US3629625A

US3629625A - Piezoelectric bender bilayer with flexible corrugated center vane

Info

Publication number: US3629625A
Application number: US72990A
Authority: US
Inventors: Hugo W Schafft
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1970-09-17
Filing date: 1970-09-17
Publication date: 1971-12-21
Anticipated expiration: 1988-12-21

Abstract

A transducer such as a bender bilayer piezoelectric device, or bimorph, in the form of a flat disc includes a pair of sheets, made for example of ceramic, separated by a center vane. The center vane is corrugated with the apices of the corrugations cemented to the sheets. The center vane has a center portion which acts as a stiff hinge permitting movement of the sheets relative to each other. The outer portion of the center vane includes corrugations at right angles to the corrugations of the center portion to prevent movement of the circumferences of the sheets with respect to each other.

Description

United States Patent Inventor Hugo W. Schaitt Des Plaines, I11.

Appl. No. 72,990

Filed Sept. 17, 1970 Patented Dec. 21, 1971 Assignee Motorola, Inc.

Franklin Park, Ill.

Continuation-in-part of application Ser. No. 773,914, Nov. 6, 1968, now abandoned. This application Sept. 17, 1970, Ser. No. 72,990

PIEZOELECTRIC BENDER BILAYER WITH FLEXIBLE CORRUGATED CENTER VANE Primary Examiner-D. F. Duggan Assistant Examiner-Mark O. Budd AttorneyMueller and Aichele ABSTRACT: A transducer such as a bender bilayer piezoelectric device, or bimorph, in the form of a flat disc includes a pair of sheets, made for example of ceramic, separated by a center vane. The center vane is corrugated with the apices of the corrugations cemented to the sheets. The center vane has a center portion which acts as a stiff hinge permitting movement of the sheets relative to each other. The outer portion of the center vane includes corrugations at right angles to the corrugations of the center portion to prevent movement of the circumferences of the sheets with respect to each other.

46 maam PATENTEDuEc21 I97! 23 PRIOR ART FIG. 3

FIG. 2

i (1 I, v I:

HUGO W. SCHAFFT ATTYS.

PIEZOELECTRIC BENDER BILAYER WITH FLEXIBLE CORRUGATED CENTER VANE CROSS-REFERENCES This application is a continuation-in-part of the application of Hugo W. Schafft, Ser. No. 773,914, filed Nov. 6, 1968, now abandoned.

BACKGROUND OF THE INVENTION In a transducer such as a bender bimorph maximum strain occurs at the surface of the structure. It is, therefore, desirable to restrict the location of the ceramic sheets of the bender bimorph to the surface portions and this is commonly accomplished by inserting a center vane of dead" material between the ceramic sheets. The use of the center vane increases the coupling factor and the output of the bender bimorph.

The ceramic sheets used in the bender bimorph have electrodes plated on the inner surfaces thereof and it is important that these electrodes be electrically connected In order to accomplish this the center vane is formed of a conductive material and is cemented to the electrodes by means of a coriductive epoxy. The use of a conductive epoxy results in less bond strength than a regular epoxy adhesive and the bond may break under high drive conditions. The relatively heavy conductive epoxy layer also introduces viscoelastic losses which show up as a lower coupling factor or a low output. During the bonding, air bubbles must be kept out of the epoxy mixture because they will affect the bond strength and this is difficult to accomplish with a solid center vane. The solid center vane construction also acts to restrict the expansion and contraction of the ceramic sheets so that the ceramic is prestressed and thereby the coupling factor and capacitance is lowered.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide an improved transducer construction in which the center vane used can be cemented to the sheets by means of a nonconductive epoxy.

Another object of this invention is to provide a bender bimorph construction having a center vane which will permit expansion and contraction of the ceramic sheets without development of stresses in the ceramic sheets.

Another object of this invention is to provide a bender bimorph construction not requiring a heavy epoxy layer.

Yet another object of this invention is to provide an improved transducer formed for example, of magnetostrictive or piezoelectric material, having a corrugated center vane connected to the outer sheets for transmitting motion therebetween.

In practicing this invention a transducer such as a bender bimorph structure is provided including a pair of circular ceramic sheets having electrodes on each side. A corrugated center vane is positioned between the sheets to space them apart at a precise distance. The apices of the corrugations are in contact with the electrodes to provide an electrical connection between the inner electrodes of the ceramic sheets. Since the center vane itself provides a conductive path between the electrodes, nonconductive epoxy is used to cement the electrodes and the ceramic sheets to the center vane. Only enough epoxy need be used to cover the area at the apices of each corrugation thereby reducing the loading effect of a heavy epoxy layer on the operation of the bender bimorph and the epoxy can be nonconductive. Since only a small amount of epoxy is used, there is no problem with air bubbles forming in the epoxy. The corrugated center vane has a center. portion which is corrugated in a direction so that it acts as a stiff hinge to permit lateral movement of the ceramic sheets. The outer circumferential portion of the center vane is corrugated at right angles to the inner portion and is likewise cemented to the two sheets. The corrugated outer portion acts to prevent the movement between the two sheets. The construction is not limited to a circular construction but a rectangular form of the bender bimorph can also be used.

The invention is illustrated in the drawings of which:

FIG. 1 is an isometric view of a circular bender bimorph transducer;

FIG. 2 is a sectional view of the prior art bender bimorph transducer construction;

FIG. 3 is an exploded view showing the center vane construction of the bender bimorph transducer of this invention;

FIG. 4 is a sectional view of the bender bimorph transducer center vane construction of FIG. 3; and

FIG. 5 is an exploded view showing the construction of a rectangular bender bimorph transducer.

DESCRIPTION OF THE INVENTION In FIG. 1 there is shown an isometric view of a circular bender bimorph transducer 10. The structure consists of a pair of ceramic sheets 12 and I3 separated by a center vane I6. An electrode 15 is shown on the top surface of a ceramic sheet 13. Electrodes are placed on the top and bottom of both the ceramic sheets but only electrode 15 can be shown in FIG. 1.

Referring to FIG. 2 there is a cross-sectional view of a prior art bender bimorph transducer structure. The structure consists of a first ceramic sheet 20 having

electrodes

23 and 24 on opposing faces thereof and a second ceramic sheet 21 having

electrodes

26 and 27 on the opposing faces thereof. The

electrodes

23, 24, 26, and 27 are extremely thin and make a negligible contribution to the thickness of the bender bimorph transducer. They are shown greatly exaggerated in thickness in the figures of this application.

The

ceramic sheets

20 and 21 are separated by a metallic center vane 29. Center vane 29 is cemented to

sheets

20 and 21 by means of layers of

conductive epoxy

30, 31. Conductive epoxy is formed by filling the epoxy with conductive material which makes the epoxy considerably heavier and weakens the holding power of the epoxy.

In a transducer structure it is desirable that the center portion of the center vane permit the outer sheets to move laterally relative to each other while securely holding the edges to prevent such lateral movement at the edges. In the structure of FIG. 2 the center vane and ceramic epoxy is uniform throughout and does not have the desirable characteristics stated above. The conductive epoxy has less load strength than a regular nonconductive epoxy adhesive and the bond may be broken under high stress conditions. The conductive epoxy, being loaded with metallic particles, is heavier and introduces viscoelastic losses which show up as a low coupling factor or a low output. During manufacture of the bender bimorph transducer structure shown in FIG. 2 epoxy must be spread evenly over the ceramic sheets and the center vane before they are joined together. In joining them together it is very difficult to prevent air bubbles from appearing in the adhesive and these air bubbles reduce the bond strength. The solid center vane construction of FIG. 2 restricts the expansion and contraction of the ceramic sheets thereby prestressing them with changes in temperature.

To reduce or eliminate the undesirable features of the solid center vane construction of FIG. 2, the center vane of this invention has been developed and is shown in FIG. 3. The bender bimorph transducer construction of FIG. 3 includes ceramic sheet 34 having an electrode 37 shown thereon and ceramic sheet 35 having an electrode 36 shown thereon. Additional electrodes are positioned on the hidden sides of

ceramic sheets

34 and 35 but are not shown in FIG. 3. Center vane 40 is provided for separating the ceramic sheets. The center vane includes a corrugated center portion 42 and a corrugated outer portion 44. The corrugations 42 can be circular or spiral and are substantially concentric with the center of the center vane. The outer portion of center vane 40 has corrugations 44 which are radial or at right angles to the corrugation of the center portion.

A sectional view of the structure of FIG. 3 is shown in FIG. 4. The same parts of the structure have the same reference numerals. Ceramic sheet 34 having

electrodes

37 and 46 thereon isseparated from ceramic sheet 35 having

electrodes

36 and 47 thereon by center vane 40. Center vane 40 has a corrugated center portion 42 and a corrugated outer portion 44. The corrugations of the outer portions are at right angles to the corrugations of the inner portion.

The corrugated inner portion has a plurality of apices, designated 49, which are in mechanical and electrical contact with

electrodes

36 and 46. Thus the metallic center vane 40 provides an electrical

path connecting electrodes

36 and 46. The corrugated outer portion 44 includes corrugations having apices, designated 50, which are also in electrical and mechanical contact with

electrodes

36 and 46. Since

apices

49 and 50 are in mechanical contact with

electrodes

36 and 46, the

ceramic sheets

34 and 35 are precisely spaced by the center vane. In contrast to this the spacing of the

ceramic sheets

20 and 21 of FIG. 2 is determined by the thickness of center vane 29 and the thickness of the epoxy layers 30 and 31. Since the thickness of the epoxy layers 30 and 31 can vary greatly during manufacture the spacing of

ceramic sheets

20 and 21 can also vary.

The

apices

49 and 50 are bonded to the

electrodes

46 and 36, and thereby the

ceramic sheets

34 and 35, by a nonconductive epoxy shown as 52. The epoxy can be nonconductive since the electrical conductivity is provided by the center vane itself. Furthermore, since the epoxy is only required where the apices contact the electrodes there is no problem with air bubbles forming in the epoxy. With the bender bimorph transducer structure of FIG. 4 there is less likelihood of getting too little or too much epoxy resin in the structure and therefore the coupling factor can be increased and the losses caused by the mechanical structure reduced.

The corrugations of the inner portion 42 act as a stiff hinge permitting movement of

ceramic sheets

34 and 35 relative to each other. The corrugations of the outer portion 44 hold the edges of the

ceramic sheets

34 and 35 in rigid relationship to each other. This center vane structure for a bender bimorph transducer results in a decrease in losses and an increase in the coupling factor.

In FIG. there is shown a rectangular version of the center vane structure of FIGS. 3 and 4. A crosssectional view of FIG. 5 would also have the same appearance and description as FIG. 4. In FIG. 5 a ceramic sheet 54 having electrode 56 on one side thereof and another electrode on the other side, not shown, is spaced apart from ceramic sheet 55 by means of center vane 59. Ceramic sheet 55 has an electrode 57 thereon and another electrode on the opposite side of the sheet, not shown. Center vane 59 includes a corrugated center portion 61 and a corrugated outer portion 59. The corrugations in the outer portions 59 are at right angles to the corrugations of the inner portion 61 in order to give sufficient rigidity to the outer portions while the inner portions are permitted to move as required by the operation of the bender bimorph transducer. The apices of the corrugations in the inner portion 61 and the outer portion 59 are cemented to the

ceramic sheets

54 and 55 by a nonconductive epoxy cement as shown in FIG. 4.

Although the transducer structure is described above with reference to a bimorph transducer, it should be recognized that the above-described features will be found of great advantage in any type of transducer employing a pair of transducer elements which produce a concave-convex shape in response to the application of an electronic signal. The ceramic sheets, 34 and 35, in FIG. 3, which are the transducer elements of the bimorph transducer structure, can be replaced by any magnetostrictive, piezoelectric, electrostrictive transducer having like concave-convex characteristics in the presence of electronic signals. These new structures will then exhibit the same improved characteristics described in the above bimorph transducer structure.

If a less efficient form of transducer is required, that is, a transducer which is not required to produce as great a mechanical response to electronic signals as those described above, a monolaminar rather than a bilaminar structure can be provided. In a bilaminar structure such' as those described above, two'transducer elements, such'as

ceramic sheets

34 and 35, in FIG. 3, are required. In a monolaminar structure, one of the two ceramic transducer sheets, as for example, sheet 35 is replaced by a rigid sheet which does not have magnetostrictive piezoelectric or electrostrictive properties. In order to function properly, the replacement sheet should have a high stiffness factor, a low mass, and a thermal expansion characteristic like the transducer element in the transducer. In the case of a structure such as shown in FIG. 3, ceramic elements 35 can be replaced by a metal sheet made of nickel. To simply construction and operation of the transducer the material used in place of a transducer element should have good electrical conduction characteristic to allow application of electronic signals to it rather than to the inner electrode of the transducer.

Thus an improved transducer such as a bender bimorph structure has been shown. The transducer structure incorporates a center vane having a corrugated center portion and a corrugated outer portion. The outer portion has its corrugations at an angle to the corrugations of the inner portion. The apices of the corrugations are in direct contact with the electrodes of the sheets forming the transducer to provide electrical conductivity therebetween. Nonconductive epoxy can be used to cement the apices to each of the sheets thus eliminating the requirement for conductive epoxy. This structure acts to reduce mechanical losses and increase the coupling factor. It provides precise spacing between the sheets and the bender bimorph transducer structure is more easily manufactured.

I claim:

1. A bender bimorph including in combination, a pair of piezoelectric elements each having a pair of flat opposing faces with electrodes thereon, resilient electrically conductive means substantially the same size as said piezoelectric elements and having a plurality of raised portions on each side thereof, said conductive means being positioned between said pair of piezoelectric elements with each of said electrodes having said plurality of raised portions in electrical and mechanical contact therewith, said conductive means acting to permit movement of said piezoelectric elements relative to each other, and cementive material bonding only said raised portions to said electrodes in contact therewith.

2. The bender bimorph of claim I wherein said cementive material is nonconductive.

3. The bender bimorph of claim 1 wherein said conductive means is corrugated with successive apex portions on alternate sides thereof forming said raised portions, said apex portions being in electrical contact with said electrodes and being mechanically fastened thereto by said cementive material.

4. The bender bimorph of claim 3 wherein said flat opposing faces of said piezoelectric elements are circular in shape, said conductive means is circular in shape with said corrugations being in the form of a spiral.

5. The bender bimorph of claim 3 wherein said flat opposing faces of said piezoelectric elements are rectangular in shape and said conductive means is rectangular in shape.

6. The bender bimorph of claim 1 wherein said conductive means includes a center portion and at least one edge portion, said center portionof said conductive means being corrugated with successive apex portions on alternate sides thereof, said apex portions being in electrical contact with said electrodes and mechanically fastened thereto by said cementive material, said conductive means being positioned to hold said piezoelectric elements in parallel relationship and further said corrugations being formed to permit movement of said piezoelectric elements relative to each other, said edge portion being corrugated with successive apex portions on each side thereof, said apex portions of said edge portion being mechanically fastened to said piezoelectric element by said cementive material, said apex portions of said edge portion further being positioned at an angle to said apex portions of said center portions whereby said edge portion is mechanically stiff in the direction-of said movement to substantially prevent the same.

7. A bender bimorph, including in combination, a pair of flat circular piezoelectric elements having opposing faces with electrodes thereon, a circular center vane positioned between said pair of piezoelectric elements and acting to space the same apart in parallel relationship, said center vane having a corrugated center portion with successive apex portions on alternate sides thereof, said apex portions of said center portion being in electrical contact with said electrodes, said corrugations further being in the form of a spiral and being formed to be resilient in the direction of parallel movement of said piezoelectric elements relative to each other to permit said parallel movement, said center vane further having a corrugated edge portion with successive apex-portions on alternate sides thereof, said apex portions of said edge portion being positioned at substantially right angles to said apex portion of said center portions, whereby said edge portion is mechanically stiff in the direction of said parallel movement to substantially prevent the same, said apex portions of said edge portion being in contact with said electrodes, and cementive material bonding only said apex portions of said center and edge portions to said electrodes.

8. The bender bimorph of claim 7 wherein said cementive material is a nonconductive epoxy cement.

9. A bender bilayer piezoelectric device, including in combination, a pair of piezoelectric elements each having a pair of fiat opposing faces with electrodes thereon, a resilient electrically conductive center vane substantially the same size as said piezoelectric elements, positioned between said pair of piezoelectric elements and acting to space the same apart in parallel relationship, said center vane having a corrugated center portion with successive apex portions on alternate sides thereof, said apex portion of said center portion being in electrical contact with said electrodes, and resilient in the direction of parallel movement of said piezoelectric elements relative to each other to permit said parallel movement, said center vane further having a corrugated edge portion with successive apex portions on alternate sides thereof, said apex portions of said edge portion being positioned at an angle to said apex portion of said center portion and in contact with said electrodes, to provide a mechanically stiff connection between said electrodes in the direction of said parallel movement to substantially prevent the same, and cementive material bonding only said apex portions of said center and edge portions to said electrodes.

10. A transducer including in combination, a first element deformable in response to an applied electronic signal to manifest a concavo-convex shape, a second element having substantially the same size and shape as said first element positioned parallel to said first element, resilient means substantially the same size as said first and second elements and having a plurality of raised portions on each side thereof, said resilient means being positioned between said first and second elements with said plurality of raised portions in mechanical contact therewith, said resilient means acting to permit movement of said first and second elements relative to one another, said cementive material bonding only said raised portions to the first and second elements in contact therewith.

11. The transducer of claim 10 wherein said first element is an electromechanical transducer selected from the group consisting of magnetostrictive, piezoelectric or electrostrictive transducers.

12. The transducer of claim 11 wherein said resilient means is corrugated metal with successive apex portions on alternate sides thereof forming said raised portions, said apex portions being in mechanical contact with said first and second elements and being mechanically fastened thereto by said cementive material.

13. The transducer of claim 11 wherein said resilient means is corrugated metal having a center portion and at least one edge portion, said center portion of said resilient means being corrugated with successive apex portions on alternate sides thereof, said apex portions being in mechanical contact with said elements and mechanically fastened thereto by said cementrve material, said resilient means being positioned to hold said elements in parallel relationship and further said corrugations being formed toperrnit movement of said elements relative to each other, said edge portion being corrugated with said successive apex portions on each side thereof, said apex portions of said edge portion being mechanically fastened to said elements by said cementive material, said apex portions of said edge portion further being positioned at an angle to said apex portions of said center portion whereby said edge portion is mechanically stiff in the direction of movement to substantially prevent the same.

14. The transducer of claim 12 wherein said first element includes, a pair of fiat parallel faces with electrodes thereon, said electrodes coupling said applied electronic signals thereto to produce said concavo-convex deformations.

15. The transducer of claim 14 wherein said resilient means is conductive and said second element is electrically conductive, said electronic signals being applied to said second element and an electrode of said first element spaced apart from said resilient means to produce said concavo-convex deformations.

16. The transducer of claim 15 wherein said second element is an electromechanical transducer substantially the same as said first element.

17. The transducer of claim 15 wherein said second element is a conductive metal of the same size and shape as said first element, and has stiffness and expansion characteristics substantially similar to said first element.

18. A transducer including in combination, a pair of substantially identical transducer elements operable in response to an applied electronic signal to produce a concave-convex deformation, said elements each having a pair of flat opposing electrically conducting faces, resilient electrically conductive means substantially the same size as said pair of elements and having a plurality of raised portions on each side thereof, said conductive means being positioned between said pair of elements with said plurality of raised portions of said conductive means being in electrical and mechanical contact therewith, said conductive means acting to permit movement of said elements relative to each other, and cementive material bonding only said raised portions to said electrically conducting faces in contact therewith.

19. The transducer of claim 18 wherein said transducer elements are selected from the group consisting of magnetostrictive, piezoelectric or electrostrictive transducers.

20. The transducer of claim 19 wherein said resilient means is corrugated metal with successive apex portions on alternate sides thereof forming said raised portions, said apex portions being in electrical and mechanical contact with said elements and being mechanically fastened thereto by said cementive material.

21. The transducer of claim 19 wherein said resilient means is corrugated metal having a center portion and at least one edge portion, said center portion of said resilient means being corrugated with successive apex portions on alternate sides thereof, said apex portion being in electrical and mechanical contact with said flat faces of said elements and mechanically fastened thereto by said cementive material, said resilient means being positioned to hold said elements in parallel relationship and further said corrugations being formed to permit movement of said elements relative to each other, said edge portion being corrugated with successive apex portions on each side thereof, said apex portions of said edge portion being mechanically fastened to said elements by said cementive material, said apex portions of said edge portion further being positioned at an angle to said apex portions of said center position whereby said edge portion is mechanically stiff in the direction of said movement to substantially prevent the.

same.

Claims

2. The bender bimorph of claim 1 wherein said cementive material is nonconductive.

6. The bender bimorph of claim 1 wherein said conductive means includes a center portion and at least one edge portion, said center portion of said conductive means being corrugated with successive apex portions on alternate sides thereof, said apex portions being in electrical contact with said electrodes and mechanically fastened thereto by said cementive material, said conductive means being positioned to hold said piezoelectric elements in parallel relationship and further said corrugations being formed to permit movement of said piezoelectric elements relative to each other, said edge portion being corrugated with successive apex portions on each side thereof, said apex portions of said edge portion being mechanically fastened to said piezoelectric element by said cementive material, said apex portions of said edge portion further being positioned at an angle to said apex portions of said center portions whereby said edge portion is mechanically stiff in the direction of said movement to substantially prevent the same.

7. A bender bimorph, including in combination, a pair of flat circular piezoelectric elements having opposing faces with electrodes thereon, a circular center vane positioned between said pair of piezoelectric elements and acting to space the same apart in parallel relationship, said center vane having a corrugated center portion with successive apex portions on alternate sides thereof, said apex portions of said center portion being in electrical contact with said electrodes, said corrugations further being in the form of a spiral and being formed to be resilient in the direction of parallel movement of said piezoelectric elements relative to each other to permit said parallel movement, said center vane further having a corrugated edge portion with successive apex portions on alternate sides thereof, said apex portions of said edge portion being positioned at substantially right angles to said apex portion of said center portions, whereby said edge portion is mechanically stiff in the direction of said parallel movement to substantially prevent the same, said apex portions of said edge portion being in contact with said electrodes, and cementive material bonding only said apex portions of said center and edge portions to said electrodes.

9. A bender bilayer piezoelectric device, including in combination, a pair of piezoelectric elements each having a pair of flat opposing faces with electrodes thereon, a resilient electrically conductive center vane substantially the same size as said piezoelectric elements, positioned between said pair of piezoelectric elements and acting to space the same apart in parallel relationship, said center vane having a corrugated center portion with successive apex portions on alternate sides thereof, said apex portion of said center portion being in electrical contact with said electrodes, and resilient in the direction of parallel movement of said piezoelectric elements relative to each other to permit said parallel movement, said center vane further having a corrugated edge portion with successive apex portions on alternate sides thereof, said apex portions of said edge portion being positioned at an angle to said apex portion of said center portion and in contact with said electrodes, to provide a mechanicaLly stiff connection between said electrodes in the direction of said parallel movement to substantially prevent the same, and cementive material bonding only said apex portions of said center and edge portions to said electrodes.

13. The transducer of claim 11 wherein said resilient means is corrugated metal having a center portion and at least one edge portion, said center portion of said resilient means being corrugated with successive apex portions on alternate sides thereof, said apex portions being in mechanical contact with said elements and mechanically fastened thereto by said cementive material, said resilient means being positioned to hold said elements in parallel relationship and further said corrugations being formed to permit movement of said elements relative to each other, said edge portion being corrugated with said successive apex portions on each side thereof, said apex portions of said edge portion being mechanically fastened to said elements by said cementive material, said apex portions of said edge portion further being positioned at an angle to said apex portions of said center portion whereby said edge portion is mechanically stiff in the direction of movement to substantially prevent the same.

14. The transducer of claim 12 wherein said first element includes, a pair of flat parallel faces with electrodes thereon, said electrodes coupling said applied electronic signals thereto to produce said concavo-convex deformations.

18. A transducer including in combination, a pair of substantially identical transducer elements operable in response to an applied electronic signal to produce a concavo-convex deformation, said elements each having a pair of flat opposing electrically conducting faces, resilient electrically conductive means substantially the same size as said pair of elements and having a plurality of raised portions on each side thereof, said conductive means being positioned between said pair of elements with said plurality of raised portions of said conductive means being in electrical and mechanical contact therewith, said conductive means acting to permit movement of said elements relative to each other, and cementive material bonding only said raised portions to said electrically conducting faces in contact therewith.

21. The transducer of claim 19 wherein said resilient means is corrugated metal having a center portion and at least one edge portion, said center portion of said resilient means being corrugated with successive apex portions on alternate sides thereof, said apex portion being in electrical and mechanical contact with said flat faces of said elements and mechanically fastened thereto by said cementive material, said resilient means being positioned to hold said elements in parallel relationship and further said corrugations being formed to permit movement of said elements relative to each other, said edge portion being corrugated with successive apex portions on each side thereof, said apex portions of said edge portion being mechanically fastened to said elements by said cementive material, said apex portions of said edge portion further being positioned at an angle to said apex portions of said center position whereby said edge portion is mechanically stiff in the direction of said movement to substantially prevent the same.