A sheet of cold rolled material having on both of its surfaces rows of projections and rows of depressions, the projections on one surface corresponding with the depressions on the other surface, the relative positions of the projections and depressions being such that lines drawn on a surface of the sheet between adjacent rows of projections are non-rectilinear, the sheet having a base gauge G, wherein each projection has a substantially continuous region of peak plastic strain at, toward or about its apex and/or is thinned by no more than 25% of its base gauge G. Methods of forming the sheet material and tools for forming the sheet material are disclosed.
1. A method of forming sheet material, the method comprising providing a sheet material having a base gauge G, providing a pair of opposed tools having rows of teeth on their outer surface, placing the sheet material between the tools and moving the tools such that rounded sheet engaging surfaces of the teeth on one tool urge portions of the sheet material into gaps between the teeth on the other tool to form projections on both surfaces of the sheet material, wherein the relative positions of the projections and corresponding depressions on the surfaces is such that lines drawn on a surface of the sheet between adjacent rows of projections are non-rectilinear, and wherein the projections have a substantially continuous region of peak plastic strain at, toward or about their apex and are thinned by no more than 25% of its base gauge G.
2. A method as claimed in claim 1 , comprising urging the material such that the apex or peak of the projections are free from contact with the other tool during forming.
3. A method as claimed in claim 1 , comprising subjecting the sheet material to a plastic strain of 0.05 or more across at least 65% of the formed area thereof.
4. A method as claimed in claim 1 , comprising subjecting the sheet material to a plastic strain of 0.05 or more across at least 80% of the formed area thereof.
5. A method as claimed in claim 1 , comprising subjecting the sheet material to a plastic strain of 0.05 or more across 90% to 100% of the formed area thereof.
6. A method as claimed in claim 1 , wherein the clearance between the teeth on one tool and the teeth on the other tool, during forming, is at least equal to the base gauge G of the plain sheet material.
7. A method as claimed in claim 6 , wherein the clearance is at least 1.1 times the base gauge G of the plain sheet material.
8. A method as claimed in claim 1 , comprising arranging the tools such that the teeth extend, during forming, into the gaps with each tooth from one tool being equidistantly spaced from each adjacent tooth from the other tool.
9. A method as claimed in claim 1 , comprising forming a material having a base gauge G of 2 mm or greater.
10. A method as claimed in claim 1 , comprising rotating the opposed tools about parallel axes.
11. A method as claimed in claim 1 , further comprising the step of shaping the formed sheet material into a profile.