The concept of deforming material to increase its rigidity is well known, and many examples can be found, such as corrugated roofing and sandwich construction cardboard. In the industry of cold rolled metal sections, to which the present invention is particularly relevant, there are many examples of processes which introduce deformations into the starting material in order to improve the material properties. This idea manifests itself in many ways, including continuous ribs along an elongate section, or dimples arrayed in various ways. The introduction of deformations into metal sheet or strip has the added benefit of locally increasing the strength of the material through work hardening. This improved strength has two main attractions: firstly it improves the screw retention properties of the material; secondly, it improves the overall yield strength and buckling resistance. However, there are two main limitations to the present methods of manufacture.

The first limitation relates to the degree of rigidity that the processes introduce into the material. Any increase in material rigidity is solely due to the geometric effects of deforming. Therefore any cross-sectional axis that can be taken through the material where the degree of deformation is reduced or absent, is therefore relatively less rigid than the rest of the material. The overall rigidity of the material is dictated by how uniform this deformation is, regardless of where the cross-section is taken. For example, Figure 1 shows, as prior art, a piece of material which has orthogonally arranged deformations 1 (i. e. dimples on a square array, which is the most common), where every row or column of dimples is separated by an element of material that is substantially less deformed, if at all. The material about an axis 2 taken through this area is much less rigid than the dimpled areas 3, which compromises the achievable overall rigidity. A continuous rib 4 or grooved pattern on the other hand, as shown for the piece of prior art material shown in Figure 2, ensures that any cross-section perpendicular to the direction of the ribs, as at 5, will always indicate the maximum deformation of material.

Therefore in this direction the material can be considered'optimally'rigid for that given degree of deformation. However, a cross-section taken parallel to the rib direction, as at 6, will not show any evidence of deformation and therefore in this direction the material is effectively only as rigid as the undeformed material i. e. much less than the deformed material.

In this respect conventional ribbing produces highly anisotropic material, which in some cases where only one axis is of concern, can be quite acceptable.

The second limitation with conventional processing relates to the degree of improvement in the strength of the material. During the deformation process a certain amount of work hardening occurs. This has no effect whatsoever on the rigidity of the material but it does increase the strength. The limitation with conventional processing is that since not all of the material is deformed, only a proportion of the material will have its strength increased.

For example the strips on axis 2 in the orthogonal dimple pattern between the dimples will be undeformed and therefore the strength will be the same as the original material. Therefore if a screw is inserted in the material, it would effectively remove the area of greatest work hardening (as the screw generally self-taps into the dimple), and the screw location is surrounded by material that has not been effectively work hardened and thus does not comprise the retention properties. It could be said that the most important area to be strengthened has been missed. With regard to the buckling and yield strength of the overall material, these locally unhardened areas will fundamentally undermine any potential improvement.

One object of the invention is to provide a method of processing sheet and strip material which improves the strength thereof. Another object is to provide a method of processing sheet and strip material which improves the rigidity thereof.

According to one aspect of the invention, a method of processing sheet or strip material comprises two separate steps, one step comprising hardening the material, thereby to increase the strength thereof, and the other step concerning deforming the material, thereby to increase the rigidity thereof.

Desirably said one step is work hardening, and precedes said other, rigidifying, step.

According to another aspect of the invention a method of processing sheet or strip material comprises producing in the material at least two continuous or discontinuous elongate deformations, each deformation having a longitudinal axis/centre line and extending to opposite sides thereof to define at each side an outer linear boundary, with at least part of one of said at least two deformations extending across an outer linear boundary of the other or another of said at least two deformations next thereto.

Thus with said another aspect of the invention deformation is present in both longitudinal and transverse cross-sections through the processed material, and preferably as deformation is present in all cross-sections, the processed material exhibits some degree of isotropy.

According to a still further aspect of the invention, a method of processing sheet or strip material comprises a combination of said one and said another aspects of the invention.

The invention also relates to strip or sheet material processed by any one of said above-mentioned aspects of the invention as well as strip material in that form produced by any other method.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1 and 2 are respective fragmentary perspective views of prior art sheets which have been deformed to increase the rigidity thereof; Figure 3 is a fragmentary perspective view of sheet or strip material deformed according to one aspect of the present invention; Figure 4 is a fragmentary plan view of sheet or strip material deformed according to a further embodiment of said one aspect of the invention; Figure 5 is a fragmentary plan view of sheet or strip material deformed according to a still further embodiment of said one aspect of the invention Figure 6 is a schematic plan view of one deformation in sheet or strip material, showing its outer linear boundaries; Figure 7 is a similar schematic view to Figure 6, showing how a pair of adjacent deformations'overlap' ; Figure 8 is a view like Figure 3, showing another form of such sheet or strip material; and Figure 9 shows the sheet or strip of Figure 8 in a panel of sandwich construction.

One aspect of the present invention relates to a method of processing sheet or strip material that results in a considerable improvement in both rigidity and strength. For the purpose of clarity, the invention will hereinafter be described in the context of cold rolled metal sections, although the described concept of rigidifying is equally applicable to any sheet or strip material, for example from cladding and sandwich construction cardboard, to advanced aerospace materials such as titanium and carbon fibre.

Two important factors have been recognised by the inventor. Firstly that the rigidity of the material is solely a function of the material geometry (for a given material) which can only be altered by deforming. Secondly that the strength is solely a function of the material modulus which can only be altered by introducing hardness (through work hardening, induction hardening etc.). Therefore any deformations which are intended to introduce strength into the material through work hardening, do not necessarily need to be evident in the final material. (Once the work hardening has been introduced, only annealing will remove it).

It is recognised that because of this independence, the optimum deformation pattern for rigidity is therefore highly unlikely to yield optimum strength and therefore, in one aspect of the invention, it is proposed that a separate stage is introduced prior to the'rigidising'process with the sole purpose of selectively introducing increased strength into the material. This would typically involve deforming 100% of the material which could require at least two pairs of sequential rolls in order to deform all the material. It would only be necessary to deform the material up to about 85 % of its original thickness as beyond that, there is relatively little contribution to the strength of the material and the chances of damaging the materai increase significantly.

If the chosen method of increasing strength is to work harden, for example by knurling, then this stage would have to precede the rigidising process.

Otherwise it would be impossible to maintain the rigidising deformations. If non-contact methods were used (such as induction or laser hardening) then as well as being able to precede the rigidising process, this method could follow it with no effect on the rigidity.

This separation of the strengthening process allows the rigidising process to be optimise in the knowledge that the desired improvement in material strength has been catered for. To combine the best features of rib deformation continuity and dimple isotropy, it is proposed in another aspect of the invention, to introduce deformations, which in a preferred embodiment are substantially continuous and characterised in that their general directions are locally non-uniform. They are also sufficiently close such that no cross- section can be taken through the material without evidence of significant deformation. In the broadest form of said another aspect of the invention an arbitrary cross-section cannot be taken through the material without evidence of such deformation. In said one aspect of the invention, the rigidifying step can be the method of said another aspect of the invention, or alternatively some other rib or knurl (dimple) producing process, e. g. stamping.

The most elegant embodiment of this concept is where all or part of the sheet surface is deformed with a plurality of continuous but waved ribs 14, as shown in Figure 3. The deformation pattern can be described in terms of its cross-sectional properties, i. e. its pitch 7, depth 8 and waveform (not limited to, but shown here as broadly triangular), and also its plan view properties i. e. wavelength 9 and pitch 10 in the plane of the material. A cross-section 11 taken perpendicular to the general direction of the ribs, will be identical to that of parallel straight ribs and consequently, so will the material rigidity.

Considering the axis parallel to the general direction of each rib, then provided that adjacent ribs are sufficiently close and that they interlink to a sufficient degree, any cross-section taken parallel to their general direction 12, will resemble the cross-section 11 taken perpendicular to it. As stated, the cross-sectional waveform need not be triangular and could thus, for example, be square. It will be appreciated that a rib 14 in one side of the sheet produces a corresponding groove 14a in the other side of the sheet.

If the pattern geometries are chosen carefully (in particular the wavelength 9 in the plane of the material) then it is possible to make the perpendicular cross-sections substantially the same. The resultant material therefore contains a high degree of isotropy and is therefore very suitable for sheet applications such as cladding, roofing, sandwich construction cardboard etc.

The potential isotropy and uniformity of the material deformation provide significant advantages over conventional rib and dimple processes.

One implication is that the wavelength 9 of the ribs will have a bearing on the amount of length by which the original material is reduced during the processing (as the ribbing process gathers in the material). Therefore consideration has to be given to the degree of rigidity one wishes to'buy'in the direction parallel to the ribs for a given application. It would be possible selectively (i. e. one axis and not the other, or both) to stretch the ribs into deformation, rather than gathering. This therefore maintains the dimensions of the original strip at the expense of the thickness of the material when formed.

The ribs need not be continuous, providing that there is sufficient 'overlapping'of the ribs to ensure that any arbitrary cross-section exhibits the deformation referred to. Figure 4 shows an example of such material.

Indeed if the ribs 14 are discontinuous, this provides for the possibility of deforming the material until it splits at the extremities of deformation as shown in Figure 5 to form perforations 13. Whilst not significantly affecting the structural performance, the perforations 13 reduce the heat-transfer across the section and could also act as ventilation slots. The improved'cold- bridging'performance of such a material would have advantages in the construction industry.

Figure 6 shows the outer linear boundaries 15,16 of a deformation (in this case a groove 14a) at respective opposite sides of its longitudinal axis/centre line 17. Figure 7 shows how an identical, adjacent/neighbouring groove 14b overlaps groove 14a to provide the avantage of the invention. It can be seen that the'peaks'of sinusoidal groove 14b at one outer linear boundary 18 thereof extend across the adjacent linear boundary 16 of groove 14a, and in a full sheet or strip this overlapping is present throughout, as for the sheets/strips shown in Figure 3 (V-shaped grooves), Figure 7 (arcuate grooves) and Figure 8 (channel-shaped grooves).

Although with the deformed sheets/strips shown in Figures 3 and 8 each pair of adjacent grooves are separated solely by a corresponding rib, it can be seen from Figure 7 that it is also possible for each pair of adjacent grooves at one side of the material (and for each pair of corresponding adjacent ribs at the other side of the material) to be separated by something other than its corresponding inverse. In Figure 7 there is an undeformed flat section 19 between the adjacent grooves 14a, 14b.

The overlapping of adjacent grooves/ribs explained with reference to Figure 7 is equally applicable to the Figures 4 and 5 arrangements where the elongate deformations, i. e. ribs or grooves, are discontinuous. Clearly also with this arrangement it is possible to identify the outer linear boundaries of each deformation and to appreciate that part of an adjacent deformation extends across one such boundary.

In Figure 8, the grooves 20 of the sheet 21 are of channel-shape in cross- section, having a flat base part and sloping channel walls. This deformed sheet is particularly suitable for use as the core of a sandwich-type panel 22 shown in Figure 9, the panel having the sheet 21 between outer flat sheets 23,24 respectively. This panel has corrugations in all directions, unlike conventional fluted corrugated panels.

The rigidifying step can be embossing, which covers rolling, stamping or pressing, or in the case of plastics material could be moulding or vacuum forming etc.

Accordingly in summary the rigidity of sheet or strip material is increased in a preferred embodiment through the introduction of elongate or continuous deformations which are not uniform in their general direction and are of sufficiently close proximity to ensure that there are no inherent axes of weakness in the material. Consequently the material attains a degree of uniformity and isotropy not found with other processes. The properties of the sheet or strip material may further be enhanced through the use of a separate process stage, which when used prior to or after the deformation (rigidifying) stage, introduces a higher degree of strength into the material than just the deformation (rigidifying) stage alone.