and 
the ribbing comprises circularly disposed plural ribbing segments lying 
radially of the center longitudinal axis of the inner of the cylindrical 
planes and between the surface structures in engaged relation with the 
ribbing mounting means on each surface structure. 
In a further preferred embodiment, the first and second surface structures 
lie in generally parallel planes, and the ribbing comprises plural ribbing 
segments disposed generally normal to at least one of the surfaces 
structures and in engaged relation with the ribbing mounting means. 
In this and like embodiments, typically, the ribbing mounting means on at 
least one of the first or second surface structures is integral with the 
surface structure; at least one of the first or second surface structures 
is locally relieved to define the ribbing mounting means; both of the 
first and second surface structures are locally relieved to define the 
ribbing mounting means; the ribbing is formed by local relief of the first 
or second surface structure; the ribbing comprises individual ribbing 
segments formed independently of both the surface structures; the mounting 
means and plural ones of the ribbing segments define interfitting 
portions; and the ribbing segments have at least one bulbous terminal 
opposite the mounting means therefor, the mounting means defining a 
cup-shaped recess congruent with the bulbous terminal and relatively sized 
with respect thereto such that at the same temperature condition the 
mounting means and the ribbing terminal engage one another in blocking 
relation against relative movement, but at relatively different 
temperature conditions of the mounting means and the ribbing terminal they 
are relatively movable in unblocked relation. 
In a further embodiment, the invention structural element comprises opposed 
first and second, axially extended surface structures each having a closed 
figure pattern of ribbing projecting therefrom toward the opposite surface 
structure in spaced relation, the ribbing being interposed between the 
first and second surface structures in freely movable relation responsive 
to a temporary first relative dimensional condition of the structures, and 
in blocking relation against separation of the structures responsive to a 
persistent second relative dimensional condition of the structures. 
In this and like embodiments, the structures are dimensionally responsive 
to heat conditions and the persistent second relative dimensional 
condition of the structures and ribbing exists at 25.degree. C. and the 
first relative dimensional condition exists independently in the first and 
second structure and the ribbing at some other temperature; the surface 
structure ribbing defines mounting means and there is also included a 
third ribbing structure bridging the space between the surface structures 
in registered, mating relation with the surface structure ribbing mounting 
means; the third ribbing structure is separately formed from the surface 
structures; the third ribbing structure defines a closed figure 
registerable with the first and second surface structure closed figure 
patterns of ribbing mounts; the third ribbing structure closed figure 
comprises linear segments; the third ribbing structure closed figure 
comprises curvilinear segments; the third ribbing structure is of 
relatively greater height than the pattern of ribbing mounts on the first 
or second surface structures; the surface structure ribbing mount of at 
least one of the first and second surface structures defines an outward 
edge groove into which the opposing outward edge of the third ribbing 
structure interfits in structural element defining relation; and/or both 
of the first and second surface structure ribbing mounts define an outward 
edge groove to receive the third ribbing structure. 
In a further embodiment, the invention provides a structural element 
comprising opposed first and second, axially extended, surface structures 
each having a closed figure pattern of ribbing mounts projecting therefrom 
toward the opposite surface structure in spaced relation, and a separately 
formed third ribbing structure shrink-fit-locked between the first and 
second surface structures. 
In this and like embodiments, typically the third ribbing structure 
comprises a closed figure of a plurality of linear segments arranged to 
enclose a space; the third structure comprises a closed figure of 
curvilinear segments arranged about a common point to enclose a space; the 
third ribbing structure comprises fiber reinforced plastic; and the first 
and second surface structures each comprise fiber reinforced plastic and 
at least one surface structure is integrally formed with the ribbing 
against unblocking except upon melting of the plastic. 
Accordingly, the invention provides a structural element comprising first 
and second surface structures, one of the surface structures defining a 
closed pattern of ribbing, the other of the surface structures defining a 
cooperating pattern of ribbing for locking together the surface structures 
in spaced relation. 
The invention further contemplates the method of supporting opposed surface 
structures in a structural element spaced by a pattern of ribbing, 
including interposing ribbing between the surface structures in a 
temporary first relative dimensional condition of the structures and 
ribbing in interfitting relation, and thereafter returning the surface 
structures and ribbing to a persistent relative dimensional condition to 
interlock the structures together across the ribbing by said dimensional 
change.

PREFERRED MODES 
The term "temperature condition" herein refers to the presence of more or 
less heat in the surface structures and/or ribbing without reference to 
any absolute level of such heat. What is important is the differential in 
temperature between the parts to be mated, not the existence of any 
particular temperature in either part. Most structural elements will be 
used at a known use temperature or temperature range, and this use 
temperature range is called ambient herein whether such temperature is 
room temperature or some other temperature. Ambient temperatures, however, 
and 
typically, are conveniently room temperature, or more precisely 25.degree. 
C. and the parts to be mated are varied from that temperature 
differentially to have a change in relative dimension so as to permit 
interfitting of parts which would not fit together were they each at the 
same temperature, but which can be dimensionally shifted by differentially 
changing their temperature. For this purpose, surface structures, mounts 
thereon and ribbing are desirably formed of materials which are so 
dimensionally shiftable. 
The term "a pattern of ribbing" herein refers to ribbing which when viewed 
in plan or elevation presents a regular or irregular even random but 
repeating arrangement of ribbing. A "closed figure pattern of ribbing" 
herein refers to a series of laterally and/or longitudinally adjacent 
closed figures of ribbing forming a pattern. A "closed figure" of ribbing 
is one in which a straight line drawn anywhere through the figure will 
intersect ribbing in at least two places, and includes a circle, triangle 
and rectangle in which adjacent segments of ribbing are contiguous, as 
well as circular and rectangular spirals and other figures wherein 
although the ribbing segments are arranged about a common point and 
adjacent segments are not always contiguous, a straight line drawn 
anywhere through the figure will intersect ribbing in at least two places. 
A figure in which all sides are in continuing contact at the same or 
different distances from a common locus within the figure is the preferred 
form of a closed figure. 
"Mechanically blocking" herein refers to a condition where one part 
interferes with the movement of another part interfitted therewith by 
bodily contacting the other part in a manner which blocks relative 
movement, as by an overhang or enclosure, full or partial, by one part of 
the other. "Unblocking" refers to the removal of a blocking condition. 
Where the establishment of a blocking condition is accomplished by 
increasing the dimensions of a receiving part relative to a received part, 
as by heating the former and cooling the latter from ambient temperature, 
the mechanical blocking results from a return to ambient by the treated 
parts. The shrunken-by-cooling received part placed within the 
expanded-by-heat receiving part expands just as the receiving part 
contracts about the received part. The received part bodily engages the 
receiving part and vice-versa such that they interfit so closely that they 
cannot be separated and still maintain the integrity of the parts. 
With reference to the drawings, in FIGS. 1 and 4 a structural element 10 is 
shown comprising a first or inner surface structure 12 comprising a 
generally cylindrical surface portion 14 and fixed thereto or preferably 
machined thereinto from a metal mass unitary with the surface portion an 
integral pattern of ribbing 16 circumferentially arranged as a series of 
axially aligned fins 17 extending outward from the surface portion. The 
structural element 10 further comprises a second or outer surface 
structure 20 comprising a generally cylindrical surface portion 22 in 
which inner surface structure 12 of lesser diameter is nested. Fixed to or 
machined into the inner face 23 of the outer surface structure 20 by 
locally relieving a metal mass unitary therewith is an integral pattern of 
ribbing mounts 24 arranged opposite and complementary to the ribbing 16 as 
a circumferential disposed series 26. The ribbing 16 and mounts 24 may be 
reversely located with the mounts located on the outer face 27 of the 
inner surface structure 12 and the ribbing 16 on the face 23 of the outer 
surface structure 20. cf. FIG. 6 to be described hereinafter. 
The ribbing 16 and mounts 24 define terminal interfitting portions, i.e. a 
radially enlarged bead 28 formed along the outward edge 30 of ribbing 16, 
bulbous in cross section, and a radially enlarged channel 32, cup-shaped 
in cross section, formed along, i.e. within the inward edge 34 of mounts 
24 inside of gap 36 in the inward edge. With reference particularly to 
Fig, 4, it will be seen that ribbing bead 28 is oversized relative to the 
gap 32 but generally congruent with the channel 32. The walls 38 forming 
gap 36 are cammed at 40 to ease the entry of bead 28 and spaced such that 
entry of the bead is a free passage or an overcenter process in which the 
bead is closed upon after its midpoint passes through the midpoint of the 
gap 32, to be seated in the channel 32. 
To effect coincidence of the channel 32 and bead 28, the bead-carrying 
ribbing 16, as the male portion of the connection, is cooled from ambient 
temperature thereby shrinking its physical dimensions. The mounts 24, as 
the female portion of the connection, are heated above ambient temperature 
to cause channel gap 36 and the channel 32 beyond to widen sufficiently to 
pass and receive respectively the ribbing bead 28. Return of the ribbing 
16 and mounts 24 to ambient temperature causes the channel 32 to close 
about the bead 28 with the channel wall 42 overlying the bead such that 
the ribbing cannot be removed without breaking the channel walls 38 
forming the gap 36. The channel 32 thus bodily interferes with separation 
of the ribbing 16 from the mounts 24 and mechanically blocks separation of 
the parts. 
The ribbing 16 and mounts 24 can be separated by reversing the assembly 
process while repeating the above-described heating and cooling steps to 
effect the necessary changes in relative dimensions of the interfitting 
parts. 
In FIG. 6 a portion of a planar structural element is shown at 10a having 
an alternative ribbing and mounting arrangement. There ribbing 16a is a 
third ribbing structure separately formed from the inner surface structure 
12a and the outer surface structure 20a. Ribbing 16a is dumbbell shaped 
with its edges 30a and 30b respectively provided with beads 28a, 28b. 
Surface structures 12a and 20a are provided with integrally formed mounts 
24a and 24b having channels 32a and 32b and channel walls 38a, 38b which 
function like mounts 24 in the FIGS. 1 and 4 embodiment to capture the 
ribbing 16a by its edges 30a and 30b and thereby secure the three element 
structure of the inner surface structure 12a, the outer surface structure 
20a and ribbing 16a together without adhesives or any bonding agents. 
In FIG. 3 in which like parts have like numerals to those in FIGS. 1 and 4, 
plus 100, structural element 110 comprises in part an inner surface 
structure 112 having a generally cylindrical surface portion 114 formed 
with integral ribbing 116 by machining away portions of the surface 
structure. It will be noted that ribbing 116 is disposed helically about 
the longitudinal axis of the structural element 110. The structural 
element 110 further comprises an outer surface structure 120 having a 
generally cylindrical surface portion 122. Ribbing mounts 124 are formed 
integrally with the surface structure 120 and define helically extended 
groove segments 132 opposed to and in registry with the ribbing 116 
segments on the inner surface portion 114. In this FIG. 3 embodiment, the 
mounting of the inner and outer surface structures 112, 120 is by reducing 
the physical dimensions of the inner surface structure and increasing the 
physical dimensions of the outer surface structure, e.g. by cooling the 
former and heating the latter to a temporary temperature condition from 
their usual persistent temperature condition to achieve thereby a change 
in relative size such that the inner surface structure can be fitted into 
the outer surface structure with sufficient mechanical clearance that the 
structures move easily past each other with the ribbing 116 segments 
sliding in the groove segments 132. Once so fitted the inner and outer 
surface structures 112, 120 and the ribbing 116 carried therebetween 
return to their normal or persistent temperature and consequently contract 
or expand depending on whether they were previously cooled or heated, and 
the edges 130 of ribbing 116 further enter the groove segments 132 binding 
there as the clearance is reduced to zero. The inner and outer surface 
structures 112, 120 are thus bound together across ribbing 116 without use 
of adhesive or any bonding agents. 
With reference now to FIGS. 2 and 5, in which like parts are given like 
numerals to those in FIGS. 1 and 4, plus 200, the structural element 210 
has ribbing 216 and inner and outer surface structures 212 and 220 which 
can any or all be formed of fiber reinforced plastic, e.g. glass or carbon 
and like filaments 211 may be embedded in epoxy, polyester, urethane, 
phenolic, polyolefin or styrene or olefin copolymer resins among other 
resins 213 and shaped into ribbing, inner and/or outer structures which 
are capable of controlled change between first and second dimensional 
conditions as by change in temperature of the ribbing, inner or outer 
structures 216, 212,220, or by passage of the resins through their glass 
transition temperatures with a resultant change in physical dimensions of 
the ribbing and/or structures to a temporary condition, enabling their 
interfitting without binding, followed by a return to their persistent 
physical dimensional condition in which latter condition the parts are 
bound together but separable by return to the temporary condition, e.g. 
along interface 215 where ribbing 216 extends integrally from inner 
surface structure 212, as in the FIGS. 1 and 4 embodiment. The bead 228 
and groove 232 arrangement described above for metal parts can be used in 
the FIGS. 2 and 5 embodiment with like effect. 
It will be noted that the respective heights of the ribbing and the mounts 
are not critically related but together they are required to bridge the 
distance by which the surface structures are spaced. The relative heights 
of the ribbing in different parts of a structural element may be varied 
for particular effects or design or machining convenience. In the design 
of structures using the invention bosses may be provided for mounting bolt 
holes for securing the structural element to other components or vice 
versa. It is to be noted that provision of the bosses does not require 
added bolt-on parts as in honeycomb, but these features may be added 
wherever desired within the capability of the machining apparatus. 
Further, the separation of the surface structures may be varied by 
changing the height of the ribbing to provide a tapered or stepped 
structural element. Desirably in manufacturing, the machining is 
accomplished to leave relatively more material at the junctions of the 
ribbing to provide increased compressive strength. Also the interior 
surface of the surface structures is suitably machined out to reduce 
material and thus weight and at the same time add rigidity by virtue of 
the resulting raised ribs. The interconnection of the surface structures 
with the ribbing is preferably by a self-locking e.g. bead and channel, 
tongue and groove, slide and slot, etc. or other interfitting/interlocking 
arrangement in which the engaged parts bodily interfere with separation. 
With the advent of structural plastics which rival metals for strength per 
pound, a portion or all of a surface structure can be formed of such 
plastic in an interlockable form or the engagement of the surface 
structure can be effected by such plastic. 
The foregoing generally described structural element can be fabricated in a 
number of ways into a wide variety of products, ranging from wings to 
walls, tanks to beams to bulkheads, and platforms to frameworks. 
As will be evident from the foregoing, materials used include metals such 
as aluminum and titanium or other metals, and plastics and plastic 
composites of fibers, fillers and resins which have suitable strength, 
machining characteristics and dimensional response. 
The foregoing objects are thus met.