Structures deform or become distorted when subjected to temperature variations. There are a number of applications where such thermal distortions create problems. One such application is in space structures which, when orbiting the earth, are subjected to very large temperature changes as they are successively exposed to the sun and hidden from the sun. Such thermally-produced distortions of the space structures can seriously affect their operation or the operation of their electronic gear. Considerable engineering effort is therefore currently being invested to eliminate, or compensate for, these thermally-produced distortions.

An object of the present invention is to provide structural or other type of members which enable structures to be constructed having less sensitivity or controlled sensitivity to deformations caused by temperature changes.

According to the present invention, there is provided a structural member including a bimaterial element comprising: a first elongated curved layer of a material having a predetermined coefficient of thermal expansion, and

a second elongated curved layer fixed to the first layer and having a different coefficient of thermal expansion than the first layer; and end connectors at the opposite ends of the bimaterial elements for connection to other structural members.

The geometry, thermal properties and mechanical properties of the two layers can be selected to produce substantially a zero, a negative, or a positive, expansivity, i.e., change in the distance between its end connectors, upon a predetermined change in ambient temperature. The structural member may include a plurality of the bimaterial elements joined at their opposite ends to the end connectors. Several embodiments of the invention are described below for purposes of example.

In one described embodiment, there are two of the bimaterial elements joined at their opposite ends to the end connectors. In this described embodiment, a transverse strut is joined to the apices of the bimaterial elements, the transverse strut being of a material selected such that its expansion is substantially equal to the change in distance between the apices of the bimaterial elements during the predetermined increase in ambient temperature.

Other embodiments are described wherein there are four bimaterial elements joined to the end connectors in an X-configuration, and three bimaterial elements joined to the end connectors in a Y-configuration.

A still further embodiment is described when the structural member is a cylinder and includes a plurality of the bimaterial elements helically wound in side-by-side relation. The bimaterial elements may be designed such that the overall length of the cylinder may undergo no change, or a controlled change, in length upon a predetermined change in the ambient temperature.

Yet another described embodiment is a composite material consisting of curved bimaterial inclusions embedded in a matrix. In such a case, the bimaterials, their number, their position and orientation, and the matrix, can be designed such that the composite material has predetermined thermal expansivity properties. An example of a possible implementation is described hereafter.

A still further embodiment is described wherein the first elongated curved layer is a continuous sheet of corrugated configuration, and the second elongated curved layer is constituted of a plurality of separate sheets each of curved configuration bonded to the continuous sheet alternatingly on opposite sides of the continuous sheet. Such an embodiment is particularly useful in constructing roofing or the like, requiring no, or a pre-controlled, change in dimensions when subjected to different ambient temperatures.

While theoretically the two materials of the bimaterial elements could be of non-metals, such as ceramics or plastics, in most applications they would both be of

metal. As one example, one of the metals may be steel, and the other may be alumimum.

Further features and advantages of the invention will be apparent from the description below.

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

Fig. 1 illustrates a typical bimaterial element constructed in accordance with the present invention;

Fig. 2 illustrates the dependency of the elongation of the chord of the bimaterial element of Fig. 1 with respect to the initial arc length for a predetermined temperature increase for a typical bimaterial element;

Fig. 3 illustrates one form of structural member constructed in accordance with the present invention;

Figs. 4 and 5 are sectional views along lines IV—IV and V--V of Fig. 3;

Fig. 6 is an enlarged fragmentary view of a part of Fig. 3;

Fig. 7 is a view similar to that of Fig. 5 but showing another structural member constructed in accordance with the invention;

Fig. 8 illustrates a further form of structural member constructed in accordance with the present invention;

Fig. 9 illustrates a helical bimaterial element constructed in accordance with the invention;

Fig. 10 illustrates the manner of using a plurality of the helical bimaterial elements of Fig. 9 to make a cylinder undergoing no length change, or- a controlled length change, upon heating;

Fig. 11 illustrates a bimaterial element in the form of a cylinder whose diameter changes upon heating;

Fig. 12 illustrates a matrix made up of a sheet and a plurality of the cylinders of Fig. 11 such that the length of the matrix undergoes no change, or a controlled change, upon heating; and Figs. 13 and 14 are side and perspective views, respectively, illustrating a further form of member constructed in accordance with the present invention.

A typical bimaterial element constructed in accordance with the present invention is shown in Fig. 1. It is a curved element of constant curvature having two elongated layers !_,_. , L_ of different materials. The outer layer L.. is of a material having a larger coefficent of thermal expansion than the inner layer L_. In most cases, the materials of such a bimaterial element would both be metals, and therefore this element will generally be hereinafter referred to as a bimetal curved element, although it will be appreciated that such elements could include two or more layers of any material, including composites, and not be of constant curvature.

As shown by the broken lines in Fig. 1, when the bimetal element is heated, both layers L.. , L_ expand, but

the outer layer L.. expands more than the inner one L_. The change in distance between the extremities A and B of the bimetal element is called δ and is given for the particular case by

where b is the chord length (AB) , R is the radius of the layers interface arc, and β is the angle spanned by the bimetal element. Subscripts o and t denote respectively the initial configuration, and the configuration due to the temperature variation.

Fig. 1 depicts a case where the chord length decreases under temperature increase,i.e. , the apparent coefficient of thermal expansion is negative. It is clear that due to heating the curvature will increase; that is, the radius will decrease, R, <Ro and βt, >βo. By selecting proper materials, thickness of the layers, initial curve

(R ), and arc intercepts (β_), a positive, zero, or negative elongation of the chord AB can be obtained. Fig. 1 illustrates the case of a negative elongation. The dependency of the elongation on these parameters can easily be calculated using Engineering Beam Theory.

Consider for instance a bimetal element with an outer alumimum layer L.. and an inner steel layer L_, both of thickness 1 cm, and with an intitial radius of Ro=30 cm.

Fig. 2 shows the dependency of the elongation of the chord δ

as a function of the initial arc intercept βo due to a temperature increase of 50°C. For β =49.4° the element will be insensitive to temperature changes. For lower values of β, the element will elongate. For higher values of β, the element will shrink (the case of Fig. 1). It should be noted that since the effective elongation is almost linear with temperature over a wide range of temperature variations, the above results remain true for large fluctuations of the temperature (hundreds of degrees C) . Note that when the outer layer has a smaller coefficient of thermal expansion, elements with super-expansive thermal properties are obtained.

Figs. 3-14 illustrate various constructions including the bimetal element of Fig. 1.

Figs. 3-6 illustrate a structural member constituted of four bimetal elements 21, 22, 23 and 24, fixed at 90° apart at their opposite ends to a pair of end connectors 25, 26, formed with openings 27, 28, for connection to other structural members. Each of the bimetal elements 21-24 is constructed as described above with respect to Fig. 1, to include the two layers L.. , L_ of different coefficients of thermal expansion. It will be appreciated that each bimetal element 21-24 can be designed such that, when heated to a predetermined temperature, it will undergo a zero change in length, or a controlled positive or negative change in length, between the two end connectors 25, 26. Such members may thus be employed in

structures to obtain dimensional stability when undergoing temperature changes.

Fig. 7 illustrates a further structural member including three curved bimetal elements 31, 32, 33, secured together in a "Y" fashion (120° apart) at their ends to connector elements, such as shown at 34 in Fig. 7.

Fig. 8 illustrates a structural member including two curved bimetal elements 41, 42, each as described with respect to Fig. 1, joined at their opposite ends to connector elements 43, 44, and joined at their apices by a transverse short strut 45. The material of strut 45 would be selected such that is expansion will be equal to the lateral relative expansions of the pair of curved bimetal elments 41, 42 at their apices. Such a structural member will therefore have the same thermal stability between the ends of the structural member, but will have substantially increased axial stiffness by virtue of the transverse strut 45. In Fig. 8, the transverse strut is composed of curved superexpansive bimaterials obtained by using an outer layer with a smaller coefficient of thermal expansion.

Fig. 9 illustrates a bimetal element, including the two layers 1__. , L_ as described above with respect to Fig. 1 , but arranged in the form of a helical strip, shown at 51. A plurality of such helical strips 51 may be assembled in side-by-side relation to produce a cylinder, as shown at 52 in Fig. 10, provided at its opposite ends with connector elements 53 and 54.

It will be seen that the design parameters can be selected such that the ends of each of the helices 51 can remain constant, can increase, or can decrease, with a rise in the ambient temperature. Thus, as the temperature increases, the linear length of the helical strip increases, but the curvature decreases to increase their transverse dimensions. A plurality of such helices may then be used for producing the cylinder 52 characterized by no length change or a controlled length change, upon heating. However, care should be exercised to maintain sufficient space between the adjacent helical bimetal elements 51 to allow for the changes in the width, as well as in the overall length, of the helical bimetal elements.

Figs. 11 and 12 illustrate a further possible application of the invention. Fig. 11 illustrates one of the bimetal elements 55 in the form of a cylinder wherein its opposite edges are in sliding contact. Thus, the outer layer L- is made of a material having a larger coefficient of thermal expansion than the inner layer L_, such that upon heating the diameter of the cylinder 55 decreases.

Fig. 12 illustrates a matrix constituted of a sheet 56 embedding a plurality of such bimetal cylinders 55 with the axes of the cylinders extending substantially perpendicularly to the faces of the sheet 56. Thus, if the sheet 56 expands upon heating, the diameters of the bimetal inserts 55 decrease, thereby reducing the thermal expansivity of the composite material. The parameters are chosen so as to produce zero expansivity, or a controlled

expansivity. Fig. 12 illustrates such a composite sheet. The curved inclusions need not be fully circular or of the same shape. These sheets, as with all lamina, can be combined to produce a composite structure.

Figs. 13 and 14 illustrate a further application of the invention. In this application, one layer 61 is in the form of a continuous sheet of corrugated configuration, and the second layer 62 is constituted of a plurality of separate sheets each of curved configuration bonded to the continuous sheet alternatingly on opposite sides of the continuous sheet. Fig. 13 illustrates the corrugated sheet used as a structural member provided with connector elements

63, 64 at its opposite ends.

It will be appreciated that the materials and dimensions of the two layers 61 , 62 can be selected so as to produce zero elongation, or a controlled positive or negative elongation, between the two connector elements 63,

64. Such a construction can be used, for example, in antennas, root coverings of equipment, and the like where thermal dimensional stability is desired.

While the invention has been described with respect to several preferred embodiments, it will be appreciated that these are set forth merely for purposes of example, and that many variations may be made. For example, the curved bimetals need not be of constant curvature. Also, they could include more than two layers. Many other variations, modifications and applications of the invention will be apparent.