U.S. patent number 3,922,899 [Application Number 05/486,428] was granted by the patent office on 1975-12-02 for method of forming sandwich materials.
This patent grant is currently assigned to Societe Nationale Industrielle Aerospatiale. Invention is credited to Jean-Francois Denis, Serve, Yvan Dzalba-Lyndis, Maurice, Henri, Louis Fremont.
United States Patent |
3,922,899 |
Fremont , et al. |
December 2, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Method of forming sandwich materials
Abstract
The invention provides a method of forming sandwich materials.
The area subjected to forming stresses is heated locally in such
manner that the strength properties of the skin metals governing
the permissible stress limits in that area are constantly monitored
without the structure as a whole being subjected to oxidation.
Inventors: |
Fremont; Maurice, Henri, Louis
(Massy, FR), Denis; Jean-Francois (Lesigny,
FR), Dzalba-Lyndis; Serve, Yvan (Villejuif,
FR) |
Assignee: |
Societe Nationale Industrielle
Aerospatiale (Paris, FR)
|
Family
ID: |
9122350 |
Appl.
No.: |
05/486,428 |
Filed: |
July 8, 1974 |
Foreign Application Priority Data
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Jul 10, 1973 [FR] |
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73.25292 |
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Current U.S.
Class: |
72/128; 428/116;
72/342.1 |
Current CPC
Class: |
B21D
47/00 (20130101); Y10T 428/24149 (20150115) |
Current International
Class: |
B21D
47/00 (20060101); B21D 047/00 () |
Field of
Search: |
;72/128,342
;29/455LM |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Larson; Lowell A.
Attorney, Agent or Firm: Flocks; Karl W.
Claims
We claim:
1. A method of forming a metallic panel sandwich comprising two
thin metal sheets maintained in mutually spaced relationship by a
spacer element of honeycomb structure, corrugated elements, or the
like, wherein the said method includes:
a. placing one sheet of a metallic panel sandwich against a
rotating former of rounded surface;
b. applying radiant heat to the immediate proximity of points where
said metallic panel sandwich is tangential to said rotating former
whereby a localized and differential heating effect is provided in
the two thin sheets passing thereby; and
c. simultaneously rotating said former to thereby effect formation
of said metallic panel sandwich.
2. The method of claim 1 wherein said radiant heat is obtained from
an iodine vapour or infrared-tube radiator.
3. The method of claim 1 wherein said heat is supplied from a
source spaced from and not contacting said metallic panel
sandwich.
4. The method of claim 1 wherein said application of radiant heat
is set to provide a temperature of 770.degree.C in one of said two
sheets and a temperature of 480.degree.C in the other of said two
sheets, where said two sheets are of titanium or titanium alloys.
Description
The present invention relates to a method of forming sandwich
materials.
The difficulties of forming the so-called sandwich materials used
in the aerospace industry, in which two thin sheets of metal are
maintained in mutually spaced relationship by a spacing element or
web, are well known. For example, this spacing element may be
either a corrugated element as used in the materials employed by
the Applicant under the trade-name "Norsial", or a honeycomb type
of element as used in the materials employed by the Applicant under
the trade-name "Nida".
These difficulties are mainly due to the fact that it is not always
possible to impart shape to such materials directly or to form them
from flat panels because local buckling phenomena tend to cause the
compressed face (or skin) to wrinkle while the stretched face (or
skin) is subjected to high tensile stresses.
However, when the neutral or zero-stress fibre passes midway
through the thickness of the material, the tensile and compressive
stresses in the two skins are of equal and opposite values, and
therefore a knowledge of these values can determine the magnitude
of the deformation which can be applied to the panel.
Indeed, by applying appropriate overall heating it is even possible
to so modify the principal characteristics of materials as to make
it possible to increase the obtainable extent of deformation.
However, although this heating method is applicable to many metals
such as stainless steel, nickel, etc., this is not so in the case
of many other special metals like titanium and its alloys,
molybdenum, magnesium, beryllium and their alloys, which metals do
not stand up well to protracted overall heating because of their
great sensitivity to oxidation.
Obviously, protection against oxidation can be obtained by carrying
out such heating in an evacuated enclosure or in a neutral
atmosphere, but this is an inconvenient and costly solution when
the items to be formed are of considerable size.
The present invention provides a method of forming sandwich
materials of the above-mentioned kind, made of oxidizable metals,
that overcomes the drawbacks of the prior art.
In the method according to this invention, the area subjected to
forming stresses is heated locally in such manner that the strength
properties of the skin metals governing the permissible stress
limits in that area are constantly monitored without the structure
as a whole being subjected to oxidation.
The description which follows with reference to the accompanying
non-limitative exemplary drawings will give a clear understanding
of how the invention can be carried into practice.
In the drawings:
FIGS. 1 and 2 are sectional side elevation views of Norsial and
Nida type sandwich panels, respectively, in which the neutral fibre
(x-y) lies in the median plane through the thickness of the
panel;
FIGS. 3 and 4 are illustrations corresponding to FIGS. 1 and 2, for
the case where the neutral fibre (x-y) is offset and does not lie
in the median plane through the thickness of the panel;
FIGS. 5 and 6 are diagrammatic illustrations showing the stress
distribution when a sandwich panel is subjected to bending and
tension respectively;
FIG. 7 is a graph for illustrating the local buckling phenomenon in
a sandwich panel;
FIG. 8 is a graph showing the effect of oxidation on a metal
oxidizable in free air, as a function of time and temperature;
FIG. 9 diagrammatically illustrates a first possible arrangement
for performing the subject method of this invention;
FIG. 10 is a graph in which the strength properties of a titanium
alloy are plotted against temperature;
FIG. 11 is a micrographic image of the effect of oxidation under
protracted heat in an assembly of titanium alloy sheets;
FIG. 12 is a micrographic image of the effect of oxidation under
heat in a titanium alloy assembly treated in accordance with the
present invention; and
FIG. 13 diagrammatically illustrates an alternative possible
arrangement for performing the subject method of this
invention.
One of the difficulties encountered in operations for forming
Norsial or Nida type sandwich panels stems from the fact that it is
almost mandatory to resort to a bending load F (see FIG. 5). The
effect of such bending is to produce a tensile stress T.sub.1 on
the stretched face (or skin) 1 and a compressive stress C.sub.1 on
the compressed face (or skin) 2, but these stresses will be of
equal magnitude provided that the neutral fibre is equidistant from
the external faces of the panel. (For greater clarity, FIGS. 1 and
2 represent equal stresses T and C for the case where the neutral
fibre x-y lies at equal distances r from the external faces 1 and 2
of Norsial and Nida type panels respectively, and FIGS. 3 and 4
represent unequal stresses T and C for the case where the neutral
fibre x-y is offset in relation to the median planes of said
panels).
The strain in each case can readily be calculated from the
elementary formula: ##EQU1## where .sigma. is the strain, M the
bending moment,
I the inertia of the material
and r the distance of the external faces (or skins) from the
neutral fibre x-y.
In practice, the tensile stress T does not present a major drawback
provided that the metal possesses adequate capacity for elongation.
On the other hand, the compressive stress in the compressed skin 2
soon results in a local buckling effect which produces increasingly
accentuated wrinkling. This phenomenon will be seen to be virtually
inevitable from and examination of the graph in FIG. 7, in which
.SIGMA. is the deformation,
.sigma. the strain,
.sigma..sub.1 the strain beyond which local buckling of the
compressed skin occurs,
and .sigma..sub.2 the strain at the elastic limit of the metal for
an 0.2% elongation.
If it is desired to deform the material permanently, it is
indispensable to greatly exceed the metal's elastic limit at 0.2%
elongation, a limit which is usually greater than the strain
.sigma..sub.1 producing the local buckling phenomenon.
This drawback can often be avoided, in particular through the use
of so-called stretch-forming methods in which a tensile force
T.sub.2 is exerted on the panel prior to deformation by bending, as
shown in FIG. 6. This stress T.sub.2 comes in deduction of the
compressive stress (C.sub.1 -T.sub.2) that appears in the internal
skin 2 and sets back the onset of the local buckling phenomenon
correspondingly. Contrariwise, it is added (T.sub.1 + T.sub.2) to
the tensile stress in the outer skin 1 and therefore limits the
deformation possibilities by reason of the high stresses involved,
which could result in rupturing of the stretched skin 1.
As is well-known, the forming of oxidizable parts, especially
titanium or titanium alloy parts, is facilitated if it can be
carried out under heat; for in addition to the fact that the rise
in temperature improves the basic strength characteristics, it
enables the elastic restoring or resilience effect, which is
particularly strong in such materials when they are cold, to be
avoided. Unfortunately however, in order to be effective, the
temperature must be high (in excess of 600.degree.C) and it is
well-known that at such temperatures all these oxidizable metals
are very seriously contaminated by the atmosphere, resulting in a
notably diminished resisting section and in the appearance of
oxidized cracks. In consequence, a conventional forming operation
would require a fairly long time during which oxidation and
contamination would develop by the process shown in FIG. 8, in
which the temperature in degrees centigrade is plotted along the
X-axis and the oxidization depth in millimeters along the Y-axis.
Curves I, II, III and IV in FIG. 8 correspond to heating times of
1/2 hour, 1 hour, 2 hours and 4 hours respectively, and it may be
noted that the depth of oxidization as a function of temperature
and time does indeed vary between about 0.03 mm and 1.1 mm.
When it is remembered that the skins of sandwich panels to be
formed are no more than a few tenths and sometimes a few hundredths
of a millimeter thick as a rule, it will be clear that with such an
operation there would be virtually no "sound" metal left, making it
impossible to treat such materials by conventional open-air
methods.
The solution consisting in placing the parts to be shaped in an
evacuated enclosure or in a neutral atmosphere, though valid for
small parts, would be difficult to apply in the case of items of
substantial size.
The forming method according to this invention enables all the
drawbacks mentioned hereinbefore to be overcome.
The subject method of the invention firstly allows of very
substantially delaying the onset of local buckling of the
compressed inner skin and therefore increases the forming
possibilities for any given panel, and secondly authorizes the
forming of oxidizable parts, and especially titanium alloy parts,
in very short times during which oxidation has very little chance
to develop, even without gaseous protection. It should be noted
that such "short-time" forming by no means implies high deformation
speeds, but quite the opposite, thereby enabling advantage to be
taken of the relaxation phenomena well-known in metallurgy.
Essentially, this method is characterized by the fact that it
consists in heating the skins of a metal sandwich panel locally and
differentially in such manner as to ensure that the tensile and
compression stresses engendered therein during forming are optimal
having regard for the strength characteristics of the metals in
question.
In accordance with further teachings of this invention:
-- the local heating zone is proximate the instantaneous
deformation zone;
-- the forming is carried out by applying the panel against a
rotatable former and by providing local heating means in immediate
proximity to the points at which the panel to be formed is
tangential to said former;
-- and in the specific case of titanium and its alloys, the
temperature of the "hotter" skin is approximately 770.degree.C,
thereby providing a modulus of elasticity of about 6200 hb, and the
temperature of the "colder" skin is approximately 480.degree.C,
thereby providing a modulus of elasticity in the region of 8500
hb.
The invention likewise relates to arrangements and means for
performing the said method, which arrangements are described
hereinbelow for exemplary purposes with reference to FIGS. 9 and
13.
Reference is first had to FIG. 9, which illustrates a first way of
performing the subject method of this invention.
The panel to be formed 3 is placed with its inner face 7 against
any convenient rotating former 4. Local heating means 5, such as an
iodine vapour or infrared-tube radiator heats the metal locally on
the outer skin 6, in proximity to the line of instantaneous
deformation, that is to say at the points where the panel to be
formed is tangential to the former. A roller-type restraining
device 8 prevents the panel from lifting, and possible tensioning
means 9 exert a traction on the panel in order to produce
additional overall stretching.
The surface of former 4 can be coated at 10 with insulating
substances such as asbestos or melted ceramic, or alternatively
with metals like copper or aluminium so that the good
heat-conducting properties thereof may ensure optimum heat
distribution through the panel.
Using the subject method of this invention and the above-described
arrangement, the Applicant has been able to make a circular
cylinder with an inner diameter of 100 mm, made of welded Norsial
sandwich material consisting of a corrugated web in 0.15 mm-thick
sheet with corrugations pitched at 6 mm and two 0.3 mm-thick skins
in TA6V4 titanium alloy (6% of aluminium and 4% of vanadium). The
panel had a total thickness of 4.3 mm and the wrapping rate was 6
mm per minute.
The local heating was provided by an iodine-vapour radiator with a
linear heating zone, positioned in such manner that the area heated
on the outer skin 6 was a generatrix of the cylinder approximately
3 mm wide.
The temperature noted on the heated outer skin was 770.degree.C and
that of the inner skin in contact with the former (which was made
of insulating material) was 480.degree.C.
The curves in FIG. 10 (obtained by plotting the temperature along
the X-axis and the strength characteristics .sigma..sub.0.2, E and
A as hereinbelow defined along the Y-axis) give the values of these
strength characteristics in the case of TA6V4 titanium alloy sheet
0.3 mm thick. It may be noted from FIG. 10, where
.sigma..sub.0.2 is the tensile strength at the conventional elastic
limit for 0.2% elongation.
E is Young's modulus of elasticity, and
A% is the ultimate elongation,
that the elastic limit and Young's modulus decrease with rising
temperature and that, conversely, the permissible elongation
increases considerably, albeit after a small transitory
decrease.
Thus when the area of the sandwich material being formed at any
given instant is uniformly heated, the surface of the neutral
fibres extends midway along the panel by reason of the thermal
symmetry achieved, and the tensile and compressive stresses are
accordingly equal in absolute value.
When however, in accordance with this invention, there is thermal
asymmetry by reason of preferential heating of the outer skin, the
latter's modulus of elasticity becomes less than that of the inner
skin and the surface of the neutral fibres shifts towards the
compressed skin and the tensile and compressive stresses are no
longer equal.
For instance, when the outer "hot" skin is tensioned to a modulus
of elasticity of 6200 hb at 770.degree.C, the inner "cooled" skin
is compressed to 8500 hb at 480.degree.C and the downward shift of
the neutral-fibre surface then corresponds to the ratio 8500
hb/6200 hb or 1.37, as schematically illustrated in FIGS. 3 and
4.
The compression in the inner skin is thus considerably less than in
the case of uniform stress referred to precedingly, and this skin
furthermore possesses high rigidity (8500 hb). It is accordingly
lightly stressed and will withstand buckling, and the unacceptable
drawbacks of local buckling are thus eliminated. Another
consequence of the shifted position of the neutral fibres is to
increase the tension in the outer skin, or in other words the
elongation required to achieve a permanent set. However, this
increase is offset by the fact that that face is at high
temperature, and that at that temperature the permissible
elongation, which is then 30% as shown in FIG. 10, is over-abundant
and easily covers most foreseeable contingencies for the sandwich
materials considered by the present invention.
FIG. 11 shows for exemplary purposes, in the case of 0.27 mm and
0.14 mm thick TA6V4 titanium alloy, the corrosion effect obtained
in air during protracted heating at 800.degree.C. The micrographic
image shows clearly, with its magnification of 125 times, that the
oxidized layer is very thick.
Conversely, the micrograph image in FIG. 12 (which shows the
joining area of other such sheet metals of similar nature forming a
sandwich panel processed by the subject method of this invention)
clearly reveals the thinness of the oxidized layer even though the
image is magnified 340 times in this case.
By way of an alternative arrangement for obtaining a panel in
accordance with this invention, FIG. 13 shows an arrangement
similar to that in FIG. 9, in which the rotating former 11 is
non-cylindrical and the panel 3 is heated by a radiator 5 having
infrared tubes and is restrained by a thrust roller 12.
All the aforementioned embodiments, regardless of whether they
involve the use of a cylindrical or non-cylindrical mandrel, employ
the same method of this invention, which provides for suitably
adapting local stresses in materials by locally heating the two
skins of a "Norsial" or "Nida" type sandwich panel
symmetrically.
It goes without saying that the present invention has been
described for non-limitative exemplary purposes only and that
changes and substitutions may be made without departing from the
scope of the invention as defined in the appended claims.
* * * * *